UNIVERSITY OF NAIROBI
DEPARTMENT OF CIVIL AND
CONSTRUCTION ENGINEERING
GEOTECHNICAL ENGINEERING – FCE 511
Teaching Lectures by:
Eng. Kenneth Amollo
COURSE OUTLINE
Foundations
 Shallow Foundations
 Introduction. Foundation loading intensities. Bearing capacity, (ultimate, safe, gross and
allowable). Influence of ground water table, sloping ground, inclined and eccentric loads on
allowable bearing capacity. Design of shallow foundations for shear strength and
settlements. Examples of foundation design (e.g. strips, pad), combined footings, raft
footings.
 Piled Foundation
 Types of piles driven and bored pile, friction and end bearing pile. Design of piles by
soil mechanics methods, end bearing, skin friction and ultimate bearing resistance.
Piles in sands. Piles in cohesive soils - total and effective stress analysis. Design from
pile tests data.
 End bearing piles on rock. Settlement of piles. Dynamic formula. Negative skin friction.
Pile groups - bearing capacity in cohesive and cohesionless soils.
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COURSE OUTLINE
Foundations
 Introduction to Earth Dams
 Design of earth embankment - homogenous and zoned dams. Definitions e.g. fetch, water
spread, shell free board etc.. Factors influencing site selection. Spillways. Settlements of
embankments. Protection of upstream and downstream slopes.
 Site Investigations
 Introduction, purpose of Site Investigation, organization of Site investigation for
different types of structures e.g. buildings, irrigation or water supply projects,
highways and airport pavements, etc.. Methods of Investigation. Sampling. Borehole
logs. Geophysical methods. Geotechnical reports.
Course Duration = 45 Hours
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COURSE REFERENCES
1. Craig FR, 1987, Soil mechanics, Van Nostrand Reinhold (International) London
2. Bowles JE , 1982, Foundation Engineering, McGraw-Hill international book
company,
Tokyo.
3. Tomlinson MJ and Boorman R (1986), Foundation and construction, Longman
scientific
and technical, England
4. Franklin JA and Dussealt MB (1989) Rock Engineering, McGraw-Hill
international
editions, London
5. Chen FH (1975) Foundations on expansive soils, Elsevier scientific Publishing
Company
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TOPIC 1 - SHALLOW FOUNDATIONS
FOUNDATIONS
The foundation is the part of the structure that transmits the loads directly to the underlying
soil. If the soil is sufficiently strong it is possible to use shallow foundation. On the other hand if
the soil is not strong enough the foundation is taken deeper into the ground and is referred to as
a deep foundation.
 Shallow Foundations
 Strip foundations for wall and closely spaced columns
 Spread or isolated footings for individual columns. In this category it is usual to consider
combined foundations for two or three closely spaced columns as spread or isolated footings
 Raft foundations covering large sections of the foundation area
 Deep Foundations
 Piles
 Caissons
In the selection of the foundations to adopt for a structure it is usually necessary to
consider the function of the structure, its loads, the subsurface conditions and the cost of the
foundation being adopted in comparison to other possible types of foundations.
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FOUNDATIONS
 Strip Foundation
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FOUNDATIONS
 Strip Foundation
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FOUNDATIONS
 Spread Foundation
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FOUNDATIONS
 Raft Foundation
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FOUNDATIONS
 Pile Foundation
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FOUNDATIONS
 Pile Foundation
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FOUNDATIONS
 Caisson Foundation
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Shallow Foundations
 Introduction to shallow foundations
 A shallow foundation as one whose depth is less or equal to its least width challenges.
 The foundation must satisfy two fundamental requirements:o The factor of safety against shear failure must be adequate. A value of 3 to 5 is usually
specified.
o The settlement of the foundation should be tolerable and in particular differential settlement
should not cause any unacceptable damage or interfere with the function of the structure.
 Allowable Bearing Capacity
 The allowable bearing capacity is defined as the pressure which may be applied to the soil to
enable the two fundamental conditions to be satisfied
 The design of the foundations is usually a two process exercise. The first is to determine the
allowable bearing capacity of the soil while the second is to size the foundation on the design
strata based on the allowable bearing capacity. These two parts are now discussed.
