APPENDIX 15 · 2019. 7. 15. · appendix 15 geotechnical studies . mbongendhlu geoenvironmental...
Transcript of APPENDIX 15 · 2019. 7. 15. · appendix 15 geotechnical studies . mbongendhlu geoenvironmental...
APPENDIX 15
GEOTECHNICAL STUDIES
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd.
Reg. no. 2005/008292/07
GEOTECHNICAL INVESTIGATIONS REPORT
MNGENI SHAFT
ZULULAND ANTHRACITE COLLIERY REPORT NO: 2019/0004
PREPARED FOR: MARUAPULA ENGINEERS
OFFICE 1B WOODHILL PARK CENTRE
948 ST BERNARD DRIVE
GARSFONTEIN 0042
PREPARED BY: E.T. NCUBE Pr. Sci. Nat DATE: MARCH 2019
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Consulting Engineering Geologists, Geohydrologists and Environmental Scientists
47 Jenner Street, Rayton, 1001
P.O. Box 83, Rayton 1001
Fax: 0866056558, Cell: 0724549247
[email protected]/[email protected]
Contact Person: Edwin Thabile Ncube
Khulani Geoenviro Consultancy
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 1
1. INTRODUCTION AND TERMS OF REFERENCE
At the request of Maruapula Engineers, Mbongendhlu GeoEnvironmental Engineers (Pty)
Ltd conducted a Geotechnical Investigation for pollution control dams; stockpile area and
office block for Mngeni Shaft - Zululand Anthracite Colliery in KwaZulu Natal. (Google
Image).
The aim of the investigation was to:
determine the geology and general mechanical properties of the soil and rock horizons
present on the site
make recommendations on the development potential of the site
comment on the excavability and usability of various materials present on site, and
comment on site water regime particularly pertaining to shallow groundwater/seepage
and surface water management.
The investigation was carried out in accordance with the current industry
guidelines listed below.
2. AVAILABLE INFORMATION AND LITERATURE USED
Information from the following literature and guidelines were used in the investigation and
assessment of the site:
1:250 000 Geological Map Series 2830 Dundee and Google Images.
“Guidelines for Urban Engineering Geological Investigations” published by South
African Institute of Engineering Geologists (1997).
“Revised guide to soil profiling for civil engineering purposes in Southern Africa” by
Jennings et al. (1973).
“Soil Survey for Engineering” by Brink A.B.A. et al. (1982).
“Presumed Allowable bearing capacities under static loading” BS 8004 Code of
Practice for Foundations, 1986.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 2
3. APPROACH AND METHODOLOGY
The investigation comprised of liaison; desk study; fieldwork; laboratory tests; data
analysis and reporting.
Liaison involved discussions with the Project Management and Locals.
Desk study concentrated on geology of the area.
Fieldwork comprised of Visual Inspection, In situ Soil Profiling, Testing and
Sampling.
Samples were submitted to Snalab, to analyze for diagnostic mechanical soil
properties.
Soil results were interpreted based on “Soil Survey for Engineering” by Brink et al
(1982).
The report does not reproduce any section of the guidelines and should be used in
conjunction with guidelines listed above and/or any other relevant current
construction industry guidelines.
4. SITE
4.1 Description
The investigated site is in a veld roughly 6km north east of the ZAC Offices. It is
polygonal within an area roughly described by:
A 280 11’ 03.30”S B 280 11’ 04.50”S
310 43’ 22.60”E 310 43’ 27.10”E
C 280 11’ 13.00”S D 280 11’ 15.30”S
310 43’ 25.40”E 310 43’ 22.90”E
E 280 11’ 13.70”S F 280 11’ 05.30”S
310 43’ 16.80”E 310 43’ 16.60”E
Roughly, +8.5 ha (estimated from Google Pro) was covered by the study.
The site is currently being used as grazing land mainly for goats – and during the
time of the investigations the area was vegetated /pristine. Plate 1 shows
parts of the site.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 3
The climate of the area is ‘sub humid dry’ with intermittent rainfall in summer.
Surface drainage is through sheet flow into dongas that cut through the western
side of the site. The dongas feed into Black Mfolozi River which is within a
kilometer radius from the site.
Plate 1: Parts of the site (ZAC, March 2019)
4.2 Geology
The site is in an area overlain by medium to coarse grained sandstone, grey
micaceous shale and coal of the Vryheid Formation that belongs to Ecca Group of
the Karoo Sequence. Also occurring in the area are dolerite intrusions of the
Jurassic (1:250 000 Geological Map Series 2830 Dundee).
On site, transported material of alluvial and hillwash origin overly residual/
weathered sandstone/shale material.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 4
5. GEOTECHNIAL INVESTIGATIONS
5.1 Liaison
The following information was collected during this exercise:
A new shaft is to be opened on site.
