Post on 06-Jul-2018
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 1/51
..
GEOTECHNICAL SUB-SURFACE INVESTIGATION
AND FOUNDATION RECCOMENDATION REPORT
FOR
G+4 and G+7 CONDOMINIUM BUILDINGS
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 2/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4and G+7 condominium Building
BEST Consulting Engineers plc April 2014
TABLE OF CONTENTS
Page
1 INTRODUCTION ................................................................................................... 4
1.1 Background ........................................................................................................4
1.2 Scope of Work and Objective..............................................................................4
1.3 Location .............................................................................................................4
2 METHODOLOGY .................................................................................................. 6
2.1 Rotary Core Drilling ............................................................................................6
2.2 In-situ Tests ........................................................................................................6
2.3 Sampling ............................................................................................................6
2.3.1 Disturbed Soil Sampling .........................................................................6
2.3.2 Undisturbed Soil Sampling .....................................................................6
2.4 Laboratory Testing..............................................................................................7
2.5 Ground water monitoring...................................................................................8
3 GEOLOGIC AND SEISMIC SETTINGS OF THE AREA ............................................... 9
3.1 Regional and Site Geology ..................................................................................9
3.1.1 Regional Geology ...................................................................................9
3.1.2 Subsurface/Site Geology ...................................................................... 10
3.2 Regional Seismicity of the area ......................................................................... 10
3.2.1 Country seismicity overview................................................................. 11
3.2.2 Region seismicity overview .................................................................. 12
4 GEOTECHNICAL INVESTIGATION AND LABORATORY TESTING .......................... 13
4.1 Introduction ..................................................................................................... 13
4.2 Summary of the Geotechnical Investigation...................................................... 14
4.3 Geotechnical characterization of the subsurface material................................. 15
4.4 In situ Field Testing........................................................................................... 16
4.5 Sampling .......................................................................................................... 20
4.5.1 Disturbed Samples ............................................................................... 214.5.2 Undisturbed Samples........................................................................... 21
4.6 Laboratory Testing............................................................................................ 21
4.7 Ground water Level Measurement ................................................................... 26
4.8 Damage due to expansive soils ......................................................................... 26
4.8.1 Mitigation measures ............................................................................ 27
4.8.2 Treatment of Expansive Soils................................................................ 27
5 FOUNDATION ANALYSIS ................................................................................... 28
5.1 Introduction ..................................................................................................... 28
5.2 Isolated Foundation.......................................................................................... 28
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 3/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4and G+7 condominium Building
BEST Consulting Engineers plc April 2014
5.2.1 Bearing Pressure Based on UCS............................................................ 28
5.2.2 Bearing Pressure Based on SPT N-Values for Isolated square Footing ... 30
5.3 Allowable Bearing capacity for Mat Foundation................................................ 37
5.4 Bearing Capacity using Settlement Criterion ..................................................... 38
6 CONCLUSION AND RECOMMENDATION ........................................................... 41
6.1 Subsurface geotechnical materials .................................................................. 41
6.2 Foundation seat and allowable bearing Capacity ............................................ 41
6.3 Material for backfill and compaction criteria .................................................. 43
6.4 Settlement consideration ................................................................................ 43
6.4.1 Seismic Consideration.......................................................................... 44
6.5 Considerations to Minimize Expansion Effect .................................................. 44
6.6 Other Consideration ........................................................................................ 45
REFERENCES ............................................................................................................ 46
LIST OF TABLES
Table 3-1: Seismic Hazard Rating (Gouin, 1976) ................................................................... 12
Table 4-1: Coordinate and Depth of Boreholes .................................................................... 13
Table 4-2: Summary of the Geotechnical Investigations Carried Out ................................... 14
Table 4-3: Distribution of the geotechnical layers in the boreholes...................................... 15
Table 4-4: Standard Penetration Test Results ...................................................................... 16
Table 4-5: Summary of UCS Test Result of undisturbed soil Samples.................................... 21
Table 4-6: Laboratory test results of disturbed soil samples................................................. 22
Table 4-7: Swelling pressure test result ............................................................................... 26
Table 4-8: Hydrometer analysis results on selected disturbed soil samples.......................... 26Table 5-1: Allowable Bearing Pressure Based on UCS Test Result of Soil Samples for Layer2.29
Table 5-2: Measured and adjusted SPT N values .................................................................. 31
Table 5-3: Allowable Bearing Pressures Based on SPT N-Value............................................. 36
Table 5-5-4: Allowable Bearing Pressures for Mat foundation Around G+7 Buildings ........... 38
Table 5-5: Bearing Pressure analysis using settlement criteria around BH-150..................... 40
Table 6-1: Summary of bearing capacity for Mat foundation ............................................... 42
APPENDICES
Appendix 1: Borehole Logs
Appendix 2: Laboratory Test Result
Appendix 3: Allowable Bearing Pressure Analysis Sheets
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 4/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
4
1 INTRODUCTION
1.1 Background
BEST Consulting Engineers private limited company has made a contract agreement with
Addis Ababa Housing Construction office to perform geotechnical investigations and provide
foundation recommendations for G+4 and G+7 Condominium buildings for Koye Feche
Project III Building site.
The building site is subdivided into different Parcels; accordingly twenty four boreholes (BH-
131 to BH-155) were sunk in Parcel 26 building area to a maximum depth of 10.0 meters and
15.0 meters below the natural ground for G+4 and G+7 buildings respectively.
The geotechnical investigations comprises of core drilling, in-situ tests such as Standard
Penetration Tests (SPT), monitoring of ground water, collection of representative samples,
and subsequent laboratory tests on representative samples to determine the engineering
properties of the sub-surface materials. Moreover, the coordinates of each borehole was
provided by the client and the ground elevation data were acquired using hand held GPS.
The field investigation was conducted from March 23 to March 24, 2013.
This report deals with the regional geology, site geology, methodology employed, laboratory
tests conducted to determine the engineering properties of the subsurface strata including
analyses and interpretation of test results. This report also encompasses foundation
recommendation including type of foundation, bearing layer, foundation depth, and
allowable bearing pressure for Parcel 26 building area.
1.2 Scope of Work and Objective
The scope of the geotechnical investigations include core drilling, in-situ tests, collection of
representative samples, subsequent laboratory testing, and ground water monitoring. The
prime objectives of the investigation are:-
a) To investigate the sub-surface geology of the proposed construction site and
identify the various soil horizons within the influence zone of foundation.
b) To carry out in-situ tests to determine the strength of the various soil horizons
within the influence zone of foundation.
c) To collect representative samples (disturbed and undisturbed) for subsequent
laboratory tests to determine the engineering properties.
d) To characterize the sub-surface materials into various geotechnical layers based
on combined parameters such as, visual description of soils/rocks, in-situ tests,and laboratory test results.
e) To provide safe and economic foundations, that is, type of foundation, bearing
layer, depth and width of foundation, and allowable bearing capacity.
1.3 Location
The project site is located in Addis Ababa, Akaki Kaliti Sub-City, around Koye Feche locality.
The project site is characterized by flat to rolling ground with an average elevation of 2205m
a.s.l.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 5/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
5
Figure 1-1: Location map of the condominium site
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 6/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
6
2 METHODOLOGY
2.1 Rotary Core Drilling
Rotary core drilling was employed using wire line rig having the capacity to perform boring
operation to the required standard and quality in accordance with ASTM D 2113 – 93, ASTM
D 1452 – 80 (95), and BS 5930: 1981.Dry drilling method was employed in soil formations using inner lining single core barrels
fitted with appropriate size tungsten carbide bits at the bottom. This will enable the drilling
to achieve good quality core recovery. In rocky section, double core barrel fitted with
diamond bit was utilized. Water was pumped down to the bit through hollow drill rods to
cool the bit and flushing the cuttings up the borehole.
Equipments to conduct in-situ tests and sampling such as SPT apparatus including split
spoon sampler, Shelby Tubes, water pump, rods, casings, and a wide range of heavy-duty
tools were used during the drilling operations. An electric water meter was utilized in
monitoring the ground water level.
Materials recovered from the boreholes were placed in core boxes, labeled, logged andphotographed by digital camera according to their depths of recoveries. Core boxes were
stored in a safe place and carefully transported to BEST Consulting Engineers Plc central
laboratory. The core box samples will be kept for the next six (6) months and then will be
disposed if the client didn’t inform the company.
2.2 In-situ Tests
Standard Penetration Test (SPT) was conducted using a standard hammer, under an impact
of an automatic sliding hammer weighing 63.5kg falling freely from a height of 760mm in
accordance with ASTM D 1586 – 99 and BS 5930: 1981. The test was carried out starting
from 1.50m depth below natural ground level (NGL).
Blow counts for a total penetration depth of 450 mm from the bottom of a cleaned boreholewere recorded. Counts for the first 150 mm penetration were discarded since the ground is
considered to be disturbed during drilling activity prior to the test. SPT N-values for the last
300mm penetration are considered for computing the bearing capacity after applying the
necessary corrections.
2.3 Sampling
Disturbed and undisturbed soil samples were collected from the drilled bore holes at the
required depths and locations. Representative soil samples were collected as per ASTM and
BS standards, using the relevant samplers. Samples were recovered from split spoon sampler
after every SPT, Shelby-Tube and from core box.
2.3.1 Disturbed Soil Sampling
At the end of each SPT operation, the sampler tube is removed and disassembled to collect
representative disturbed sample for further laboratory tests. The disturbed samples were
properly sealed in plastic bags or small containers for NMC (Natural Moisture Content)
determination and other index tests. When the split spoon sample is in sufficient and not
found for a particular geotechnical layer, disturbed samples are also taken from core boxes.
2.3.2 Undisturbed Soil Sampling
Undisturbed Soil Samples are taken from cohesive materials encountered during drilling by
means of Shelby-Tube sampler. The samples are taken by applying static force and pressing
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 7/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
7
a Shelby Tube having an internal diameter of 80mm and length of 600mm. The top and
bottom of the Shelby tube samples were immediately wax sealed and covered with
polyethylene bags and labeled with necessary information for subsequent laboratory testing
to determine the engineering properties which are essential for providing the foundation
recommendations. All undisturbed samples were taken after dry boring and before SPT tests
to avoid disturbances.2.4 Laboratory Testing
BEST Consulting Engineers PLC (Private Limited Company) has a material testing laboratory
staffed with well-trained technician and engineers, in Addis Ababa. The laboratory is well
equipped by calibrated and certified ELE branded laboratory equipment to conduct various
index and engineering laboratory tests.
The following laboratory tests were conducted on different type of samples recovered from
boreholes in accordance with acceptable standards (such as, ASTM, AASHTO and BS
Standards).
Classification Tests
Classification tests are performed on collected representative samples for verification of the
field classification of the major soil types encountered during the investigation. A minor soil
type, if not critical, may be given a visual classification, instead of performing classification
test for reference. The classification tests performed for this project includes:
- Hydrometer and Sieve Analysis: - consist of determining the gradation of a
sample in accordance with AASHTO T-88.
- Atterberg Limit:- consist of the determination of the liquid limit, Plastic Limit
and Plasticity Index in accordance with AASHTO T89 and T90. If the soil is found
to be non-plastic, then the liquid limit shall not be performed, and the AASHTOgroup index shall be reported as zero.
Special Tests
These tests are performed on undisturbed soil samples, and/or split-spoon samples to
obtain additional information about the soils and their condition. In addition special tests
also include the analysis water samples. This information is used in analysis of conditions
and preparation of recommendations for design and construction. The special tests
performed for this project includes:
- Moisture content Test: determination of moisture content in accordance with
AASHTO T265, on representative samples of soil from each major stratum in each
boring.
- Unit Weight Determination: consist of the determination of the unit weight by
measurement of the length and diameter as performed in accordance with the
appropriate part of ASRM D-2937.
