GCE Kannur

1. SOIL EXPLORATION
Module 1 -2K6 -March 2013 -GCEK
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2. WHY?
To determine the type of foundation required
for the proposed project at the site, i.e.
shallow foundation or deep foundation.
To make recommendations regarding the safe
bearing capacity or pile load capacity.
Ultimately, it is the subsoil that provides the
ultimate support for the structures.
Geotechnical Investigation
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3. HOW?
The three important aspect are planning,
execution and report writing.
Geotechnical Investigation
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4. Methods of Investigation
 Probing or sounding methods:
 Drive a pipe or rod into the soil.
 Measure the resistance offered by the soil. Ex.
CPT, SPT
 Geophysical methods:
 Seismic refraction method
 Electrical resistivity method.
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SOIL BORING

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Depth of Boring
The approximate required minimum depth of
the borings should be predetermined. The
estimated depths can be changed during the
drilling operation, depending on the subsoil
encountered. To determine the approximate
minimum depth of boring, engineers may use
the following rule:

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Depth of Boring
1. Determine the net increase of stress, under a
foundation with depth as shown in the Figure.
2. Estimate the variation of the vertical effective stress, ',
with depth.
3. Determine the depth, D = D1, at which the stress
increase  is equal to (1/10) q (q = estimated net stress
on the foundation).
4. Determine the depth, D = D2, at which /' = 0.05.
5. Unless bedrock is encountered, the smaller of the two
depths, D1 and D2, just determined is the approximate
minimum depth of boring required.

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Depth of Boring
Determination of the minimum depth of boring

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Depth of Boring
Depth of Boring

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Depth of Boring
For hospitals and office buildings, the following
rule could be use to determine boring depth

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Depth of Boring
When deep excavations are anticipated, the
depth of boring should be at, least 1.5 times
the depth of excavation. Sometimes subsoil
conditions are such that the foundation load
may have to be transmitted to the bedrock.
The minimum depth of core boring into the
bedrock is about 3m. If the bedrock is
irregular or weathered, the core borings may
have to be extended to greater depths.

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Spacing Boring
There are no hard and fast rules for the
spacing of the boreholes. The following
table gives some general guidelines for
borehole spacing. These spacing can be
increased or decreased, depending on the
subsoil condition. If various soil strata are
more or less uniform and predictable, the
number of boreholes can be reduced.

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Spacing Boring
Approximate Spacing of Boreholes

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SOIL BORING
The earliest method of obtaining a test hole
was to excavate a test pit using a pick and
shovel. Because of economics, the current
procedure is to use power-excavation
equipment such as a backhoe to excavate
the pit and then to use hand tools to remove
a block sample or shape the site for in situ
testing. This is the best method at present for
obtaining quality undisturbed samples or
samples for testing at other than vertical
orientation.

15. Methods of Boring
• Wash Boring:
– A casing is driven with a drop hammer. A hollow drill rod
with chopping bit is inserted inside the casing.
– Soil is loosened and removed from the borehole using
water or a drilling mud jetted under pressure.
– The water is jetted in the hole through the bottom of a
wash pipe and leaves the hole along with the loose soil,
from the annual space between the hole and wash pipe.
– The water reaches the ground level where the soil in
suspension is allowed to settle and mud is re-circulated.
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16. Methods of Boring
Another example of wash boring is called mud rotary
drilling (soil) or core drilling (rock).
 Mud rotary
 Hollow drill rods with a drill bit is rotated into the soil.
Drilling mud is continuously pumped into the hole. The bit
grinds the soil and the return flow brings the cuttings to the
surface.
 Core drilling
 Used for obtaining rock cores.
 A core barrel is fitted with a drill bit is attached to hollow
drill rods.
 Examples: diamond coring, calyx or shot core drilling
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Boring tools
Auger boring Power drills

18. Hand operated augers
Power driven augers
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19. Augering
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Boring tools

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Boring tools

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WASH BORING

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SOIL SAMPLING
Two types of soil samples can be obtained during
sampling disturbed and undisturbed. The most
important engineering properties required for foundation
design are strength, compressibility, and permeability.
Reasonably good estimates of these properties for
cohesive soils can be made by laboratory tests on
undisturbed samples which can be obtained with
moderate difficulty. It is nearly impossible to obtain a
truly undisturbed sample of soil; so in general usage the
term "undisturbed" means a sample where some
precautions have been taken to minimize disturbance or
remolding effects. In this context, the quality of an
"undisturbed" sample varies widely between soil
laboratories.