Two pollution control dams, a stockpile area and a single storey office block
will be built on site.
Mfolozi River rarely runs dry.
Clayey material overlies thick porous sandstone (Consultant Mine Geologist).
5.2 Desk Study
The study concentrated on the geology as no previous geotechnical reports of the
area were obtained.
5.3 Fieldwork
Fieldwork comprised of Visual Inspection, In situ Soil Testing, Profiling and
Sampling for laboratory analysis.
5.3.1 Visual Inspection
The area was thoroughly inspected and the following observations were
made:
The site is currently used as grazing area.
There is a village within 1km radius from the site.
They are dongas that expose sandstone/shale below transported
clayey cover soil.
There are some ant holes on site.
There is also hillwash material in parts of the site.
Plate 2 is a collection of photos randomly picked showing different
sceneries that affect the geotechnical aspects of the area.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 5
Plate 2: Randomly picked photos showing different surface scenic (ZAC, March 2019).
5.3.2 In Situ Soil Profiling, Testing and Sampling
Fourteen test pits, dug by a TLB, were used to test, profile and sample the
soil. The position and the physical properties of the pits are shown in
Google Image and Table 2 respectively. Their locality was mostly dictated
by accessibility. Care was taken not to destroy vegetation and donga
banks. Minimal vegetation clearance in thick bush was done along an
existing path used by locals. On the main, testpits were dug on open and
easily accessible places.
The testpits were dug as deep as possible up to refusal/considerable
resistance. The profiles were recorded using 6 descriptors, viz.: moisture
condition, colour, consistency, structure, soil texture and origin (i.e.
MCCSSO for short) according to the guidelines of Jennings et al. (1973).
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 6
Photos of profiles were taken and are included in the text or clipped on
detailed soil profile sheets in Appendix.
Seven disturbed samples - taken on different dates - were submitted to
Snalab for compaction and foundation indicator analysis. No undisturbed
samples were taken as the bulk of the site was generally fissured.
Table 1: Physical properties of Testpits
TESTPIT
I.D.
POSITION
Latitude Longitude
DEPTH OF
PIT(m)
SAMPLING
DEPTH
(+ m) Pit 1
Pit 2
Pit 3
Pit 4
Pit 5
Pit 6
Pit 7
Pit 8
Pit 9
Pit 10
Pit 11
Pit 12
Pit 13
Pit 15
Pos 1
Pos 2
280 11’ 04.00”
280 11’ 04.80”
280 11’ 05.90”
280 11’ 07.70”
280 11’ 09.70”
280 11’ 10.30”
280 11’ 12.20”
280 11’ 12.80”
280 11’ 13.90”
280 11’ 14.70”
280 11’ 10.50”
280 11’ 09.80”
280 11’ 08.40”
280 11’ 08.30”
280 11’ 08.40”
280 11’ 08.30”
310 43’ 23.50”
310 43’ 22.00”
310 43’ 21.10”
310 43’ 19.50”
310 43’ 20.60”
310 43’ 19.50”
310 43’ 18.20”
310 43’ 21.40”
310 43’ 23.20”
310 43’ 22.10”
310 43’ 22.00”
310 43’ 23.10”
310 43’ 24.40”
310 43’ 21.50”
310 43’ 24.40”
310 43’ 21.50”
1.20
1.00
0.90
0.70
0.60
1.10
0.60
0.70
0.50
0.60
0.80
0.80
1.10
0.30
0.40
0.30
0.40
0.40
0.30
0.60
Table 2: In situ consistency test results
SAMPLE I.D. MOISTURE CONTENT AND CONSISTENCY
Pit 1
Pit 2
Pit 3
Pit 4
Pit 5
Pit 6
Pit 7
Pit 8
Dry – stiff
Dry – stiff
Dry - stiff
Dry – stiff
Dry – stiff
Dry - stiff
Dry – stiff
Dry – stiff
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 7
Pit 9
Pit 10
Pit 11
Pit 12
Pit 13
Pit 14
Dry - stiff
Dry – stiff
Dry – stiff
Dry - stiff
Dry – stiff
Dry – stiff
From the in situ profiling and testing, the following observations were
made:
The site is covered by clayey alluvium overlying weathered
shale/sandstone.
The profile has more or less uniform texture across the site.
Consistency was generally stiff becoming stiffer/denser with depth.
Clayey material is dominantly greyish brown with visible ground cracks in
some places.
Shale/sandstone is well exposed along donga beds.
No groundwater seepage was encountered in all testpits.
Plates 3 are randomly picked photos showing textural uniformity across the site.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 8
Plate 4: Uniform texture across the site (ZAC, March 2019)
5.4 Laboratory Test Results
Laboratory results are summarized in Tables 3. Detailed Test Results Sheets are
presented as Appendix.
