CE3350 Soil Eigxploration

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SITE INVESTIGATION AND SUBSOILEXPLORATION

“ Subsurface material properties cannot be specified; they must be

deduced through exploration.” Charles Dowding (1979)



Definition and Objectives A detailed investigation for site is essential before a design

can be finalized.

The process of determining the layers of natural soil androck deposits that underlie a proposed structure and theirGeotechnical, Geological and Hydrological properties is

referred to as site investigation.

The objective of subsurface and related site investigation isto provide the engineer or architect with as much

information as possible about the existing conditions suchas:

Exposed overburden

Course of a stream nearby

Rock outcrop or a hillock

Vegetation, and other geological features of the area

It is equally important to know the subsoil conditions below

a proposed structure



Objectives of Exploration To select suitable site for the proposed project

(Earth/Concrete dam; Nuclear power plant; Engineered

landfil l and ………..)

Assess the fundamental properties of the sub layers suchas shear strength, volume compressibility and hydraulic

characteristics.

To decide type of foundation required for the proposed

project at the site, i.e. shallow foundation or deepfoundation and to recommend safe bearing capacity or pile

load capacity.

The program should be planned so that the maximum

amount of information can be obtained at minimum cost.



Scope of the Subsoil Investigation Includes.

Sequence and extent of each soil and rock stratum in the

region likely to be affected by the proposed work

Nature of each stratum and engineering properties of soil

and rock which may affect design and mode of constructionof proposed structures and their foundations

Location of ground water and possible corrosive effects of

soil and water on foundation materials

In areas which have already been developed, advantageshould be taken of existing local knowledge, records of trialpits, bore holes, etc, in the vicinity, and the behaviour of

existing structures, particularly those of a nature similar tothat of the proposed structure



Scope of the Subsoil Investigation Includes. In such cases, exploration may be limited to checking that,

the expected soil conditions are those as in the

neighbourhood. If the existing information is not sufficient or is inconclusive

the site should be explored in detail so as to obtain a

knowledge of the type, uniformity, consistence, thickness,sequence and dip of the strata and of the ground water

conditions.

Site Reconnaissance- Site reconnaissance would help indeciding future programme of field investigations? that is,

to assess the need for Preliminary or Detailed

Investigations. This would also help in determining scope of work, methods

of exploration to be adopted, field tests to be carried outand administrative arrangements required for the

investigation.



Site Reconnaissance Includes.. A study of local topography, excavations, ravines, quarries,

escarpments; evidence of erosion or landsl ides

Behaviour of existing structures at or near the site; waterlevel in streams, water courses and wells; flood marks from

topographical maps, geological maps, pedology and soilsurvey maps, and aerial photographs.

Data regarding removal of overburden by excavation,

erosion or land slides should be obtained. This gives anidea of the amount of preconsolidation the soil strata hasundergone.

Similarly, data regarding recent fills is also important to

study the consolidation characteristics of the fill as well asthe original strata.

The type of flora affords at times provides some indication

of the nature of the soil. The extent of swamp andsuperficial deposits and peats wil l usually be obvious.



Site Reconnaissance Includes..Ground-water conditions: The ground-water level fluctuates

and will depend upon the permeability of the strata and thehead causing the water to flow.

The water level in streams and water courses, if any, in the

neighbourhood, should be noted, but it may be misleadingto take this as an indication of the depth of the water table in

the ground.

Wells at the site or in the vicinity gives useful indications ofthe ground-water condit ions.

Flood marks of rivers may indicate former highest waterlevels.

Tidal actuations may be of importance. It is also apossibility of there being several water tables at differentlevels, separated by impermeable strata, and some of this

water may be subject to artesian head.



Site Reconnaissance Includes.. Enquiries Regarding Early Use of the Site: In certain cases

the earlier uses of the site may have important bearing onproposed new works. This is particularly so in areas, where

there have been underground workings, such as worked-outballast pits, quarries, old brick fields, coal mines and

mineral workings.

