Unit 1 Sub Surface Exploration

8/13/2019 Unit 1 Sub Surface Exploration

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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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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.



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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.



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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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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


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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.





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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.





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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.




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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

Documents

Unit 1 Sub Surface Exploration