- Unconfined Compression Strength Test of soil: consist of performing the
unconfined compression test in accordance with ASTM D-2166. The test include
initial and final moisture content test, unit weight determination, visual description
of the soil, average strain at failure and average rate of strain of failure. This test
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 8/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
8
shall be performed on 3-inch undisturbed samples unless other types are specifically
approved in advance.
2.5 Ground water monitoring
The ground water level in each borehole was monitored before starting and after
completion of every day drilling activity. Presence of drilling water in boreholes, particularlyin cased ones, is often misleading with actual ground water level. Ground water level
measurements will only be reliable if measured for a reasonable period of times after
completion of the borehole.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 9/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
9
3 GEOLOGIC AND SEISMIC SETTINGS OF THE AREA
3.1 Regional and Site Geology
3.1.1 Regional Geology
Addis Ababa city is situated in the western margin of the main Ethiopian Rift and representsa transitional zone between the Ethiopian Plateau and the rift with poorly defined
escarpment.
The geology of Addis Ababa area is represented by four volcanic units dominated in the
lower part by basaltic lava flows (Addis Ababa basalt), followed by a pyroclastic sequence,
mainly formed by ignimbrites (Addis Ababa Ignimbrite), followed by central composite
volcanoes (Central Volcanoes unit), and finally small spatter cones and lava flows (Akaki
unit).
Based on the Geologic Map of Addis Ababa City (Mulugeta H/Mariam et. al 2007), the
following volcanic formations are found in the project and surrounding area:-
1. Quaternary Olivine phyric Basalt (Qb): this unit is exposed in the northerncentral and southern part of Addis Ababa geologic map. It is grey in color on fresh
outcrop and becomes reddish brown up on weathering.
2. Quaternary Scoria (QSc): These scoria cones are found as either cones or simple
domes. Mostly, they are layered and sometimes contain grey Scoraceous basalt
bombs. This unit is mainly cut by basaltic dyke of different orientation.
3. Quaternary black cotton soil (Qs):
4. Chelekleka BASALT (Tb2): the oldest geological unit, found along the river course
(e.g. Akaki River and its tributaries). It is represented by layered BASALT
intercalated with scoria pyroclastic rock.
5. Tertiary sediments (Ts): Out crops are mainly observed at the banks of the river
and small creeks. It generally forms very gentle slope and lower topography. It is
overlain by the young Quaternary basalt and overlay the Repi basalt. The
maximum thickness is about 9 m which is around Akaki area.
6. Wechecha Yerer-Furi IGNIMBRITE (Ti3): locally covers the products of the
composite central volcanoes of Wechecha and Furi. The sequence is constituted
by different flow units, consisting of pale-green to pale-yellow welded and crystal
rich ignimbrites.
7. Lower ignimbrite and pyroclastic rock (Ti2): it is grey and black colored and
shows columnar jointing. The rock is medium to fine grained and is composed ofsanidine phenocrysts and fine grained ground mass. The top layer is very loose
massive ash deposit which is whitish in color.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 10/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
10
Figure 3-1: Regional Geology of the area
3.1.2 Subsurface/Site Geology
The sub-surface geology of the proposed building sites is simple and fairly correlated in allthe boreholes sunk. Visual description of core samples was made following widely used and
practiced international procedures (such as, ASTM D 2488 – 93, BS 5930: 1981).
The top part of the project site is represented predominantly by soft to medium stiff, dark
grey, highly plastic CLAY with thickness ranging from 0.70m to 2.40m. This unit is underlain
in all the boreholes investigated by, dominantly, medium stiff to stiff, grayish brown, moist
and highly plastic Silty CLAY soil. The bottom part of the area is covered by light gray, slightly
weathered, dominantly closely to medium jointed, fine grained BASALT; this unit is
encountered from BH-151 to BH-154 only. Detailed descriptions of the sub-surface geology
encountered in all the boreholes are presented in the log sheets and cross sections attached
with this report (Appendix 1).
3.2 Regional Seismicity of the area
Stability and foundation of any civil engineering structures should be evaluated for seismic
stability. Information on the seismicity can be obtained from different sources that are
either from seismicity history of Ethiopia (seismicity zone map), regional location of the
country or localized or site specific study if it is needed. To do site specific earthquake hazard
analysis it demands detail study of faults by measuring slip rate, rupture length and depth of
energy release which are non-existence for this particular case.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 11/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
11
3.2.1 Country seismicity overview
Earthquake is a common phenomenon that occurred daily in different magnitude and
frequency all over the world. In Ethiopia the afar depression and the Main Ethiopian Rift
(MER) which is part of the East African rift is where these earthquake epicenters were
aligned. Among them the 1960 Awasa earthquake (M=6.1), the 1961 Kara Kore earthquake,the 1969 Serdo earthquake (M=6.3), the 1983 Wendo Genet earthquake, the 1985 Langano
earthquake and the 1989 Dobi graben earthquake (M=6.5) were significant ones and some
of them were fatal. The current volcanic activities and the resulting geologic phenomena’s in
Afar and Main Ethiopian Rift (MER) are good manifestations for tectonically dynamic nature
of the zone.
Figure 4.1: Seismic zoning map of Ethiopia
This zone is also under earthquake magnitude (I100) of 7.4 to 6.5 on Richter scales and with
ground acceleration 10.0 to 4.6% g (Table 3-1). On both scales show that with thismagnitude seismic motion has minor damage.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 12/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
12
Table 3-1: Seismic Hazard Rating (Gouin, 1976)
3.2.2 Region seismicity overview
From regional point of view the Global Seismic Hazard Assessment Program (GSHAP) which
was effective from 1992 to 1999 has produced digital data of Peak ground Acceleration
(PGA) and reports specific for several test areas including East African rift system. As per
GSHAP the project site is also located on the moderate seismic area of the country
characterized by a PAG of 0.8to 1.0m/s2 (Fig.4-2) over 50 years’ time.
Figure 3-2: Map that shows Peak ground Acceleration (After GSHAP, 1992-1999).
To generalize, the project site is located within the western rift margin of the country with
moderate seismic activity. Based on the Ethiopian Seismic Hazard Map (Gouin P 1976), the
area falls under Zone 2 corresponding moderate damage with VII MM intensity scale and
based on GSHAP it is located within a Peak Ground Acceleration (PGA) zone ranging from 0.8
to 1.0m/s2which is classified as seismically moderately vulnerable for potential damage.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 13/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
13
4 GEOTECHNICAL INVESTIGATION AND LABORATORY TESTING
4.1 Introduction
The field geotechnical investigation had been performed with the help of core drilling,
sampling, insitu and laboratory testing. A total of twenty four (24) boreholes were drilled in
the building area and the co-ordinates and depths of the drilled boreholes are presented inTable 4-1.
Table 4-1: Coordinate and Depth of Boreholes
Sr.
No.BH-ID Easting Northing
Elevation
(m a.s.l)
Depth drilled
(m)
Boreholes at G+4 Building Sites
1 BH-143 480709.8739 985034.2030 2205 10
2 BH-144 480682.4774 985020.2660 2204 10
3 BH-145 480656.3469 985030.2021 2204 10
4 BH-146 480642.9868 985056.2095 2205 10
5 BH-147 480621.5830 985045.5805 2206 106 BH-148 480633.1591 985018.9585 2205 10
7 BH-149 480626.4520 984993.2795 2205 10
8 BH-150 480599.2936 984979.6320 2205 10
Boreholes at G+7 Building Sites
1 BH-131 480534.9863 985010.7899 2207 15
2 BH-132 480550.7011 985038.8251 2207 15
3 BH-133 480576.0159 985051.8397 2207 15
4 BH-134 480601.3875 985064.4364 2207 15
5 BH-135 480633.6238 985081.9554 2206 15
6 BH-136 480659.0453 985095.7982 2206 15
7 BH-137 480681.7767 985105.4902 2206 15
8 BH-138 480712.6727 985104.3926 2206 15
9 BH-139 480725.1287 985081.4795 2206 15
10 BH-140 480739.2475 985054.0753 2205 15
11 BH-141 480750.5019 985029.5674 2205 15
12 BH-142 480723.4073 985015.7022 2204 15
13 BH-151 480602.9246 984951.7569 2204 15
14 BH-152 480576.4438 984937.2897 2203 14
15 BH-153 480599.2936 984979.6320 2204 15
16 BH-154 480599.2936 984979.6320 2206 15
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 14/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
14
Figure 4-1: Location of the boreholes on Google image
From the above Google image, symbols with red circle represent borehole locations for G+7
buildings and the blue circles are borehole locations for G+4 buildings.
4.2 Summary of the Geotechnical Investigation
The detailed geotechnical investigations carried out including drilling, in-situ tests, and
laboratory tests were summarized and presented in Table 4-2.
Table 4-2: Summary of the Geotechnical Investigations Carried Out
Geotechnical investigations carried out Quantity
Inter borehole movement and setup of drilling equipment 24
Core drilling in ALL formation for G+4 80
Core drilling in ALL formation for G+7 239
Standard Penetration Tests (SPT) 186
Disturbed soil samples 149
Undisturbed samples 24
Ground water level measurement 24
Relative surface elevation of boreholes using hand held GPS 24
Core boxes and photographing of cores in core boxes 65
Laboratory Tests
Grain size analysis 149
Hydrometer analysis 12
Atterberg Limits 149
Free Swell 149
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 15/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
15
Geotechnical investigations carried out Quantity
Natural Moisture Content (NMC) 149
Unit weight 102
Specific gravity 149
Swelling pressure 4
Unconfined Compressive Strength of soil 24
4.3 Geotechnical characterization of the subsurface material
Based on visual description, in-situ and laboratory test results, the sub-surface geology is
sub-divided into various geotechnical layers. Accordingly, the geotechnical investigation
reveals the occurrence of three homogenous geotechnical layers.
Layer 1: Soft to Medium stiff, highly plastic CLAY
The top most part of the building site is covered by soft to medium stiff, dark grey,
highly plastic CLAY with a maximum thickness of 2.40 around BH-140 and BH-141
(Table 4-3).
Layer 2: Medium stiff to stiff, Silty CLAY
This layer is characterized by medium stiff to stiff, grayish brown, moist and highly
plastic Silty CLAY soil. It is encountered in all the boreholes underlying the top layer
1; the average field SPT N-values/300mm is 9.7 (Table 4-4).
Even if the soil is class is MH in USCS, after having discussion with the client and by
considering the nature of the soil type and the hydrometer analysis result, it has
been decided to set the soil in CH soil class.
Layer 3: Moderately to slightly weathered, fine grained BASALT
This layer is characterized by light gray, dominantly slightly weathered to fresh,
closely to medium spaced joints, fine grained BASALT. It is encountered in few of the
boreholes drilled (Table 4-3).
