Soil exploration by abhishek sharma

A detailed investigation for site is essential before a design
can be finalized. The object of subsurface and related
site investigation is to provide the engineer or
architect with as much information as possible about
the existing conditions,
for example, the exposed overburden, the course of a
stream nearby, a rock outcrop or a hillock, vegetation, and other
geological features of the area. It is equally important to know
the subsoil conditions below a proposed structure.
The field and laboratory investigations required to
obtain the necessary data of the soils for proper design and
constructions of any structure at the site are collectively called
Soil Exploration.
Abhishek sharma 661/15

Investigation of the underground conditions at a site for
the economical design of the substructure elements.
Purpose of Exploration
To determine the general suitability of the site.
To find the nature of each stratum and engineering properties of the
soil and rock, which may affect the design and mode of construction
of proposed structureand foundation.
To find out the sources of construction material.
To ensure the safety of surrounding existing structures
To locate the ground water level and possible corrosive effect of soil
and water on foundation material.
To predict the settlements
Selection of suitable construction technique.
Selection of type and depth of foundation.
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A SAMPLE
There are 2 types of samples :
1. Disturbed Sample
2. Undisturbed Sample
In first one, structure of the soil get disturbed
And actual results can not be determined. But in
latter case we can get the actual result because
the soil sample is not disturbed and hence the
structure of the soil remains same. In INDIA Soil
Exploration Should be done as per
IS 1892.1979. Abhishek sharma 661/15

 Foundations of Multi-storeyed Buildings
(IS: 1892,1979)
 Earth and rockfill Dams (IS: 6955, 1973)
 Power House Sites (IS: 10060, 1981)
 Canals and Cross Drainage Works (IS: 11385, 1985)
 Portsand Harbours (IS: 4651 – Part 1, 1974)

Exploration program involves location and depth of
borings, test pits or other methods to be used, and methods
of sampling and tests to be carried out to determine
the stratification and engineering properties of the
soils underlying the site. The principal properties of interest
Will the shear strength, deformation and hydraulic
characteristics of soil
The program should be planned so that the maximum
amount of information can be obtained at minimum
cost. The actual planning of a subsurface exploration
program includes the following steps:
 Gather all available information
 Reconnaissance
 Preliminary exploration
 Detailed exploration

Assemble all information on dimensions,
column spacing, type and use of
structure, basement requirements and any
special architectural consideration of the
proposed building For bridges the
soil engineer should have access to type and span
lengths as well as pier loadings. This information will
indicate any settlement limitations, and can be used to
estimate foundation loads.

Site reconnaissance would help in deciding future
programme of field investigations ?
that is, to assess the need for
preliminary or detailed investigations. This would also help in determining
scope of work, methods of exploration to be adopted, field tests to be carried
out and administrative arrangements required for the investigation. The
main Objective of site reconnaissance is enquiries regarding earlier use of
site.
• Site reconnaissance includes a study of local topography, excavations,
ravines, quarries, escarpments; evidence of erosion or landslides.
• It Includes study of behaviour of existing structures at or near the site;
water level in streams, water courses and wells; flood marks.
• It includes study of Information on some of these may be obtained from
topographical maps, geological maps, pedological and soil survey maps, and
aerial photographs.
(as per clause 2.2.1)

In this step a few borings are made to establish in a general
manner.
To know the stratification, types of soil to be expected,
andpossibly the location of thegroundwater table.
If the initial borings indicate the upper soil is loose
or highly compressible, One or more borings should be
taken to rock, or hard strata,
A feasibility exploration program should include collection
of enough site data and sample recovery to approximately
determine the properties of soil, foundation design
and identify theconstruction procedures.
To find the thickness and composition of each soil layer.

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.
The report is prepared with clear description of the soils
at the site, methods of exploration, soil profile, test
methodsand 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.

