SITE INVESTIGATION/SUBSURFACE EXPLORATION

1.1 Introduction
Earthwork forms the largest activity of a Civil Engineer. It is well understood that irrespective of
the type of civil engineering structure on earth –
✓ It has to be rested either in soil (e.g., foundations)
✓ Rested on soil (e.g., pavements) or
✓ The structure is itself constructed making use of soil (e.g., Earthen dams).
This implies that a better knowledge of the spatial variation of the soils encountered is essential.
Therefore, before construction of any civil engineering work a thorough investigation of the site
is essential. Site investigations constitute an essential and important engineering program which,
while guiding in assessing the general suitability of the site for the proposed works, enables the
engineer to prepare an adequate and economic design and to foresee and provide against
difficulties that may arise during the construction phase. Site investigations are equally necessary
in reporting upon the safety or causes of failures of existing works or in examining the suitability
and availability of construction materials.

Site investigation refers to the methodology of determining surface and subsurface features of the
proposed area.

Information on surface conditions is necessary for planning the accessibility of site, for deciding
the disposal of removed material (particularly in urban areas), for removal of surface water in
water logged areas, for movement of construction equipment, and other factors that could affect
construction procedures.

Information on subsurface conditions is more critical requirement in planning and designing the
foundations of structures, dewatering systems, shoring or bracing of excavations, the materials of
construction and site improvement methods.

1.2 Soil Exploration

The knowledge of subsoil conditions at a site is a prerequisite for safe and economical design of
substructure elements. The field and laboratory studies carried out for obtaining the necessary
information about the surface and subsurface features of the proposed area including the position
of the ground water table, are termed as soil exploration or site investigation.

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 1.3 Objectives of soil exploration program
The information from soil investigations will enable a Civil engineer to plan, decide, design, and
execute a construction project. Soil investigations are done to obtain the information that is useful
for one or more of the following purposes.
1. To know the geological condition of rock and soil formation.
2. To establish the groundwater levels and determine the properties of water.
3. To select the type and depth of foundation for proposed structure
4. To determine the bearing capacity of the site.
5. To estimate the probable maximum and differential settlements.
6. To predict the lateral earth pressure against retaining walls and abutments.
7. To select suitable construction techniques
8. To predict and to solve potential foundation problems
9. To ascertain the suitability of the soil as a construction material.
10. To determine soil properties required for design
11. Establish procedures for soil improvement to suit design purpose
12. To investigate the safety of existing structures and to suggest the remedial measures.
13. To observe the soil the soil performance after construction.
14. To locate suitable transportation routes.

The objectives of soil investigations from various requirements point of view is summarized in
Table 1.1
Table 1.1 Objectives of soil investigations
Design requirements ✓ define stratigraphy/geology
✓ to determine soil properties required for design
✓ aid material selection
✓ to determine the type and depth of foundation
Construction requirements ✓ to select suitable construction techniques
✓ define equipment and techniques needed
✓ to locate suitable transportation routes
Auditing ✓ checking a site prior to sale/purchase

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 ✓ to establish procedures for soil improvement to suit design
purpose
Monitoring ✓ to observe the soil performance after construction
✓ determine reasons for poor behaviour
✓ document performance for future reference

1.4 Scope of soil investigation

The scope of a soils investigation depends on the type, size, and importance of the structure, the
client, the engineer's familiarity with the soils at the site, and local building codes. Structures that
are sensitive to settlement such as machine foundations and high-use buildings usually require a
thorough soils investigation compared to a foundation for a house. A client may wish to take a
greater risk than normal to save money and set limits on the type and extent of the site
investigation. If the geotechnical engineer is familiar with a site, he/she may undertake a very
simple soils investigation to confirm his/her experience. Some local building codes have
provisions that set out the extent of a site investigation. It is mandatory that a visit be made to the
proposed site.

In the early stages of a project, the available information is often inadequate to allow a detailed
plan to be made. A site investigation must be developed in phases.

1.5 Phases of a Soils Investigation

The soil investigation is conducted in phases. Each preceding phase affects the extent of the next
phase. The various phases of a soil investigation are given below:
Phase I. Collection of available information such as a site plan, type, size, and importance of the
structure, loading conditions, previous geotechnical reports, topographic maps, air photographs,
geologic maps, hydrological information and newspaper clippings.

Phase II. Preliminary reconnaissance or a site visit to provide a general picture of the topography
and geology of the site. It is necessary that you take with you on the site visit all the information
gathered in Phase I to compare with the current conditions of the site. Here visual inspection is
done to gather information on topography, soil stratification, vegetation, water marks, ground
water level, and type of construction nearby.

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Phase III. Detailed soils exploration. Here we make a detailed planning for soil exploration in
the form trial pits or borings, their spacing and depth. Accordingly, the soil exploration is carried
out. The details of the soils encountered, the type of field tests adopted and the type of sampling
done, presence of water table if met with are recorded in the form of bore log. The soil samples
are properly labeled and sent to laboratory for evaluation of their physical and engineering
properties.

Phase IV. Write a report. The report must contain a clear description of the soils at the site,
methods of exploration, soil profile, test methods and results, and the location of the groundwater.
This should include information and/or explanations of any unusual soil, water bearing stratum,
and soil and groundwater condition that may be troublesome during construction.

1.6 Soil Exploration Methods

1) Trial pits or test pits
2) Boring
3) Probes (in situ test) and geophysical methods

1.6.1 Trial pits or test pits

✓ Applicable to all types of soils

✓ Provide for visual examination in their natural condition
✓ Disturbed and undisturbed soil samples can be conveniently obtained at different depths
✓ Depth of investigation: limited to 3 to 3.5 m.
Advantages
i) Cost effective
ii) Provide detailed information of stratigraphy
iii) Large quantities of disturbed soils are available for testing
iv) Large blocks of undisturbed samples can be carved out from the pits
v) Field tests can be conducted at the bottom of the pits
Disadvantages
i). Depth limited to about 6m
ii). Deep pits uneconomical
iii). Excavation below groundwater and into rock difficult and costly
iv). Too many pits may scar site and require backfill soils.

