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.

iv

Acknowledgements

Enter acknowledgements here.

v

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

vii

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

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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.