unit 1 sub surface exploration
TRANSCRIPT
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SUBSURFACE EXPLORATION
UNIT - 1
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SOIL EXPLORATION
The field and laboratory studies carried out for obtaining the
necessary information about the surface and subsurface
features of the proposed area including the position of groundwater table, are termed as soil exploration or site
investigation.
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IMPORTANCE OF EXPLORATION PROGRAM
Before construction of any civil engineering work a thoroughinvestigation of the site is essential.
Site investigations constitute an essential and important
engineering program which, while guiding in assessing thegeneral suitability of the site for the proposed works, enables the
engineer to prepare an adequate and economic design and to
foresee and provide against difficulties that may arise during the
construction phase.
Site investigations are equally necessary in reporting upon the
safety or causes of failures of existing works.
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OBJECTIVES OF SOIL EXPLORATION PROGRAM
To establish the groundwater levels
To select the type and depth of foundation for proposed structure
To determine thebearing capacity of the site
To estimate the probable maximum and differential settlements
To select suitable construction techniques
To determine soil properties required for design
Topredict and to solve potential foundation problems
To investigate the safety of existing structures and to suggest the
remedial measures
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STAGES IN SUBSURFACE INVESTIGATION
Phase I. Collection of available information such as a site plan,
type, size, and importance of the structure, loading
conditions, previous geotechnical reports, newspaper
clippings etc.
Phase II.
Preliminary reconnaissance or a site visit : Here visual
inspection is done to gather information on topography,
vegetation, water marks, ground water level, and type ofconstruction nearby.
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STAGES IN SUBSURFACE INVESTIGATION
Phase III.
Detailed soil exploration : Here we make a detailed planning
for soil exploration in the form trial pits or borings. 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. The soil samples are properly labelled and
sent to laboratory for evaluation of their physical and
engineering properties.
Phase IV.
Report: The report must contain a clear description of the
soils at the site, methods of exploration, soil profile, test
methods and results, and the location of the groundwater. This
should include information on soil and groundwater condition
that may be troublesome during construction.
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METHODS OF EXPLORATION
1. OPEN EXCAVATIONS
Pits and trenches
Cost increases rapidly with depth
Considered suitable for shallow depths (upto 3m)
2. BORING
Auger boring:
Hand augers are used in boring holes
about 1520 cm in diameter and upto
a depth of 36 m in soft soils. A post hole
auger is used for collecting soil samples
from already driven holes.
Mechanical augers are used for making
holes in hard strata to a greater depth (upto 12m)
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METHODS OF EXPLORATION
Auger borings are particularly useful for subsurface investigations of
highways, railways etc, where the depth of exploration is small.
Rapid and economical.
Soil samples are highly disturbed.
Wash boring:
First , a casing about 2 - 3 m long is driven. Then, a hollow drill rod with a chisel
shaped chopping bit at its bottom is inserted
Water is pumped down the hollow drill rod,
which emerges as a strong jet through a
small opening of the chopping bit
The hole is advanced by a combination of
chopping action and jetting action.
The wash water is collected in a tub.
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Wash samples are of little practical use
Once a hole has been drilled upto a desired depth, a sampler is inserted to
obtain soil samples.
It cannot be used effectively in hard soils, rocks and soils containing
boulders.
It is not suitable for taking good quality undisturbed soil samples above
ground water table as the wash water enters the strata and causes an increase
in water content.
Rotary drilling:
Bore hole is advanced by rotating a hollow drill rod which has a cutting bit
at its lower end. The cutting bit shears off chips of the material penetrated.
A drilling fluid under pressure is introduced through the drill rod which
carries the cuttings from the bottom of the hole.
The drilling fluid also cools the drilling bit.
METHODS OF EXPLORATION
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When the soil sample is required to be taken the drilling rod is raised
and the drilling bit is replaced by a sampler.
This method can be used in clay, sand etc
This method is not suitable for materials containing large % of gravel
size particles. These particles start rotating beneath the drill rod and it
becomes difficult to advance the hole.