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Limitations
i) Undisturbed sampling is difficult
ii) Collapse in granular soils or below ground water table
1.6.2 Exploratory borings
Boring is carried out in the relatively soft and uncemented ground (engineering ‘soil’) which is
normally found close to ground surface. The techniques used vary widely across the world.
1.6.2.1 Location, spacing and depth of borings
It depends on:
i). Type of structure
ii). Size of the structure
iii). Weight coming from the structure
1.6.2.2 General guidelines for location and depth of bore holes
Boreholes are generally located at:
✓ The building corners
✓ The centre of the site
✓ Where heavily loaded columns or machinery pads are proposed.
✓ At least one boring should be taken to a deeper stratum, probably up to the bedrock if
practicable
✓ Other borings may be taken at least to significant stress level.
1.6.2.3 Spacing of Bore Holes – Check BS 5930 Codal Recommendations

1.6.2.4 Borehole Spacing- Guidelines

Table 1.2 gives the general guidelines for the spacing of boreholes
Type of project Spacing (m) Spacing (ft)

Multi-storeys building 10-30 30-100

Industrial plant 20-60 60-200

Highway 250-500 800-1600

Residential subdivision 250-500 800-1600

Dams and dikes 40-80 130-260

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1.6.3 Depth of Investigation
The depth of investigation depends on:
✓ The size and type of proposed structure
✓ Sequence of proposed strata.
The depths of boreholes should cover the zone of soil that will be affected by the structural loads.
There is no fixed rule to follow. In most cases, the depths of boreholes are governed by experience
based on the geological character of the ground, the importance of the structure, the structural
loads, and the availability of equipment.

1.6.3.1 Guidelines for depth of investigation:

1. At least one boring should be taken to deeper stratum, probably up to the bedrock if
practicable. Borings should penetrate at least 3 m into rock.
2. Other borings may be taken at least to significant stress level.
3. In compressible soils such as clays, the borings should penetrate at least between 1 and 3
times the width of the proposed foundation or until the stress increment due to the heaviest
foundation load is less than 10%, whichever is greater.
4. In very stiff clays, borings should penetrate 5-7 m to prove that the thickness of the stratum
is adequate.
5. Borings must penetrate below any fills or very soft deposits below the proposed structure.
6. The minimum depth of boreholes should be 6 m unless bedrock or very dense material is
encountered.

1.6.4 Significant depth

The investigation shall be carried out to the point at which the vertical stress due to proposed
structure is equal to or less than 10% of original effective stress at the point before the structure
is constructed – significant depth

1.6.5 Methods of borings

i). Auger boring – preferred for shallow depths, low ground water table
ii). Wash boring: high water table, deeper soil deposit
iii). Rotary drilling: high quality boring, also for rock drilling
iv). Percussion drilling: fast drilling, not taking samples, gravel

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1.6.5.1 Hand Auger
Enables quick assessment of the soils present in the top few metres of the profile. It is limited by
depth of water table in sandy soils and the presence of strong layer.

Fig. 1.1 Augers

Manual boring
It is suitable up to depths of 6 to 8 m. The soil samples obtained from auger borings are highly
disturbed. In some non-cohesive soils or soils having low cohesion, the wall of the bore holes will
not stand unsupported. In such circumstance, a metal pipes are used as a casing to prevent the soil
from caving in.

1.6.6 Wash Boring

Wash boring relies on relatively little drilling action and can form a hole primarily by jetting. This
can be undertaken with light equipment without the need for a drilling rig.

Fig.1. 2 Wash Boring set up

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1.7 Soil Sampling
1.7.1.1 Need for sampling
Sampling is carried out in order that soil and rock description, and laboratory testing can be carried
out. Laboratory tests typically consist of:
i). Index tests (for example, specific gravity, water content)
ii). Classification tests (for example, Atterberg limit tests on clays); and
iii). Tests to determine engineering design parameters (for example strength,
compressibility, and permeability).

1.7.1.2 Factors to be considered while sampling soil

i). Samples should be representative of the ground from which they are taken.
ii). They should be large enough to contain representative particle sizes, fabric, and
fissuring and fracturing.
iii). They should be taken in such a way that they have not lost fractions of the in-situ
soil (for example, coarse or fine particles) and, where strength and compressibility
tests are planned, they should be subject to as little disturbance as possible.

1.7.1.3 Type of soil samples

Fig. 1.3 Types of soil samples

1.7.1.4 Non-Representative samples

Non-Representative soil samples are those in which neither the in-situ soil structure,
moisture content nor the soil particles are preserved.
✓ They are not representative

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 ✓ They cannot be used for any tests as the soil particles either gets mixed up
or some particles may be lost.
✓ e.g., Samples that are obtained through wash boring or percussion drilling.

1.7.1.5 Disturbed soil samples

Disturbed soil samples are those in which the in-situ soil structure and moisture content
are lost, but the soil particles are intact.
✓ They are representative
✓ They can be used for grain size analysis, liquid and plastic limit, specific
gravity, compaction tests, moisture content, organic content determination
and soil classification test performed in the lab
✓ e.g., obtained through cuttings while auguring, grab, split spoon (SPT), etc.

1.7.1.6 Undisturbed soil samples

Undisturbed soil samples are those in which the in-situ soil structure and moisture content
are preserved.
✓ They are representative and also intact
✓ These are used for consolidation, permeability or shear strengths test (Engineering
properties)
✓ More complex jobs or where clay exist
✓ In sand is very difficult to obtain undisturbed sample
✓ Obtained by using Shelby tube (thin wall), piston sampler, surface (box), vacuum,
freezing, etc.,

1.7.1.7 Causes of Soil disturbances

✓ Friction between the soil and the sampling tube
✓ The wall thickness of the sampling tube
✓ The sharpness of the cutting edge
✓ Care and handling during transportation of the sample tube
To minimize friction:
✓ The sampling tube should be pushed instead of driven into the ground
✓ Sampling tube that are in common use have been designed to minimize sampling
disturbances.

