Chapter-I-Soil Investigation or Soil Exploration
Chapter-I-Soil Investigation or Soil Exploration
Chapter-I-Soil Investigation or Soil Exploration
SUBJECT:FOUNDATION ENGINEERING-I
SUBJECT CODE:Ceng-3141
CLASS:III-YEAR
SEMESTR-II
INSTRUCTOR:VIJAYKUMAR
(GEOTECHNICAL ENGINEERING)
COURSE OUTLINE
20% Assignments
30% Mid-exam
50% Final-exam
REFERENCES
Bowles, J.E., (1982). Foundation Analysis abd Design, 3rd
Edition, McGraw-Hill intl. Book company, Auckland.
Chen, F.H., (1988). Foundations of expansive soils,
Elsevier, Oxford.
Cheng Liu & Jack B. Evett, (1998). Soils and Foundations,
Prentice Hall, New Jersey.
Robert, W. Day, (2006). Foundation Engineering Hand
book- Design and construction with the 2006 international
building code, McGraw-Hill, New York
Tomelson A.J., (1980). Foundation Design and
Construction, Pitman, Boston.Tomelson A.J., (1980).
Foundation Design and Construction, Pitman, Boston.
Zeevart, L., (1983). Foundation engineering for difficult
subsoil conditions, 2nd edition, Van Nostrand Reinhold
company, New York
Foundation engineering by C Venkatramaya
Foundation engineering by VNS Murthy
CHAPTER-I-SOIL INVESTIGATION OR SOIL
EXPLORATION
INTRODUCTION
A fairly accurate assessment of the characteristics and
engineering properties of the soils at a site is essential for
proper design and successful construction of any structure at
the site.
The field and laboratory investigations required to obtain
the necessary data for the soils for this purpose are
collectively called soil exploration.
The choice of the foundation and its depth, the bearing
capacity, settlement analysis and such other important
aspects depend very much upon the various engineering
properties of the foundation soils involved.
Objectives of soil exploration:
(i) Determination of the nature of the deposits of soil,
(ii) Determination of the depth and thickness of the various
soil strata and their extent in the horizontal direction,
(iii) The location of groundwater and fluctuations in GWT,
(iv) Obtaining soil and rock samples from the various strata,
(v) The determination of the engineering properties of the soil
and rock strata that affect the performance of the structure, and
(vi) Determination of the in-situ properties by performing field
tests.
The subsoil exploration should enable the engineer to draw the soil
profile indicating the sequence of the strata and the properties of the
soils involved.
In general, the methods available for soil exploration may be
classified as follows:
1. Direct methods ... Test pits, trial pits or trenches
2. Semi-direct methods ... Borings
3. Indirect methods ... Soundings or penetration tests and geophysical
methods
In an exploratory programme, one or more of these methods may be used
to yield the desired information
1. Direct method of soil Exploration
Test Pits, trial pits or Trenches
Test pits or trenches are open type or accessible exploratory methods.
Soils can be inspected in their natural condition.
The necessary soils samples may be obtained by sampling techniques
and used for ascertaining strength and other engineering properties by
appropriate laboratory tests.
Test pits will also be useful for conducting field tests such as the plate-
bearing test.
Test pits are considered suitable only for small depths—up to 3m; the
cost of these increases rapidly with depth. For greater depths,
especially in pervious soils, lateral supports or bracing of the
excavations will be necessary.
Ground water table may also be encountered and may have to be
lowered.
Hence, test pits are usually made only for supplementing other
methods or for minor structures.
2. Semi Direct Methods
Boring
Making or drilling bore holes into the ground with a view to
obtaining soil or rock samples from specified or known
depths is called ‘boring’.
The common methods of advancing bore holes are:
1. Auger boring
2. Auger and shell boring
3. Wash boring
4. Percussion drilling- more commonly employed for
sampling in rock strata
5. Rotary drilling - more commonly employed for sampling in
rock strata
1. Auger Boring
‘Soil auger’ is a device that is useful for advancing a bore
hole into the ground.
Augers may be hand-operated or power-driven; the former
are used for relatively small depths (less than 3 to 5 m),
while the latter are used for greater depths.
The soil auger is advanced by rotating it while pressing it
into the soil at the same time.
It is used primarily in soils in which the bore hole can be
kept dry and unsupported. As soon as the auger gets filled
with soil, it is taken out and the soil sample collected.
Two common types of augers, the post hole auger and the
helical auger, are shown in Fig.
2.Auger and Shell Boring
If the sides of the hole cannot remain unsupported, the soil
is prevented from falling in by means of a pipe known as
‘shell’ or ‘casing’.
The casing is to be driven first and then the auger; whenever
the casing is to be extended, the auger has to be withdrawn,
this being an impediment to quick progress of the work.
An equipment called a ‘boring rig’ is employed for power-
driven augers, which may be used up to 50 m depth (A hand
rig may be sufficient for borings up to 25 m in depth).
Casings may be used for sands or stiff clays. Soft rock or
gravel can be broken by chisel bits attached to drill rods.
Sand pumps are used in the case of sandy soils.
3.Wash Boring
Wash boring is commonly used for exploration below
ground water table for which the auger method is
unsuitable.
This method may be used in all kinds of soils except those
mixed with gravel and boulders.
Initially, the hole is advanced for a short depth by using an
auger. A casing pipe is pushed in and driven with a drop
weight.
The driving may be with the aid of power.