26. Soil disturbance can occur during drilling, during
sampling, during transportation and storage, or
during preparation for testing. Any sample of soil
being taken from the ground, transferred to the
laboratory, and prepared for testing will be subject
to disturbance. The mechanisms associated with
this disturbance can be classified as follows:
1. changes in stress conditions;
2. mechanical deformation;
3. changes in water content and voids ratio;
4. chemical changes.
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Disturbed vs Undisturbed
Good quality samples necessary.
AR<10%
sampling
tube
soi
l
(%)100
..
....
2
22



DI
DIDO
AR
area
ratio
Thicker the wall, greater the disturbance.

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Disturbed vs Undisturbed

30. Methods of Sampling
 Requirements to minimize
disturbance to samples
 Area ratio = (D2
w-D2
e) x 100%
D2
e
 Area ratio should be as low as
possible. (<10%).
 It represents the amount of soil
displaced.
 Inside clearance = (Ds-De) x 100%
ratio De
.
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31. Methods of Sampling
 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.
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32. Split spoon sampler IS:9640-1980
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34. Sketch of split spoon sampler
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Common Sampling Methods

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ROCK SAMPLING
 Rock cores are necessary if the
soundness of the rock is to be
established.
 small cores tend to break up
inside the drill barrel.
 Larger cores also have a
tendency to break up (rotate
inside the barrel and degrade),
especially if the rock is soft or
fissured.

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Rock coring

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ROCK SAMPLING - Definition

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Rock Core Drilling
 Done with either
tungsten carbide or
diamond core bits
 Use a double or triple
tube core barrel when
sampling weathered or
fractured rock
 Used to determine Rock
Quality Designation
core barrel

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43. Diamond Drill Bit
Tricone drill bit
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Rock Quality Designation RQD

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Rock Quality Designation
RQD
Rock Quality Designation (RQD) is defined as the percentage of rock
cores that have length equal or greater than 10 cm over the total drill
length.

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Example on Core Recovery & RQD
 Core run of 150 cm
 Total core recovery =
125 cm
 Core recovery ratio =
125/150 = 83%
 On modified basis,
95 cm are counted
RQD = 95/150=63 %

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GROUND WATER TABLE LEVEL
Groundwater conditions and the potential for
groundwater seepage are fundamental factors in
virtually all geotechnical analyses and design
studies. Accordingly, the evaluation of
groundwater conditions is a basic element of
almost all geotechnical investigation programs.
Groundwater investigations are of two types as
follows:
 Determination of groundwater levels and
pressures.
 Measurement of the permeability of the
subsurface materials.

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FIELD STRENGTH TESTS
The following are the major field tests for
determining the soil strength:
1. Vane shear test (VST).
2. Standard Penetration Test (SPT).
3. Cone Penetration Test (CPT).
4. The Borehole Shear Test (BST).
5. The Flat Dilatometer Test (DMT).
6. The Pressure-meter Test (PMT).
7. The Plate Load Test (PLT).

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Field Strength Tests

50. SPT (Standard Penetration Test)
 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.
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51. SPT Procedure
 Procedure:
1. Drive a section of casing pipe.
2. Complete wash boring and clean the hole
3. Replace driving bit by split spoon sampler at the bottom end of the
driving rod
4. Drive the sampler by dropping a hammer of 63.5kg weight through a
height of 75 cm
5. The number of blows required to penetrate three successive lengths
of 15 cm are noted.
6. 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.
7. Raise the sampler to the surface, open it and extract the sample.
8. Drive the next length of casing and repeat the process until required
depth is reached.
9. At the end of the borehole, note the gwt after it stabilizes and then
backfill the borehole.
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Standard Penetration Test (SPT)

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Standard Penetration Test (SPT)