Table 4-3: Distribution of the geotechnical layers in the boreholes
BH-IDDepth of occurrence (m)
Layer 1 Layer 2 Layer 3
Around G+4 Buildings
BH-143 0.00 – 2.00 2.00 – 10.00 -
BH-144 0.00 – 2.00 2.00 – 10.00 -
BH-145 0.00 – 1.50 1.50 – 10.00 -
BH-146 0.00 – 1.50 1.50 – 10.00 -
BH-147 0.00 – 1.50 1.50 – 10.00 -
BH-148 0.00 – 1.50 1.50 – 10.00 -
BH-149 0.00 – 1.50 1.50 – 10.00 -
BH-150 0.00 – 2.00 1.30 – 10.00 -
Around G+7 Buildings
BH-131 0.00 – 1.50 1.50 – 15.00 -
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 16/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
16
BH-IDDepth of occurrence (m)
Layer 1 Layer 2 Layer 3
BH-132 0.00 – 1.50 1.50 – 15.00 -
BH-133 0.00 – 1.50 1.50 – 15.00 -
BH-134 0.00 – 1.50 1.50 – 15.00 -
BH-135 0.00 – 0.70 0.70 – 15.00 -
BH-136 0.00 – 1.40 1.40 – 15.00 -
BH-137 0.00 – 1.50 1.50 – 15.00 -
BH-138 0.00 – 1.50 1.50 – 15.00 -
BH-139 0.00 – 2.20 2.20 – 15.00 -
BH-140 0.00 – 2.40 2.40 – 15.00 -
BH-141 0.00 – 2.40 2.40 – 15.00 -
BH-142 0.00 – 2.30 2.30 – 15.00 -
BH-151 0.00 – 1.50 1.50 – 13.45 13.45-15.00
BH-152 0.00 – 1.00 1.00 – 11.00 11.00-14.00
BH-153 0.00 – 1.50 1.50 – 11.80 11.80-15.00
BH-154 0.00 – 1.50 1.50-12.80 12.80-15.00
4.4 In situ Field Testing
The only insitu test conducted in the drilled boreholes is Standard Penetration Test (SPT)
using a standard hammer, under an impact of an automatic sliding hammer. The test was
carried out starting from 1.5m depth below natural ground level (NGL). Accordingly, a total
of one hundred eighty six (186) SPT tests were carried out. Summary of the SPT test results
are given in Table 5-4 below.
Table 4-4: Standard Penetration Test Results
Sr.
No BH-ID Depth (m)Material Description
Measured
SPT values
SPT N-
values/
300mm
Around G+4 Buildings
1 BH-143
1.55-2.00 CLAY 2/1/2 3
3.10-3.55
Silty CLAY
2/2/3 5
4.55-5.00 3/4/5 9
6.00-6.45 4/5/6 11
7.55-8.00 4/6/9 15
9.00-9.45 5/7/8 15
2 BH-144
1.50-1.95 CLAY 2/1/3 4
3.00-3.45
Silty CLAY
1/2/3 5
4.50-4.95 2/2/3 5
6.00-6.45 3/4/4 8
7.50-7.95 3/4/5 9
9.00-9.45 4/4/5 9
3 BH-145
1.55-2.00
Silty CLAY
2/2/3 5
3.10-3.55 1/2/4 6
4.55-5.00 2/3/5 8
6.00-6.45 3/4/6 10
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 17/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
17
Sr.
No BH-ID Depth (m)Material Description
Measured
SPT values
SPT N-
values/
300mm
7.50-7.95 3/4/8 12
9.00-9.45 3/5/8 13
4BH-146
1.55-2.00
Silty CLAY
2/2/2 4
3.10-3.55 2/2/3 5
4.55-5.00 2/3/4 7
6.00-6.45 3/3/5 8
7.55-8.00 3/4/5 9
9.00-9.45 4/5/6 11
5 BH-147
1.55-2.00
Silty CLAY
1/2/3 5
3.10-3.55 2/2/2 4
4.55-5.00 3/4/6 10
6.00-6.45 3/4/3 7
7.55-8.00 3/5/7 129.00-9.45 4/5/8 13
6 BH-148
1.55-2.00
Silty CLAY
2/1/3 4
3.10-3.55 2/2/4 6
4.55-5.00 2/4/6 10
6.00-6.45 3/4/5 9
7.55-8.00 3/3/4 7
9.00-9.45 3/3/5 8
7 BH-149
1.55-2.00
Silty CLAY
1/2/2 4
3.10-3.55 2/2/4 6
4.55-5.00 3/5/6 11
6.00-6.45 4/5/7 12
7.55-8.00 4/6/8 14
9.00-9.45 5/7/8 15
8BH-150
1.55-2.00 CLAY 3/2/11 13
3.10-3.55
Silty CLAY
2/3/5 8
4.55-5.00 3/5/7 12
6.00-6.45 5/7/10 17
7.55-8.00 6/8/10 18
9.00-9.45 6/7/11 18
Around G+7 Buildings
1 BH-131
1.55 – 2.00
Silty CLAY
1/1/2 33.00 - 3.45 2/3/4 7
4.50 – 4.95 2/3/6 9
6.00 - 6.45 2/4/6 10
7.50 – 7.95 3/5/6 11
9.00 - 9.45 3/4/5 9
10.5 - 10.95 3/4/6 10
12.00 - 12.45 3/5/6 11
13.5 - 13.95 4/5/5 10
2 BH-1321.50-1.95
Silty CLAY 1/2/3 5
3.10-3.552/3/3
6
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 18/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
18
Sr.
No BH-ID Depth (m)Material Description
Measured
SPT values
SPT N-
values/
300mm
4.50-4.95 2/2/4 6
6.00-6.45 2/3/3 6
7.55-8.00 Clayey Silty SAND 3/3/4 7
9.00-9.45
Silty CLAY
2/4/4 8
10.50-10.95 2/3/6 9
12.00-12.45 3/3/6 9
13.50-13.95 3/4/7 11
3 BH-133
1.55-2.00
Silty CLAY
2/1/2 3
3.00-3.45 2/3/4 7
4.50-4.95 2/2/4 6
6.00-6.45 2/2/3 5
7.50-7.95 2/3/4 7
9.00-9.45 2/3/5 810.50-10.95 3/3/6 9
12.00-12.45 3/4/6 10
13.50-13.55 4/4/7 11
4 BH-134
1.55-2.00
Silty CLAY
1/2/3 5
3.00-3.45 2/3/4 7
4.50-4.95 2/3/6 9
6.00-6.45 6/5/10 15
7.55-7.95 3/3/5 8
9.00-9.45 3/3/4 7
10.55-11.00 2/3/5 8
12.00-12.45 3/3/5 8
13.55-14.00 3/4/6 10
5 BH-135
1.50-1.95
Silty CLAY
1/2/2 4
3.00-3.45 2/2/3 5
4.50-4.95 3/3/3 6
6.00-6.45 3/3/5 8
7.55-7.95 3/4/6 10
9.00-9.45 3/6/7 13
10.50-10.95 2/3/5 8
12.00-12.45 3/4/6 10
13.50-13.95 4/6/6 12
6 BH-136
1.55-2.00
Silty CLAY
2/2/2 4
3.00-3.45 2/3/5 8
4.50-4.95 3/4/5 9
6.00-6.45 4/5/5 10
7.55-7.95 4/4/6 10
9.00-9.45 3/5/6 11
10.50-10.95 3/5/7 12
12.00-12.45 4/5/6 11
13.50-13.95 4/6/7 13
7 BH-137 1.50-1.95 Silty CLAY 1/1/2
3
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 19/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
19
Sr.
No BH-ID Depth (m)Material Description
Measured
SPT values
SPT N-
values/
300mm
3.00-3.45 2/3/3 6
4.50-4.95 2/2/5 7
6.00-6.45 2/4/5 9
7.50-7.95 3/3/5 8
9.00-9.45 3/5/5 10
10.50-10.95 2/5/7 12
12.00-12.45 3/6/6 12
13.50-13.95 4/5/6 11
8 BH-138
1.50-1.95
Silty CLAY
1/1/2 3
3.00-3.45 2/3/4 7
4.50-4.95 2/4/7 11
6.00-6.45 2/4/7 11
7.50-7.95 3/4/6 109.00-9.45 3/3/3 6
10.55-11.00 3/4/6 10
12.00-12.45 4/5/6 11
13.50-13.95 5/6/7 13
9 BH-139
1.50-1.95 CLAY 2/2/2 4
3.00-3.45
Silty CLAY
2/2/4 6
4.50-4.95 2/3/4 7
6.00-6.45 2/4/8 12
7.50-7.95 2/3/4 7
9.00-9.45 3/4/6 10
10.50-10.95 3/5/6 11
12.00-12.45 4/5/6 11
13.50-13.95 3/6/7 13
10 BH-140
1.50-1.95 CLAY 1/2/3 5
3.00-3.45
Silty CLAY
2/2/3 5
4.50-4.95 2/3/5 8
6.00-6.45 3/4/5 9
7.50-7.95 3/4/6 10
9.00-9.45 2/4/6 10
10.50-10.95 3/5/6 11
12.00-12.45 Silty CLAY 4/4/6 1013.50-13.95 4/5/7 12
11 BH-141
1.50-1.95 CLAY 1/1/2 3
3.00-3.45
Silty CLAY
2/3/3 6
4.50-4.95 3/3/4 7
6.00-6.45 2/4/6 10
7.50-7.95 3/5/5 10
9.00-9.45 3/5/6 11
10.50-10.95 4/4/5 9
12.00-12.45 3/5/6 11
13.50-13.954/6/6
12
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 20/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
20
Sr.
No BH-ID Depth (m)Material Description
Measured
SPT values
SPT N-
values/
300mm
12 BH-142
1.55-2.00 CLAY 1/2/2 4
3.00-3.45
Silty CLAY
2/3/3 6
4.50-4.95 2/3/5 8
6.00-6.45 3/4/6 10
7.50-7.95 3/3/5 8
9.00-9.45 3/5/6 11
10.50-10.95 3/4/5 9
12.00-12.45 4/4/6 10
13.50-13.95 4/5/6 11
13 BH-151
1.55-2.00
Silty CLAY
1/2/3 5
3.10-3.55 2/4/5 9
4.50-4.95 3/5/6 11
6.00-6.45 3/6/8 147.55-8.00 3/7/9 16
9.00-9.45 5/8/10 18
10.55-11.00 6/9/10 19
12.00-12.45 6/8/11 19
14 BH-152
1.55-2.00
Silty CLAY
2/2/3 5
3.10-3.55 3/3/5 8
4.55-5.00 4/5/6 11
6.00-6.45 3/5/7 12
7.55-8.00 4/7/9 16
9.00-9.45 6/8/9 17
10.55-11.00 R 50
15 BH-153
1.55-2.00
Silty CLAY
1/2/3 5
3.10-3.55 3/5/6 11
4.55-5.00 4/5/6 11
6.00-6.45 4/6/6 12
7.55-8.00 2/3/4 7
9.00-9.45 3/3/5 8
10.55-11.00 4/7/9 16
16 BH-154
1.50-1.95
Silty CLAY
2/3/4 7
3.10-3.55 2/3/5 8
4.50-4.95 3/4/6 106.00-6.45 4/5/9 14
7.50-7.95 5/6/9 15
9.00-9.45 7/8/10 18
10.55-11.00 4/5/6 11
12.00-12.45 3/6/8 14
4.5 Sampling
A total of one hundred seventy three (173) representative samples were collected from the
drilled boreholes for subsequent laboratory tests.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 21/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
21
4.5.1 Disturbed Samples
A total of one hundred forty nine (149) representative disturbed soil samples were collected
from split spoon sampler and core box. At the end of each SPT operation, the sampler tube
is removed and disassembled to collect representative disturbed sample for further
laboratory tests.
4.5.2 Undisturbed Samples
Twenty four undisturbed samples were collected by applying static force and pressing a
Shelby Tube having an internal diameter of 80mm and length of 600mm (in accordance with
ASTM D 1587 – 94 and BS 5930: 1981). The top and bottom of the Shelby Tube samples
were immediately wax sealed and covered with polyethylene bags and labeled with all
relevant information for subsequent laboratory testing to determine their geotechnical
properties. To avoid disturbances, all undisturbed samples were taken after dry boring and
before SPT.
4.6 Laboratory Testing
Representative disturbed and undisturbed soil samples collected from the boreholes were
brought to BEST Consulting Engineers Plc Central Laboratory and subjected to different kind
of quantitative and qualitative tests.
The laboratory testing on the disturbed soil samples include Atterberg Limits, sieve analyses,
hydrometer analysis, moisture content, specific gravity and free swell tests. Undisturbed
samples collected were also subjected to Unconfined Compressive Strength (UCS) tests,
Swelling pressure and Bulk density measurements.
Summary of all the laboratory test results are presented in Table 4-5 to Table 4-8 and the
details in Appendix 2.