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1.7.2 Design Features affecting the sample disturbance
✓ Area ratio
✓ Inside Clearance
✓ Outside Clearance
✓ Recovery Ratio
✓ Inside wall friction
✓ Design of non-return value
✓ Method of applying force
✓ Sizes of sampling tubes

1.7.2.1 Area ratio

Where,
D1 = inner diameter of the cutting edge

D2 = outer diameter of the cutting edge

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 Fig. 1.4 Typical Sampling tube
✓ To obtain good quality undisturbed samples, the area ratio should be less than or equal to 10%.
✓ It may be high as 110% for thick wall sampler like split spoon sampler and may be as low as
6 to 9% for thin wall samples like Shelby tube

1.7.2.2 Inside Clearance

Where D3 = inner diameter of the sample tube

✓ The inside clearance allows elastic expansion of the sample when it enters the sampling tube.
✓ It helps in reducing the frictional drag on the sample, and also helps to retain the core.
✓ For an undisturbed sample, the inside clearance should be between 0.5 and 3%.

1.7.2.3 Outside Clearance

Where D4 = outer diameter of the sample tube

Outside clearance facilitates the withdrawal of the sample from the ground. For reducing the
driving force, the outside clearance should be as small as possible. Normally, it lies between zero
and 2%. Co Should not be more than Ci.

1.7.2.4 Recovery Ratio

Where
L = length of the sample within the tube and
H = Depth of penetration of the sampling tube
Rr = 96 – 98 % for getting a satisfactory undisturbed sample
1.7.2.5 Inside wall friction
✓ The friction on the inside wall of the sampling tube causes disturbances of the sample.
✓ Therefore, the inside surface of the sampler should be as smooth as possible.
✓ It is usually smeared with oil before use to reduce friction.

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1.7.2.6 Design of non-return value
✓ The non – return value provided on the sampler should be of proper design.
✓ It should have an orifice of large area to allow air, water or slurry to escape quickly when
the sampler is driven.
✓ It should close when the sample is withdrawn.

1.7.2.7 Method of applying force

The degree of disturbance depends upon the method of applying force during sampling and
depends upon the rate of penetration of the sample.
For obtaining undisturbed samples, the sampler should be pushed and not driven.

Numerical Example on sampling disturbance

1. Following sizes of sampling tubes are used to collect the samples
Outer Dia (mm) 75 110 50
Inner Dia ( mm ) 72 107 35
Length (mm) 600 600 600

Which is the least disturbed sample in the above?

Solution:
To find out which sample is least disturbed, we have to calculate the area ratio (Ar) which is given
by:

1st sampler – Ar = 8.51 %

2nd sampler – Ar = 5.69 %

3rd sampler – Ar = 104.08 %

Since the area ratio of the 2nd Sampler is the least, the sample obtained from it will be the least
disturbed.
1.8 Type of Soil Samplers

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 Fig. 1.5 Type of samplers

1.8.1.1 Split Spoon Sampler

✓ Has an inside diameter of 35mm and an outside diameter of 50mm.
✓ Has a split barrel which is held together using a screw-on driving shoe at the bottom end
and a cap at the upper end.
✓ The thicker wall of the standard sampler permits higher driving stresses than the Shelby
tube but does so at the expense of higher levels of soil disturbances.
✓ Split spoon samples are highly disturbed.
✓ They are used for visual examination and for classification tests.

Fig.1.6 Split spoon sampler

1.8.1.2 Shelby Tube Sampler
✓ Thin-walled seamless steel tube of diameter 50 or 75mm and length of 600-900mm.
✓ The bottom end of the tube is sharpened.
✓ The tubes can be attached to drilling rods.
✓ The drilling rod with the sampler attached is lowered to the bottom of the borehole and the
sampler is pushed into the soil.
✓ The soil sample inside the tube is then pulled out.
✓ The two ends of the sampler are sealed and sent to the lab. The samples can be used for
consolidation or shear tests.

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 Plate 1.1 Shelby tube sampler
Fig.1.7 Shelby tube sampler

1.8.1.3 Piston Sampler

✓ When sampling very soft or larger than 76.2mm in diameter to get high quality undisturbed
samples, they tend to fall out of the sampler. Then piston samplers are used.
✓ They consist of a thin wall tube with a piston.
✓ Initially, the piston closes the end of the thin wall tube. The sampler is lowered to the bottom
of the borehole and then the thin wall tube is pushed into the soil hydraulically past the piston.
✓ Later the pressure is released through a hole in the piston rod. To a large extent, the presence
of the piston prevents distortion in the sample by not letting the soil squeeze into the sampling
tube very fast and by not admitting excess soil.
✓ Consequently, samples obtained in this manner are less disturbed than those obtained by
Shelby tubes.

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 Fig.1.8 Piston sampler

1.9 Rock Coring

It may be necessary to core rock if bedrock is encountered at a certain depth during drilling. It is
always desirable that coring be done for at least 3 m. If the bedrock is weathered or irregular, the
coring may need to be extended to a greater depth. For coring, a core barrel is attached to the
drilling rod. A coring bit is attached to the bottom of the core barrel. The cutting element in the bit
may be diamond, tungsten, or carbide. The coring is advanced by rotary drilling. Water is
circulated through the drilling rod during coring, and the cuttings are washed out. Figure shows a
diagram of rock coring by the use of a single-tube core barrel.
Rock cores obtained by such barrels can be fractured because of torsion. To avoid this problem,
one can use double-tube core barrels.
On the basis of the length of the rock core obtained from each run, the following quantities can
be obtained for evaluation of the quality of rock:

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 𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑟𝑜𝑐𝑘 𝑐𝑜𝑟𝑒 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑
1. Recovery ratio = 𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑜𝑟𝑖𝑛𝑔

∑𝐿𝑒𝑛𝑔𝑡ℎ𝑠 𝑜𝑓 𝑖𝑛𝑡𝑎𝑐𝑡 𝑝𝑖𝑒𝑐𝑒𝑠 𝑜𝑓 𝑐𝑜𝑟𝑒>100 𝑚𝑚𝑛

2. Rock quality designation (RQD) = 𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑜𝑟𝑖𝑛𝑔

Table 1.3 Classification of rocks based on RQD

RQD (%) Rock quality

<25 Very poor

25-50 Poor

50-75 Fair

75-90 Good

>90 Excellent

1.10 Bore Hole Support or Stabilization of Bore Hole

This necessary to prevent cohesion less soils against caving while drilling bore hole. Either of the
following is used for bore hole support.
✓ Steel casing – hydraulically pushed
✓ Drilling mud – Circulation bentonite slurry

1.10.1 Stabilization of bore holes using drilling mud

Bentonite mud or Drilling mud’ is a thin mixture of water and bentonite clay, which can be mixed
in powder form to the drilling water to create higher density suspension.
Advantages
✓ It is advantages over water.
✓ Firstly, it is more viscous and can therefore lift cuttings adequately at a lower velocity.
✓ Secondly it will cake the edges of the borehole, and the outside of the core, and will largely
eliminate the seepage of water out of the borehole, thus reducing problems of loss of return.
✓ Hence, smaller volumes of flush fluid will be required and the fluid may he recirculated
via a settling tank (where the cuttings are allowed to drop out of suspension).
✓ The cake formed on the outside of the borehole has the effect of considerably improving.
Borehole stability and the prevention of softening of weak rock cores.

