Post on 23-Dec-2015
description
1st Chapter
Foundation and Geotechnical Engineering
6th
Semester
Compiled by
Dr. Irshad Ahmad
Department of Civil Engineering
N-W.F.P. University of Engineering & Technology
Peshawar
Peshawar, N-W.F.P., Pakistan, 2008
©Irshad 2008
ii
AUTHOR'S DECLARATION
These notes are complied for the 6th
semester students in the department of civil engineering, NWFP
UET Peshawar. I highly acknowledge the authors of the books on this subject from where the
material has been compiled.
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Abstract
Enter abstract here.
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Acknowledgements
Enter acknowledgements here.
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Dedication (if included)
Enter dedication here (if included-if no dedication page is included, the Table of Contents should start
at page v). If there is no dedication, delete this page; when updating the table of contents, this page
will no longer appear in the table of contents (if this page has been deleted).
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Table of Contents
AUTHOR'S DECLARATION...................................................................................................... ii
Abstract ..................................................................................................................................... iii
Acknowledgements .....................................................................................................................iv
Dedication (if included) ................................................................................................................v
Table of Contents ........................................................................................................................vi
List of Figures ............................................................................................................................vii
List of Tables ........................................................................................................................... viii
Chapter 1 INTRODUCTION.........................................................................................................1
1.1 Setting tone for the course....................................................................................................1
1.2 Foundation..........................................................................................................................2
1.3 Foundation Engineer............................................................................................................2
1.4 Types of foundations ...........................................................................................................2
1.4.1 Shallow foundations ......................................................................................................2
Spread/Single/ Isolated footing: ......................................................................................2
Strip/wall footing: ..........................................................................................................3
Combined footing: .........................................................................................................3
Mat/Raft foundation .......................................................................................................8
1.4.2 Deep foundations ........................................................................................................10
1.5 Requirement of foundation system......................................................................................10
1.5.1 Safety requirement ......................................................................................................10
1.5.2 Depth requirement.......................................................................................................10
1.5.3 Spacing requirement....................................................................................................11
1.5.4 Economic and functional requirement...........................................................................11
1.6 Steps for Designing a Foundation .......................................................................................11
1.7 Foundation Selection Process .............................................................................................12
1.8 Type of loads ....................................................................................................................15
1.8.1 Dead loads..................................................................................................................15
1.8.2 Live loads...................................................................................................................15
1.8.3 Environmental loads....................................................................................................15
Appendix A Sample Appendix ....................................................................................................16
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List of Figures
Insert List of Figures here.
viii
List of Tables
Insert List of Tables here.
1
CHAPTER 1
INTRODUCTION
1.1 Setting tone for the course
Consider a frame structure in figure 1.1. The middle column (1ft 1ft cross section) carries a load (P)
of 40 tons which has to be transmitted to soil safely. For this example consider that
The ultimate pressure that will cause shear failure of the soil is 10 tsf. If a factor of safety
equal to 4 against shear failure is assumed, the allowable pressure intensity on the soil is 10/4
= 2.5 tsf. The pressure on the soil should not be more than this to prevent shear failure of the
soil.
However, the allowable pressure intensity to limit the foundation settlement to 1 inch, for this
example, is 1.8 tsf. Any pressure intensity more than this will cause unacceptable foundation
settlement (i.e. more than 1 inch)
If we place the column directly on ground, the bearing area of the column that transmits the
load to the soil is only A = 1 x 1 = 1 sft. The pressure on the soil is P/A = 40/1 = 40 tsf. This
is huge pressure. This pressure exceeds both limits of shear failure (2.5 tsf), and 1 inch
foundation settlement (1.5 tsf).
One way to deal with the problem is to increase the area of the column, but this is clearly not
a wise solution. Another way is to enlarge the area of the column only at its base. This
enlarged portion of the column used to transmit the load of structure and of itself is called
foundation.
Let us increase the area of the footing by (5 ft 5ft), A =25 sft. The pressure intensity on the
soil now is P/A = 40/25 = 1.6 tsf. This pressure is less the allowable pressure to prevent shear
failure, and also less than the pressure intensity (1.8 tsf) that will cause 1 inch foundation
settlement.
The factor of safety against shear failure is now increased to FOS = 10/1.6 = 6.25 and the
foundation settlement is within 1 inch.
In this example clearly the settlement criteria governs. That is the design of footing is
governed by settlement criteria and not the shear failure of the soil.
How to calculate the ultimate pressure intensity that will cause the shear failure of the soil is the
theme of chapter-2 (for shallow foundation), and chapter-4 (pile foundation). How to estimate the
foundation settlement is covered in chapter-3 for shallow foundation and chapter-4 for deep
foundation.