The depth of exploration required depends on the type of 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 a half times the width
of the footing below foundation level.
In certain cases, it may be necessary to take at
least one bore hole or cone test or both to twice the
width of the foundation.
If a number of loaded areas are in close proximity the effect of
each is additive. In such cases, the whole of the area may be considered
as loaded and exploration should be carried out up to one and a half times
the lower dimension. In weak soils, the exploration should be continued
to a depth at which the loads can be carried by the stratum in question
without undesirable settlement and shear failure.
(As per clause 2.3.2)

NUMBER AND DISPOSITION OF TRIAL PITS AND BORINGS :
The disposition and spacing of the trial pits and borings
should be such as to reveal any major changes in thickness, depth or
properties of the strata over the base area of the structure and its
immediate surroundings.
For a compact building site covering an area of
about 0.4 hectare, one bore hole or trial pit in
each corner and one in the centre should be adequate.
For very large areas covering industrial and residential
colonies, the geological nature of the terrain will help in deciding the
number of bore holes or trial pits.
(as per clause 2.3.1)

Depth up to which stress
intensity is 0.2 times the
stress near application of
load is said to be significant
depth of exploration.

Sl.No. TypeOfFoundation DepthOfExploration
1 Isolated spread
footing
Or Raft
1.5B
2 Adjacent footings
with
Clear spacing less
than
2B
1.5L
3 Pile foundation 10to30mOR1.5B
4
Base of retaining
wall
1.5B(Basewidth)
1.5H(Exposed height of wall face)
[whichever is Greater]
5 Floating basement Depth of construction
6 Dams
1. 1.5 times of bottom width of earth dams
2. 2 times of height from stream bed to
crest for concrete dams, for dams less than
30m high
3. Up to bed rock, in all soft, unstable and
Permeable strata.

Sl.No. TypeOfFoundation DepthOfExploration
7 RoadsCuts
1. 1m little cut or fill is
required
2.In cut sections, 1. below
Formation level
3. In deep cuts, equal to the
bottom
Width or depth of the cut
8 RoadFill
2m Below ground
Level or equal to the height
Of the fill which ever is
Greater
 B = Width of the foundation
 L = Length of the foundation

❖ Split spoon Sampler
❖ Scraper Bucket Sampler
❖ Shelby tube or Thin
Walled Sampler
❖ Piston Sampler

It has an inside diameter of 35mm and an
outside diameter of 50mm.
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Has a split tube which is held together using a screw-
on driving shoe at the bottom end and a cap at the
upperend.
4 vent posts are provided to improve recovery of
sample.
The thicker wall of the standard sampler
permits higher driving stresses than the Shelby tube but
with higher levelsof soil disturbances. It is used in SPT
test.
Splitspoon samplesare highlydisturbed.
They are used for visual examination and for
classification tests.

 Scraper bucket can be used in case of Sandy soil
containing pebbles (gravels) also below water table, it
is difficult to use splitspoon sampler.
 Driving point is attached at the end.
 It hasvertical slit in the upperportionof the sampler.
 As the sampler rotates the cutting of disturbed sample
is collected in slit.

It is a thin-walled seamless steel tube of inside diameter
50 to 76.2mm, outer diameter upto 125mm 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, when the required depth is reached it is
twisted to 360 degree twice.
For sandy soil length of tube = 5xdia to 10xdia
For Clayey soil length of tube = 10xdia to 15xdia
The sheared soil sample inside the tube at the bottom is
then pulled out and the two ends of the sampler are sealed and
sent to the lab.
The samples can be used for consolidation and shear tests as it is
undisturbed.

 When sampling very soft and sensitive clays 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.

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 beextended toa greaterdepth.
 For coring, a coring bit is attached to the core barrel
and core barrel is attached to the drilling rod. The
cutting element in the bit may be diamond, tungsten,
or carbide.
 The coring is done by rotary drilling. Water
is circulated and cuttings are washed out

 Rock cores obtained by such barrels can be fractured
because of torsion. To avoid this problem, one can use
double-tubecore barrels.
On the basis of the length of the rock coreobtained the
following quantities can be obtained for evaluation of
the qualityof rock

RQD was developed in 1964 by D. U. Deere*. It is
determined by measuring the core recovery percentage
of core chunks that are greater than 100 mm in length.
Core that is not hard or sound should not be counted
even if they are 100 mm in length. RQD was introduced
for use with core diameters of 54.7 mm. It is a leading
indicator for low-quality rock zones. Today RQD is used
as a standard parameter in drill core logging and forms a
basic element value of the major mass classification
systems.

In areas which have already been developed, advantage
should be taken of existing local knowledge, records of trial pits,
bore holes, etc, in the vicinity, and the behaviour of existing
structures, particularly those of a nature similar to that of the
proposed structure. In such cases, exploration may be limited to
checking that the expected soil conditions are those as in the
neighborhood.

Trial pits are applicable to all types of soils, which
Provides visual inspection of soil in their natural
condition in either disturbed or undisturbed state.
Here depth of investigation is limited to 3 to 3.5m.There are 2 ways
1. Pits and trenches
2. Drifts and Shafts

1. Pits and Trenches
Pits: They are excavated at site for inspection of strata
so as t0 provide necessary working space. According
to IS 4453 1967, a clear working space at the bottom
of the
pit should be 1.2 m x 1.2 m. Shallow pits (upto 3m) do
not require lateral support. For depth greater than
3m and GWT arises then lateral support in the
form of sheeting and bracing is required.

Trenches:
Theycan bedefined as long shallow pits. It is continuous
overa considerable length and provides exposure along a
line. On slopes trenches are more suitable than pits.

Drifts:
They are the horizontal tunnels made in the hill sides to
determine the nature and structure of the geological strata.
According to IS 4453-1980 a drift should be 1.5m wide and
2m height in hard rock. In soft rock arched roof can
Be provided.
Shafts:
Large sized vertical holes made in the geological formation
are called as shafts
For Circular Diameter = 2.4m (min)
For Rectangular Width = 2.4m
Usuallydone fordepth greater than 4m.

This method consist of excavating trial pits at the site and thereby
exposing the subsoil surface thoroughly, enabling undisturbed samples to
be taken.
The undisturbed sample can be obtained by sharp-edged thin
wall tubes into the ground by generally hammering or pressure. Hand cut
samples known as chunk samples.
This method is generally used for depth upto 3m
Deep trial pits are used to investigate open fissures.
If a soil is easily disturbed a firmly constructed wooden box, with
a lid and bottom removed, is kept around the protuding sample block so as
to leave a space of about 25 mm between the sample and sides of the box.
The space between the sample and side of the box is filled with moist saw
dust or similar packing material.

Auger boring - An auger may be used for boring holes to a depth of
about 6 m in soft soil which can stand unsupported but it may also be
used with lining tubes if required. Mechanically operated augers are
suitable for gravelly soils or where a large number of holes are to be
made.
Shell and Auger Boring
A hand rig may be used for vertical
boring up to 200 mm in dia. and
25m in depth.
In alluvial deposit depth of the
bore hole may be extended up to
50 m with a mechanized rig.
Augers for soft to stiff clay.
Shell for very stiff and hard clay.

Hand operated
Helical Auger
Mechanical Operated
Helical Auger
Hand operated
post hole
auger

This method consists of breaking up of the formation
by repeated blows from a bit or a chisel. Water should be added
to the hole at the time of boring, and the debris baled out at
intervals. The bit may be suspended by a cable or rods from a
walking beam or spudding device.
Where the boring is in soil or into soft rocks and provided
that a sampler can be driven into them, cores may be obtained at
intervals using suitable tools; but in soils, the material tends to
become disturbed by the action of this method of boring and for this
reason, the sample may not be as reliable as by the shell and auger
method. As these machines are devised for rapid drilling by
pulverizing the material, they are not suitable for careful
investigation. However, this is the only method suitable for drilling
bore holes in boulderous and gravelly strata.

In this method, water is forced under pressure through an inner
tube which may be rotated or moved up and down inside a casing pipe.
The lower end of the tube, fixed with sharp edge or a tool, cuts the
soil which will be floated up through the casing pipe around the tube.
The slurry flowing out gives an indication of the soil type. In this
method heavier particles of different soil layers remain under
suspension in the casing pipe and get mixed up, and hence this method
is not suitable for obtaining samples for classification. Whenever a
change in strata is indicated by the slurry flowing out, washing
should be stopped and a tube sampler should be attached to the end
of the drill rod or the inner tube. Samples of the soil should be
obtained by driving the sampler into the soil by hammering or
jacking. Jacking or pulley method should be used when
undisturbed samples are required. Initially fish-tail bit or pistol
bits are used for drilling bore hole up to weathered material. These bits
should be replaced by tungsten carbide or diamond bits. Double
tubecore barrels are recommended for drilling in weathered rock
stratum, with seaming shells and core catcher as required.

