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SUBJECT:-Building Construction
TOPIC:DIFFERENT
METHODS OF
SURFACE
INVESTIGATION
Group no:-1
Henil Shah(10)
Vishwam Sheth(49)
Karan Joshi(12)
Bhumil Shah(46)
Abhay Nainani(01)
Dharamraj Vaghela(57)
METHODS
OF SUBSOIL
INVESTIGATION
DIRECT METHODS
1)
2)
3)
4)
TEST PITS
TRIAL PITS
TRENCHES
DRIFTS,SHAFTS
SEMI
DIRECT
METHODS
1) BORINGS
INDIRECT METHODS
1) SOUNDINGS OR
PENETRATION
TESTS
2) GEOPHYSICAL
METHODS
DIRECT METHOD
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Trial pits and trenches are excavated at the site to
inspect the strata. As per IS:4453-1980 a clear
working space of 1.2m x 1.2m is required at the
bottom of the pit
Shallow pits upto 3m depth can be made without
any lateral support. For deep pits,the lateral
support in the form of sheeting and bracing
system is required. For depths greater than 6 m,
bore holes are more economical than open pits.
Trenches are long shallow pits. As a trench is
continuous over a considerable length, it provides
exposure over a line. The trenches are more
suitable than pits for exploration on slopes.
Test Pit
2)Drifts and shafts
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Drifts are horizontal tunnelsmade in the hill side to determine the
nature and structre of the geological formation
Drift should have minimum clear dimension of 1.5m width and
2.0m height in hard rock. In soft rock,an arch roof is more
advantageous,than a flat roof.
Drifts are useful for establishing the minimum excavation limits to
reach sound rock and for locating faults and shear zones and
buried channels in the river section
Drifts are also known as audits
Shafts are large vertical holes made in the geological formation.
These may be rectangular or circular in section dimensions for
shafts are 2.4 m and for circular shaft diameter is 2.4 m
Shafts are used to reach a particular strata at a depth of 4 m or
more
Shafts are also used to extend the exploration below the river bed
already done by the means of tunnels
Semi direct methods
Borings
 Anger
boring
 Auger snd shell boring
 Wash boring
 Rotary drilling
 Percussion drilling
1)Auger boring
An auger is a tool used for drilling a
bore hole into the ground
 Auger are of two types : Hand operated augers

 Mechanical augers
 auger boring is generally used in soils
which can stay open without casing
 Clays,silts and partially satuarted
sands can stand unsupported
For soils which cannot stand
unsupported, especially for sandy soils
below water table, a casing is normally
required.
 Auger boring is fairly satisfactory where
the depth of exploration is small, such as
for highways, railways ,air fields,borrow
pits etc.
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2)Auger and shell boring
If the sides of the bore hole cannot remain
unsupported, the soil is prevented from
falling by the means of cylindrical “shell”
or “casing” used along with the auger
 The equipment used for drilling bore holes
is generally known as ‘boring rig’
 The hand operated boring rigs may be used
for boring holes upto 25 m and power
driven or mechanical boring rigs for boring
holes upto depth of 50 m
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3)Wash boring
Wash boring is a fast and simple method for boring
holes into the ground. This method may be used in all
types of soil except those gravel, boulders,rocky and
strata
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Process:
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A hole is drilled for a short depth by using an auger. A
casing pipe is pushed into the hole and driven with a
drop weight or with the aid of the power
A hollow drill bit screwed to the lower end of a hollow
drill rod connected to a rope passing over a pulley and
supported by a tripod is inserted into the casing pipe.
Water jet under pressure is forced into the hole through
the rod and the bit, which is alternatively raised and
dropped, and rotated as well
The resulting chopping and jetting action
of the bit and water loosens the soil at the
lower end and forces the water-slurry
upwards through the annular space
between the drill rod and the casing.
 The soil water slurry is led into a settling
tank where the soil particles settle where
the water overflows into a sump
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The soil particles collected represent a
very disturb sample and is not very useful
for the evaluation of the engineering
properties of the soil. The changes in soil
strata may be indicated by the change in
the rate of progress and the change in the
colour of the wash water.