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55. Geotechnical Investigation
•Borings - cont’d
•Drill rig – Hollow Stem Auger - SPT
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56. Geotechnical Investigation
•Borings - cont’d
•Drill rig – Hollow Stem Auger - SPT
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Standard Penetration Test (SPT)
Corrections are normally applied to the SPT
blow count to account for differences in:
• energy imparted during the test (60%
hammer efficiency)
• the stress level at the test depth
The following equation is used to compensate
for the testing factors (Skempton, 1986):

58. Correlations between N values and soil
properties
N Compactness
Relative
Density (%)
fo
0 to 4 Very Loose 0-15 < 28
4 to 10 Loose 15 - 35 28 -30
10 to 30 Medium Dense 25 - 65 30 -36
> 50 Very Dense > 85 > 41
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Standard Penetration Test (SPT)

60. Corrections to N value
 Overburden Correction applied to N value
based on chart by Peck, et. al. (varies from
0.45 to 2)
 N’=Cn * N where Cn is correction factor
 Correction due to dilatancy for fine sand and
silt below gwt having N’ > 15
 N” = 15 + 0.5*(N’-15)
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61. Correlations between N values and soil
properties
Consistency N qu (kPa)
Very Soft 0 to 2 < 25
Soft 2 to 4 25 to 50
Medium Stiff 4 to 8 50 to 100
Stiff 8 to 12 100 to 200
Very Stiff 15 to 30 200 to 400
Hard > 30 > 400
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Standard Penetration Test (SPT)

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64. Other test methods
 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)
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Cone Penetration Test (CPT)

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The Plate Load Test (PLT)

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The Plate Load Test (PLT)

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The Plate Load Test (PLT)

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The Plate Load Test (PLT)
Scale Effect in Foundation Design

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Geotechnical Design Reports
 At the end of all subsoil exploration programs, the soil
and/or rock specimens collected from the field are
subjected to visual observation and appropriate
laboratory testing. After the compilation of all of the
required information, a soil exploration report is prepared
for the use of the design office and for reference during
future construction work. Although the details and
sequence of information in the report may vary to some
degree is depending on the structure under
consideration and the person compiling the report.

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Preparation of Boring Logs
1. Name and address of the drilling company
2. Driller’s name
3. Job description and number
4. Number, type, and location of boring
5. Date of boring
6. Subsurface stratification, which can he obtained by visual
observation of the soil brought out by auger, split-spoon sampler,
and thin-walled Shelby tube sampler
7. Elevation of water table and date observed, use of casing and mud
losses, and so on
8. Standard penetration resistance and the depth of SPT
9. Number, type, and depth of soil sample collected
10. In case of rock coring, type of core barrel used and, for each run,
the actual length of coring, length of core recovery, and ROD

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Subsoil Exploration Report
1. A description of the scope of the investigation
2. A description of the proposed structure for which the subsoil exploration has
been conducted
3. A description of the location of the site, including any structures nearby,
drainage conditions, the nature of vegetation on the site and surrounding it,
and any other features unique to the site
4. A description of the geological setting of the site
5. Details of the field exploration—that is, number of borings, depths of borings,
types of borings involved, and so on
6. A general description of the subsoil conditions, as determined from soil
specimens and from related laboratory tests, standard penetration
resistance and cone penetration resistance, and soon
7. A description of the water-table conditions
8. Re commendations regarding the foundation, including the type of
foundation recommended, the allowable hearing pressure, and any special
construction procedure that may he needed; alternative foundation design
procedures should also be discussed in this portion of the report
9. Conclusions and limitations of the investigations

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Subsoil Exploration Report
The following graphical presentations should he
attached to the report:
1. A site location map
2. A plan view of the location of the borings with
respect to the proposed structures and those
nearby
3. Boring logs
4. Laboratory test results
5. Other special graphical presentations

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Example Table
of Contents for
a Geotechnical
Investigation
(Data) Report

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BORE LOG

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85. Geophysical testing
 stratification of subsurface materials, the profile
of the top of bedrock, the depth togroundwater,
the boundaries of various types of soil deposits,
the rippability of hard soil and rock, and the
presence and depth of voids, buried pipes, and
existing foundations. Data from geophysical
testing should always be correlated with
information from the direct methods of
exploration discussed previously
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