Table 4-5: Summary of UCS Test Result of undisturbed soil Samples
BH ID. Depth (m)
Bulk Unit
Weight
(KN/m3)
Dry Unit
Weight
(KN/m3)
Moisture
Content (%)
UCS
(KPa)
Cu
(KPa)
Around G+4 Buildings
BH - 143 2.50-3.10 17.19 11.66 47.39 51.11 25.56
BH - 144 2.50-3.00 17.41 11.31 53.90 53.80 26.90
BH - 145 2.50-3.10 16.41 10.35 58.51 58.98 29.49
BH - 146 2.50-3.10 16.08 10.84 48.43 42.12 21.06
BH - 147 2.50-3.10 15.95 10.45 52.68 55.30 27.65
BH - 148 2.50-3.10 16.77 11.23 49.30 49.44 24.72
BH - 149 2.50-3.10 16.12 10.63 51.66 64.50 32.25
BH - 150 2.50-3.10 17.50 11.44 52.90 61.87 30.94
Around G+7 Building
BH - 131 2.50-3.00 16.53 10.85 52.44 57.12 28.56
BH - 132 2.50-3.00 17.49 12.20 43.33 96.95 48.48
BH - 133 2.50-3.10 17.76 12.20 45.62 45.19 22.60
BH - 134 2.50-3.10 16.02 11.02 45.27 83.40 41.70
BH - 135 2.50-3.00 17.50 11.61 50.75 56.27 28.14
BH - 136 2.50-3.00 17.09 12.16 40.56 62.86 31.43
BH - 137 2.50-3.00 17.22 11.50 49.77 58.30 29.15
BH - 138 2.50-3.00 16.69 10.69 56.08 70.16 35.08
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 22/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
22
Table 4-6: Laboratory test results of disturbed soil samples
SrNo
BH-ID Depth (m) NMC GsWet Sieve Analysis
(AASHTO T27)
AtterbergLimit
(AASHTOT89&90) USCS
FreeSwell(%)
2.mm0.425mm
0.075mm
LL PL PI
Around G+4 Buildings
1 BH-143 1.50-2.10 52.17 2.62 99.7 99.0 98.6 93 51 42 CH 200
2 BH-143 3.10-3.70 45.90 2.63 99.7 99.6 99.0 91 51 40 CH 190
3 BH-143 4.50-5.10 51.35 2.60 98.3 97.3 95.8 89 50 39 CH 150
4 BH-143 9.00-9.60 40.77 2.57 96.3 95.5 94.6 98 52 46 CH 180
5 BH-144 1.50-2.10 42.92 2.63 77.2 75.3 74.3 81 48 33 CH 160
6 BH-144 3.00-3.60 47.19 2.61 99.5 98.8 98.3 84 50 34 CH 170
7 BH-144 4.50-5.10 35.92 2.58 99.7 99.1 97.5 86 50 36 CH 150
8 BH-144 9.00-9.60 43.81 2.60 84.7 83.7 83.2 95 51 44 CH 190
9 BH-145 1.50-2.10 53.61 2.63 95.4 95.1 94.7 93 50 43 CH 180
10 BH-145 3.10-3.70 51.15 2.60 97.4 97.3 97.1 112 58 54 CH 200
11 BH-145 4.50-5.10 49.89 2.58 99.9 99.7 98.9 71 49 22 CH 130
12 BH-145 6.00-6.60 43.96 2.61 97.5 97.0 96.5 89 51 38 CH 150
13 BH-145 9.00-9.60 41.96 2.62 99.0 98.7 98.1 93 52 41 CH 190
14 BH-146 3.10-3.70 50.00 2.6 93.6 92.9 91.5 97 52 45 CH 180
15 BH-146 4.50-5.10 50.78 2.58 99.8 99.7 99.3 105 55 50 CH 160
16 BH-146 6.00-6.60 43.18 2.60 97.0 96.0 95.4 87 48 39 CH 190
17 BH-146 9.00-9.60 40.12 2.57 93.2 90.7 88.7 71 38 33 CH 140
18 BH-147 1.50-2.10 44.84 2.60 92.5 90.2 89.3 91 47 44 CH 190
19 BH-147 3.10-3.70 49.82 2.62 96.7 95.0 94.0 102 44 58 CH 180
20 BH-147 4.50-5.10 44.14 2.59 97.0 95.8 94.3 93 51 42 CH 140
21 BH-147 6.00-6.60 42.49 2.58 95.6 94.7 93.9 84 47 37 CH 170
22 BH-147 9.00-9.60 40.66 2.61 96.2 95.0 93.6 78 47 31 CH 150
23 BH-148 3.10-3.70 46.22 2.60 98.8 98.4 98.1 90 56 34 CH 190
24 BH-148 4.50-5.10 52.45 2.58 98.0 96.7 95.7 72 52 20 CH 140
25 BH-148 6.00-6.60 42.54 2.63 95.2 93.9 92.9 81 43 38 CH 160
26 BH-148 9.00-9.60 44.98 2.61 99.2 98.7 97.9 98 53 45 CH 170
27 BH-149 1.50-2.10 31.15 2.64 94.8 93.9 93.4 86 47 39 CH 190
28 BH-149 3.10-3.70 25.90 2.58 99.3 97.8 96.8 85 49 36 CH 140
29 BH-149 4.50-5.10 31.62 2.62 99.6 99.0 98.4 82 45 37 CH 150
30 BH-149 9.00-9.60 46.62 2.60 99.8 99.3 97.6 90 49 41 CH 180
BH - 139 2.50-3.00 18.66 13.09 42.55 48.98 24.49
BH - 140 2.50-3.00 18.14 12.78 41.92 94.37 47.19
BH - 141 2.50-3.00 16.63 10.73 54.94 92.98 46.49
BH - 142 2.50-3.00 16.85 12.07 39.57 102.40 51.20
BH - 153 2.50-3.10 16.63 10.96 51.68 79.11 39.56
BH - 154 2.50-3.10 17.02 11.80 44.25 62.02 31.01
BH - 151 2.50-3.10 16.55 10.45 58.27 75.36 37.68
BH - 152 2.50-3.10 17.61 10.87 62.03 80.04 40.02
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 23/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
23
SrNo
BH-ID Depth (m) NMC GsWet Sieve Analysis
(AASHTO T27)
AtterbergLimit
(AASHTOT89&90) USCS
FreeSwell(%)
2.mm0.425mm
0.075mm
LL PL PI
31 BH-150 3.10-3.70 45.60 2.61 98.7 98.3 98.0 102 66 36 CH 190
32 BH-150 4.50-5.10 46.36 2.64 99.5 99.3 98.6 99 51 48 CH 170
33 BH-150 6.00-6.60 29.57 2.59 96.5 95.7 95.1 91 50 41 CH 150
34 BH-150 9.00-9.60 41.69 2.61 96.8 96.2 95.1 86 45 41 CH 140
Around G+7 Buildings
35 BH-131 1.50-2.10 41.77 2.62 90.7 88.0 87.5 83 46 37 CH 130
36 BH-131 3.00-3.60 43.47 2.59 98.7 98.4 97.9 89 48 41 CH 150
37 BH-131 4.50-5.10 39.50 2.61 98.66 98.3 97.0 84 49 35 CH 160
38 BH-131 6.00-6.60 48.74 2.60 98.4 97.8 96.3 72 44 28 CH 140
39 BH-131 7.50-8.10 43.19 2.59 97.5 97.3 96.7 88 47 41 CH 17040 BH-131 9.00-9.60 38.16 2.58 99.7 99.4 99.0 90 52 38 CH 150
41 BH-131 10.5-11.10 37.60 2.59 99.5 98.6 97.9 87 50 37 CH 160
42 BH-131 12.0-12.60 22.22 2.61 99.3 99.0 97.9 87 46 41 CH 180
43 BH-131 13.5-14.10 39.64 2.57 98.8 97.9 96.8 72 45 27 CH 120
44 BH-132 1.50-2.10 43.61 2.58 99.6 99.5 99.0 94 50 44 CH 160
45 BH-132 3.00-3.60 24.06 2.61 96.6 95.9 95.4 106 57 49 CH 150
46 BH-132 4.50-5.10 40.97 2.63 99.5 98.9 98.0 105 55 50 CH 180
47 BH-132 6.00-6.60 41.47 2.61 99.5 99.3 98.5 96 51 45 CH 140
48 BH-132 7.50-8.10 33.99 2.58 28.3 20.0 11.6 36 26 10 SM 30
49 BH-132 9.00-9.60 33.49 2.62 90.6 89.7 88.4 71 41 30 CH 12050 BH-132 10.5-11.10 31.67 2.58 99.5 98.9 98.5 101 52 49 CH 190
51 BH-132 12.0-12.60 35.58 2.57 99.4 98.7 97.9 84 47 37 CH 130
52 BH-132 13.5-14.10 20.48 2.56 99.5 98.9 98.0 94 50 44 CH 190
53 BH-133 1.50-2.10 41.33 2.61 91.4 90.4 89.9 104 54 50 CH 180
54 BH-133 3.10-3.70 45.70 2.64 99.6 98.9 98.0 95 50 45 CH 160
55 BH-133 4.50-5.10 35.93 2.60 85.8 84.8 84.2 93 49 44 CH 200
56 BH-133 6.00-6.60 39.47 2.57 93.9 92.4 91.4 100 55 45 CH 180
57 BH-133 7.50-8.10 43.28 2.59 84.8 83.5 82.3 71 43 28 CH 140
58 BH-133 9.00-9.60 35.97 2.60 99.8 99.3 98.8 80 49 31 CH 160
59 BH-133 10.5-11.10 27.46 2.57 94.6 94.0 92.7 96 51 45 CH 13060 BH-133 12.0-12.60 29.17 2.59 98.5 97.9 96.3 98 52 46 CH 150
61 BH-133 13.5-14.10 41.92 2.57 99.6 98.9 93.4 72 39 33 CH 120
62 BH-134 1.50-2.10 48.45 2.60 81.4 79.6 78.8 90 49 41 CH 180
63 BH-134 3.10-3.70 37.59 2.61 99.6 99.2 98.6 95 50 45 CH 160
64 BH-134 4.50-5.10 41.43 2.58 99.6 98.9 98.2 94 51 43 CH 150
65 BH-134 6.00-6.60 38.26 2.62 93.6 93.0 91.5 89 48 41 CH 160
66 BH-134 7.50-8.10 40.10 2.60 98.5 97.9 97.0 101 53 48 CH 180
67 BH-134 9.00-9.60 35.08 2.58 99.9 99.7 98.4 87 48 39 CH 150
68 BH-134 10.5-11.10 39.23 2.59 99.5 98.9 98.0 98 50 48 CH 170
69 BH-134 12.0-12.60 35.66 2.57 99.7 99.2 98.8 91 50 41 CH 150
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 24/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
24
SrNo
BH-ID Depth (m) NMC GsWet Sieve Analysis
(AASHTO T27)
AtterbergLimit
(AASHTOT89&90) USCS
FreeSwell(%)
2.mm0.425mm
0.075mm
LL PL PI
70 BH-134 13.5-14.10 31.49 2.59 98.3 97.9 97.0 93 48 45 CH 130
71 BH-135 1.50-2.10 38.57 2.63 95.5 94.8 93.7 80 50 30 CH 190
72 BH-135 3.00-3.60 43.78 2.61 99.6 99.0 98.4 98 55 43 CH 160
73 BH-135 4.50-5.10 43.29 2.59 99.4 98.6 97.8 89 49 40 CH 190
74 BH-135 6.00-6.60 34.98 2.61 99.4 98.6 98.0 93 52 41 CH 170
75 BH-135 7.50-8.10 30.37 2.60 99.5 98.4 97.6 81 44 37 CH 180
76 BH-135 9.00-9.60 38.52 2.57 99.8 99.6 98.7 79 44 35 CH 140
77 BH-135 10.5-11.10 41.93 2.59 95.6 93.9 93.1 82 48 34 CH 170
78 BH-135 12.0-12.60 38.75 2.58 99.7 99.2 98.3 79 43 36 CH 150
79 BH-135 13.5-14.10 31.89 2.57 99.5 98.9 98.1 99 53 46 CH 130
80 BH-136 1.50-2.10 40.18 2.62 93.8 93.2 92.3 64 38 26 CH 160
81 BH-136 3.00-3.60 38.46 2.63 99.5 98.2 96.9 83 53 30 CH 170
82 BH-136 4.50-5.10 41.77 2.60 99.5 98.8 98.2 90 49 41 CH 130
83 BH-136 6.00-6.60 43.69 2.58 92.6 90.9 90.0 83 47 36 CH 150
84 BH-136 7.50-8.10 36.84 2.56 97.8 97.3 96.5 74 46 28 CH 170
85 BH-136 9.00-9.60 30.26 2.59 92.1 91.4 90.3 78 40 38 CH 190
