Foundation Engineering

1. CE3503 – FOUNDATION ENGINEERING
3rd Year – 5th Semester - Civil

3. ‘Site investigation’ refers to the procedure of
determining surface and sub-surface conditions in
the area of proposed construction.
- for assessing the suitability of the site
- for preparing adequate and economic designs
- for selecting the construction materials
- for deciding the construction methods to be
adopted.

4. The primary objectives of soil exploration are:
1) Determination of the nature of the deposits of soil,
2) Determination of the depth and thickness of the various soil
strata and their extent in the horizontal direction,
3) the location of groundwater and fluctuations in GWT,
4) obtaining soil and rock samples from the various strata,
5) the determination of the engineering properties of the soil and
rock strata that affect the performance of the structure, and
6) determination of the in-situ properties by performing field
tests.

5. Site investigations may involve one or more of the
following preliminary steps:
1. Reconnaissance
2. Study of maps
3. Aerial photography

6. Reconnaissance involves an inspection of the site and study of the
topographical features.
- soil and ground-water conditions
- plan the programme of exploration
- The topography, drainage pattern, vegetation and land use provide
valuable information.

7. Study of Maps
Information on surface and subsurface conditions - in the form of maps.
Such sources in India are the Survey of India and Geological
Survey of India, which provide topographical maps, often
called ‘toposheets’.
- A geological study is essential.
- to establish the nature of the deposits underlying the site.
- The types of soil and rock - the method of exploration most suited
- Faults, folds, cracks, fissures, dikes, sills and caves, and such other
defects in rock and soil strata may be indicated.
- natural resources such as oil, gas and minerals - considered carefully
during the evaluation of a site.

8. Aerial Photography
Aerial photography is now a fairly well-developed method
by which site investigation may be conducted for any major
project.
Air photo interpretation is the estimation of underground
conditions by relating landform development and plant
growth to geology as reflected in aerial photographs.

9. SUB-SOIL EXPLORATION
- The objective of sub-soil exploration is to provide reliable, specific and
detailed information about the soil and groundwater conditions of the
site,
Should yield precise information about the following:
1. The order of occurrence and extent of soil and rock strata;
2. The nature and properties of the soil and rock formation; and
3. The location of groundwater and its variation.

10. Methods of Sub-soil Exploration
The methods available for sub-soil exploration may be classified as
follows:
1. Direct methods : Test pits, trial pits or trenches
2. Semi-direct methods : Borings
3. Indirect methods : Soundings or penetration tests and
geophysical methods

11. Test Pits
- Test pits or trenches are the open type of accessible exploratory
methods.
- can be used for all types of soils.
- can be inspected in soil’s natural condition.
- disturbed and undisturbed samples - appropriate laboratory tests.
- also useful for conducting field tests such as the plate bearing test.
- suitable only for small depths-up to 3m
- For greater depths, especially in pervious soils, lateral supports or
bracings will be necessary for the excavations.
- Further in such cases groundwater table may be encountered which
may have to be lowered.
Hence test pits are usually made only for minor structures, or
for supplementing other methods.

12. Borings
When the depth of exploration is large, borings are used for exploration.
Making or drilling bore holes into the ground with a view to obtain
soil or rock samples from specified or known depths is called boring.
The common methods of boring are as indicted below:
1. Auger boring
2. Auger and shell boring
3. Wash boring
4. Percussion drilling
5. Rotary drilling

13. Auger Boring.
An auger is a tool used for drilling a bore hole into the
ground.
- Used for drilling Cohesive and other soft soils above
GWT
- hand-operated and power-driven
- Hand-operated augers - depths upto 6 m.
- Power-driven augers - mechanical augers - greater
depths -used in gravelly soils also.
- auger is advanced by rotating it while pressing it into the
soil at the same time - auger gets filled with soil - soil
sample is collected.
- very much disturbed - useful for identification
purposes only.
- Auger boring is fairly satisfactory for sub-soil
explorations where the depth of exploration is small,
such as for highways, railways, air fields, borrow pits, etc.