Enquiries should be made regarding the location of shafts

and workings, particularly shallow ones, where there maybe danger of collapse, if heavy new structures aresuperimposed.

The possibility of damage to sewers, conduits and drainage

systems by subsidence should also be investigated.

Geophysical investigations: to provide a simple and quick

means of getting useful information about stratifications.

Electrical resistivity method

Seismic method.



Methods of InvestigationsOpen Pits

Bore Holes

Geophysical Investigation (non destructive)

Electrical Resistivity Survey Seismic Survey

Gravity Survey

Magnetic Survey

Temperature Logging

Radioactivity Logging



Truck-Mounted Drill Rig

Typical Equipment Used for

Geotechnical Drilling

Truck Mounted Drill Rig &Support Truck (Water Tank)



Angle Drilling

Assess geologic features(dip, strike, joints, etc.)

Foundation testing for

bridge abutments.

M th d f B i (A B i )



Methods of Boring (Auger Boring) Auger Boring

Hand auger may be useful up to 6 m depth

Soil can stand itself

Yield disturbed sample

C i Fli h A (D illi )



Continuous Flight Auger (Drill ing)

H ll St A



Casing with outer spiral

Inner rod with plug or pilot assembly

For sampling, remove pilot assembly andinsert sampler

Typically 5 ft sections, keyed, box & pin

connections

Maximum depth 60-150 ft

Hollow Stem Auger (Drill ing & Sampling)

Hollow Stem Auger



Hollow Stem Auger (Drilling & Sampling)

M th d f B i (W h B i )



Methods of Boring (Wash Boring)

Wash Boring

Casing pipe is inserted

Jet of water is forced through drillrods

Cutting the fines

For all geomaterials except rock

Methods of Boring (Percussion Boring)



Methods of Boring (Percussion Boring) Percussion Boring

Suitable for formation ofboulders and gravels

Methods of Boring (Rotary Boring)



Air or Mud Rotary Drilling

Methods of Boring (Rotary Boring)

Offshore Drill ing/Barge Rig



Exploration for abutments, bridges, docks, etc.

Offshore Drill ing/Barge Rig

Type of samples



Type of samples Disturbed samples

Soil structure gets altered (or) modified dur ing sampling operation

If natural moisture content and proport ion of mineral is preserved

then these samples are called “ Representative Samples”

If the above conditions is not met, then we call them as

“ non-representative Samples”

Undisturbed samples

If any sample is not falling under the criteria mentioned above is

considered undisturbed sample

The extent of disturbance to the sample due to the sampler

depends on its design features Cutting edge

Inside wall friction

Non-return valve

Design Features of Sampler (f di t b d l



Area ratio = (D22-D1

2) x 100 %

D12

Should be < 10 % for sensitive clays< 20 % for stif f formations

Inside clearance = (D3-D1) x 100 %

D1

Should be between 1- 3 %

Is meant to reduce the friction between

sample and inner surface of the sampler

Is also intended to allow elastic expansion ofthe sample

Outside clearance = (D2-D4) x 100 %D4

Help in reducing the friction when sampler isbeing driven and being withdrawn

D2

D1

D3

D4

Design Features of Sampler (for undisturbed sample

Types of Sampler



Types of Sampler Standard Split Spoon Sampler

Shelby Tube Sampler

Open Drive and Piston Sampler Double Tube Core Barrel

Piston Type Sampler

Standard Spli t Spoon Sampler

Shelby Tube Sampler (D ill i & S li )



Suitable for undisturbed soil sampling offine grained material

Thin-wall Steel Tubes 3.0" OD, 2.875" ID, 30.0" long, 7.2 lbs

Shelby Tube Sampler (Drill ing & Sampling)

Undisturbed Samples from



Undisturbed Samples from

Block sample of Clay

Open drive Sampler

Rock Coring (Drill ing & Sampling)



Rock Coring (Drill ing & Sampling)

A typical double-tube core barrel

Diamond or tungsten-carbide tooth

bit Size of core samples varies (NX-54

mm, NQ-47.6 mm, HQ-63.5 mm, etc.)