86 BH-136 10.5-11.10 38.24 2.61 99.4 98.5 97.7 82 47 35 CH 180
87 BH-136 12.0-12.60 36.18 2.58 99.4 99.1 98.4 81 50 31 CH 150
88 BH-136 13.5-14.10 39.84 2.60 99.7 99.5 98.8 80 50 30 CH 130
89 BH-137 1.50-2.10 30.13 2.64 95.1 94.1 93.2 94 49 45 CH 150
90 BH-137 3.00-3.60 56.06 2.61 94.0 93.6 92.5 92 48 44 CH 180
91 BH-137 4.50-5.10 47.68 2.59 99.4 98.4 96.0 82 43 39 CH 140
92 BH-137 7.50-8.10 42.19 2.61 98.7 97.8 95.7 85 47 38 CH 180
93 BH-137 10.5-11.10 43.66 2.57 98.4 98.0 97.5 104 54 50 CH 160
94 BH-137 13.5-14.10 44.45 2.62 99.8 99.3 97.2 85 50 35 CH 100
95 BH-138 1.50-2.10 48.23 2.63 94.6 93.6 93.1 89 50 39 CH 170
96 BH-138 3.00-3.60 38.23 2.60 99.5 98.7 98.1 84 47 37 CH 160
97 BH-138 4.50-5.10 42.47 2.62 100.0 99.9 99.2 92 54 38 CH 160
98 BH-138 7.50-8.10 46.72 2.60 99.5 98.6 97.6 84 49 35 CH 180
99 BH-138 10.5-11.10 38.27 2.58 88.0 87.0 86.4 77 45 32 CH 140
100 BH-138 13.5-14.10 40.16 2.57 100.0 99.8 99.1 85 49 36 CH 100
101 BH-139 1.50-2.10 47.18 2.64 91.4 87.0 84.4 75 39 36 CH 160
102 BH-139 3.00-3.60 50.47 2.61 91.8 91.4 91.3 96 49 47 CH 160
103 BH-139 4.50-5.10 35.14 2.62 94.9 93.8 93.1 97 50 47 CH 170
104 BH-139 7.50-8.10 51.97 2.58 98.0 97.5 96.4 71 41 30 CH 40
105 BH-139 10.5-11.10 44.65 2.62 98.4 97.8 96.1 93 43 50 CH 150
106 BH-139 13.5-14.10 37.64 2.64 99.6 98.6 97.4 91 43 48 CH 190
107 BH-140 1.50-2.10 48.24 2.64 89.6 88.7 87.7 84 46 38 CH 160
108 BH-140 3.00-3.60 49.38 2.60 95.6 94.2 92.6 82 43 39 CH 180
109 BH-140 4.50-5.10 46.86 2.58 91.7 90.9 90.4 90 48 42 CH 160
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 25/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
25
SrNo
BH-ID Depth (m) NMC GsWet Sieve Analysis
(AASHTO T27)
AtterbergLimit
(AASHTOT89&90) USCS
FreeSwell(%)
2.mm0.425mm
0.075mm
LL PL PI
110 BH-140 7.50-8.10 40.79 2.61 99.5 99.3 98.6 101 52 49 CH 150
111 BH-140 10.5-11.10 40.98 2.58 98.8 98.4 97.6 88 46 42 CH 170
112 BH-140 13.5-14.10 36.91 2.60 92.6 90.5 88.5 75 40 35 CH 140
113 BH-141 1.50-2.10 47.27 2.63 95.0 93.5 91.6 91 48 43 CH 150
114 BH-141 3.00-3.60 34.18 2.60 88.2 87.6 87.4 107 57 50 CH 200
115 BH-141 4.50-5.10 34.12 2.61 99.6 98.7 97.9 85 46 39 CH 150
116 BH-141 7.50-8.10 45.62 2.58 97.6 96.8 95.7 83 48 35 CH 180
117 BH-141 10.5-11.10 35.33 2.61 93.1 92.4 91.1 85 43 42 CH 170
118 BH-141 13.5-14.10 26.62 2.64 96.6 94.6 91.8 74 39 35 CH 140
119 BH-142 1.50-2.10 34.71 2.64 99.5 98.8 98.2 95 50 45 CH 130
120 BH-142 3.00-3.60 42.74 2.59 90.5 89.2 88.4 92 49 43 CH 160
121 BH-142 4.50-5.10 42.24 2.58 99.6 98.9 98.2 99 53 46 CH 190
122 BH-142 7.50-8.10 37.82 2.60 99.6 98.9 98.4 90 54 36 CH 180
123 BH-142 10.5-11.10 35.21 2.61 99.8 99.0 98.2 78 41 37 CH 150
124 BH-142 13.5-14.10 39.96 2.58 98.6 97.2 95.7 88 49 39 CH 170
125 BH-151 1.50-2.10 42.70 2.64 99.7 99.1 98.5 96 51 45 CH 170
126 BH-151 3.10-3.70 55.91 2.60 99.6 99.0 98.4 87 47 40 CH 150
127 BH-151 4.50-5.10 42.15 2.62 99.7 99.5 98.8 98 55 43 CH 190
128 BH-151 7.50-8.10 44.44 2.60 98.6 98.2 97.8 90 54 36 CH 140
129 BH-151 12.0-12.60 41.49 2.60 93.9 92.6 91.6 82 47 35 CH 130
130 BH-152 1.50-2.10 50.26 2.62 98.9 98.4 97.7 93 53 40 CH 170
131 BH-152 3.10-3.70 53.54 2.63 99.6 98.9 98.1 99 52 47 CH 190
132 BH-152 4.50-5.10 42.25 2.61 93.3 91.5 90.9 95 42 53 CH 160
133 BH-152 7.50-8.10 42.31 2.58 99.0 98.5 97.9 90 49 41 CH 180
134 BH-152 10.4-11.00 44.89 2.60 72.8 69.2 67.6 59 30 29 CH 120
135 BH-153 1.50-2.10 47.90 2.63 99.3 99.1 98.1 103 54 49 CH 190
136 BH-153 3.10-3.70 49.42 2.60 99.8 99.3 97.7 83 46 37 CH 130
137 BH-153 4.50-5.10 35.56 2.58 99.5 98.9 98.3 92 49 43 CH 160
138 BH-153 6.00-6.60 37.92 2.60 94.3 94.2 93.9 97 51 46 CH 190
139 BH-153 7.50-8.10 31.72 2.59 99.5 98.7 97.8 90 53 37 CH 180
140 BH-153 9.00-9.60 36.46 2.61 99.2 98.6 98.0 99 51 48 CH 200
141 BH-153 10.5-11.10 35.35 2.61 99.4 98.9 98.2 91 51 40 CH 140
142 BH-154 1.50-2.10 36.32 2.63 95.4 94.8 94.1 96 39 57 CH 160
143 BH-154 3.10-3.70 34.56 2.64 98.6 98.4 97.5 93 53 40 CH 130
144 BH-154 4.50-5.10 35.14 2.59 99.4 98.6 97.7 88 52 36 CH 180
145 BH-154 6.00-6.60 38.90 2.61 99.6 98.8 97.8 92 50 42 CH 150
146 BH-154 7.50-8.10 34.17 2.59 99.2 99.0 98.1 75 46 29 CH 120
147 BH-154 9.00-9.60 25.72 2.57 99.5 98.4 97.0 82 45 37 CH 190
148 BH-154 10.5-11.10 27.10 2.62 99.1 98.7 97.5 98 51 47 CH 170
149 BH-154 12.0-12.60 36.19 2.60 98.2 97.8 96.5 70 38 32 CH 160
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 26/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
26
Table 4-7: Swelling pressure test result
Sr. No BH-ID Depth, m NMC Swelling Pressure, Kpa
1 BH - 131 2.50-3.10 52.44 37.0
2 BH - 135 2.50-3.10 50.75 42.2
3 BH - 142 2.50-3.00 39.57 45.7
4 BH - 146 2.50-3.10 48.43 39.5
Table 4-8:
Hydrometer analysis results on selected disturbed soil samples
Sr No BH-ID Depth (m)
HYDROMETER TYPE 152H
Sand%
(2.00 – 0.075mm)
Silt%
(0.075 – 0.002mm)
Clay%
(< 0.002)
1 BH-131 3.00-3.60 3.1 34.9 62.0
2 BH-132 3.00-3.60 5.4 31.6 63.0
3 BH-136 3.00-3.60 4.0 39.0 57.0
4 BH-137 3.00-3.60 7.9 28.1 64.05 BH-139 3.00-3.60 10.2 30.8 59.0
6 BH-141 3.00-3.60 14.0 30.0 56.0
7 BH-143 4.50-5.10 4.8 31.2 64.0
8 BH-145 3.10-3.70 4.0 30.0 66.0
9 BH-148 3.10-3.70 2.9 31.1 66.0
10 BH-149 4.40-5.00 2.4 29.6 68.0
11 BH-151 4.50-5.10 2.5 29.5 68.0
12 BH-154 3.10-3.70 3.2 28.8 68.0
4.7 Ground water Level Measurement
When encountered, ground water level is measured every day before and after 24 hour
from completion of drilling activity. There was no groundwater occurrence in all the drilled
boreholes up to the target depth.
4.8 Damage due to expansive soils
Potentially expansive soils were identified in the building site during the geotechnical
investigation. Expansive soils are prone to change in volume because of the presence or
absence of moisture, which can cause the soils to shrink or swell, resulting in damage to
structures or infrastructure. The change in volume exerts stress on building foundations andother loads placed on these soils.
The most obvious way in which expansive soils can damage foundations is by uplift as they
swell with moisture increases. Swelling soils lift up and crack lightly-loaded, continuous strip
footings, and frequently cause distress in floor slabs and because of the different building
loads on different portions of a structure's foundation, the resultant uplift will vary in
different areas; such differential movement of the foundation can also cause distress to the
framing of a structure. Besides, Shallow pipes buried in the zone of seasonal moisture
fluctuation, are exposed to enormous stresses by shrinking soils. If water or sewage pipes
break, then the resultant leaking moisture can aggravate swelling damage to the nearby
structures.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 27/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
27
4.8.1 Mitigation measures
The best way to avoid damage from expansive soils is to extend building foundations
beneath the zone of water content fluctuation. The reason is twofold: first, to provide for
sufficient skin friction adhesion below the zone of drying; and second, to resist upward
movement when the surface soils become wet and begin to swell.
Another way of mitigating expansive soil problems is to collect surface runoff and to limit
surface infiltration during the rainy season; proper design and construction of surface
drainage systems will be crucial.
Soils shrink and swell - because the moisture content changes from dry to moist and vice
versa. Thus, shrinking and swelling can be reduced if the moisture content is kept stable.