Note the higher factor of safety generally used in foundation engineering because there is natural
variation in the soil properties, our sampling and testing techniques are not perfect, the theories to
find bearing capacity have limitations, and any foundation failure are difficult and expensive to
repair.
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1.2 Foundation
It is the interface between superstructure (or other load carrying component like
machinery/tower/pipes tanks) and soil. The function of foundation is to transmit to, and into, the
underlying soil or rock the loads supported by it and its self weight.
Figure 1-1 Bringing the pressure intensity within allowable limits
1.3 Foundation Engineer
The title foundation engineer is given to that person who by reason of training and experience is
sufficiently versed in scientific principles and engineering judgment (often termed “art”) to design a
foundation.
The necessary scientific principles are acquired through formal educational courses in geotechnical
(soil mechanics, geology, foundation engineering) and structural (analysis, design in reinforced
concrete and steel, etc) engineering and continued self-study via short courses, professional
conferences, journal reading, and the like.
1.4 Types of foundations
Foundations may be classified as Shallow foundations and Deep foundations.
1.4.1 Shallow foundations
For shallow foundations, the depth to width ratio of footing is (D/B) 1 but may be somewhat more.
Different types of shallow foundations are:
Spread/Single/ Isolated footing:
A footing carrying a single column is called spread footing, since its function is to spread the load
laterally to the soil so that the stress intensity is reduced to a value that the soil can safely carry.
P=40 tons P=40 tons
3
Figure 1-2 Types of spread footings
Single footings may be of constant thickness (figure 1-2a) or either stepped (figure 1-2b) or sloped
(figure 1-2c). Stepped or sloped footings are most commonly used to reduce the quantity of concrete
away from the column where the bending moments are small and when the footing is not reinforced.
Spread footings are most widely used because they are economical. Construction of footings requires
a least amount of equipment and skill and no heavy or special equipment is necessary. Furthermore,
the conditions of the footing and the supporting soil can be readily examined.
Strip/wall footing:
A wall footing is simply a strip of reinforced concrete or brick masonry wider than the wall (figure 1-
2 d). The function of wall footing is also to distribute (spread) the load laterally as in isolated
footings. A pedestal may be used to interface metal columns with spread or wall footings that are
located at the depth in the ground. This prevents possible corrosion of metal through direct contact
with the soil (figure 1-2e)
Combined footing:
It may not be possible to place columns at the center of a spread footing if they are at the property
line, near mechanical equipment locations, or irregularly spaced. Columns located off-center will
usually result in a nonuniform soil pressure. To avoid the nonuniform soil pressure, an alternative is
to enlarge the footing and place one or more of the adjacent columns in the same line on it (figure
1.3). These types of footings are called combined footing. A combined footing may have either
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rectangular or trapezoidal shape or be a series of pads connected by narrow rigid beams called a strap
footing (figure 1.4).
The footing can be rectangular if the column that is eccentric with respect to a spread footing carries
a smaller load than the interior columns (figure 1.4a). The footing geometry is made such that the
resultant of the several columns is in the center of the footing area. This footing and load geometry
allows the designer to assume a uniform soil pressure distribution (figure 1.5).
A combined footing will be trapezoid-shaped if the column that has too limited a space for a spread
footing carries the larger load (figure 1.4b). In this case the resultant of the column loads (including
moments) will be closer to the larger column load, and doubling the centroid distance as done for the
rectangular footing will not provide sufficient length to reach the interior column. In most cases
1-3 typical layout of combined footings for column loads as shown; more than two columns can
be used
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Figure 1-4 Types of combined footings (a) Rectangular (b) Trapezoidal (c) Strap
Figure 1-5 Rectangular footing when P1<P2
trapezoidal footing would be used with only two columns, however, more than two columns can also
be supported on trapezoidal footing. The forming and reinforcing steel for a trapezoid footing is
somewhat awkward to place. For these reason it may be preferable to use strap footing where
possible, since essentially the same goal of producing a computed uniform soil pressure is obtained.
A strap footing is used to connect an eccentrically loaded column footing to an interior column
(figure 1.4c). The strap is used to transmit the moment caused from eccentricity to interior column
footing so that a uniform soil pressure is computed beneath both footings. The strap serves the same
purpose as the interior portion of a combined footing but is much narrower to save materials.
A strap footing may be used in lieu of a combined rectangular/trapezoidal footing if the distance
between columns is large and /or the allowable soil pressure is relatively large so that the additional
footing area is not needed.
A strap footing should be considered only after a careful analysis shows that spread footings-even if
oversize-will not work. The extra labor and forming cost for this type of footing make it one to use as
last resort.