Rock Core Quality (Drill ing & Sampling)



Rock Core Quality (Drill ing & Sampling)

Core recovery percentage

Rock quality designation (RQD)Calculated as the ratio of the sum of

length of core fragments greater than

4 inches to the total drilled footage perrun, expressed as a percentage

Methods of Sampling and Resulting Samples



Disturbed samples are used for determining index properties(ex. PSD, Consistency, etc.)

Undisturbed samples are used for determining engineeringproperties (ex. Density, water content, shear strengthparameters, etc.)

Wash samples are obtained from wash boring water or mud.

Representative sample retains all constituents of the soil, but

is disturbed from natural state and structure. (ex. Split spoonsampler)

Block samples are carved out from sides or bottoms ofexcavations, sealed in a box and taken to lab.

Open drive samplers consist of thin walled tubes which are

driven or pushed into the soil at the bottom of the hole. (ex.Shelby Tube sampler)

Methods of Sampling and Resulting Samples

Typical Bore Log Contains….



1. Details of drilling company

2. Driller’s name

3. Job description

4. Number, type, and location of boring

5. Date of boring

6. Subsurface stratification, which canbe obtained by visual observation of

the soil brought out by auger, split-

spoon sampler, and thin-walled

Shelby tube sampler

7. Elevation of water table and date

observed, use of casing and mud

losses, and so on

8. Standard penetration resistance and

the depth of SPT

9. Number, type, and depth of soilsample collected

10.In case of rock coring, type of core

barrel used and, for each run, the

actual length of coring, length of

core recovery, and RQD

Typical Bore Log Contains….

Depth of Investigation




Stress Distribution below the footing

based on reconnaissance study




The approximate required minimum depth of the borings should be

predetermined. The estimated depths can be changed during the drilling

operation, depending on the subsoil encountered.

To determine the approximate minimum depth ofboring, engineers may use the following cr iteria:

1. Determine the net increase of stress, σ under afoundation with depth as shown in the Figure.

2. Estimate the variation of the vertical effectivestress, σ'v, with depth.

3. Determine the depth, D = D1, at which the stressincrease σ is equal to 0.1q ( where q = estimatednet stress at foundation level due to thestructure).

4. Determine depth, D = D2, at which σ/σ’v = 0.05.

5. Unless bedrock is encountered, the smaller of thetwo depths, D1 and D2, just determined is theapproximate minimum depth of boring required.


Stress distribution belowthe shallow foundation

Number of Bore Holes &Extent of Investigation



g

For a compact building site covering an area of about 0.4hectare, one bore hole or trial pit in each corner and one in

the centre should be adequate

For smaller and less important buildings even one bore hole

or tr ial pit in the centre will suffice.

For very large areas covering industrial and residentialcolonies, the geological nature of the terrain will help in

deciding the number of bore holes or trial pits. Conepenetration tests may be performed at every 50 m by

dividing the area in a grid pattern and number of bore holesor trial pits decided by examining the variation in the

penetration curves.

The cone penetration tests may not be possible at siteshaving gravelly or boulderous strata.

Extent of Investigation



Approximate spacing of boreholesType of project Spacing (m)

Multi storey Building 10-30

One storey industr ial plants 20-60

Highways 250-500

Residential subdivision 250-500

Dams and Dykes 40-80

g There are no hard and fast rules for the spacing of the

boreholes. The following table Provides some generalguidelines for borehole spacing.

These spacing can be increased or decreased, depending on

the subsoil condition. If various soil strata are more or lessuniform and predictable, the number of boreholes can be

reduced.