A hollow drill bit is screwed to a hollow drill rod connected
to a rope passing over a pulley and supported by a tripod.
Water jet under pressure is forced through the rod and the
bit into the hole.
This loosens the soil at the lower end and forces the soil-
water suspension upwards along the annular surface
between the rod and the side of the hole.
This suspension is led to a settling tank where the soil
particles settle while the water overflows into a sump.
The water collected in the sump is used for circulation
again.
The soil particles collected represent a very disturbed
sample and is not very useful for the evaluation of the
engineering properties.
The set-up for wash boring is shown in Fig.
Different drill bits
4.Percussion Drilling
A heavy drill bit called ‘churn bit’ is suspended from a
drill rod or a cable and is driven by repeated blows.
Water is added to facilitate the breaking of stiff soil or
rock.
The slurry of the pulverised material is bailed out at
intervals.
The method cannot be used in loose sand and is slow in
plastic clay.
Percussion Drilling at Site
5. Rotary Drilling
This method is fast in rock formations.
A drill bit, fixed to the lower end of a drill rod, is rotated
by power while being kept in firm contact with the hole.
Drilling fluid or bentonite slurry is forced under pressure
through the drill rod and it comes up bringing the cuttings
to the surface.
Even rock cores may be obtained by using suitable
diamond drill bits.
This method is not used in porous deposits as the
consumption of drilling fluid would be prohibitively high.
Rotary Drilling & Drill bits
Boring Log
Information on subsurface conditions obtained from the
boring operation is typically presented in the form of a
boring record, commonly known as “boring log”.
A continuous record of the various strata identified at
various depths of the boring is presented.
Description or classification of the various soil and rock
types encountered, and data regarding ground water level
have to be necessarily given in a pictorial manner on the
log.
A “field” log will consist of this minimum information,
while a “lab” log might include test data presented
alongside the boring sample actually tested.
The following spacing are recommended in planning an
exploration programme:
Soil Sampling
Laboratory test results are mainly dependent on
the quality of soil samples.
There are two main types of soil samples which
can be recovered from bore holes or trial pits.
Disturbed and
Undisturbed samples
Soil Sampling for Geo-stratification
Disturbed Samples
These are samples where the structure of the natural
soil has been disturbed to a considerable degree by
the action of the boring tolls or excavation
equipment.
However, these samples represent the composition
and the mineral content of the soil.
Disturbed samples are satisfactory for performing
classification tests such as, sieve analysis, Atterberg
limits etc.
Undisturbed Samples
These are samples, which represent as closely as is
practicable, the true in-situ structure and water
content of the soil.
Undisturbed samples are required for determining
reliable information on the shearing resistance and
stress-deformation characteristics of a deposit.
It is virtually impossible to obtain totally
undisturbed samples.
This is due to that:
The process of boring, driving the coring
tool, raising and withdrawing the coring tool
and extruding the sample from the coring
tool, all conspire to cause some disturbance.
In addition, samples taken from holes may
tend to swell as a result of stress relief.
Samples should be taken only from a newly- drilled or newly
extended hole, with care being taken to avoid contact with
water.
As soon as they are brought to the surface:
Core tubes ends should be sealed with wax and
capped to preserve the loss of moisture content.
Core tubes should properly be labeled to indicate
the number of bore holes and the depth at which
they are taken and then stored away from extremes
of heat or cold and vibration.
Types of tube samplers
Split Spoon Sample
Thin-Walled Tube Sampler
Piston Samplers
INDIRECT METHODS—GEOPHYSICAL METHODS
The determination of the nature of the subsurface materials
through the use of borings and test pits can be time-
consuming and expensive.
Considerable interpolation between checked locations is
normally required to arrive at an area-wide indication of the
conditions.
Geophysical methods involve the technique of determining
subsurface materials by measuring some physical property
of the materials, and through correlations, using the values
obtained for identifications.
Most geophysical methods determine conditions over large
distances and can be used to obtain rapid results. Thus, these
are suitable for investigating large areas quickly, as in
preliminary investigations.
1. Seismic Refraction method
When a shock or impact is made at a point on or in the
earth, the resulting seismic (shock or sound) waves travel
through the surrounding soil at speeds related to their elastic
characteristics.
The magnitude of the velocity is determined and is utilised
to identify the material.
A shock may be created with a sledge hammer hitting a
strike plate placed on the ground or by detonating a small
explosive charge at or below the ground surface.
The radiating shock waves are picked up by detectors,
called ‘geophones’, placed in a line at increasing distances,
d1, d2, ..., from the origin of the shock (The geophone is
actually a transducer, an electromechanical device that
detects vibrations and converts them into measurable
electric signals).
The time required for the elastic wave to reach each
geophone is automatically recorded by a ‘seismograph’.
Some of the waves, known as direct or primary waves,
travel directly from the source along the ground surface or
through the upper stratum and are picked up first by the
geophone.
If the sub soil consists of two or more distinct layers, some
of the primary waves travel downwards to the lower layer
and get refracted as the surface.
If the underlying layer is denser, the refracted waves travel
much faster.
As the distance from the source and the geophone increases,
the refracted waves reach the geophone earlier than the
direct waves.
The distance of the point at which the primary and refracted
waves reach the geophone simultaneously is called the
‘critical distance’ which is a function of the depth and the
velocity ratio of the strata.
Figure shows the diagrammatic representation of the travel
of the primary and the refracted waves.
The results are plotted as a distance of travel versus time
graph, known as the ‘time travel graph’.
The velocity is given by: