Soil Exploration Part II
T.E. Civil 2009-2010
G.E.- I
Topics Covered
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Introduction
Methods of investigation
Methods of boring
Soil samplers and sampling
Location and number of pits and borings
Penetrometer tests
Borehole logs
Geophysical methods
Methods of Sampling
Samples
Disturbed
Undisturbed
(The structure of the soil is disturbed
(The true in-situ structure and water content
to a considerable degree)
is retained as closely as possible)
Remoulded
Wash
Representative
Block
Drive
Methods of Sampling
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Disturbed samples used for determining index properties (ex.
gsd, consistency, etc.)
Undisturbed samples used for determining engineering
properties (ex. Density, water content, shear strength
parameters, etc.)
Wash samples obtained from wash boring water or mud.
Representative sample retains all constituents of the soil, but
is disturbed from natural state and structure. (ex. Split spoon
sampler)
Block samples are carved out form sides or bottoms of
excavations, sealed in a box and taken to lab.
Open drive samplers consist of thin walled tubes which are
driven or pushed into the soil at the bottom of the hole. (ex.
Shelby Tube sampler)
Undisturbed Samples
Methods of Sampling
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Thin-wall open-tube samplers (Shelby Tube
Samplers) are used for soils that are
particularly sensitive to sampling disturbance.
They are suitable for fine soils up to a firm
consistency, and free from large particles.
Shelby Tubes are available in carbon steel
and in stainless steel. Usual diameters are 3"
or 4" OD.
Shelby Tube
Split spoon sampler IS:9640-1980
Sketch of split spoon sampler
Methods of Sampling
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Requirements to minimize.
disturbance to samples
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Area ratio = (D2w-D2e) x 100%
D2e
Area ratio should be as low as
possible. (<10%).
It represents the amount of soil
displaced.
Inside clearance = (Ds-De) x 100%
ratio
De
Methods of Sampling
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The inside clearance allows elastic expansion of the
sample and minimizes frictional drag on the sample.
The inside clearance should be between 0.5% to 3%.
The outside clearance = (Dw-Dt) x 100%
ratio
Dt
Outside clearance is necessary to reduce the driving force
and resistance to withdrawal.
The outside clearance should be between 0% and 2%.
Diameter of samples should not be less than 38mm.
(Generally between 50-150mm).
Piston sampler (Osterberg sampler) not covered. Please
refer to any book.
Rock coring
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Sampler consists of core barrel and bit is coring
bit.
Core barrel – single tube or double tube of
various dia: EX, AX, BX, NX, etc.
Recovery = Length of sample recovered x 100%
Length of run
RQD = Length of samples >= 4” x 100 %
Length of run
SPT (Standard Penetration Test)
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IS:2131 – 1981 Method for Standard
Penetration Test for Soils.
Most important and most commonly used
field test
Typical equipment: drill rig, split spoon
sampler, hammering equipment, casing
pipe.
SPT Procedure
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Procedure:
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Drive a section of casing pipe.
Complete wash boring and clean the hole
Replace driving bit by split spoon sampler at the bottom end of the
driving rod
Drive the sampler by dropping a hammer of 63.5kg weight through a
height of 75 cm
The number of blows required to penetrate three successive lengths
of 15 cm are noted.
The first 15 cm drive is considered as seating load and is ignored.
The total number of blows required to penetrate the remaining 30 cm
is called the blow count or penetration number N.
Raise the sampler to the surface, open it and extract the sample.
Drive the next length of casing and repeat the process until required
depth is reached.
At the end of the borehole, note the gwt after it stabilizes and then
backfill the borehole.
Corrections to N value
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Overburden Correction applied to N value
based on chart by Peck, et. al. (varies from
0.45 to 2)
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N’=Cn * N where Cn is correction factor
Correction due to dilatancy for fine sand and
silt below gwt having N’ > 15
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N” = 15 + 0.5*(N’-15)
Correlations between N values and soil
properties
N
Compactness
0 to 4
4 to 10
10 to 30
> 50
Very Loose
Loose
Medium Dense
Very Dense
Relative
Density (%)
0-15
15 - 35
25 - 65
> 85
fo
< 28
28 -30
30 -36
> 41
Correlations between N values and soil
properties
Consistency
Very Soft
Soft
Medium Stiff
N
0 to 2
2 to 4
4 to 8
qu (kPa)
< 25
25 to 50
50 to 100
Stiff
Very Stiff
Hard
8 to 12
15 to 30
> 30
100 to 200
200 to 400
> 400
Other test methods
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DCPT – Dynamic cone penetration test
IS:4968 -1976 (Method for subsurface
sounding for soils: Parts I and II)
CPT – Static Cone Penetration Test (Method
for subsurface sounding for soils: Part 3
Static cone penetration test)
Plate Load Test (Method of Load Test on
Soils)
Borelog and geotechnical investigation
report
.