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Disadvantages
✓ The bentonite mud-soil cakes are difficult to dispose of, at the end of drilling a borehole.
The mud cannot simply be tipped on the site, and it cannot be discharged into nearby
sewers.
✓ Bentonite mud must be properly mixed, using appropriate equipment, in order to ensure
that it is of the correct consistency and does not contain unmixed dry bentonite lumps,
capable of clogging flush ports in the core barrel.
1.11 In Situ Testing

There is a wide variety of different tests that can be used for evaluating the properties of the ground.
It is often preferable to do an in-situ test in an attempt to measure a particular parameter, rather
than obtain a sample and do a laboratory test.
✓ sampling results in disturbance (reduces strength and stiffness).
✓ sometimes only best (strongest) material is recovered and is not representative of overall
in situ material.
1.11.1.1 Parameters obtained from In Situ Testing
Typical parameters that may be obtained either directly, or indirectly from in situ tests:
✓ strength
✓ stiffness
✓ permeability
✓ relative density
1.11.1.2 In-situ Tests
In situ testing is a division of field testing corresponding to the cases where the ground is tested
in-place by instruments that are inserted in or penetrate the ground. In-situ tests are normally
associated with tests for which a borehole either is unnecessary or is only an incidental part of the
overall test procedure, required only to permit insertion of the testing tool or equipment. The role
of specialized in-situ testing for site characterization and the research and development of in-situ
techniques have received considerable attention over the last 15 years or so. The use of specialized
in-situ testing in geotechnical engineering practice is rapidly gaining increased popularity. In
Europe, specialized in-situ testing has been commonly used for more than 25 years. Improvements
in apparatus, instrumentation, and technique of deployment, data acquisition and analysis
procedure have been significant. The rapid increase in the number, diversity and capability of in-

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situ tests has made it difficult for practicing engineers to keep abreast of specialized in-situ testing
and to fully understand their benefits and limitations. Table below summarizes the primary
advantages and disadvantages of in-situ testing.
Table 1.4 Summary of advantages and disadvantages of in-situ testing
Advantages Disadvantages
✓ Tests are carried out in place in the natural ✓ Samples are not obtained; the soil tested
environment without sampling disturbance, cannot be positively identified.
which can cause detrimental effects and ✓ The exception to this is the SPT in which a
modifications to stresses, strains, drainage, sample, although disturbed, is obtained.
fabric and particle arrangement. ✓ The fundamental behaviour of soils during
✓ Continuous profiles of stratigraphy and testing is not well understood.
engineering properties/characteristics can be ✓ Drainage conditions during testing are not
obtained. known.
✓ Detection of planes of weakness and defects are ✓ Consistent, rational interpretation is often
more likely and practical. difficult and uncertain.
✓ Methods are usually fast, repeatable, produce ✓ The stress path imposed during testing may
large amounts of information and are cost bear no resemblance to the stress path
effective. induced by full-scale engineering structure
✓ Tests can be carried out in soils that are either Most push-in devices are not suitable for a
impossible or difficult to sample without the wide range of ground conditions.
use of expensive specialized methods. ✓ Some disturbance is imparted to the ground
✓ A large volume of soil may be tested than is by the insertion or installation of the
normally practicable for laboratory testing. instrument.
This may be more representative of the soil ✓ There is usually no direct measurement of
mass. engineering properties.
✓ Empirical correlations usually have to be
applied to interpret and obtain engineering
properties and designs

The in-situ tests that are most commonly used in practice are:
1) Standard penetration test (SPT)

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 2) Cone – penetration test (CPT)
3) Piezo-cone penetration test (CPTU)
4) Field vane shear test (FVT)
5) Pressure meter test (PMT)
6) Dilatometer test (DMT)
7) Becker Penetration Test (BPT) and
8) Iowa Bore hole shear test (BHST)
9) Plate load test

1.11.2 Standard Penetration Test (SPT):

One of the most common in-situ tests is the standard penetration test or SPT. This test which was
originally developed in the late 1920s, is currently the most popular and economical means to
obtain subsurface information (both inland and offshore).
It offers the advantage of low cost, applicability to many soil types, samples are obtained (although
disturbed) and a large database from which many useful correlations have been developed.
Procedure:
The standard penetration test is conducted in a borehole using a standard split-spoon sampler.
1) When the borehole (55 to 150 mm in dia) has been drilled to the desired depth, the drilling
tools are removed and the split-spoon sampler, attached to standard drill rods of required length
is lowered to the bottom of the borehole and rested at the bottom.
2) The split-spoon sampler is then driven into the soil for a distance of 450 mm in three stages of
150 mm each by blows of a drop hammer of 63.5 kg mass falling vertically and freely through
a height of 750 mm at the rate of 30 blows per minute (IS 2131 – 1981). The number of blows
required to penetrate every 150-mm is recorded while driving the sampler. If full penetration
is obtained, the blows for the first 150 mm is retained for reference purposes, but not used to
compute the SPT value because the bottom of the boring is likely to be disturbed by the drilling
process and may be covered with loose soil that may fall from the sides of the boring. The
number of blows required for the next 300 mm of penetration is recorded as the SPT value.
The number of blows is designated as the “Standard Penetration Value” or “Number” N.
3) The split-spoon sampler is then withdrawn and is detached from the drill rods. The split barrel
is disconnected from the cutting shoe and the coupling. The soil sample collected inside the
split barrel is carefully collected so as to preserve the natural moisture content and transported