Sl.No. Typeofproject Spacing(m)
1 Multi-storeybuilding 10-30
2 IndustrialPlant 20-60
3 Highway 250-500
4 ResidentialSubdivision 250-500
5 DamsandDikes 40-80

In this system, boring is effected by the cutting action of a
rotating bit which should be kept in firm contact with the bottom of
the hole.
The bit is carried at the end of hollow, jointed drill rods which
are rotated by a suitable chuck.
A mud-laden fluid or grout is pumped continuously down the
hollow drill rods and the fluid returns to the surface in the annular
space between the rods and the side of the hole, and so the protective
casing may not be generally necessary.
In this method cores may be obtained by the use of coring
tools. In case gravel and kankar are encountered, a gravel trap
fitted with Stays around the drill rod, a little above the cutter, may
be used. The trap consists of 80 to 100 cm long hollow cylinder
having a conical shape at bottom. Holes of 3 mm diameter are also
drilled in the drill rod within the trap as well as in the conical portion
of the trap. During boring, gravel and kankar rise a little and then
settle into the trap. With the provision of holes, no finer materials
settle in the trap.

Core drilling-core drills shall be so designed that in sound ‘rock,
continuous recovery of core is achieved.
Water is circulated down the hollow rods, which returns
outside them, carrying the rock cuttings to the surface as sludge. These
shall be retained as samples in traversing friable rock where cores cannot
be recovered.
It is important to ensure that boulders, or layers of cemented
soils are not mistaken for bed rock. This necessitates core drilling to a
depth of at least 3 m in bed rock in areas where boulders are known to
occur.
For shear strength determination, a core with diameter to
height ratio of 1 : 1 is required. Rock pieces may be used for
determination of specific gravity and classification.

A pressure meter (see Fig. 5 ) applies a uniform radial
stress to the bore hole at any desired depth and measures
consequent deformation.
The test involves lowering of an inflatable cylindrical probe to
the test depth in a bore hole. The probe is inflated by applying water
pressure from a reservoir.
Under pressure it presses against the unlined wall of the bore
hole and causes volumetric deformation. The stress on the bore-hole
wall is the pressure of water applied.
The deformation of the bore hole is read in terms of
volume corresponding to fall in water level of the reservoir. The
readings are plotted as shown in Fig. 6.

Geo-physical methods are used when the depth
Of exploration is very large, and also when the
speed of investigation is of primary importance.
The major method of geo-physical investigations are:
gravitational methods, magnetic methods, seismic
Refraction method, and electrical resistivity method.
Out of these, seismic refraction method and
electrical resistivity methods are the most commonly
used for Civil Engineering purposes. It is a non-
intrusive method of “seeing” into the ground.
Geophysical methods includes surface and down-
hole measurement techniques which provide details
about subsurface hydro-geologic and geologic
conditions. These methods have also been applied to
detecting contaminant plumes and locating buried
waste materials. some methods are quite site specific in
their performance.

In this method, shock waves are created into the soil at their ground level or a
certain depth below it by exploding small charge in the soil or by striking a
plate on the soil with a hammer. The radiating shock waves are picked
up by the vibration detector (also called geophone or seismometer) where
the time of travel of the shock waves gets recorded. A number of geophones
are arranged on surface , The shock waves travels directly from the shock point
along the ground surface and are picked first by the geophone. The other
waves which travel through the soil get refracted at the interface of two soil
strata. The refracted rays are also picked up by the geophone. If the underlying
layer is denser, the refracted waves travel much faster. As the distance between
the shock point and the geophone increases, the refracted waves are able to
reach the geophone earlier than the direct waves. By knowing the time of
travel primary and refracted waves at various geophones, the depth of
various strata can be evaluated, by preparing distance-time graphs and
using analytical methods. Seismic refraction method is fast and reliable in
establishing profiles of different strata provided the deeper layer have
increasingly greater density and thus higher velocities and also increasingly
greater thickness. Different kinds of materials such as gravel, clay hardpan,
or rock have characteristic seismic velocities and hence they may be
identified by the distance-time graphs.