Table 3 Summary of the Laboratory Indicator Test – Disturbed Samples
SAMPLE I.D.
FINES
%
PI LS PE AASTHO Classification
Pos 1 (Layer 1)
Pos 2 (Layer 2)
Pit 1 Layer 1
Pit 4
Pit 9
Pit 10
Pit 15
43
46
31
31
37
40
29
10
14
14
8
10
16
8
4.8
7.0
6.9
3.9
5.4
7.7
4.3
Low
Low
Low
Low
Low
Low
Low
A-4(1)
A-6(2)
A-2-6(1)
A-2-4(0)
A-4(0)
A-6(2)
A-2-4(0)
PI– Plasticity Index, LS–Linear Shrinkage, PE Potential Expansion,
Table 4 Compaction Tests
SAMPLE I.D. OMC
(%)
MMD
(kg/m3)
CBR
(100% Rel Comp)
Pos 1
Pos 2
4.8
6.3
2055
2034
9.7
12.3 MDD – Maximum Dry Density, OMC – Optimum Moisture Content, CBR – California Bearing Ratio
From the lab test results, the following deductions could be made:
The site is generally covered by clayey sand with varying proportions of silt.
The Plasticity Index ranges from +8 to +16.
The material can be compacted to a high density at low optimum moisture
content.
The soils range from excellent to poor subgrade material according to PRA
ratings in Brink 1982 {A-2-6(4) and A-6(2)}.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 9
6. SITE ASSESSMENT
The site is in an area covered by clayey soils believed to be predominantly of alluvial origin.
These are underlain by shale/sandstone and coal. Clayey material suffers volumetric
changes when subject to moisture variation. The extent of volume change/expansion
depends on clay type, proportions of coarser material and layer thickness.
Laboratory results show a generally low activity – Plasticity Index (PI) ranging from 8 to
16. This can be attributed to high sand content and lean silt. However, this does not rule
out occurrences of pockets with higher PI - suggesting concerns for swelling and
shrinkage problems. This is also demonstrated by shrinkage cracks/shatter in some parts
of the site.
The typical permeability value of clayey sand is +5.50 x 10-9m/s. On site, this figure may
be affected by fissures, roots, ant cavities, exploration holes and sand proportions. No one
figure can describe the whole area. The pH is slightly acidic – at 6.49 and electrical
conductivity at 0.171S/m. This adds to the corrosive nature of fine material.
The typical presumed bearing capacity for stiff clayey soils ranges between 150 to
300kPa. However, when softened by water, it losses its strength to a bearing capacity of
less than 75kPa. Laboratory CBR at 100% relative compaction is 9.7% - indicative of
relatively low strength, though producing high maximum dry density.
The shale/sandstone underlying clayey material has proportions of clay, silt and sand that
suggest a potential problem of settlement and/or collapse.
Settlement might be caused by:
compaction due to the rearrangement of particles responding to new load
closure of voids where there is open texture
Collapse occurs due to sudden weakening of the ‘gluing effects’ of fine material when
saturated with water. This is typical of relatively loose profiles that show stability/pseudo
strength during excavation as shown by the sidewalls of all test pits. However, the high
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 10
content of fine material and shallow shale/sandstone to some degree, mitigate sever collapse.
The varying depths to refusal across the site and the occurrence of hillwash indicate
consistency variation both laterally and vertically. It is therefore logical that some structures
will straddle hard and soft ground resulting in differential settlement.
The occurrence of anthills presents a potential problem of collapse when resultant
underground cavities yield to load. The position and sizes of anthills that are on site might
not be representative of the underground cavities as they might have been destroyed and
eroded overtime. Also, where hard ground is shallow the cavities might not be confined to
where the holes manifest on the ground but could be spread laterally underground. This
exacerbates the collapse potential.
Permeability of shale and sandstone are in the region of 1.00 x 10-13 3.00 x 10-10m/s
respectively. The figures can be affected by bedding fissures/joints, roots, ant cavities and
exploration holes. No solid figure can be a representative of the whole area.
The typical presumed allowable presumed bearing capacity for loose sand to strong shale
ranges between <100 to 2000kPa. Shale generally losses integrity when soaked in water for
long periods. Laboratory CBR at 100% relative compaction is 12.3% - indicative of
relatively low strength.
The site is within a kilometer radius from a semi to perennial Black Mfolozi River.
Topography gentle slopes southwards towards the river. Clayey material has poor
percolation, leading to run off and erosion as testified by the numerous dongas on the
western side of the site. The soil on the eastern side is protected by vegetation.
No slope instability is foreseen.
Though no groundwater seepage was encountered, soil discolouration is evidence of past or
seasonal groundwater inundation. Clay and shallow shale inhibit water from percolating
downwards and might result in perched ground- and surface water during prolonged periods
of heavy rain. Clayey soils are slippery during rainy seasons. With seasonal moisture
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 11
changes, corrosiviness is anticipated – due to high percentage of fine and slightly acidic
material.