14. Wash boring is commonly used for boring
holes.
• Soil exploration below the GWT – difficult
by means of pits or auger holes.
• Wash boring - a very convenient method -
the soil is either sand, silt or clay.
• The method is not suitable if the soil is mixed
with gravel or boulders.
• Short depth by auger - then a casing pipe - a
chopping bit fixed at the end of a string of
hollow drill rods.
• Water under pressure is forced through the rod
and the bit into the hole, which loosens the soil
as the water flows up around the pipe.
• The loosened soil in suspension in water is
discharged into a tub.
• Whenever an undisturbed sample is required at a particular depth, the boring
is stopped, and the chopping bit is replaced by a sampler.
• The sampler is pushed into the soil at the bottom of the hole and the sample is withdrawn.

15. Rotary Drilling
• Cutter bit or a core barrel with a coring bit
attached to the end of a string of drill rods is
rotated by a power rig.
• Cutting bit shears the material and is
washed out of the hole by a stream of
• Coring bits, cut an annular hole around an
intact core - is used primarily in rocky strata
to get rock samples.
• drilling fluid under pressure is introduced -
dual function of cooling the bit and removing
the cuttings from the bottom
• In an uncased hole, the drilling fluid also
serves to support the walls of the hole.
• In soils the drilling bit is removed and
replaced by a sampler when sampling is
required.
• but in rocky strata the coring bit is used to obtain continuous rock samples.

16. Coring Bits
• Three basic categories of bits are in use.
• diamond, carbide insert, and saw tooth.
• Diamond coring bits - surface set or diamond impregnated type - most versatile -
high quality cores in rock materials ranging from soft to extremely hard.
• Carbide bits - use tungsten carbide in lieu of diamonds - core soft to medium
hard rock. less expensive but slower than with diamond bits.
• Saw-tooth bits, the cutting edge comprises a series of teeth. The teeth are faced and
tipped with a hard metal alloy such as tungsten carbide to provide wear resistance
and thereby increase the life of the bit - less expensive but normally used to core
overburden soil and very soft rocks only.

17. Percussion Drilling :
• used for boring holes in rocks, boulders and other hard strata.
• heavy drill bit called ‘churn bit’ suspended from a drill rod or a
cable is alternately lifted and dropped in the vertical hole.
• the repeated blows - material in the hole gets pulverised.
• If the point where the drill bit strikes is above the GWT, water is
added to the hole to facilitate the breaking of stiff soil or rock.
• The water forms a slurry with the pulverised material which is
removed by a bailer at intervals.
• The formation gets very much disturbed by the impact. Moreover
the method cannot be used in loose sand and is slow in plastic clay.

18. Planning a Sub-soil Exploration Programme
• depends on the type and importance of the structure and the nature of
the soil strata.
• the depth, thickness, extent and composition of each of the
strata, the depth of the rock and the depth of the ground
water table.
• Further, approximate idea of the strength and compressibility of the
strata is necessary to make preliminary estimate of the safety and
expected settlement of the structure.
• include a site plan of the area, a layout plan of the proposed structures
with column locations and expected loads and the location of the bore
holes and other field tests.
• The two important aspects of boring programme are ‘spacing of
borings’ and ‘depth of borings’.

19. Spacing of Borings
“IS: 1892–1979–Code of Practice for Subsurface Investigation for Foundations”
recommendations:
• -for a compact building - area of about 0.4 hectare, one bore hole or trial pit
in each corner and one in the centre should be adequate.
• For smaller and less important buildings even one bore hole or trial pit in
the centre will be sufficient.
• For very large areas covering industrial and residential colonies, the
geological nature of the terrain will help in deciding the number of the bore holes
or trial pits. Cone penetration test must be performed at every 50 m by
dividing the area in a grid pattern.
The cone penetration test may not be possible at sites having gravelly or
boulders strata. In such cases geophysical methods (Seismic refraction
method and electrical resistivity method) may be suitable.