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 to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at
the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its
ends before it is taken away to the laboratory.
Usually SPT is carried out at every 0.75-m vertical interval or at the change of stratum in a
borehole. This can be increased to 1.5 m if the depth of borehole is large. Due to the presence of
boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a
case, the N value can be recorded for the first 300-mm penetration. The boring log shows refusal
and the test is halted if:
1) 50 blows are required for any 150 mm penetration
2) 100 blows are required for 300 penetrations
3) 10 successive blows produce no advance

Plate 1.2 Standard Penetration testing

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 Fig.1.9 Stages of Standard Penetration testing

a) Donut hammer b) Safety Hammer c) Automatic hammer

Fig. 1.10 Types of SPT hammers

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Precautions:
Some of the precautions to be observed to avoid some of the pitfalls of the test are as follows:
1) The drill rods should be of standard specification and should not be in bent condition.
2) The split spoon sampler must be in good condition and the cutting shoe must be free from
wear and tear.
3) The drop hammer must be of right weight and the fall should be free, frictionless and
vertical.
4) The height of fall must be exactly 750 mm. Any change in this will seriously affect the N
value.
5) The bottom of the borehole must be properly cleaned before the test is carried out. If this
is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed
soil.
6) When a casing is used in borehole, it should be ensured that the casing is driven just short
of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a
soil plug enclosed at the bottom of the casing.
7) When the test is carried out in a sandy soil below the water table, it must be ensured that
the water level in the borehole is lower than the ground water level in the borehole is always
maintained slightly above the ground water level. If the water level in the borehole is lower
than the ground water level, ‘quick’ condition may develop in the soil and very low N
values may be recorded.
In spite of all these imperfections, SPT is still extensively used because the test is simple and
relatively economical. It is the only test that provides representative soil samples both for visual
inspection in the field and for natural moisture content and classification tests in the laboratory.
Because of its wide usage, a number of time-tested correlations between N value and soil
parameters are available, mainly for cohesionless soils. Even design charts for shallow foundations
resting on cohesionless soils have been developed on the basis of N values. The use of N values
for cohesive soils is limited, since the compressibility of such soils is not reflected by N values.

SPT values obtained in the field for sand have to be corrected before they are used in empirical
correlations and design charts. The code recommends that the field value of N corrected for two
effects, namely,
(a) effect of overburden pressure, and

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 (b) effect of dilatancy
(a) Correction for overburden pressure:
Several investigators have found that the overburden pressure influences the penetration resistance
or the N value in a granular soil. If two granular soils possessing the same relative density but
having different confining pressures are tested, the one with a higher confining pressure gives a
higher N value. Since the confining pressure (which is directly proportional to the overburden
pressure) increases with depth, the N values at shallow depths are underestimated and the N values
at larger depths are overestimated. Hence, if no correction is applied to recorded N values, the
relative densities at shallow depths will be underestimated and at higher depths, they will be
overestimated. To account for this, N values recorded (NR) from field tests at different effective
overburden pressures are corrected to a standard effective overburden pressure.
The corrected N value is given by:

N 'c = CN NR
Where, N′c = corrected value of observed N value
CN = correction factor for overburden pressure
NR = Recorded or observed N value in the field

The correction proposed by Peck, Hanson and Thornburn (1974) is given by the equation:

Where, σ′ = Effective overburden pressure at the depth at which N value is recorded, in kPa.

(b) Correction for dilatancy:

Dilatancy correction is to be applied when N′c obtained after overburden correction, exceeds 15 in
saturated fine sands and silts. The code incorporates the Terzaghi and Peck recommended
dilatancy correction (when N′c > 15) using the equation
Nc = 15 + 0.5 (N′ − 15)
Where Nc = final corrected value to be used in design charts.
N′c > 15 is an indication of a dense sand, based on the assumption that critical void ratio occurs at
approximately N′c = 15. The fast rate of application of shear through the blows of a drop hammer
is likely to induce negative pore water pressure in saturated fine sand under undrained condition

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of loading. Consequently, a transient increase in shear resistance will occur, leading to a SPT value
higher than the actual one.
Note: The overburden correction is applied first. This value is used as observed N value and then
the dilatancy correction is applied.
Correlation of ‘N’ with engineering properties:
The value of standard Penetration number depends upon the relative density of the cohesionless
soil and the UCC strength of the cohesive soil.
The angle of shearing resistance (ϕ) of the cohesionless soil depends upon the number N. In
general, greater the N-value, greater is the angle of shearing resistance. Table below gives the
average values of ϕ for different ranges of N:
Table 1.5 Correlation between N value and angle of shearing resistance
N Denseness ϕ
0-14 Very loose 25o – 32o 27o
4-10 Loose – 35o
10-30 Medium
30o – 40o
30-50 Dense
35o – 45o
>50 Very dense
> 45o
The consistency and the UCC strength of the cohesive soils can be approximately determined from
the SPT number N. Table gives the approximate values of UCC strength for different ranges of N.

Table 1.6: Correlation between N value and UCC strength

N Consistency qu (kN/m2)
0-2 Very soft < 25
2-4 Soft 25 – 50
4-8 Medium 50 – 100
8-15 Stiff 100 – 200
15-30 Very stiff 200 – 400
>30 Hard > 400

It can also be determined from the following relation

qu = 12.5 N
Where, qu = UCC strength (kN/m2)