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Shallow Foundations
 Modes of Failure in Strip Footing
Three distinct modes of failure have been identified for the strip footing which generally
depend on the compressibility of the soil and the depth of the foundation. They include:
 General shear failure :- As the pressure increases on the foundation layer the state of
plastic equilibrium is reached initially in the soil around the edges of the footing and
then spreads downwards and outwards. Ultimately the state of plastic equilibrium is
reached throughout above the failure surfaces. The soil around the footing heaves
on both sides. At the moment of failure one side continues to settle at a higher rate
and the strip footing tilts. This behavior is exhibited by soils of low compressibility
(Figure a)
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Shallow Foundations
 Modes of Failure in Strip Footing
 local shear failure :- This is characterized by local development of plastic conditions
usually below the foundation. The plastic conditions do not reach the surface and
only slight heaving is expected. This kind of failure is expected with soils of high
compressibility and is associated with large settlements (Figure b). These soils
include dense and stiff soils.
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Shallow Foundations
 Modes of Failure in Strip Footing (Cont..d)
 Punching shear failure :- This occurs when shearing takes place directly below the
footing under compression from load. No heaving is of the ground is expected by the
side of the footing. Large settlements are characteristics of this mode of failure and
are typical of soils of high compressibility and foundations at considerable depth
(Figure c).
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Shallow Foundations
 Bearing Capacity of Soils
Definitions:
 The Ultimate bearing capacity is the value of the average contact pressure between the
foundation and the soil which will produce shear failure in the soil.
 The net foundation pressure is the increase in the pressure at the foundation level due to
the structure loads
 The safe net foundation pressure is the net foundation pressure divided by a suitable factor
of safety.
 Allowable bearing pressure is the maximum allowable net loading intensity on the soil
allowing for both shear and settlement effects.
 Determination of Bearing Capacity of Soils
The bearing capacity can be determined by calculation from known values or by using field tests.
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Shallow Foundations
 Bearing capacity Calculation
 The Terzaghi Equation
 Meyerhoff-Hensen Equation.
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Shallow Foundations
Field methods for the determination of bearing capacity of soils
 Standard Penetration Test (SPT)
 The SPT is one of the most widely used methods. It involves driving a split-spoon
sampler into the ground and recording the number of blows required to advance it
for every 1.5m of penetration. The N-value, which is the number of blows per foot,
can be correlated with bearing capacity.
 Cone Penetration Test (CPT)
 The CPT involves pushing a cone-shaped penetrometer into the soil while measuring
the resistance to penetration. It provides continuous data on soil properties,
including shear strength, which can be used to estimate bearing capacity.
 Plate Load Test
 In this test, a rigid plate (usually 30 inches or 762 mm in diameter) is loaded
incrementally on the soil's surface, and the resulting settlement is measured. The
bearing capacity can be calculated from the applied load and settlement data.
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Shallow Foundations
 Dynamic Cone Penetrometer (DCP)
 The DCP test involves dropping a hammer with a standard mass from a known height
onto a steel cone that is driven into the ground. The penetration depth per blow is
recorded and used to estimate bearing capacity.
 Vane Shear Test
 The vane shear test is used to determine the undrained shear strength of finegrained soils. A vane with a known cross-sectional area is inserted into the ground
and rotated to measure the torque required for shearing. This torque is related to
shear strength, which can be used to estimate bearing capacity.
 Pressuremeter Test
 The pressuremeter test involves inserting a cylindrical probe into the soil and
applying radial pressure to expand it. The resulting pressure-volume relationship can
be used to estimate bearing capacity and the deformation characteristics of the soil.
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Shallow Foundations
 Hand Auger Test
 A hand auger is used to collect disturbed or undisturbed soil samples at various depths. The
collected samples can be tested in a laboratory to determine their properties, including
shear strength, which can be used to estimate bearing capacity.
 Borehole Shear Test
 This test involves pushing a shear vane into a borehole and measuring the torque required to
shear the soil. It is often used for estimating the bearing capacity of soils at greater depths.
 Ground Penetrating Radar (GPR)
 GPR can be used to assess soil density and stratigraphy by sending radar waves into the
ground. This information can help in understanding the soil's bearing capacity characteristics.
 Load Cells and Settlement Plates
 Placing load cells or settlement plates beneath foundations or structures can help monitor
real-time load and settlement data, providing valuable information for assessing bearing
capacity over time.
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