Percussion drilling:
It is used for making holes in rocks, boulders and other hard strata.
A heavy chisel is alternatively lifted and dropped in a vertical hole.
If the point where the chisel strikes is above the water table, water is
added to the hole.
Water forms a slurry with the pulverised material, which is removed at
intervals.
The material at the bottom of the hole is disturbed by the heavy blows of
the chisel and it is not possible to get good quality undisturbed samples.
METHODS OF EXPLORATION
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Soil samples in general can be classified as disturbed samples and
undisturbed samples.
Disturbed samples are those in which the natural soil structure
gets modified or destroyed during the sampling operation.
With suitable precautions the natural moisture content and the
proportion of mineral constituents can be preserved. These are
called representative samples.
Where, in addition to alteration in the original soil structure, soil
from other layers get mixed up or the mineral constituents get
altered, the samples are called non - representative samples.
Representative samples are used for identification tests but non
representative samples are virtually of no use.
Samples that are obtained through wash boring, percussion
drilling etc are non representative samples.
TYPES OF SOIL SAMPLES
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Undisturbed samples are those where the original soil structure
is preserved and the material properties have not undergone any
alteration or modification.
Perfectly undisturbed samples are practically impossible to
obtain.
Undisturbed soil samples can be used for laboratory tests such as
shear strength and consolidation tests.
TYPES OF SOIL SAMPLES
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The disturbance of soil depends mainly upon the following
design features:
Area ratio
For obtaining good quality undisturbed samples in soft
clays the area ratio should be less than or equal to 10%.
Inside clearance
For an undisturbed sample the inside clearance should be between 0.5 and
3%
SAMPLE DISTURBANCE
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Outsideclearance
Normally it lies between 02%
Recovery ratio
Recovery ratio should be equal to 9698% for getting a satisfactory
undisturbed sample
SAMPLE DISTURBANCE
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Inside wall friction
Friction on the side walls of the sampling tube causes disturbances to the
soil sample. The inside surface of the sampler is usually smeared with oil
before use to reduce friction.
Design of non return valve
Non return valve on the sampler should have an orifice of large area to
allow air, water or slurry to escape quickly when the sampler is driven. It
should immediately close when the sampler is withdrawn.
Method of applying forceFor obtaining undisturbed soil samples, the sampler should be pushed and
not driven.
SAMPLE DISTURBANCE
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Split spoon sampler
Can be used to obtain disturbed but representative soil samples.
It consists of :
A driving shoe at the bottom
A steel tube which can be split longitudinally
Coupling head on top
The sampler is attached to a drill rod and the sample is
collected by forcing the sampler into the soil by repeated
blows of a drop hammer.
The sampler is then withdrawn and the split tube is separated
after removing the shoe and the coupling. The sample is placed in a container sealed and transported
to laboratory .
SOIL SAMPLERS
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Shelby tube sampler
They are thin walled tube samplers made of seamless steel.
The bottom of the tube is sharpened and beveled, which acts
as a cutting edge.
The sampler which is attached to the drill rod is pushed into
the soil by a continuous rapid motion without impact ortwisting.
At least 5 minutes after pushing the tube into its final position,
the tube is turned 2 revolutions to shear the sample off at the
bottom before it is withdrawn.
The tube is taken out and its ends are sealed before
transportation.
Good quality undisturbed samples can be obtained.
SOIL SAMPLERS
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Standard penetration test (SPT)
Most commonly used field test.
When the bore hole has been drilled to a desired depth, the drilling tools
are removed and the split spoon sampler is lowered to the bottom of the
hole.
The sampler is driven into the soil by a drop hammer of 63.5 kg mass
falling through a height of 750mm at the rate of 30 blows per minute.
The no: of blows required to drive 150mm of sample is counted.
The sampler is further driven by 150mm and the no: of blows counted.
Once again for the third time, sampler is driven by 150mm and the no: of
blows recorded. The no: of blows for the first 150mm penetration is disregarded as the
bottom of the bore hole is likely to be disturbed by the drilling process.