21. Depth of Borings
• for important and heavy structures such as bridges and tall buildings,
borings should extend to rock.
• for small structures - previous investigations in the vicinity of the site,
and from geologic evidence.
• According to the Indian Standard “IS: 1892–1979–Code of Practice for Subsurface
Investigation for Foundations”
 the depth of sub-soil exploration required depends on the type of the proposed
structure, its total weight, the size, shape and disposition of the loaded
areas, soil profile and the physical properties of the soil that
constitutes each individual stratum.
 Normally it should be one and half times the width of the footing
below foundation level.

22. SOIL SAMPLING
- is the process of obtaining samples of soil from the desired depth at the
desired location in a natural soil deposit - to assess the engineering
properties of the soil for ensuring a proper design of the foundation.
- The devices used for obtaining soil samples are known as ‘soil samplers.’
- Determination of ground water level is also considered as a part of the
process of soil sampling.

23. Types of Samples
Disturbed samples, and undisturbed samples, depending upon the
degree of disturbance
• Disturbed sample - the natural structure of the soil gets modified partly or fully
during sampling
• Undisturbed sample - the natural structure and other physical properties remain
preserved – but disturbance is absolutely inevitable.
• Disturbed samples - (i) Non-representative samples, and (ii) Representative samples
• Non-representative samples consist of mixture of materials from various soil or rock strata
• Samples from auger borings and wash borings are non-representative samples - suitable
only for providing qualitative information such as major changes in subsurface strata.
• Representative samples contain all the mineral constituents of the soil, but the structure of
the soil may be significantly disturbed. The water content may also have changed.
 suitable for identification and for the determination of Atterberg limits and grain
specific gravity
• Undisturbed samples - minimum disturbance - suitable for strength tests and consolidation
tests - Tube samples and chunk samples - ‘chunks’ of soil with some cohesion.

24. Types of Samplers
• Soil samplers are classified as ‘thick wall’ samplers and ‘thin wall’ samplers.
• Split spoon sampler (or split tube sampler) is of the thick-wall type,
• ‘shelby’ tubes are of the thin-wall type.

25. Depending upon the mode of
operation, samplers may be
classified as the open drive
sampler, stationary piston
sampler and rotary
sampler.

27. Area ratio - most critical factor - which affects sample disturbance;
- it indicates the ratio of displaced volume of soil to that of the soil
sample collected.
If Ar is less than 10%, disturbance is supposed to be small
-it may be as high as 30% for a thick wall sampler like split spoon
may be as low as 6 to 9% for thin wall samplers like shelby tubes.
The inside clearance, CI, should not be more than 1 to 3%, the outside
clearance Co should also not be much greater than CI.

28. The recovery ratio Rr = L/H
where, L = length of the sample within the tube, and
H = depth of penetration of the sampling tube.
This value should be 96 to 98% for a satisfactory
undisturbed sample.
This concept is more commonly used in the case of
rock cores.

30. Ground Water Level
• Determination of the location of ground water is an essential part of
every exploratory programme
• Necessary to make borings purely for this purpose, when artesian or
perched ground water is expected, or the use of drilling mud
obscures ground water.
• A correct indication of the general ground water level is found by
allowing the water in the boring to reach an equilibrium level. In
sandy soils, the level gets stabilised very quickly— within a few hours at
the most. In clayey soils it will take many days for this purpose.

32. Perched groundwater
is unconfined groundwater
separated from an underlying
body of groundwater by an
unsaturated zone.
It occurs when subsurface
water percolating
downward is held by a bed
or lens of low-
permeability material.

33. Standpipes or piezometers are
used in clays and silt.
A piezometer is an open-ended tube
(may be about 50 mm in diameter)
perforated at its end.
The tube is packed around with
gravel and sealed in position with
puddle clay. Observations must
be taken for several weeks until
the water level gets stabilised.