In-situ (Field Strength) Tests



( g )

When it is difficult to obtain “ undisturbed” samples

Cohesionless soils, Sensitive clays Vane shear test (VST)

Standard Penetration Test (SPT)

Cone Penetration Test (SCPT/CPT; DCPT)

The Pressure-meter Test (PMT)

The Plate Load Test (PLT)

The Borehole Shear Test (BST)

The Flat Dilatometer Test (DMT)

Field (Strength) Tests



( g )

Vane Shear Test



Determine in-place shear strength for soft clay

Good for loose cohesionless soils, that lose part

of their strength when disturbed (with Caution)

How to use?

Push the vane tester into the soil and apply a

torque to the vertical shaft

+

=

62

32d hd

T

c

π

where

c = cohesion of the clay

T = torque required to shear the soil

d = diameter of vane tester

h = height of vane tester

Correction factor for Vane Shear Test



Plasticity Index (%)

C o r r e c

t i o n

F a c

t o r

( μ )

Standard Penetration Test (SPT)



As per IS:2131 –1981 for soils.

Most important andmost commonly

used field test

Typical equipment:drill rig, split spoon

sampler, hammerand casing pipe.

Standard Penetration Test (SPT)- Procedure1 D i ti f i i



1.Drive a section of casing pipe.

2.Complete wash boring and clean the hole

3.Replace driving bit by spl it spoon sampler at the bottom end of

the driving rod4.Drive the sampler by dropping a hammer of 63.5 kg weight

through a height of 75 cm

5.The number of blows required to penetrate three successive

lengths of 15 cm are noted.6.The first 15 cm drive is considered as seating load and is

ignored. The total number of blows required to penetrate the

remaining 30 cm (15 cm +15 cm) is called the blow count orpenetration number N (N2+N3).

7.Raise the sampler to the surface, open it and extract thesample.

8.Drive the next length of casing and repeat the process until

re uired de th is reached.

SPT- Standard Split Spoon Sampler



Types of SPT Hammers



SPT- Manually Operated Hammer



SPT- Automatic Trip Hammer and SPT-Refusal



The bore log shows refusal andtest is halted if :

1. More than 50 blows per 150 mm

penetration (i.e., from 150 mm to

300 mm)

2. More than 100 blows per 300 mm

penetration (i.e., from 300 mm to

450 mm)

3. Ten successive blows produce noadvance

Correction to SPT Blow Counts



Energy Correction (Due to the Hammer Efficiency)---- 60 %

Borehole diameter

Type of sampler

Rod length60.0

60

N C C E N

r bm=

SPT-Overburden Correction



(kPa) 2000log77.0

)(`

110

60601

×

=

V

N

N

C

N C N N

σ

Or)

Applied to N value based on chart by Peck, et. al. (varies from 0.45 to 2)

Correction due to di latancy for f ine sand and sil t below GWThaving N`=(N1)60 > 15 (due to the liquefaction effect)

N” = 15 + 0.5*(N’-15)

SPT- Dilatancy Correction

Correlations between N Values and Soil Property



Consistency qu (kPa)

Very Soft Soft Medium Stiff Very Stiff Hard

SPT N-value <2 2-4 4-8 8-15 15-30 >30qu <25 25-50 50-100 100-

200

200-400 >400

Relation of Consistency of Clay and Unconfined Compressive (qu)

NCompactness

of sand

Relative

Density (%)

0 to 4 Very Loose 0-15 < 284 to 10 Loose 15 - 35 28 -30

10 to 30 Medium Dense 25 - 65 30 -36

> 50 Very Dense > 85 > 41

Relevance of SPT for Bearing Capacity



Bearing capacity factors (Nq and Nγ)depends on fr iction angle ( )

vs. SPT N-value

Cone Penetration Test (SCPT/CPT and DCPT)



Originally Developed in Netherlands 1930s

Further developments in 1950s

Popularly known as “ Dutch Cone” ASTM D3441, IS 4968-Part3

Base Area= 10 cm2, Appex Angle =60°

Methods

Static cone test-CPT (when cone is pushed)

Dynamic cone test-DCPT (when cone is driven)

Types of CPT devices

Mechanical cone

Electric cone

Piezocone

P T )