The electrical resistivity is resistance of the material
to the passage of electrical current.
D = Distance beween theelectrodes (m)
E = Current Flowing between outerelectrodes (amps)
I = Potential drop between innerelectrodes (Volts)
Each soil has its own resistivitydepending upon its water
content, compaction and composition; for example, it is low
For saturated silt and high for loose drygravel or solid rock.
The test is conducted by driving four metal spikes to serve
As electrodes into the ground along a straight lineat equal
distance. A directvoltage is imposed between the two outer
electrodes, and the potential drop is measured between the
innerelectrodes. The mean resistivity Q (ohm-m) is
Computed from the expression:
I
Ώ = 2П D E Ohm-m

To take undisturbed samples from bore holes properly
designed sampling tools are required. These differ for cohesive
and non-cohesive soils and for rocks.
The fundamental requirement of a sampling tool is that
on being forced into the ground it should cause as little
displacement, remoulding and disturbance as possible. The
degree of disturbance is controlled by the following three
features of its design:
• Cutting edge
• Inside wall friction
• Non return value

Cutting Edge- A typical cutting edge is shown in Fig. 7. It should
embody the following features:
a) Inside clearance ( CI ) - The internal diameter (Dc) of the cutting
edge should be slightly less than that inside dia. of the sample tube (Ds)
to give inside clearance. The inside clearance, calculated as follows,
should be between 1 percent and 3 percent of the internal
diameter of the sample tube. This allows for elastic expansion of
the soil as it enters the tube, reduces frictional drag on the sample from
the wall of the tube and helps to retain the core.
CI = Ds – Dc x 100
Dc
b) Outside clearance (Co) = The outside diameter (Dw) of the cutting
edge should be slightly larger than the outside diameter (Dt) of the
tube to give outside clearance. The outside clearance should not
be much greater than the inside clearance. This facilitates the
withdrawal of the sampler from the ground. The outside clearance
should be calculated as follows:
Co = Dw – Dt x 100
Dt

Area ratio (Ar) - The area ratio, calculated as follows, should be kept as
low as possible consistent with the strength requirements of the sample
tube. Its value should not be greater than about 20 percent for stiff
formations; for soft sensitive clays an area ratio of 10 percent or less
should be preferred Where it is not possible to provide sufficient inside
clearance, piston sampler should preferably be used:
Ar = D2
w – D2
c x 100
D2
c
Dw - outside diameter of the cutting shoe
DC - inside diameter of the cutting shoe.
Dt – outside diameter of sample tube
Ds – inside diameter of sample tube

Wall Friction –
This can be reduced by:
a) suitable inside clearance,
b) a smooth finish to the sample tube, and
c) oiling the tube properly.
Non-return Valve –
The valve should have a large orifice to allow the air and
water to escape quickly and easily when driving the sampler.
Recovery Ratio - For a satisfactory undisturbed sample, taking
into consideration the influence of the inside clearance [ see
4.1.1 (a) ] when excess soil is prevented from entering the tube,
the recovery ratio calculated as follows should be between 98 to
96 percent.
Rr = L/H
L = length of sample within tube
H = depth of penetration of sample tube

Generally used for cohesionless soils
To determine relative density , angle of shearing
resistance, UCC
A bore hole is made using drilling tools
After reaching the specified depth, the drilling tool
is replaced by a split spoon sampler to collect soil
sample.

First 150 mm penetration is taken as seating drive and
the no. of blows required for that penetration is
discarded
No. of blows required for next 300mm penetration after
seating drive is taken as standard penetration number
(N). But the condition is that hammer weight should be
63.5kg and height of free fall should be 750 mm
No of blows greater than 50 are taken as refusal and
the test is discontinued
Corrections are applied to the observed N value

• The sum of the blows for second and third
increment of 0.15 m penetration is termed
"penetration resistance or "N-value".
• If the split spoon sampler is driven less than 45
cm (total), then the penetration resistance shall
be for the last 30 cm of penetration (if less than
30 cm is penetrated, the logs should state
the number of blows and the depth penetrated).
• If the no. of blows for 15cm drive exceeds 50, it
is taken as a refusal and the test is discontinued.
• Tests shall be made at every change in stratum
or at intervals of not more than l-5 m whichever
is less. Tests may be made at lesser intervals if
specified or considered necessary.

• Dilatancy Correction
• Overburden correction
Of these, overburden correction is applied first
and to that corrected value, dilatancy Correction is
applied

• In granular soils, overburden pressure affects the
penetration resistance
• If two soils, having same relative density but
different confining pressures are tested, the one
with a higher confining pressure gives a higher
penetration number as the confining pressure in
cohesion less soils increases with the depth, the
penetration number for soils at shallow depths is
underestimated and that at greater depths is
overestimated.
• For uniformity, the N- values obtained from field
tests under different effective overburden
pressures are corrected to a standard effective
overburden pressure.