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1.11.3 Cone Penetration Test (CPT)
(a) Dynamic Cone Penetration Test (DCPT)
In this test, a cone, which has an apex angle of 60o and attached to drill rods is driven into the soil
by blows of a hammer of 63.5 kg, falling freely from a height of 750 mm. The blow count for every
100-mm penetration of the cone is continuously recorded. The cone is driven till refusal or up to
the required depth and the drill rods are withdrawn, leaving the cone behind in the ground.
The number of blows required for 300-mm penetration is noted as the dynamic cone resistance,
Ncd. The test gives a continuous record of Ncd with depth. No sample, however, can be obtained in
this test.
Dynamic cone penetration tests are performed either by using a 50 mm diameter cone without
bentonite slurry or a 65 mm diameter cone with bentonite slurry. When bentonite slurry is used,
the set-up has an arrangement for the circulation of slurry so that friction on the drill rod is
eliminated.
The dynamic cone test is a quick test and helps to cover a large area under investigation rather
economically. It helps in identifying the uniformity or the variability of the subsoil profile at the
site and reveals local soft pockets, if any. It can also establish the position of rock stratum, when
required. The test is much less expensive and much quicker than the SPT. If the tests are carried
out close to a few boreholes, the data from DCPT can be compared with the SPT data and
correlation between the two established for the particular site conditions. The correlation can then
be used to obtain N values from Ncd values.
Some approximate correlations between Ncd and N, applicable for medium to fine sands are given
below:
When a 50 mm diameter cone is used,
Ncd = 1.5 N for depths upto 3 m
Ncd = 1.75 N for depths from 3 m to 6 m
Ncd = 2.0 N for depths greater than 6 m
(b) Static cone penetration test (CPT)
The static cone penetration test, simply called the cone penetration test (CPT), is a simple test that
is presently widely used in place of SPT, particularly for soft clays and fine to medium sand
deposits. The test was developed in Holland and is, therefore, also known as the Dutch cone test.

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The test assembly is shown in Fig. 1.12. The penetrometer that is commonly used is a cone with
an apex angle of 60o and a base area of 10 cm2.

(a) (b)
Fig.1.11 Cone penetration set up
The sequence of operations of the penetrometer is as follows:
1) Position 1: The cone and the friction jacket is in a stationary position.
2) Position 2: The cone is pushed into the soil by the inner drill rod/sounding rod to a depth
‘a’, at a steady rate of 20 mm/s, till a collar engages the cone. The tip resistance q c called
the cone or point resistance, can be calculated by the force Qc read on a pressure gauge.
The tip resistance, qc = Qc / Ac
Where Ac is the base area
Normally the value of a = 40 mm

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 Fig. 1.12 Sequence of operations of CPT

3) Position 3: The sounding rod is pushed further to a depth ‘b’. This has the effect of pushing
the friction jacket and the cone assembly together. The total force Qt required for this is
again read on the pressure gauge. The force required to push the friction jacket along,
Qf is then obtained as Qt – Qc
The side or the skin friction, fs = Qf / Af
Where, Af is the surface area of the friction jacket.
Normally the value of b = 40 mm
4) Position 4: The outside mantle tube is pushed down to a distance (a+b), bringing the cone
and the friction jacket to position 1.
The procedure illustrated above is continued till the proposed depth of sounding is reached. CPT
gives a continuous record of variation of both cone resistance and friction resistance with depth.
Unlike the SPT and the DCPT, this test measures the static resistance of the soil. From CPT soil

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sample is not obtained. The test is not suitable in gravels and very dense sands owing to the
difficulty experienced in pushing the cone and the anchorage system.
Data from CPT is often used to estimate the point bearing resistance of a pile foundation. In
granular soils, correlations have been established between qc and N. table below shows the
correlations.
Table 1.7 Correlations between qc and N
Type of soil qc / N (kPa)
a) Sandy gravels and gravels 800 to 1000
b) Coarse sands 500 to 600
c) Clean, fine to medium sands and slightly silty sands 300 to 400
d) Silts, sandy silts, slightly cohesive silt-sand mixtures 200

c) Piezo-cone Penetration Test

The piezo-cone penetration test (CPTU) is an example of improved in-situ techniques that have
been developed for site investigations. It is an extension to the standard cone penetration test
(CPT). The standard cone penetrometer consists of a 35 – mm diameter rod with a 60o conical-
shaped tip equipped with electronic sensing elements to measure tip resistance and the local side
friction on a sleeve. The piezo-cone penetrometer in addition incorporates a transducer to measure
pore water pressure. The penetrometer is pushed at a constant rate of 2 cm/s into the ground. In
general, no pre-boring is required. In addition to the standard three-channel piezo-cone
penetrometer, new penetrometers have been developed that include additional sensing elements
for measurement of temperature, inclination, lateral stress and seismic waves. In particular, the
seismic cone penetrometer (CPTS), which houses an accelerometer, has proven itself most
beneficial in the assessment of liquefaction and determining dynamic soil properties.
The CPTU is one of the most commonly used specialized in-situ tests and is ideal for use in sands,
silts, clays and mine-tailings materials. A reliable, continuous stratigraphic profile, together with
important engineering properties, can be interpreted from its results. Further, because the data are
collected on a continuous basis, detection of thin, weak layers and /or more pervious layers is
possible. This is important because these weaker zones ultimately control the behaviour and
performance of the soil mass and, in particular, are significant to slope stability and seepage
considerations, liquefaction and dynamic stability analysis, and foundation design. These weaker
layers often go undetected by conventional borehole drilling and sampling operations. The CPTU

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also avoids the disturbance effects commonly encountered when drilling boreholes and sampling
below the water table. Thus, CPTU greatly enhances the quality of data gathered in a sub-surface
investigation. In addition, because of its relative simplicity, ease of operation and high rate of
production in terms of depth of soil investigated, the CPTU has proven itself to be very cost-
effective.
The interpreted data provide information on: stratigraphy in terms of thickness, gradation and soil
type; density (void ratio) and /or state (state parameter) of the more sandy or coarser materials;
friction angle (strength) of the more sandy or coarser materials; undrained strength and stress
history (OCR) of the more clayey or fine-grained materials; and liquefaction potential evaluation
and cyclic resistance. Further, as the cone penetrometer is pushed through the ground, excess
porewater pressure is induced, the magnitude of which is controlled by the hydraulic conductivity
of the material. If the pushing of the cone is interrupted and the cone is held stationary for a
sufficient period of time, the porewater pressure measured by the cone will stabilize to the in-situ
piezometric pressure at the cone tip. Monitoring the rate of porewater pressure change to this
stabilized pressure provides data from which the in-situ hydraulic conductivity can be calculated.
From the above operations, the piezometric pressure, gradients and hydraulic conductivity within
the ground can also be obtained.

