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 The surface and subsurface investigation or site exploration is carried
out to collect the information about physical properties and
characteristics of the subsoil material as well as the details of other
geological features of the site area .
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Purpose of site exploration
Choice of method of exploration
Soil samples and samplers
Methods of site exploration
SBC of soil
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 The site exploration or investigation is carried out to collect the
complete details of the site for the following purposes :
i. To fix the depth of foundation up to which it must be taken
inside the ground.
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ii. To fix the value
of safe bearing
capacity of the
soil.
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iii. To predict the likely
s e t t l e m e n t o f t h e
selected foundation and
to make the allowance
for the same in the
design .
iv. T o k n o w t h e
underground water level.
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v. To select an economical and
safe type of foundation.
vi.To evaluate the earth pressure
against the walls ,basements ,
abutments , etc. and to make the
provision against difficulties
during construction.
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 The major three factors are:
 Nature of ground
 Topography
 Cost of exploration
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 Two types
Disturbed samples
Undisturbed samples
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 A disturbed sample is a sample in which
the natural structure of soil gets partly
or fully modified and destroyed.
 However, by taking proper precautions,
natural water content in soil sample can
be preserved
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 Here, for a particular sample, the natural structure and
properties remain preserved
 The sample disturbance depends upon the design of the
samplers and method of sampling.
 Properly designed samplings are required to get undisturbed
sample from bore holes as when it is forced in to ground, it
causes little remoulding and disturbance
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Although boring and test pits provide definite results but
they are time consuming and expensive.
Subsurface conditions are known only at the bore or test pit
location.
The subsurface conditions between the boring need to be
interpolated or estimated.
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Geophysical methods are more quick and cheaper.
They provide thorough coverage of the entire area.
The results of Geophysical testing however are less definitive and require
subjective interpretation.
Therefore both methods are important. In case geophysical testing in
major in scope, few borings and sampling will be required for accurate
determination of soil properties.
If boring is major in scope then few geophysical lines will be required to
know the conditions in-between the borings
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ANIMATION OF SEISMIC METHOD
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  A series of seismic
receivers, geophones
(right) are laid out
along the survey line
at regular intervals
and receive the
reflected wave
energy. 24

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1) Trial pits
2) Probing
3) Subsurface
soundings
4) Boring
 Auger boring
 Auger and shell
boring
 Wash boring
 Percussion boring
 Rotary boring 29

 Trial pits are the cheapest method of exploration in
shallow deposits, since these can be used in all
types of soils. In this method, pits are excavated at
the site, exposing the sub-soil surface thoroughly.
 Soil samples are collected at various levels. The
biggest advantage of this method is that soil strata
can be inspected in their natural condition and
samples (disturbed or undisturbed) can be
conveniently taken.
30

The method is generally
considered suitable for shallow
depths, say upto 3 m. The cost
of open excavation increases
rapidly with depth. For greater
depths and for excavation
below ground water table,
specially in pervious soils,
measures for lateral support
and ground water lowering
becomes necessary.
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1
2
3
4
Trial pits video
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 This method is suitable for soft soils such as clay ,
gravel and sand.
 In this method , a steel bar of 25 to 40mm diameter
having a pointed end is driven in the ground until a
hard strata is met . The bar is allowed to fall vertically
under its own weight or it is driven by drop hammer .
 The bar is drawn out at some interval , so that the
rough idea of the nature of strata is obtained from the
soil sticking to the sides of the bar.
 The experienced persons can be engaged in the
execution of the driving operation . Fig. shows various
types of probing stones:
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 This method consists of measuring the variation in the resistance
offered by the soil with the depth by means of a tool known as
penetrometer.
 The penetrometer may consist of a 50 mm diameter mild steel
cone fitted loosely to a steel rod or it may be a tool known as
standard split spoon sampler.
 The resistance offered by the soil to penetration is co-related
with the engineering properties of soil , such as – density ,
consistency , permeability and bearing capacity.
Kit of subsurface soundings
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 The various boring methods adopted for soil exploration
are as follows :
 Auger boring
 Auger and shell boring
 Wash boring
 Percussion boring
 Rotary boring
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 This type of augers are commonly used for clayey or sandy
soils or soft soils .
 It can be operated manually or mechanically .
 The hand operated augers are used up to 6 m depth and the
mechanically operated augers are used for greater depth up to
15 m .
 The auger is vertically driven into the ground by rotating its
handle and is pressed down during the process of rotation .
 At every 30 cm depth of penetration , the auger is drawn out
and the samples of soils are collected separately for testing .
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Manually operated
Mechanically operated
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Samples recovered from the soil brought up by the augers
are badly disturbed and are useful for identification
purposes only. Auger boring is fairly satisfactory by
explorations at shallow depths and for exploratory borrow
pits.
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 In this method ,different types of tools are used for boring.
 In case of soft clay , cylindrical auger having 75 to 200 mm
diameter hollow tube with a cutting edge at its bottom is used .
 In case of hard clay , the shells with cutting edge or teeth at its
lower end are used ; while in case of sandy soil or sand , the pumps
are used for boring .
 The hand operated rigs are used 25 m depth up to 200 mm
diameter and the mechanical rigs are used for 50m depth.
 The samples of soil are collected at regular interval for testing .
Auger and shell operated
46