4)Rotary drilling
This method is a fast method of drilling
holes in rock formations
 In this method by using suitable diamond
studded drill bits or steel bits with shots,
the rock cores may be obtained. The
method is then known as core drilling or
core boring
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5)Percussion drilling
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The percussion drilling method is used for making holes
in rocks, boulders and other hard strata. In this method
a heavy drilling bit called ‘churn bit’ suspended from a
drill rod or a cable is alternatively raised and dropped
in the vertical hole
By the repeated blows of the drill bit the material in the
hole gets pulverised. if the point where the drill bit
strikes is above the ground water table, water is added
to the hole to faciliate the breaking of stiff soil or rock
One of the major disadvantages is that the material at
the bottom of the hole is disturbed by the heavy blows of
the chisel
Indirect methods
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Geophysical methods:-
1)
Seismic refraction method
Electrical resistivity method
2)
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Geophysical techniques:
◦ Advantages Relatively low cost;
 Obtain results quickly;
 Can be undertaken in rough , inhospitable terrains
by small teams; and
 Can assist planning of expensive drilling programs.
◦ Limitations Techniques all identify boundaries between two
layers with appreciably different properties. Little
contrast - poor definition of
layers.
 Requires confirmation by independent means.
◦ Techniques Seismic reflection & refraction;
 Electrical resistivity (ER);
◦ Seismic Methods involve propagation of waves through
earth materials
◦ Electrical methods involve measurement of electrical
properties of earth materials either measurement of natural earth currents, or
 the resistance to induced electrical flow.
◦ Natural earth current flow generated under geological
conditions in which anode & cathode develop
naturally.
Measurement of strength & extent of current
helps establish geologic conditions.
◦ Electrical resistivity is resistance to electrical flow
through earth materials. Current induced &
resistivity measured- identifies basic property of
earth material.
Seismic
refraction:
◦ Theoretical treatments of theory of Elasticity,&
wavelength of seismic waves confirms that velocity of
P waves > than S waves;
◦ Also velocities of seismic waves dependent on rock
density & Young’s modulus. Both increase with
depth, so wave velocity also increases;
◦ P waves behave like visible spectrum waves- obey
Snell’s law;
◦ Man-made seismic wave created& times of arrival of
P waves sensed by regularly spaced geophones;
◦ Both refracted & reflected events measured on
same seismic signal;
◦ Engineering relies on P wave (  rock strength)
◦ Cannot rip apart material whose seismic velocity
exceeds 2,500 m/sec.(compacted sand with 40%
porosity has P wave velocity = 1800 m/sec).
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Used for many years to predict ease of excavation of earth
materials;
Changes in jointing, dipping beds & cementation changes will affect
seismic velocity;
◦ Limitation- geologic units must increase in velocity with depth to
ensure that refracted ray can return to the Earth’s surface.
Seismic reflection:
◦ provides a detailed picture of sub surface structure & interfaces;
◦ depths determined by observing travel times of P waves
generated near surface & reflected back from deep formations;
◦ comparable to echo sounding of water depths.
◦ Advantages- permits mapping of many horizons
for each shot;
◦ can determine depths to dipping interfaces, as well
as angle of dip;
◦ Not used as much as refraction, but refraction will not work
where a high velocity layer overlies a low one;
◦ Reflection profiling in permafrost areas is not affected by the
high velocity permafrost, whereas refraction techniques can be
nullified completely.
Electrical
Resistivity:
◦ The range of resistivities( ) of rocks is
enormous:
◦ Amount of ground water& dissolved ions
in rocks & water of great importance;
◦ Dry rock has virtually no electrical
conductance:
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Water-bearing rock resistivity is function of amount of
groundwater present & salinity;
◦ Resistivity of saturated fine-grained sedimentary rocks
tends to be lower than coarse-grained sedimentary
rocks because of greater porosity;
◦ Gravels have more ground water recharge & less total
dissolved solids (TDS) than fine-grained material such
as colluvium or till;
◦ Resistivity used to map overburden thickness, faults,
fractures, specific rock units, etc;
◦ Difficult to relate resistivity value directly to rock
type. Fortunately show profound anomalies;
◦ Therefore should be compared to drill log data;
◦ In practice use Werner array- constant spacing,
and moving whole array- This is resistivity
profiling;