Uniform pressure distribution
M1 M2
P1 P2 R
S
Col (1)
Col (2)
L
c w w
xw
cL
PP
MMSPx
M col
22
0
21
212
1
x
6
7
8
Mat/Raft foundation
A mat/raft foundation is a large concrete slab used to interface one column, or more than one column
in several lines, with the base soil (figure 1.6). It may encompass the entire foundation area or only a
portion. A mat or raft foundation is used where 50% of the area is covered by conventional spread
footings or in soils with extremely erratic characteristics. It is common to use mat foundations for
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deep basements both to spread the column loads to a more uniform pressure distribution and to
provide floor slab for the basement. A particular advantage for basements at or below the GWT is to
provide water barrier. Depending on local costs and noting that a mat foundation requires both +ive
and –ive reinforcing steel, one may find it more economical to use spread footings-even if the entire
area is covered. Spread footings avoid the use of –ive reinforcing steel and can be accomplished as in
figure 1.7 by pouring alternate footings, to avoid formwork, and using fiber spacer boards to separate
the footings poured later.
Figure 1-6 Common types of mat foundations. (a) Flat plate (b) plate thickened under columns
(c) waffle slab (d) plate with pedestals (e) basement walls as part of ma
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Figure 1-7 Mat Versus possible use of spread footings to save labor, forming costs, and negative
reinforcement
1.4.2 Deep foundations
For deep foundations the length L/B ratio i.e. (length or depth of foundation to its width or diameter)
4. For types of deep foundations and uses please refer to chapter 5.
1.5 Requirement of foundation system
1.5.1 Safety requirement
1. Factor of safety against shear failure of the soil should be adequate (FOS 2.5-3)
2. Settlement (total or differential): The settlement should not cause any damage to the
structure or interfere with the function of the structure
3. Factor of safety against “structural failure” of foundation should be adequate.
1.5.2 Depth requirement
1. Prevent movement due to soil volume changes by seasonal freezing and thawing of the
ground.
2. Footing should be below zones of high volume changes due to moisture fluctuation. Many
soils particularly with those of high plasticity shrink greatly on drying and swell upon the
addition of moisture.
3. Prevent wind or water erosion.
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4. By pass unsuitable soil layer such as peat, expensive clay, soft unconsolidated deposit, and
old soil layer
5. Prevent footing movement or distortion by plant or tree root growth.
Figure 1-8 Approximate frost-depth contours in meters for the United States
1.5.3 Spacing requirement
The foundation must be spaced appropriately in order to prevent distress in adjacent foundation.
1.5.4 Economic and functional requirement
The foundation should be economical and performs satisfactorily the intended function.
1.6 Steps for Designing a Foundation
Following minimum steps are required for designing a foundation:
1. Locate the site and the position of load. A rough estimate of the foundation load(s) is usually
provided by the client or made in-house. Depending on the site or load system complexity, a
literature survey may be started to see how others have approached similar problems.
2. Physically inspect the site for any geological or other evidence that may indicate a potential
design problem that will have to be taken into account when making the design or giving a design
recommendation supplement the inspection with any previously obtained soil data.
3. Establish the field exploration program and, on the basis of discovery (or what is found in the
initial phase), set up the necessary supplemental field testing and any laboratory test program.
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4. Determine the necessary soil design parameters based on integration of test data, scientific
principles, and engineering judgment. Simple or complex computer analyses may be involved.
5. Design the foundation using the soil parameters from step 4. The foundation should be
economical and be able to be built by the available construction personnel. Take into account
practical construction tolerances and local construction practices. Interact closely with all
concerned (client, engineers, architect, contractor) so that the substructure system is not
excessively overdesigned and risk is kept within acceptable levels. A computer may be used
extensively (or not at all) in this step.
1.7 Foundation Selection Process
The rational selection of a safe foundation involves a systematic process of evaluation of many
factors, including structural design load, environmental effects, subsurface condition, performance
requirement, construction methods and economics. A suggested sequence of steps in this process is
outlined in figure 1.9 and discussed briefly below. Additional discussion of the various phases of the
process is presented in subsequent of this synthesis.
The foundation selection must be based on information about the proposed structure and the site
conditions. Ideally, a preliminary evaluation of the subsurface conditions and potential foundation
problems should be included in the preliminary site location studies. However, foundation conditions
frequently are overlooked in site selection. Similarly, the type of structure usually is established prior
to the foundation investigations. Thus, the type and site of structure, the foundation design loads and
the required performance criteria often are specified by the structural engineers with little or no
geotechnical input.