Damage from shrinking and swelling soils can also be reduced or prevented with proper
foundation design. Several design alternatives are:
–Drilled pier and beam: Drilled pier and beam systems are designed to isolatethe structure from expansive soil movements.
– Stiffened slab-on-grade: Designed to provide a rigid foundation to protect
the structure from differential soil movement.
– Monolithic wall and slab: Designed to provide a rigid foundation to resist
differential soil movement.
– Modified continuous footings, walls, and basement construction. Design to
provide a rigid foundation to resist differential soil movement.
4.8.2 Treatment of Expansive Soils
To avoid damage from the expansive soils, soils can also be treated in different ways, both
before and after construction. The different treatment techniques are:
• Removal of expansive soil and replacement with a non-expansive material is a
common method of reducing shrink-swell risk. If the expansive soil or stratum is
thin, then the entire layer can be removed.
• Pre-wetting a site can eliminate an expansive soil problem if the high moisture
content can be maintained.
• Chemical treatment: Lime stabilization can be used.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 28/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
28
5 FOUNDATION ANALYSIS
5.1 Introduction
Foundation analysis refers to the determination of the bearing layer and depth, allowable
bearing pressure and type of foundation that could be adopted safely and economically.
Factors such as the load to be transmitted to the foundation and the subsurface condition ofthe soil have been considered in selecting the foundation type.
As can be observed from the detailed geotechnical logging, the subsurface formation of the
project site comprises of three different geotechnical layers:
The top most part of the building site is covered by medium stiff, dark grey, highly
plastic Silty CLAY with a maximum thickness of 2.40m (Layer 1).
Medium stiff to very stiff, grayish brown, moist and highly plastic Silty CLAY soil. It is
encountered in all the boreholes underlying the top layer 1 (Layer 2).
Light gray, moderately to slightly weathered, dominantly with closely spaced joints,
fine grained BASALT; this layer is encountered in four of boreholes only, i.e. BH-151to BH-154 (Layer 3).
Layer 1 is the high plastic soil that is unsuitable as foundation soils as far as the nature of the
material is concerned. Among the three layers the possible seat of the foundation footings
is Layer 2, which is highly plastic Silty CLAY soil; but, the impact of the expansive soil on the
foundation shall be considered and appropriate mitigation measures and treatments shall be
met.
Allowable bearing pressures for the selected foundation layers shall be discussed based on
correlation of the relative compaction of the insitu ground as indicated from SPT and
laboratory UCS tests.
5.2 Isolated Foundation
Isolated footings are the simplest to construct and economical type of foundations. The
allowable bearing capacity of these types of footings can be determined using different
methods; in the different methods insitu tests (SPT N-Values), laboratory tests and visual
identification can be used to determine the allowable bearing capacities for this project.
5.2.1 Bearing Pressure Based on UCS
Unconfined compressive tests were conducted on twenty four undisturbed soil samples (8
on G+4 and 16 on G+7 Building sites) taken from all the boreholes sunk. Unconfined
compression tests are conducted to determine the undrained shear strength value, Cu of thesoil. The undrained shear strength of the soil, Cu, can be determined from unconfined
compressive strength (UCS) of soil as follows:
Cu
The net ultimate bearing pressure for vertical loads on clay soils is normally computed as a
simplification of either the Meyerhof or Hansen bearing capacity equations (Bowles, 1997).
For cohesive soils, changes in ground water levels do not affect theoretical ultimate bearing
capacity. For the most critical stability state (Ø = 0), which is created when the foundation
load is applied so rapidly, the immediate bearing capacity is independent of the location of
the water table. This is in contrast to the long term stability in which the value of the drained
= ½ UCS
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 29/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
29
shear strength cd, and drained friction angle Ød should be considered. The ultimate bearing
capacity of the footings can be calculated using:
qult = 5.14Cu(l + s'c + d'c) + q
Where qult = Ultimate bearing capacity in unit of Cu
Cu = Undrained shear strength of soil
s'c = Shape factor = 0.2(B/L)
d'c = Depth factor = 0.4(D/B)
q = Overburden pressure which is neglected since there will
probably be footing excavation.
B = Width of the foundation
L = Length of the foundation
D = Depth of the foundation
Designing a foundation on the basis of ultimate bearing capacity, a suitable factor of safety
should be used to determine the allowable pressure so that the foundation system may besafe against shear failure. For isolated footing foundations, a factor of safety of 2 to 3 is
commonly used under normal loading conditions. Thus, for the project buildings we have
taken a factor of 3.0. The allowable bearing capacity is determined using:
qall = qult
Table 5-1: Allowable Bearing Pressure Based on UCS Test Result of Soil Samples for Layer2.
/FS
The allowable bearing capacities calculated from UCS results are presented in Table 5-1
below. Here, the bearing capacity is computed for different widths of foundation at a depth
of 2.5m on Layer 2; average Cu value, within the blocks, has been taken for the bearing
capacity analysis.
BH-ID BLOCK
NO Width,
B in m
Depth
( m)
OB,
Mean
Ƴbulk
KN/m3
Mean Cu
(KPa)
OB.
Press.q,
KPa
qa (KPa),
Hansen
Around G+4 Buildings
Around BH-
143, & BH-144B-299
2
2.5
17.30 26.23 43 84
2.5 17.30 26.23 43 86
3.0 17.30 26.23 43 83
Around BH-
145 &BH-146 B-303
2
2.5
16.24 25.27 41 81
2.5 16.24 25.27 41 833.0 16.24 25.27 41 80
Around BH-
147 &BH-148B-304
2
2.5
16.34 26.18 41 84
2.5 16.34 26.18 41 85
3.0 16.34 26.18 41 82
Around BH-
149&BH-150B-308
2
2.5
16.81 28.26 42 89
2.5 16.81 28.26 42 91
3.0 16.81 28.26 42 88
Around G+7 Buildings
Around BH-
131 &BH-132 B-306
2
2.5
17.01 38.52 43 117
2.5 17.01 38.52 43 120
3.0 17.01 38.52 43 115
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 30/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
30
Around BH-
133 &BH-134B-305
2
2.5
16.89 32.15 42 100
2.5 16.89 32.15 42 102
3.0 16.89 32.15 42 99
Around BH-
135 &BH-136 B-302
2
2.5
17.29 29.78 43 94
2.5 17.29 29.78 43 96
3.0 17.29 29.78 43 93Around BH-
137 &BH-138 B-301
2
2.5
16.95 32.11 42 100
2.5 16.95 32.11 42 102
3.0 16.95 32.11 42 98
Around BH-
139, BH-140 B-300
2
2.5
18.40 35.84 46 111
2.5 18.40 35.84 46 114
3.0 18.40 35.84 46 109
Around BH-
141 &BH-142B-298
2
2.5
16.74 48.84 42 144
2.5 16.74 48.84 42 148
3.0 16.74 48.84 42 142
Around BH-151 &BH-152
B-309
2
2.5
17.08 38.85 43 118
2.5 17.08 38.85 43 121
3.0 17.08 38.85 43 116
Around BH-
153 &BH-154 B-307
2
2.5
16.82 35.28 42 108
2.5 16.82 35.28 42 111
3.0 16.82 35.28 42 107
5.2.2 Bearing Pressure Based on SPT N-Values for Isolated square Footing
The SPT N-values/300mm should be adjusted for different factors before employing them
for computing the allowable bearing pressure. The SPT N-values are converted to N70
standard energy ratio value (Bowles, 1988) using:
N'70 = CN x N x n1 x n2 x n3 x n4
Where N'70 = adjusted N
CN = adjustment for overburden pressure
(p''o/p'o)1/2
p'o = overburden pressure
p''o = reference overburden pressure (95.76kPa or
1.0kg/cm2)
n1 = Er/Erb (where Er is average energy ratio that depends onthe drill system and Erb is the standard energy ratio).
n2 = Rod length correction
Rod length > 10 m = 1,
Rod length 6-10 m = 0.95,
Rod length 4-6 m = 0.85,
Rod length 0-4 m = 0.75
n3 = sampler correction (1.00 in this case)
n4
= borehole diameter correction (1.00 in this case)
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 31/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
31
The depths below NGL, SPT N-values, adjusted N-values (i.e., N’70
Table 5-2: Measured and adjusted SPT N values
) and the calculated design
N-values are given below.
Sr.
NoBH-ID Depth (m)
SPT N-values/ 300mm Adjusted N-values
Around G+4 Building
1 BH-143
1.55-2.00 3 3
3.10-3.55 5 5
4.55-5.00 9 9
6.00-6.45 11 10
7.55-8.00 15 12
9.00-9.45 15 12
2 BH-144
1.50-1.95 4 4
3.00-3.45 5 5
4.50-4.95 5 56.00-6.45 8 7
7.50-7.95 9 7
9.00-9.45 9 7
3 BH-145
1.55-2.00 5 5
3.10-3.55 6 6
4.55-5.00 8 8
6.00-6.45 10 9
7.50-7.95 12 10
9.00-9.45 13 10
4BH-146
1.55-2.00 4 4
3.10-3.55 5 5
4.55-5.00 7 7
6.00-6.45 8 7
7.55-8.00 9 7
9.00-9.45 11 9
5 BH-147
1.55-2.00 5 5
3.10-3.55 4 4
4.55-5.00 10 10
6.00-6.45 7 6
7.55-8.00 12 10
9.00-9.45 13 10
6 BH-148
1.55-2.00 4 4
3.10-3.55 6 6
4.55-5.00 10 10
6.00-6.45 9 8
7.55-8.00 7 6
9.00-9.45 8 6
7BH-149
1.55-2.00 4 4
3.10-3.55 6 6
4.55-5.00 11 11
6.00-6.45 12 11
7.55-8.00 14 12
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 32/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
32
Sr.
No BH-ID Depth (m) SPT N-values/ 300mm Adjusted N-values
9.00-9.45 15 12
8BH-150
1.55-2.00 13 13
3.10-3.55 8 84.55-5.00 12 12
6.00-6.45 17 16
7.55-8.00 18 15
9.00-9.45 18 14
Around G+7 Building
1 BH-131
1.55 – 2.00 3 3
3.00 - 3.45 7 7
4.50 – 4.95 9 9
6.00 - 6.45 10 9
7.50 – 7.95 11 9
9.00 - 9.45 9 7
10.5 - 10.95 10 7
12.00 - 12.45 11 7
13.5 - 13.95 10 6
2 BH-132
1.50-1.95 5 5
3.10-3.55 6 6
4.50-4.95 6 6
6.00-6.45 6 6
7.55-8.00 7 6
9.00-9.45 8 6
10.50-10.95 9 612.00-12.45 9 6
13.50-13.95 11 7
3 BH-133
1.55-2.00 3 3
3.00-3.45 7 7
4.50-4.95 6 6
6.00-6.45 5 5
7.50-7.95 7 6
9.00-9.45 8 6
10.50-10.95 9 6
12.00-12.45 10 6
13.50-13.55 11 7
4 BH-134
1.55-2.00 5 5
3.00-3.45 7 7
4.50-4.95 9 9
6.00-6.45 15 14
7.55-7.95 8 7
9.00-9.45 7 6
10.55-11.00 8 6
12.00-12.45 8 5
13.55-14.00 10 6
5 BH-135 1.50-1.95 4 4
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 33/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
33
Sr.