1.12 Geophysical Methods

It is a non-intrusive method of “seeing” into the ground. Unlike direct sampling and analysis, such
as obtaining a soil or water sample and sending it to a laboratory, the geophysical methods provide
non-destructive, in situ measurements of physical, electrical or geochemical properties of the
natural or contaminated soil and rock.
Geophysical methods encompass a wide range of surface and down-hole measurement techniques
which provide a means of investigating subsurface hydrogeologic and geologic conditions. These
methods have also been applied to detecting contaminant plumes and locating buried waste
materials.
All of the geophysical methods, like any other means of measurements, have advantages and
limitations. There is no single, universally applicable geophysical method, and some methods are
quite site specific in their performance. Thus, the user must select the method or methods carefully
and understand how they are applied to specific site conditions and project requirements.

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1.12.1 Usefulness of geophysical methods
i). To provide a greater volume of measurement
Data obtained from borings or monitoring wells come from a very localized area and are
representative of material conditions at the bore-hole. Geophysical methods, on the other hand,
usually measure a much larger volume of the subsurface material
ii). As anomaly detectors
As a result of geophysical measurements being relatively rapid, a larger number of measurements
can be taken, for a given budget. With a greater number of measurements plus the fact that the
measurement encompasses a larger volume of subsurface material, the geophysical methods can
detect anomalous areas which may pose potential problems, and thus are essentially anomaly
detectors.
Once an overall characterization of a site has been made using geophysical methods and anomalous
zones identified, drilling and sampling programmes are made more effective by:
✓ locating boreholes and monitoring wells to provide samples that are representative of both
anomalous and background conditions;
✓ minimizing the number of borings, samples, piezometers and monitoring wells required
to characterize accurately a site;
✓ reducing field investigation time and cost; and
✓ significantly improving the accuracy of the overall investigation.
✓ This approach yields a much greater confidence in the final results, with fewer borings
or wells, and an overall cost savings.
✓ It makes good sense to minimize the number of monitoring wells at a site while optimizing
the location of those installed.
✓ If the wells are located in the wrong position, they do not provide representative data and a
large amount of relatively useless data would accrue.
✓ Using the geophysical method in a systematic approach, drilling is no longer being used for
hit-or-miss reconnaissance, but is being used to provide the specific quantitative
assessment of subsurface conditions.
✓ Boreholes or wells located with this approach may be thought of as smart holes because
they are scientifically placed, for a specific purpose, in a specific location, based on
knowledge of site conditions from geophysical data.

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1.12.2 Geophysical measurements
1) Mechanical Wave Measurements
2) Electromagnetic Wave Techniques
Mechanical Wave Measurements
✓ Crosshole Tests (CHT)
✓ Downhole Tests (DHT)
✓ Spectral Analysis of Surface Waves
✓ Seismic Methods (Reflection and Refraction)
✓ Suspension Logging
Electromagnetic Wave Techniques
✓ Ground Penetrating Radar (GPR)
✓ Electromagnetic Conductivity (EM)
✓ Surface Resistivity (SR)
✓ Magnetometer Surveys (MT)
1.12.3 Seismic Methods – Principle Involved
It is based on fact that compression waves or shear waves travel at different speeds in the ground,
and that waves reflect off at interfaces between materials of different density or stiffness.
There are two methods based on seismic waves. They are:
✓ Seismic Reflection Method
✓ Seismic Refraction Method
1.12.4 Seismic Reflection
It is well known that reflections of sound waves (compression waves) from the subsurface arrive
at the geophones at some measurable time after the source pulse. If we know the speed of sound
in the earth and the geometry of the wave path, we can convert that seismic travel time to depth.
By measuring the arrival time at successive surface locations, we can produce a profile, or cross-
section, of seismic travel times. In practice, the speed of sound in the earth varies enormously. Dry
sand might carry sound waves at 250 m/s or less. At the other extreme, unfractured granite might
have a velocity in excess of 6,000 m/s. More is the number of layers between the surface and the
layer of interest, more complicated is the velocity picture. Various methods are used to estimate
subsurface velocities including refraction analysis, borehole geophysical measurements, estimates
from known lithologic properties, and analysis of reflection times at increasing offsets. Generally,
a combination of velocity estimation methods will give the best results.

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 Fig. 1.13 Seismic Reflection method

1.12.5 Seismic Refraction

When a sound wave crosses an interface between layers of two different velocities, the wave is
refracted. That is, the angle of the wave leaving the interface will be altered from the incident
angle, depending on the relative velocities. Going from a low-velocity layer to a high-velocity
layer, a wave at a particular incident angle (the "critical angle") will be refracted along the upper
surface of the lower layer. As it travels, the refracted wave spawns upgoing waves in the upper
layer, which impinge on the surface geophones. Sound moves faster in the lower layer than the
upper, so at some point, the wave refracted along that surface will overtake the direct wave. This
refracted wave is then the first arrival at all subsequent geophones, at least until it is in turn
overtaken by a deeper, faster refraction. The difference in travel time of this wave arrival between
geophones depends on the velocity of the lower layer. If that layer is plane and level, the refraction
arrivals form a straight line whose slope corresponds directly to that velocity. The point at which
the refraction overtakes the direct arrival is known as the "critical distance” and can be used to
estimate the depth to the refracting surface.

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 Seismograph b) Spectrum Analyzer

c) Portable Analyzer d) Velocity Recorder

Plate 1.3 Geophysical Equipment

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 Plate 1.4 Geophones

Fig.1. 14 Seismic refraction method

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 Fig. 1.15 Seismic refraction method

Fig. 1.16 Plot of Travel time versus distance obtained from seismic refraction method
Now at critical distance d1(or dc), both the primary wave and refracted wave arrive simultaneously.
Therefore,
Time taken by primary wave to travel distance d1 = time taken by refracted wave to travel
distance (2H1+ d1)

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This equation gives reliable results when the waves are produced by sinusoidal force and not by
impact.
When impact loads are used the following empirical relation gives more reliable results:

------------ (1.2)
1.12.6 Applicability of Seismic refraction method
The seismic refraction method is commonly applied to shallow investigations up to about 100m.
However, with sufficient energy, surveys to several hundred meters are possible.