The field and laboratory geotechnical investigations should be planned by a geotechnical engineer or
engineering geologist who understands the type of information that will be needed in the foundation
selection studies. Thus, this individual must recognize the requirements of various types of structures,
the foundation alternatives that may be considered and the types of analyses that will be required to
make a rational selection among these alternatives.
The analyses of behavior of various potential foundation systems in reponse to design loads and
environmental factors are the responsibility of the geotechnical or foundation engineer. The predicted
behavior of each alternative then is compared with the performance requirements established by the
structural engineer. For foundations that appear to provide satisfactory performance, potential
construction problems and cost are considered. Maintenance costs also should be considered. Finally,
the foundation system that will provide satisfactory performance at least cost is recommended.
As noted in figure 1.9 normally shallow foundation should be evaluated first.
If shallow foundations will perform satisfactorily, they usually will be the most economical
alternative. If the response of a shallow foundation appears to be satisfactory or marginal, other
alternatives must be considered. Various types of deep foundation and/or ground modification
techniques may be evaluated. Ideally, modification of the primary structure to reduce performance
criteria also should be considered. However, this option is seldom used in current practice.
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The foundation investigation and recommendations are presented in a foundation report, which is
prepared by the geotechnical engineer.
The report should include,
Site description
Boring logs and subsurface profile
Results of laboratory and field tests for identification, classification and relevant engineering
properties of strata
Review of design loads
Analyses of behavior of each foundation alternative
Evaluation of predicted performance in relation to performance requirements
Discussion of potential construction problems (excavation, dewatering, etc.)
Discussion of relative costs
Recommendations
Foundation type
Foundation design criteria (allowable loads, depth, etc.)
Special construction methods
Construction monitoring where required
The geotechnical engineer’s recommendations are submitted to the structure designers, who
ultimately must approve the design recommendations and prepare the detailed structural design and
the construction plans and specification for the foundation. Finally, the geotechnical engineer must be
prepared to respond to problems that may develop during construction. Because of the inherent
variability of subsurface conditions, it is not uncommon to encounter unanticipated conditions which
may significantly affect the foundation design. Minor and occasionally major design revisions may be
necessary to accommodate the unforeseen conditions. In other instance, the geotechnical report may
have recommendation monitoring of field behavior during and/or after construction.
In summary, the selection, design and construction of an adequate cost affective foundation require
coordination among geotechnical engineers. It is desirable for the agency to be organized in a manner
that permits the geotechnical engineer to be directly involved in all phases of the foundation work
from preliminary planning through construction.
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Figure 1-9 Flow chart of foundation selection process
After selection Acceptable
Not Acceptable
OBTAIN SITE INFORMATION
Surface Subsurface
-Topography -Soil/Rock Strata -Hydrology -Soil/Rock properties
-Climate -Ground water table
OBTAIN STRUCTURE DATA
Type Performance criteria
Foundation loads
EVALUATION FOUNDATION ALTERANTIVE
1. Shallow foundations 2. Deep foundations 3. Ground modification for
shallow foundation
PREDICT BEHAVIOR
-Settlement -Bearing capacity -Lateral stability -Environmental factor
Select Another Foundation
Alternative
DETERMING FEASIBILITY
-Predicted vs. Required performance -Potential construction problems -Cost estimate
RECOMMENDATION
-Foundation type -Design data
-Construction procedures
Process completed
Prepare Detailed Design plans and Specification, Monitor
Construction
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1.8 Type of loads
A structure may be subjected to a combination of some or all of the following loads and forces.
1.8.1 Dead loads
Dead loads are those that are constant in magnitude and fixed in location through out the lifetime of
the structure. Usually the major part of the dead load is the weight of the structure itself.
1.8.2 Live loads
Live loads consist chiefly of occupancy loads in buildings and traffic loads on bridges. They may be
either fully or partially in place or not present at all and may also change in location. Their magnitude
and distribution at any given time are uncertain, and even their maximum intensities thorugho0ut the
lifetime of the structure are not known with precision.
1.8.3 Environmental loads
These mainly consists of snow loads, wind pressure and suction, earthquake loads (i.e. inertia forces
caused by earthquake motions), soil pressures on subsurface portions of structures, water pressure
acting laterally against basement walls and vertically against base slabs, loads from possible ponding
of rainwater on flat surfaces, and forces caused by temperature differentials. Like live loads,
environmental loads at any given time are uncertain both in magnitude and distribution.
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Appendix A
Sample Appendix
This is a sample Appendix. Insert additional appendices with the “Start New Appendix” command.
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Bibliography
Vogt, C. 1999. Creating Long Documents using Microsoft Word. Published on the Web at the
University of Waterloo.