No BH-ID Depth (m) SPT N-values/ 300mm Adjusted N-values
3.00-3.45 5 5
4.50-4.95 6 6
6.00-6.45 8 77.55-7.95 10 8
9.00-9.45 13 10
10.50-10.95 8 6
12.00-12.45 10 6
13.50-13.95 12 7
6 BH-136
1.55-2.00 4 4
3.00-3.45 8 8
4.50-4.95 9 9
6.00-6.45 10 9
7.55-7.95 10 8
9.00-9.45 11 9
10.50-10.95 12 9
12.00-12.45 11 7
13.50-13.95 13 8
7 BH-137
1.50-1.95 3 3
3.00-3.45 6 6
4.50-4.95 7 7
6.00-6.45 9 8
7.50-7.95 8 7
9.00-9.45 10 8
10.50-10.95 12 912.00-12.45 12 8
13.50-13.95 11 7
8 BH-138
1.50-1.95 3 3
3.00-3.45 7 7
4.50-4.95 11 11
6.00-6.45 11 10
7.50-7.95 10 8
9.00-9.45 6 5
10.55-11.00 10 7
12.00-12.45 11 7
13.50-13.95 13 8
9 BH-139
1.50-1.95 4 4
3.00-3.45 6 6
4.50-4.95 7 7
6.00-6.45 12 11
7.50-7.95 7 6
9.00-9.45 10 8
10.50-10.95 11 8
12.00-12.45 11 7
13.50-13.95 13 8
10 BH-140 1.50-1.95 5 5
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 34/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
34
Sr.
No BH-ID Depth (m) SPT N-values/ 300mm Adjusted N-values
3.00-3.45 5 5
4.50-4.95 8 8
6.00-6.45 9 87.50-7.95 10 8
9.00-9.45 10 8
10.50-10.95 11 8
12.00-12.45 10 6
13.50-13.95 12 7
11 BH-141
1.50-1.95 3 3
3.00-3.45 6 6
4.50-4.95 7 7
6.00-6.45 10 9
7.50-7.95 10 8
9.00-9.45 11 9
10.50-10.95 9 6
12.00-12.45 11 7
13.50-13.95 12 7
12 BH-142
1.55-2.00 4 4
3.00-3.45 6 6
4.50-4.95 8 8
6.00-6.45 10 9
7.50-7.95 8 7
9.00-9.45 11 9
10.50-10.95 9 612.00-12.45 10 6
13.50-13.95 11 7
13 BH-151
1.55-2.00 5 5
3.10-3.55 9 9
4.50-4.95 11 11
6.00-6.45 14 13
7.55-8.00 16 13
9.00-9.45 18 14
10.55-11.00 19 13
12.00-12.45 19 12
14 BH-152
1.55-2.00 5 53.10-3.55 8 8
4.55-5.00 11 11
6.00-6.45 12 11
7.55-8.00 16 13
9.00-9.45 17 13
10.55-11.00 50 32
15 B-153
1.55-2.00 5 5
3.10-3.55 11 11
4.55-5.00 11 11
6.00-6.45 12 11
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 35/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
35
Sr.
No BH-ID Depth (m) SPT N-values/ 300mm Adjusted N-values
7.55-8.00 7 6
9.00-9.45 8 6
10.55-11.00 16 11
16 B-154
1.50-1.95 7 7
3.10-3.55 8 8
4.50-4.95 10 10
6.00-6.45 14 13
7.50-7.95 15 12
9.00-9.45 18 14
10.55-11.00 11 8
12.00-12.45 14 9
After adjusting the N-values based on the above formula, the design N-values are calculated
as the average of N-values which are found in between ½ B above and 2B below the
proposed foundation depth. B is the width of the foundation.
The bearing capacity for the soil layer is calculated from the SPT N- values using Meyerhof’s
equation as follows (Bowles, 1997):
qa = N'/F2(1 + F3/B)2Kd , B>F4
Where qa = Allowable bearing pressure for
Settlement limited to 25 mm.
Kd = 1+0.33D/B < 1.33
F2 = 0.06
F3 = 0.3
F4
= 1.2
B = Width of foundation
D = Depth of foundation
The following allowable bearing pressures are calculated from a depth of 2.0 to 3.0m below
the ground level for different width for settlement limited to 25mm. Here, the SPT on rock
head is considered as refusal and given N value of 50 for bearing capacity computation, for
other depths see Annex 3.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 36/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
36
Table 5-3: Allowable Bearing Pressures Based on SPT N-Value
BH-IDBlockNumber
Depth offoundation
belowNGL (m)
Width of foundation (B), m
2 2.5 3 4 5
Al lowable Bearing Capaci ty (Qall) in Kpa
Around G+4 Building
Average ofBH-143,144 B-299
2.0 169 160 168 163 154
2.5 178 190 175 169 159
3.0 203 213 195 175 163
4.0 224 223 215 205 172
Average ofBH-145,146
2.0 179 170 169 164 155
B-303
2.5 189 191 177 170 160
3.0 208 208 196 176 164
4.0 219 219 211 202 173
Average ofBH-147,148
2.0 187 177 171 161 152
B-304
2.5 197 193 178 166 156
3.0 219 217 192 172 161
4.0 221 214 206 197 170
Average ofBH-149,150
B-308
2.0 242 230 222 207 196
2.5 255 251 238 215 202
3.0 257 255 242 222 207
4.0 269 261 252 240 219
Around G+7 Building
Average ofBH-131,132 B-306
2.0 176 168 161 148 140
2.5 186 182 168 154 145
3.0 209 200 177 159 148
4.0 211 196 191 178 154
Average ofBH-133,134
2.0 193 183 167 149 137
2.5 203 188 174 152 141
B-305 3.0 232 208 178 157 145
4.0 219 199 192 174 152
Average ofBH-135,136
2.0 183 174 170 164 153
2.5 193 192 180 170 160
B-302
3.0218 212 197 175 162
4.0 223 222 214 197 172
Average ofBH-137,138
2.0 194 185 178 162 160
B-301 2.5 205 202 186 173 165
3.0 244 231 193 179 169
4.0 244 224 216 210 181
Average ofBH-139,140
2.0 192 183 173 165 161
B-3002.5 203 196 181 176 166
3.0 226 214 198 182 171
4.0 225 221 213 206 182
Average of 2.0 182 173 165 158 147
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 37/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
37
BH-IDBlockNumber
Depth offoundation
belowNGL (m)
Width of foundation (B), m
2 2.5 3 4 5
Al lowable Bearing Capaci ty (Qall) in Kpa
BH-141,142B-298
2.5 192 188 173 162 151
3.0 222 209 185 168 1554.0 221 216 208 190 164
Average ofBH-151,152
B-308
2.0 234 222 221 212 232
2.5 246 250 237 223 238
3.0 280 277 254 262 245
4.0 292 288 278 304 259
Average ofBH-153,154
B-307
2.0 244 232 211 193 186
2.5 257 238 220 199 185
3.0 284 256 231 206 190
4.0 270 254 245 227 200
The above bearing capacity analysis is computed for different depths and widths of
foundation footings for Layer 2 around G+4 and G+7 Buildings.
5.3 Allowable Bearing capacity for Mat Foundation
A mat foundation is commonly used where the base soil has a low bearing capacity and/or
the column loads are so large that more than 50 percent of the area is covered by
conventional spread footings. It is common to use mat foundations having basements both
to spread the column loads to a more uniform pressure distribution and to provide the floor
slab for the basement.
The bearing capacity values obtained for isolated foundation for the G+7 buildings may smallfor the proposed design load; in such cases, mat foundation will be the best choice. The
bearing capacity for the soil layer is calculated from the SPT N-values using Meyerhof’s
equation as follows (Bowles, 1997):
qall = (N55/0.08)( Ha/25)Kd For Mat foundation
Where qa = Allowable bearing pressure.
Kd = 1+0.33D/B < 1.33
∆Ha = Allowable settlement (In our case 75mm)
F2 = 0.08
F3 = 0.3F4 = 1.2
B = Width of foundation
D = Depth of foundation
The following allowable bearing pressures are calculated at a depth of 3.00m below the
natural ground level for different widths for settlement limited to 75mm. A permissible
settlement of 50, 75 and 100mm are recommended by different authors and standard
(Bowles, EBCS, U.S. Army Corps etc).
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 38/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
38
Table 5-5-4: Allowable Bearing Pressures for Mat foundation Around G+7 Buildings
BH-ID
Width, m
10 15 20 25 30 35 40 45 50 55 60
Allowable Bearing Capacity
BH-131&132 288 280 275 273 271 270 269 268 268 267 267
BH-133&134 288 280 275 273 271 270 269 268 268 267 267
BH-135&136 288 280 275 273 271 270 269 268 268 267 267
BH-137&138 288 280 275 273 271 270 269 268 268 267 267
BH-139&140 288 280 275 273 271 270 269 268 268 267 267
BH-141&142 288 280 275 273 271 270 269 268 268 267 267
BH-151&152 495 480 472 468 465 463 461 460 459 458 457
BH-153&154 371 360 354 351 349 347 346 345 344 344 343
5.4 Bearing Capacity using Settlement Criterion
As far as the properties of the project soils concerned, as depicted from laboratory tests
settlement shall have to be addressed properly.
Compressibility and stiffness of cohesive soil is strongly strain level dependent. But in
addition, it is also influenced by the relative rates of loading and drainage of excess pore
pressure. Compressibility and stiffness of cohesive soil is commonly expressed in a number
of ways:
• Compression Index (Cc)
• Coefficient of volume compressibility (mv)
• Undrained Young’s Modulus (Eu)
• Drained Young’s Modulus (E’)
The Compression Index (Cc) is routinely used in the calculation of settlements of normally
and lightly over-consolidated clays. The predicted compression of such materials is strongly
dependent on the value of pre-consolidation pressure used in the calculation.
In the design of any foundation, one must consider the safety against bearing capacity
failure as well as against excessive settlement of the foundation. In the design of mostfoundations, there are specifications for allowable levels of settlement.
The settlement of a foundation can have three components: (a) elastic settlement Se, (b)
primary consolidation settlement Sc, and (c) secondary consolidation settlement Ss. The
total settlement St can be expressed as:
St= St + Sc + Ss
For any given foundation, one or more of the components may be zero or negligible.
Consolidation settlement, Sc, is a time-dependent process that occurs due to the expulsion
of excess pore water pressure in saturated clayey soils below the groundwater table and is
created by the increase in stress created by the foundation load.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 39/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 & G+7 Condominium Building
BEST Consulting Engineers plc April 2014
39
The consolidation settlement Sc due to this average stress increase can be calculated as
follows:
Dh= Cc'H[log(Pf/Po)]/(1+eo)
Consolidation test had been conducted on soil sample collected from the surrounding area
for consolidation settlement analysis. Table below show total settlement estimated basedon consolidation test result for different square footings with width, B located within Layer
2. The settlement is computed for Allowable Bearing Capacities ranging from 125 to 350Kpa
around BH-112 as shown in the Table below. If maximum total settlement of 50mm is
considered, it can be selected any footing widths for any required load excreted on the
foundation soil without causing unwanted settlement.
From the settlement analysis of the foundation soil (See table below), the shaded areas are
permitted both in terms of shear and settlement for the given depths and widths (taking the
maximum settlement limit i.e. 50mm).
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 40/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+4 and G+7 Condominium Building
BEST Consulting Engineers plc January 2014
40
Table 5-5: Bearing Pressure analysis using settlement criteria around BH-150
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 41/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
41
6 CONCLUSION AND RECOMMENDATION
As a result of field and laboratory activities carried out and the analysis of the available data
and test results, the following engineering recommendations can be made:
6.1 Subsurface geotechnical materials
Sub-surface geotechnical investigation was conducted for G+4 and G+7 Condominium
Buildings that includes drilling of twenty four (24) boreholes, sampling, insitu and laboratory
testing. Based on visual description, in-situ and laboratory test results, the sub-surface
geology is sub-divided into three geotechnical layers. Accordingly, the geotechnical
investigation revealed the occurrence of three quasi homogenous geotechnical layers.
Layer 1: Soft to Medium stiff, highly plastic CLAY
The top most part of the building site is covered by soft to medium stiff, dark grey,
highly plastic CLAY with a maximum thickness of 2.40 around BH-140 and BH-141
(Table 4-3).Layer 2: Medium stiff to stiff, Silty CLAY
This layer is characterized by medium stiff to stiff, grayish brown, moist and highly
plastic Silty CLAY soil. It is encountered in all the boreholes underlying the top layer
1; the average field SPT N-values/300mm is 9.7 (Table 4-4).