The no: of blows recorded for the last two 150mm intervals are added to
give the standard penetration no: (N)
If the no: of blows exceeds 50 for 150mm drive, it is taken as refusal andthe test is discontinued.
FIELD TESTS
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Corrections for standard penetration no: (N)
1. Overburden pressure correction:
In granular soils the overburden pressure affects the penetration
resistance.
If two granular soils having same relative density but having different
confining pressures are tested, the one with higher confining pressuregives higher N values.
To account for this N values recorded from the field (NR)are corrected
to a standard effective overburden pressure
The correction factor (CN) given by Peck, Hanson and Thornburn is
given by:
FIELD TESTS
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2. Dilatancy correction
In saturated fine sands and silts, the pore pressure
developed is not easily dissipated. The pore pressure
increases the resistance of the soil and hence the
penetration no: (N)
Dilatancy correction is to be applied if the NC obtained
after overburden pressure correction exceeds 15.
The correction proposed by Terzaghi and Peck is given
by:
FIELD TESTS
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Static cone penetration test (CPT)
It is widely used in soft clays and fine to medium sand deposits.
A cone having an apex angle of 600 and a base area of 10cm2is used.
Position 1: Cone and friction jacket are stationary.
Position 2: Cone is pushed into the soil to a depth a
at a rate of 20mm/s. The tip resistance qccalled the
cone or point resistance = Qc/Ac
Qc= applied force ; Ac = base area of the cone
Position 3: Sounding rod is pushed to depth b. This
has the effect of pushing both the cone and the friction
jacket together. The total force required can be noted(Qt). The force required to push friction jacket alone,
Qf = QtQc .The side or skin friction fs = Qf / Af
Af = surface area of the friction jacket
Position 4: The outside mantle tube is pushed to a distance of (a+b), bringing the
cone and the friction jacket to position 1
FIELD TESTS
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Dynamic cone penetration test (DCPT)
The cone with an apex angle of 600 is attached to drill rods and is driven
into the soil by means of a drop hammer of 63.5 kg, falling freely from a
height of 750mm.
The blow count for every 100 mm penetration is recorded.
The no: of blows required for 300mm penetration is known as dynamiccone resistance Ncd .
This test gives a continuous record of Ncd values with depth
It helps to identify the uniformity or variability of soil profile and reveals
local soft pockets if any.
It can establish the position of rock stratum. The test is much less expensive and quicker than SPT
FIELD TESTS
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For determination of thickness of different layers a distance time
graph is plotted
Upto a certain distance X1, the direct waves in layer 1 reach first.
X1 represents critical distance.
The empirical equation for the depth H1is given as:
FIELD TESTS
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Electrical resistivity method:
The electrical resistivity method is based on the
measurement and recording of changes in the mean
resistivity.
Mean resistivity is given by:
FIELD TESTS
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The depth of exploration is roughly proportional to the
electrode spacing.
For knowing the horizontal changes in the subsoil, the
electrodes kept at constant spacing are moved as a group
along the line of test - Resistivity mapping
For studying the vertical changes in the soil stratum, the
electrode system is expanded, about a fixed point by
increasing the spacing graduallyResistivity sounding
FIELD TESTS
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Steel casing
A steel casing can be used for bore hole support.
Water
Water if used as a drilling fluid is circulated to remove the cuttings
from the bottom of the hole.
Water exerts a thrust downwards and laterally and thereby
counteracts soil and pore water pressure.
Water alone cannot be used to prevent caving in of soft and
cohessionless soils but it can be used in rock and stiff cohesive soils
Drilling mud
Bentonite mud which is a thin mixture of water and bentonite clay
is used to create a higher density suspension.
It is more viscous than water.
STABILIZATION OF BORE HOLES
NO AND DEPTH OF BORING FOR VARIOUS
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Spacing of borings: It depends mainly on the variation of the strata in the
horizontal direction.