 Wash boring is a fast and simple method for advancing holes
in all types of soils. Boulders and rock cannot be penetrated
by this method. The method consists of first driving a casing
through which a hollow drilled rod with a sharp chisel or
chopping bit at the lower end is inserted.
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Schematic for wash boring
Video of wash boring
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Water is forced under pressure through the dril rod which is
alternatively raised and dropped, and also rotated. The
resulting chopping and jetting action of the bit and water
disintegrates the soil. The cuttings are forced upto the ground
surface in the form of soil-water slurry through the annular
space between the drill rod and the casing. The change in soil
stratification could be guessed from the rate of progress and
colour of wash water. The samples recovered from the wash water
are almost valueless for interpreting the correct geo-technical
properties of soil
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 This method is used for rocks and soils having boulders ; which
are broken up by repeated blows from a bit or chisel .
 Thus , the pulverised material is converted into slurry by pouring
water in the bore and this slurry is bailed out at regular interval
and dried for testing.
Video of percussion boring
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 The rig consists of a derrick, power unit, winch, pump and a
drill head to apply high-speed rotary drive and downward
thrust to the drilling rods.
 Primarily intended for investigation in rock, but also used in
soils.
 The drilling tool, (cutting bit or a coring bit) is attached to
the lower end of hollow drilling rods
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• The fluid (bentonite slurry) also provides some support to the sides of
the hole if no casing is used
•The drilling fluid cools and lubricates the drilling tool and carries
the loose debris to the surface between the rods and the side of the
hole.
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•The coring bit is fixed to the lower end of a core .
•Water or drilling fluid is pumped down the hollow rods and passes under
pressure through narrow holes in the bit or barrel.

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There are two forms of rotary drilling, open-hole drilling and core
drilling.
Open- hole drilling, which is generally used in soils and weak rock,
just for advancing the hole
The drilling rods can then be removed to allow tube samples to be
taken or in-situ tests to be carried out.
In core drilling, which is used in rocks and hard clays, the diamond or
tungsten carbide bit cuts an annular hole in the material and an
intact core enters the barrel, to be removed as a sample. Typical core
diameters are 41, 54 and 76mm, but can range up to 165 mm.

 Capacity of soil to support
the loads applied to the
ground is knows as SBC of
soil.
 The bearing capacity of soil
is the maximum average
contact pressure between the
foundation and the soil which
should not produce shear
failure in the soil.
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The stability of foundation
depends on:
1. The bearing capacity of
soil
2. The settlement of soil
beneath the foundation
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 Plate load test
 Penetration test
 SBC values given in IS (1904-
1978)
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 The following techniques can be used for improving
bearing capacity
 Increasing depth of foundation
 Draining the soil
 Compacting the soil
 Confining the soil
 Replacing the poor soil
 Using grouting material
 Stabilizing the soil with chemicals
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