Even if the soil is class is MH in USCS, after having discussion with the client and by
considering the nature of the soil type and the hydrometer analysis result, it has
been decided to set the soil in CH soil class.
Layer 3: Moderately to slightly weathered, fine grained BASALT
This layer is characterized by light gray, dominantly slightly weathered to fresh,closely to medium spaced joints, fine grained BASALT. It is encountered in few of the
boreholes drilled (Table 4-3).
6.2 Foundation seat and allowable bearing Capacity
Among the three geotechnical layers identified, the possible seat of the foundation footings
is Layer 2, which is grayish brown, highly plastic Clayey SILT.
The bearing capacity of the bearing layer is computed based on both the SPT N-Value and
using the laboratory UCS value. The allowable bearing capacity results obtained for isolated
foundation are given in tables Table 5-1 and 5-3. Since SPT reflects the bearing capacity ofthe whole materials under the influence depth of the foundation layer i.e. 0.5B above and
2B below the foundation depth and normally reflects the actual site condition of the
foundation layer , the bearing capacity computed from SPT are more reliable for this
project.
For the G+4 buildings: Based on the geotechnical site investigation and bearing capacity
analysis, it is recommended that the foundation footings shall seat on layer 2 starting from
2.0m from the surface after considering the bearing capacities appropriate for the design
load according to table 5.3; however, the effect of the expansive soil shall be considered
and appropriate mitigation measures and/or treatment techniques (discussed in section
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 42/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
42
4.8) shall be adopted to protect the buildings from damage due to the expansive nature of
the subsurface material.
To avoid the damage due to the expansive soils on the building, it is highly recommended
to improve the ground by replacing at least 1.5m of the subsurface material, starting from
the bearing depth (about 2.0m from the surface), with suitable non-expansive material
with a compaction of 95% standard proctor density for every 25cm fill or to extend the
depth of the foundation footings below 3.0m from the bearing depth (bearing depth is
about 2.0m from the surface) to pass the moisture fluctuation zone. In addition, proper
drainage system shall be constructed to collect surface runoff and to limit surface
infiltration during the rainy season.
For the G+7 buildings: Since the allowable bearing capacity values obtained are small (as
seen in Table 5-3 above) and the foundation material is also highly expansive soil, mat
foundation shall be used or the foundation ground shall be improved by imported non-
expansive granular material; mat foundation will be the best choice in order to avoid any
differential settlements that may happen in the building area.
A mat foundation is commonly used where the base soil has a low bearing capacity and/or
the column loads are so large that more than 50 percent of the area is covered by
conventional spread footings. Improving the foundation ground and to use mat
foundation on top of it will also be the best choice in order to avoid any differential
settlements that may happen in the building area and future failure of the buildings.
The swelling pressure test result (Table 4-8) shows that, the value of the swelling pressure is
small; the smaller value could be because of the current condition of the soil, i.e. the soil was
saturated during the test and it may already been expanded soil. Therefore , if it is possible
to maintain the current moisture content of the soil, during and after construction, the
building will be safe from the impact of swelling pressure from the subsurface material.
Table 6-1: Summary of bearing capacity for Mat foundation
BH-ID
Width, m
10 15 20 25 30 35 40 45 50 55 60
Allowable Bearing Capacity
BH-131&132 288 280 275 273 271 270 269 268 268 267 267
BH-133&134 288 280 275 273 271 270 269 268 268 267 267BH-135&136 288 280 275 273 271 270 269 268 268 267 267
BH-137&138 288 280 275 273 271 270 269 268 268 267 267
BH-139&140 288 280 275 273 271 270 269 268 268 267 267
BH-141&142 288 280 275 273 271 270 269 268 268 267 267
BH-151&152 495 480 472 468 465 463 461 460 459 458 457
BH-153&154 371 360 354 351 349 347 346 345 344 344 343
Remark: the building area is characterized by the presence of highly expansive, thick clay
soil; therefore, proper mitigation measures and/or treatment technique shall be
implemented to avoid any differential settlement that may happen in the building area
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 43/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
43
6.3 Material for backfill and compaction criteria
In general, materials for the backfilling should be non expansive granular, not containing
rocks or lumps over 150mm in greatest dimension, free from organic matter, with plasticity
index (PI) not more than 10. The backfill material should be laid in lifts not exceeding 250mm
in loose thickness and compacted to at least 95% of the maximum dry density at optimum
moisture content as determined by modified compaction test (Proctor) (ASTM D-1557).
In case of improving the foundation by imported selected material, following any excavation
activity, and prior to any fill placement, proof rolling should be performed. It is commonly
recommend to a vibratory roller use with appropriate static weight. Compaction of the fill
materials should continue until the roller has made at least ten passes over all areas of the
site and the soils appear to be relatively firm and unyielding. Half of the roller passes should
be perpendicular to the direction of travel of the other passes. Proof rolling should be
closely monitored by the concerned engineers to observe for unusual deflection of the soils
beneath the compacting equipment. If unusual or excessive deflection is observed, then the
areas should be undercut to firm soils and backfilled with structural fill placed in maximum
one-foot thick lifts. Backfill soils should be of the same composition and be compacted to
the same criteria as structural fill soils.
In confined construction areas, proof rolling and compaction of fill materials can be
compacted with manually operated vibratory compaction equipment. But, it should meet
the compaction criteria.
The following issues should also be addresses in the compaction processes:
- The compaction work shall be checked by inspecting or testing in order to insure
that the nature of the fill material, its placement water content and the
compaction procedures are consistent with those prescribed. The commoninsitu compaction checking tests are dry density and moisture content.
- The procedures for fill placement and compaction shall be selected in such a
way that stability of the fill is ensuring during the entire construction period and
the natural subsoil is not adversely affected.
- The source of fill material shall be appropriately tested to ensure that it is
suitable and adequate for the intended purpose. The type, number and
frequency of the tests shall be selected according to the type and heterogeneity
of the material.
6.4 Settlement consideration
In the design of any foundation, one must consider the safety against bearing capacity
failure as well as against excessive settlement of the foundation. In the design of most
foundations, there are specifications for allowable levels of settlement. Here, the maximum
settlement is recommended not to exceed 50mm.
Settlement analysis was done around BH-150 based on consolidation test result. Based on
the settlement analysis of the foundation soil (tables 5-5 in previous chapter), around BH-
150, for foundation depth less than or equal to 2m, the foundation fails totally by
settlement before shear, i.e. settlement is critical; for foundation depth greater than 2.0m,
the settlement will be permissible taking the maximum settlement limit i.e. 50mm;
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 44/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
44
however, settlement becomes critical for higher contact pressures. Generally, it can be
concluded that the foundation fails by settlement before shear; settlement is critical.
6.4.1 Seismic Consideration
In Ethiopia the afar depression and the Main Ethiopian Rift (MER) which is part of the East
African rift is where earthquake epicenters were aligned. The current volcanic activities and
the resulting geologic phenomena’s in Afar Depression and Main Ethiopian Rift (MER) are
good manifestations for tectonically dynamic nature of the zone.
Though seismic activity in the region (Addis Ababa) has not witnessed any serious
earthquakes, the project site is situated in a seismically medium dangerous part of the
country. So that it cannot rule out of the possibility of damaging earthquake from the
adjacent rift.
To generalize, the project site is located within the western rift margin of the country with
moderate seismic activity. Based on the Ethiopian Seismic Hazard Map (Gouin P 1976), the
area falls under Zone 2 corresponding earthquake magnitude (I100) of 7.4 to 6.5 on Richterscales and with ground acceleration 10.0 to 4.6% g and based on GSHAP it is located within
a Peak Ground Acceleration (PGA) zone ranging from 0.8 to 1.0m/s2
6.5 Considerations to Minimize Expansion Effect
which is also classified
as seismically moderately vulnerable for potential damage.
To minimize Expansion effects where it is not economically feasible to remove expansive
materials or to support foundations below depths of possible expansion, the effects can be
minimized as follows:
Since large seasonal changes in soil moisture are responsible for swelling, schedule
construction during or immediately after a prolonged rainy period when there will
be less potential volume change in the future.
Grade beams should contain sufficient steel reinforcement to resist the horizontal
and vertical thrust of swelling soils.
Provide impervious blankets and surface grading around the foundations to prevent
infiltration of surface water.
Locate water and drainage lines so that if any leakage occurs, water will not be
readily accessible to foundation soils thereby causing damage.
Construct proper drainage system to collect surface runoff and to limit surface
infiltration during the rainy season.
Avoid planting deep rooted trees since they will extract the moisture of the
subsurface material and cause differential settlement.
Maintenance programs shall be directed toward promoting uniform soil moisture
beneath the foundation during and after construction.
Consider stabilization of the foundation soils.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 45/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
45
6.6 Other Consideration
This report has been prepared for the exclusive use of Addis Ababa Housing Construction
office for G+4 and G+7 Condominium Buildings in Koye Feche III project area (Parcel 26
Building area) specific application for the geotechnical aspects. Our conclusions and
recommendations have been rendered using generally accepted standards of geotechnicalengineering and geological practices.
As a general remark, the following supplementary consideration shall be considered during
foundation construction:
• It is advisable to verify the nature and actual depth of occurrence of the bearing
layers when construction of the building starts and make adjustments if
necessary. Our conclusion and recommendation do not reflect variations in the
subsurface conditions that are likely to exist in the region of our borings and in
unexplored areas of the site. These variations are due to the inherent variabilityof the subsurface conditions of the geology of the area. If variations become
apparent during construction, it will be necessary to re-evaluate our conclusions
and recommendations based upon our on-site observations of the conditions.
• Exposure to the environment may weaken the subsurface material at the
foundation bearing level if the foundation excavations remain open for long
time.
• It is recommended to design an effective rainwater drainage system to get rid of
the consequences of the rainwater percolation into the layers. The site should
be graded so as to direct rainwater and water away from all planned structures.
If drastic changes are found on the subsurface geology and also if there is a change in the
design or the location of the proposed substructures, the recommendations presented in
this report must not be considered valid unless the changes are reviewed whether the
changes are consistent with the intent of our recommendations.
Finally, it should be noted that the results and recommendations of this report are solely
based on the site geotechnical investigation through core drilling of 24 boreholes including
insitu SPT test, collected samples and laboratory testing and assuming that the subsurface
conditions do not significantly deviate from those encountered.
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 46/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
46
REFERENCES
1. Arora, 2003. Soil Mechanics and Foundation Engineering, 6th
2. ASSHTO, 1996, Standard Specification for Highway Brdiges, 16
edition
th
3. ASTM, American Society for Testing and Materials
edition, American
Association of States Highway and Transportation officials, D.C
4. Bowels, 1997, Foundation Analysis and Design
5. CIRIA, 1995. Construction Industry Research Information Association Report No. 143,
SPT Methods and Use
6. Donald, P. Coduto, Foundation Design Principles and Practice. Second edition
7. NAVFAC DM7-02 Foundations and Earth Structure
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 47/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
47
APPENDICES
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 48/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
48
Appendix 1
Borehole Logs and Cross Sections
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 49/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
49
Appendix 2
Laboratory Test Results
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 50/51
Addis Ababa Housing Construction office
Geotechnical Sub-Surface Investigations and Foundation Recommendations for G+ 4 and G+7 Condominium Building
BEST Consulting Engineers plc April 2014
50
Appendix 3
Allowable Bearing Pressure Analysis
Sheets
8/18/2019 Geotechnical Report and foundation recomendation
http://slidepdf.com/reader/full/geotechnical-report-and-foundation-recomendation 51/51
Addis Ababa Housing Construction office