34. SOUNDING AND PENETRATION TESTS
• The methods of subsurface sounding normally consists of driving or pushing a
standard sampling tube or a cone.
• Penetrometers - made to penetrate the subsoil with a view to measure the
resistance to penetration of the soil strata, and thereby try to identify the
soil and some of its engineering characteristics. The necessary field tests
undertaken for this purpose are also called ‘penetration tests’.
• If a sampling tube is used to penetrate the subsoil, the test is known as
‘Standard Penetration Test’ (SPT).
• If a cone is used to penetrate the subsoil the test is called ‘Cone Penetration
Test’.
• ‘Static Cone Penetration Test’ (Dutch Cone Test) and ‘Dynamic Cone Penetration
Test’, depending on the mode of penetration being static or dynamic.
• A field test called ‘Vane Shear Test’ is used to determine the shear strength of the
soil located at a depth below the ground surface.

35. Standard Penetration Test (SPT)
• widely used to determine the parameters of the soil in-situ.
• especially suited for cohesionless soils as a correlation has been
established between the SPT value and the angle of internal friction
of the soil.
• driving a split-spoon sampler into the soil through a bore hole 55 to
150 mm in diameter at the desired depth.
• A drop hammer of 640 N (65 kg) weight - free fall of 750 mm - used
to drive the sampler.
• The number of blows for a penetration of 300 mm is designated as the
“Standard Penetration Value” or “Number” N.
• three stages - The blow count for every 150 mm penetration.
• the first 150 mm are ignored as those required for the seating drive.
•

36. • The number of blows required for the next 300 mm of
penetration is recorded as the SPT value.
• The test procedure is standardized by ISI and set out in “IS:
2131-1986— Standard Penetration Test”.
• Usually SPT is conducted at every 2 m depth or at the
change of stratum. If refusal is noticed at any stage, it should
be recorded.

39. S.
No.
Type of Test Type of sample required
1. Natural Water Content Undisturbed (or) SPT sample
2. Density Undisturbed
3. Specific gravity Representative (or) undisturbed
4. Grain Size Distribution Representative (or) undisturbed
5. Atterberg limits Representative (or) undisturbed
6. Coefficient of permeability Undisturbed
7. Consolidation parameters Undisturbed
8. Shear Strength parameters Undisturbed

40. Unit II – Bearing Capacity of Shallow foundation

48. BASIC DEFINITIONS
Foundation - It is the lowest part of the structure which is in contact
with the soil or bed lying below and transmits loads to it.
Foundation soil or bed - The soil or bed to which loads are
transmitted from the base of the structure.
Footing - It is the lowest portion of the foundation of a structure
which transmits loads directly to the foundation soil or bed. It is
constructed for the purpose of distributing the load over a larger area.

49. General Requirements of foundations
For a satisfactory performance, a foundation must satisfy the following three basic criteria
a) Location and Depth criterion
b) Shear failure criterion or bearing capacity
c) Settlement criterion
a) Location and Depth criterion
- lateral expulsion of soil
- seasonal volume changes (freezing and thawing)
- Prescence of adjoining structures

50. b) Shear failure criterion or bearing capacity
- adequate FOS is provided to prevent bearing capacity failure
c) Settlement criterion
- settlement of a foundation especially the differential settlement,
must be within the permissible limit.
- excessive settlement affects  the utility of structure
 spoil the appearance
 may even cause damage to the
structure

51. Location and Depth of Foundation
- IS : 1904-1986 recommends – a foundation should be located at a minimum
depth of 50 cm below the natural ground surface.
- Must be placed below the zone of volume change, where volume change is
expected. (expansive clay soil, swelling and shrinkage of soil mass will occur due to
rise or lowering of GWT, following seasonal weather changes)

53. General Shear Failure
• A gradual increase in the load on the foundation will increase the settlement of
the footing and increases the pressure under the foundation.
• When the pressure under the foundation reaches the ultimate pressure that soil
can bear, the foundation will fail suddenly.
• These types of foundation failures occur in low compressible soils.
The ultimate pressure that soil can
bear is the ultimate bearing
capacity of the foundation. As
indicated in the above, when the load
reaches the ultimate bearing capacity
qu, it fails.
• This types of shallow foundation failure occur in dense sand or stiff cohesive soil.
• Dense and stiff soils are low compressible leads to shear failure.
• Tilting of foundation occurs when fail.