According to IS 1892; Code of practice for
subsurface investigation,
For a small building, one bore hole at the centre can give
necessary data.
For a building covering not more than 4000 sq m, one
bore hole at each corner and one at centre is adequate.
For a large project, generally a grid of 50m spacing
should be used with a combination of bore holes and
sounding tests.
NO: AND DEPTH OF BORING FOR VARIOUS
CIVIL ENGINEERING STRUCTURES
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NO AND DEPTH OF BORING FOR VARIOUS
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Building foundation:
For footings, the min depth of exploration is given by:
Isolated or spread footings :D = 1.5B ; A4B
Adjacent footings:- D = 1.5L ; A2B
D = 1.5B; A>4B
A = spacing between footings
B = width of footing
L = Length of footings
For pile foundations, the depth of exploration should be
atleast equal to 1.5 times the width of the structure, unless a
good bearing stratum is encountered at a higher level.
NO: AND DEPTH OF BORING FOR VARIOUS
CIVIL ENGINEERING STRUCTURES
NO
AND DEPTH OF BORING FOR VARIOUS
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Roads, cuts and fills:
Roads
Exploration is carried out along the centre line of the road and
along the proposed ditch lines.
The spacing should be about 100m to begin with, which may be
reduced to 30m or less if frequent changes in soil profile are
found.
All borings should be carried out to a min depth of 1m below
the existing ground level.
Cuts Borings should be made to a min depth of 1m below the
formation level.
Deep cutsborings should be carried below the deepest part of
the proposed cut equal to its bottom width or depth of the cut,whichever is smaller.
NO: AND DEPTH OF BORING FOR VARIOUS
CIVIL ENGINEERING STRUCTURES
NO AND DEPTH OF BORING FOR VARIOUS
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Fills
The min depth of borings below the ground level should be 2m orequal to the height of the fill above the ground, whichever is
greater.
Dam sites:
Preliminary exploration may be carried out by drilling four or fiveholes in a line at right angles to the direction of flow of water and
across one or both abutments.
A few holes, widely scattered, are also required in the bottom of the
reservoir on the upstream side of the dam.
Detailed exploration is carried out by drilling additional holes in the
valley floor, on the abutments and other connected structures.
The depth of exploration should ordinarily reach bedrock.
The min depth of preliminary exploration is equal to 1.5 times the
bottom width for earth dam and equal to twice the height from streambed to crest for concrete dams having height < 30m.
NO: AND DEPTH OF BORING FOR VARIOUS
CIVIL ENGINEERING STRUCTURES
NO AND DEPTH OF BORING FOR VARIOUS
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Borrow areas:
Single bore holes or pits are located in likely locations to verify the
existence of materials of required characteristics.
On finding a potential source supplementary holes are located.
When a comparatively large quantities of materials are required,
say for the body of an earth dam, the area should be covered withbore holes and test pits on a grid system so that subsurface profile
may be plotted and quantities calculated.
It is better to start with a wide spacing of 150 to 300m and put
additional holes in between as the variability of the deposit mayrequire.
The depth of exploration should be decided by convenience of
excavation and the thickness of the available suitable materials.
NO: AND DEPTH OF BORING FOR VARIOUS
CIVIL ENGINEERING STRUCTURES
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During soil exploration all the details are recorded and
presented in a boring log.
A typical boring log consists of the following details:
The soil profile
Ground water level
Termination level of the bore hole
The depth at which samples were taken or insitu tests were performed
The type of soil samples
N-values at different depths Results of important lab tests
BORING LOG
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Any soil investigation report should consist of the following
details:
Scope of investigation
General description of the proposed structure
Drainage conditions of the site
Details of boring
Description of sub soil conditions determined from samples
collected.
Ground water table Details of foundation recommendations
Any anticipated construction problems
Limitations of investigation.
SOIL EXPLORATION REPORT
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The following graphic presentations should also be attached:
Site location map
Location of borings
Boring logs Laboratory test results etc..
SOIL EXPLORATION REPORT