The site is within a coal mining region - hence seismic activities are highly anticipated.
Excavation will need an excavator as the TLB struggled on stiff soil and hard country rock.
As construction material, the soil in generally classifies as excellent to poor subgrade, {A-
2-4(0) and A-6(2)} – according to the PRA ratings in Brink et al. (1982).
7. CONCLUSION
On the basis of the information presented above, the profile is generally considered as not
good founding material at its natural state. Heaving, Settlement and/or Collapse problems
are anticipated across the site. It is, however, believed that the site is suitable for the
proposed development if subjected to stringent and appropriate precautionary measures. The
precautionary measures should at all cost minimize negative effects on the prevailing
ecosystem. The site should be regarded as very sensitive with respect to ground- and surface
water pollution – as it is within a kilometer radius from a river that passes through many
villages, game reserve and farms downstream.
Even though the potential expansiveness of the sand clay layer is expressed as low in the
laboratory test results, precautionary measures must accommodate a safety factor. The
thickness of the layer is on average around 1m. Such material is also known to be susceptible
to long-term settlement. Exploration logs can be used to get precise thickness and depth to
groundwater, therefore H2/S2 should be the minimum design on clayey layer.
The integrity of underlying shale and sandstone can be affected by differential weathering,
loose loamy beds and ant activities. These can trigger some differential settlement,
settlement and/or collapse of structures. C1/S2 should be the minimum design on
shale/sandstone; unless proven otherwise during foundation excavation. The main donga
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 12
can be sealed to curb erosion and divert clean water away from contamination sources.
Haulage routes, driveways, walkways and parking should be designed and engineered in a
way that totally quells potential slipperiness and minimizes contamination of water
resources.
It should be emphasized that the evaluation is based on interpolation of information
between testpits that are unevenly distributed, it is therefore possible that variations from
the described conditions might occur; also taking into account that footprint investigations
couldn’t be conducted on all structures.
It is however believed that the information presented exhausts the bulk of the problem and
the site can be engineered - within the confines of legislation - to suite the proposed
development.
8. GROUND PREPARATION AND CONSTRUCTION RECOMMENDATIONS
Planning should highly consider preserving the prevailing ecosystem.
A 1:100 year flood line should be determined and a buffer zone added to protect the
river/valley.
Where vegetation is cleared, all roots and humus must be removed from site.
To minimize erosion; the main donga within the site can be concrete sealed and used
as storm water conduit/tunnel.
Silt traps must also be placed strategically along the tunnel.
Infrastructure:
Though there are many solutions, a raft foundation is highly recommended for office
block (SANS10400H:2012).
All surface water must be directed away from the structure and stringent precautionary
measures on wet services must be applied as stipulated in guidelines.
Remove up to half a meter depth of clay along ‘haulage tracks’, drive ins, walkways
and packing.
Refill with G5 to G7 material and pave according to anticipated loads.
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 13
Pollution control dams and stockpile:
Remove all clay up to shale/sandstone across the site. (This will reduce ground activity
and add to storage capacity.)
Engineer some of the clayey material for use as core/seal to curb permeability and
improve attenuation capacity below and around the dams and the stockpile.
Thorough compaction of replacement or blended material - coupled with shallow
shale/sandstone- should provide adequate bearing capacity.
An impermeable reinforced concrete seal for dams and stockpile is highly
recommended due to high sensitivity of the site.
On the occurrence of conditions that totally vary from those described above, a
geotechnical competent person should be consulted for a qualified opinion.
Mbongendhlu GeoEnvironmental Engineers cannot be held liable for any damages to any
structure should the recommendations contained herein be not properly executed.
EDWIN THABILE NCUBE Pr. Sci. Nat ENGINEERING GEOLOGIST
Mbongendhlu GeoEnvironmental Engineers (Pty) Ltd. Reg. No. 2005/008292/07 Page 14
REFERENCES
Brink A.B., Partridge T.C. and Williams A.A.B. (1982), Soil Survey for Engineering, Caledon
Press, Oxford.
Bryne G., Everett J.P. and Schwartz K. (1995), A Guide to Practical Geotechnical Engineering
in Southern Africa, Franki, Johannesburg.
Jennings J.E., Brink A.B.A. and Williams A.A.B. (1973) Revised guide to soil profiling for civil
engineering purposes in Southern Africa, Transactions of the South African Institute of Civil
Engineers, Vol. 15, No 1.
South African Institute of Engineering Geologists (1997), Guidelines for Urban Engineering
Geological Investigations.
BS 8004 Code of Practice for Foundations (1986) “Presumed Allowable bearing capacities under
static loading”