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The process of determining the layers of

natural soil deposits that will underlie a proposed

structure and their physical properties is generally

referred to as sub surface investigationg

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For proper design it is very necessary to have a

l t i t f th f & b f diticomplete picture of the surface & sub surface conditions.

Samples are tested on the site or in a laboratory for

determining shear strength parameters, bearing capacity

f il bilit t t bl t l ifi ti dof soil, permeability, water table, type, classification and

other geophysical information in the field.

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Nature of Ground

Topography

Cost

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When the depth of exploration is larger borings are

used for exploration and for shallow depth trial pits

are used

Depending upon the type of soil and purpose of

boring following methods are used for drilling

h lholes

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1. Augur boring- these are siutable for advancing holes up to a depth of 3 to 6m in soft soils

2. Wash boring- Used for advancing hole in graoundgraound

3. Rotory drilling- Can be used in clay sand and rocks

4. Percussion drilling- used for drilling holes in rocks boulders and other hard strata

5. Core drilling- used for drilling holes in rock g gand for obtaining rock cores( for identification and tests)

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The location and number of boreholes should be inaccordance with the type of the structure.

For check dams at abutment points and at 20 m intervals in riverFor check dams at abutment points and at 20 m intervals in river

course (minimum one number in river course).

For regulators borehole should be taken at all pier points and

abutments. (Usually the centre to centre distance between the

pier is standardized to 14m).

If d i f id t ti k b h lIf design of side protection works are necessary, bore holes

should be taken on the banks at the upstream and downstream

of the structure at 30 m intervals along the required length.

At positions of cut off walls.

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For check dams, boring should be done until an N value

of 30 – 35 is obtained.

For Regulators,Check Dam cum Bridge and RCB, boring

should be done up to rock level.

if rock is not available with in a reasonable depth, the

depth of boring should be up to dense sand with N value

greater than 50.

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When rock is encountered during drilling, depth of drill

holes may be taken up to 3m in rock.

If th i b d i t f b ld i d f tiffIf the river bed consists of boulders or is made up of stiff

soil, feasibility of driving sheet piles to act as cut off walls

must be investigated properly.

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The following are the major field tests for

d t i i th il t thdetermining the soil strength:

Vane shear test (VST)- to determine shear strenth of

cohesive soil in its natural condition

Standard Penetration Test (SPT)- to determine N-value

Cone Penetration Test (CPT) to determine type of soilCone Penetration Test (CPT)- to determine type of soil

using electronic measuring system that records tip

resistance and friction

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Samples are of two types

Disturbed samples:- These are taken by methods which

modify or destroy the nature structure of the material, thoughy y g

with suitable precautions natural moisture content can be

preserved- are used to determine the index properties of soil

such as grain size, plasticity characteristics and specific

gravity.etc

Undisturbed Samples:- These are taken by methods whichUndisturbed Samples:- These are taken by methods which

preserve the structure and properties of the material- are

used for determining basic engineering properties such as

compressibility, shear strength and permeability

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Auger boring Power drills

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Nature of Ground Type of sample Method of sampling

Soil Disturbed Hand samples, Auger

samples(in clays), shell

samples in sand

Undisturbed Chunk samples and

tube samples

Rock Disturbed Wash samples from

Undisturbed

percussion or Rotary

drilling

Cores

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1. DisturbedSamples – placed in cloth bag/tin (glass jar of 0 5 kgSamples placed in cloth bag/tin (glass jar of 0.5 kg capacity, should be air tight with minimum air space)Tins containing samples should be packed in stout wooden box ( saw dust / suitable material to prevent damage during transit)

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Contd...

2. Undisturbedi) Samples that are retained in tube sampler

Ends should be cut to about 2.5 cm thick.For every porous sample, a layer of waxed papery p p y p pshould be placed over the ends of the sample.Any left space between the end of liner or tube and the top of the wax tightly packed with saw dust / other suitable material

ii) Samples that are not retained in tube samplerShould be covered with several layers of molten paraffin wax, tightly packed with saw dust or other suitable material in metal container.container. If the sample is very porous, cover with waxed paper before

applying the molten wax.