• One of the most commonly used corrections
• According to them,

FACTORS
Attitude of operators
Overdrive sampler
Sampler plugged by gravel
Plugged casing
COMMENTS
Blow counts for the same soil using the same
rig can vary, depending on who is operating
the rig, and perhaps the mood of operator
and time of drilling.
Higher blow counts usually result from an
overdriven sampler.
Higher blow counts result when gravel plugs
the
sampler, resistance of loose sand could be
highly overestimated.
High N-values may be recorded for loose sand
when sampling below groundwater table.
Hydrostatic pressure can cause sand to rise
within the casing. Abhishek sharma 661/15

FACTORS COMMENTS
Inadequate cleaning of the SPT is only partially made in original soil. Sludge may be
borehole
Not seating the sampler
spoon on undisturbed
material
Driving of the sample spoon
above the bottom of the
trapped in the sampler and compressed as the sampler
is driven, increasing the blow count (This may even
prevent sample recovery.)
Incorrect N-values obtained.
N-values are increased in sands and reduced in
cohesive soils.
hydrostatic head in boring
casing
Failure to maintain sufficient The water table in the borehole must be at least equal
to the piezometric level in the sand, otherwise the sand
at the bottom of the borehole may be transformed into
a loose state thereby decreasing the blow countsAbhishek sharma 661/15

FACTORS COMMENTS
Overwashing ahead of
casing
Low blow count may result for dense sand since
overwashing loosens sand.
Drilling method
Free fall of the drive
Weight is not attained
Not using correct weight
Drilling technique (e.g., cased holes vs. mud
stabilized holes) may result in different N-values for
the same soil.
Using more than 1-1/2 turns of rope around the
drum and or using wire cable will restrict the fall of
the drive weight.
Driller frequently supplies drive hammers with
weights varying from the standard by as much as 10
lbs.

FACTORS COMMENTS
Weight does not strike the
drive cap concentrically
Impact energy is reduced, increasing N-values.
Not using a guide rod Incorrect N-value obtained.
Not using a good tip on the
sampling spoon
If the tip is damaged and reduces the opening or
increases the end area the N-value can be increased.
Use of drill rods heavier than
standard
With heavier rods more energy is absorbed by the
rods causing
an increase in the blow count.

- Relative Density
- Effective Stress Friction Angle
- Unconfined Compressive Strength
*Some correlations require the raw N-values whereas others
use the corrected N-values.

• Relatively quick and simple to perform.
• Provides a representative soil sample.
• Provides useful index of relative
strength and compressibility of the soil.
• Able to penetrate dense layers, gravel, and fill.
• Numerous case histories of soil liquefaction
during past earthquakes are available with SPT
N-values. The method based on this history
can reflect actual soil behavior during
earthquakes, which cannot be simulated in
the laboratory.

• The SPT is an in situ test that reflects soil density,
soil fabric, stress and strain history effects, and
horizontal effective stress, all of which are
known to influence the liquefaction resistance
but are difficult to obtain with undisturbed
samples.
The SPT equipment is rugged, and the test can be
performed in a wide range of soil conditions.
There are numerous correlations for predicting
engineering properties with a good degree of
confidence.

PRECAUTIONS
• The drill rods should be of standard specification and should
not be in bent condition.
• The split spoon sampler must be in good condition and the
cutting shoe must be free from wear and tear.
• The drop hammer must be of the right weight and the fall
should be free, frictionless and vertical.
• The height of fall must be exactly 750 mm. Any change from
this will seriously affect the N value.

• 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.
• 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.
• 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 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.

REFRENCES:
C. VENKATARAMAIAH
GEOTECHNICAL ENGINEERING
THIRD EDITION
( NEW AGE INTERNATIONAL (P) LTD.
PUBLISHERS)
A.S.R RAO & GOPAL RANJAN-
BASIC AND APPLIED SOIL MECHANICS
(NEW AGE INTERNATIONAL (P) LTD.,
PUBLISHERS)
80ABHISHEK SHARMA 661

Soil exploration by abhishek sharma

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