1.12.7 Limitations of Seismic Methods

✓ The Method cannot be used where a hard layer overlies a soft layer, because there will be
no measurable refraction from a deeper soft layer. Test data from such an area would tend
to give a single-slope line on the travel-time graph, indicating a deep layer of uniform
material.
✓ The method cannot be used in an area covered by concrete or asphalt pavement, since these
materials represent a condition of hard surface over a soft stratum.

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 ✓ A frozen surface layer also may give results similar to the situation of a hard layer over a
soft layer.
✓ If the area contains some underground features, such as buried conduits, irregularly dipping
strata, discontinuities such as rock faults or earth cuts, irregular water table, and the
existence of thin layers varying materials, the interpretation of the results becomes very
difficult.
✓ The method requires sophisticated and costly equipment.
✓ For proper interpretations of the seismic survey results, the services of an expert are
required.
1.13 Surface Resistivity Methods
These methods make use of Electromagnetic Wave Geophysics. These are non-destructive
methods, non-invasive and are conducted across surface. Here measurements of electrical &
magnetic properties of the ground namely, resistivity (conductivity), permittivity, dielectric, and
magnetic fields are measured. They cover wide spectrum in frequencies (10 Hz < f < 1022 Hz).
The popular and most widely used surface resistivity methods are:
✓ Electrical Profiling Method
✓ Electrical Sounding Method

1.13.1 Electrical Resistivity

“Electrical Resistivity is the physical property of a material, which is defined as the resistance of
the material to the passage of electrical current”.
It is expressed as: ρ= RA
L

Where R = Electrical Resistance (Ohms)

A = area of cross section (m2)
L = Length of Conductor (m)
Ρ = Electrical Resistivity (Ohm-m)

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1.13.2 Electrical Profiling Method
1.13.2.1 Test Procedure
Four electrodes are placed in a straight line at equal distances as shown in Fig. 1.17. The two outer
electrodes are known as current electrodes and the inner electrodes are known as potential
electrodes. The mean resistivity of the strata is determined by applying a direct current of 50 to
100 milli amperes between the two inner electrodes (here a null-point circuit is used that requires
no flow of current at the instant of measurement).

Fig. 1.17. Wenner arrangement

In a semi-infinite homogeneous isotropic material, the electrical resistivity, is given by the formula:
ρ π= 2 a ∆iV

Where,
a = distance between electrodes (m)
V = potential drop between the inner electrodes
i = current flowing between the outer electrodes (Amps)

= Mean resistivity (Ohm-m)

The calculated value is the apparent resistivity, which is the weighted average of all materials
within the zone created by the electrical field of the electrodes. The depth of material included in
the measurement is approximately the same as the spacing between the electrodes. The electrodes

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are moved as a group, and different profile lines are run across the area. The test is repeated after
changing the spacing (‘a’) and again determining the mean resistivity with the new spacing.
It is necessary to make a preliminary trial on known formations, in order to be in a position to
interpret the resistivity data for knowing the nature and distribution of soil formations.

1.13.2.2 Applications of Electrical Profiling method

This method is useful for establishing boundaries between different strata. The method is generally
used for locating sand and gravel deposits (or ore deposits) within a fine-grained soil deposit.

1.13.3 Electrical Sounding Method

1.13.3.1 Test procedure
In this method a centre location for the electrodes is selected. A series of resistivity readings is
obtained by gradually increasing the electrode spacing. As the depth of the current penetration is
equal to the electrode spacing, the changes in the mean resistivity is correlated to the changes in
strata at that location.
Table 1.8 Representative Resistivity values (after Peck, et. Al., 1974)
S. No Material Resistivity, (Ohm-cm x 103)

1 Clay and saturated silt 0-10

2 Sandy clay and wet silty sand 10-25

3 Clayey sand and saturated 25-50

sand
4 Sand 50-150

5 Gravel 150-500

6 Weathered rock 100-200

7 Sound rock 150-4000

1.13.3.2 Applications of Electrical Sounding method

This method is useful in studying the layering of materials. The method is capable of indicating
sub-surface conditions where a hard layer underlies a soft layer and also the situation of a soft
layer underlying a hard layer.
1.13.4 Limitations of Electrical Resistivity Methods
✓ The methods are capable of detecting only the strata having different electrical resistivity.

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 ✓ The results are considerably influenced by surface irregularities, wetness of the strata and
electrolyte concentration of the ground water.
✓ As the resistivity of different strata at the interface changes gradually and not abruptly as
assumed, then the interpolation becomes difficult.
✓ The services of an expert in the field are needed
1.14 Boring Log
During soil exploration all suitable details are recorded and presented in a boring log. Additional
information consisting mainly of lab and field test result is added to complete the boring log.
1.14.1 Details of Boring Log
The ground conditions discovered in each borehole are summarized in the form of a bore log. The
method of investigation and details of the equipment used should be stated on each log. The
location, ground level and diameter of the hole should be specified. The names of the client and
the project should be mentioned.

1.14.2 Other Details of Boring Log

✓ The soil profile with elevations of different strata.
✓ Ground water level.
✓ Termination level of the bore hole.
✓ The depth at which samples were taken or at which in-situ tests were performed.
✓ The type of soil samples.
✓ N-values at the measured elevation.
✓ The results of important laboratory tests

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 1.14.3 Typical Bore Log

1.15 Soil Exploration Report

At the end of the soil exploration program, the soil and rock samples, collected from the field are
subjected to visual observation and laboratory tests. Then, a soil exploration report is prepared for

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use by the planning and design office. Any soil exploration report should contain the following
information:
1. Scope of investigation
2. General description of the proposed structure for which the exploration has been
conducted
3. Geological conditions of the site
4. Drainage facilities at the site
5. Details of boring
6. Description of subsoil conditions as determined from the soil and rock samples
collected
7. Ground water table as observed from the boreholes
8. Details of foundation recommendations and alternatives
9. Any anticipated construction problems
10. Limitations of the investigation
The following graphic presentations also need to be attached to the soil exploration report:
1. Site location map
2. Location of borings with respect to the proposed structure
3. Boring logs
4. Laboratory test results
5. Other special presentations
The boring log is the graphic representation of the details gathered from each borehole.

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