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Contd...

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Properties such as soil classification, unit weight, angle of internal friction, void ratio, specific gravity, grain size di t ib ti t h ld b d t i d b i l idistribution etc. should be determined by sieve analysis and other laboratory tests.For clayey strata – samples should be tested to determine cohesion, unconfined compressive strength, moisture content, dry density, Atterberg limits (such as Liquid Limit, Plastic Limit, Shrinkage Limit) and consolidationLimit, Plastic Limit, Shrinkage Limit) and consolidation characteristics.

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i) Geological systemi) Geological systemii) Depth of rock and its variation over the siteiii) Whether isolated boulder or massive rock formationiv) Extent and character of weathered zonev) The structure of rock-including bedding planes, faults, etc.;vi) Properties of rock material like strength geological vi) Properties of rock material, like, strength, geological

formation, etc.vii) Quality and quantity of returning drill water, andviii) Erodibility of rock to the extent possible, where relevant

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Site investigation boreholes

Percussion drilling a) soils/soft clay rocks b) core recovery b) core recovery Rotary coring a) soil or rock >100m deep b) core recovery Rock probing a)rotary percussion rig

b) soil or rock ) c)no core recovery

A standard modern diamond coring string consists of:1 Drill rods in 1 5 3 or 6 m lengths1. Drill rods in 1.5, 3 or 6 m lengths,2. A diamond impregnated or surface set

coring bit3. A core barrel

“Core” is retrieved using wire line techniquesg q

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Designation AQ BQ NQ NQ2 NX HQ PQ

Dia (mm) 27 36 5 47 6 50 5 54 7 63 5 85 0Dia. (mm) 27 36.5 47.6 50.5 54.7 63.5 85.0

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i) Visual identification fora) Texturea) Textureb) Structurec) Compositiond) Coloure) Grain size

ii) Laboratory tests may be done fora) Specific gravitya) Specific gravityb) Porosityc) Water absorptiond) uniaxial compressive strength e) unconfined compression

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1) Rock Quality DesignationAll bl B i St R k M2) Allowable Bearing Stresses on Rock Masses

The RQD is a modified core recoveryMeasure of the degree of fractures joints and discontinuities Measure of the degree of fractures, joints, and discontinuities of rock mass RQD = sum of pieces > 100 mm (4 inches) divided by total core run. Generally performed on NX-size core

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RockQualityDesignation

In case of rock mass with favorable discontinuities, the net allowable bearing pressure may be estimated from:g p y

qa = qc * NjWhere qc = average uniaxial compressive strength of rock coresNj = empirical coefficient depending on the spacing of the discontinuities

3+ S / B= 10 [1+ 300(δ / S)]

δ = thickness of discontinuityyS = spacing of discontinuitiesB = width of footingThe above relationship is valid for a rock mass with spacing of continuities > 0.3 m, δ < 10 mm (15 mm if filled with soil) andB >0.3 m.

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Pile Foundations:The allowable bearing capacity of socketed piles is given by:The allowable bearing capacity of socketed piles is given by:

qa = qc * Nj * NdWhereNd = 0.8 + 0.2 h/dh = depth of socket in rockd = diameter of socketd diameter of socket

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The following details should be attached to the report:Lattitude and longitude of locationA site location mapA site location mapA plan view of the location of the borings with distancebetween them with respect to the proposed structureBoring logsLaboratory test resultsSBC and N value of each layer

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For rocky strata, the RQD value should also be includedRecommendation of type of foundation by the soil consultantReport should be authenticated by field officers.

The investigation should be conducted by the firms which

CONCLUSION

have adequate facilities and experience to do the investigation

at any depth of water and in difficult conditions.

Boring operation and other investigations should be done

under the presence and guidance of the concerned Assistant

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Engineer to ensure that all the required data are obtained.

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1. IS -1892 Code and practice for subsurface investigation for foundationg

2. IS 6966 (part 1) – 1989 Hydraulic design of barrages and weirs-guidelines

3. Manual on Barrages and weirs on permeable foundation.

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