M e c h a n

i c a

l C o n e

P e n e

t r o m e

t e r

( C P

T )



E l e c t r i c a

l C o n e

P e n e

t r o m e

t e r

( C P T

P T )



P i e z o

c o n e

P e n

e t r o m e t e

r ( C P

Sequential Steps to Perform CPT1 “ Cone” and “ Friction Jacket”



1. “ Cone” and “ Friction Jacket”

are in stationary posit ion

(Position 1)

2. Cone pushed by innersounding rod to a depth of

a=40mm at a steady rate of

20 mm/sec till the collarengage the cone (Position 2)

3. The tip resistance qc

(QC/Ac)called cone or pointresistance

4. The sounding rod pushed

further to a depth of b=40mm, this will push the

“friction jacket and cone”together (Position 3)

5. The skin frict ion f s= Qf /Af

= (Qt-QC)/Af

Sequential Steps to Perform CPT6 The outside mantle is



6. The outside mantle is

pushed down to a distance

of (a+b), which brings thefriction jacket and cone to

posit ion 1 (Position 4)

7. CPT provides continues

record of variation of coneresistance,friction resistance

and pore pressure with

depth but CPT does not yieldsample

8. The test is unsuitable for

gravels and very dense sand

9. The data from CPT often

used to estimate the Pointbearing resistance and

friction resistance of pile

foundation

Pictorial View of Cone Penetrometer



Cone Penetrometer Mounted on Truck



Typical Results from CPT



CPT Correlations With Engineering Properties



Type of Soil qc/N (qC in kg/cm2)

Sandy gravels and gravels 8 to 10

Coarse sand 5 to 10

Clean fine to medium sand and slight ly silty-sand 3 to 4

Silts. Sandy silts , slight ly cohesive silt -sand mixture 2

Angle of internal fr ic tion, Ø (Degrees

C o n e r

e s

i s t a n c e

( q c ,

k g / c

m 2 )

Correlations With Engineering Properties



Friction ratio, f R (f s/qc)

C o

n e

R e s

i s t a n c e

q C

( k g

/ c m

2 )

CPT versus SPT



CPT: provides much better resolut ion, reliabili ty

CPT: Versatile and provides pore water pressure, dynamicsoil properties

CPT : Does not provide a sample

CPT :Will not work with soi l with gravel

CPT: Need to mobilize a special rig

Self study from IS4968 Part1 and 2 Gopal Ranjan Text Book pp 683

Dynamic Cone Penetration Test (DCPT)



Self study from IS4968-Part1 and 2, Gopal Ranjan Text Book pp. 683

Pressure Meter Test (PMT)

Type of load test



Type of load test

Load is applied by uniform radial pressure to the sides of bore hole in

which it is placed

Types of pressure meters

Menard Pressure meter (MPM), which is lowered in preformed bore hole

Self-boring pressure meter (SBP), which form its own bore hole and

causes much less disturbance to the soil

Menard Pressure meter (MPM)

Is an inline three cell probe

Middle part is the test or measuring cell

Two cells called guard cell (to protect the test cell from end effects)

Water is used to fill the test cell and compressed gas is to fill guardcells

Sequential Steps to Perform MPM1. The test is conducted in



predrilled bore hole normally

at 1m interval

2. Oversized bore hole up to 10% is allowed

3. The pressure of water in the

test cell is increased

incrementally till the soil fails

(till limiting pressure, pl)

4. The failure is consideredwhen the total expanded

volume of test zone reaches

twice the volume of originalcavity

5. Each increment of pressure isheld for a minute and

corresponding volume isrecorded

Salient Features of MPM Data1. Init ial part “ OA” is the



process of pushing bore

hole sides to their originalposition

2. The volume of expansion of

cavity considered to begin

only from “ A”

3. The straight line portion

“AB” is considered as

“ Pseudoelastic stage” of thesoil. The point “B” indicates

end of this stage and

corresponding pressure is

called “ Creep Pressure”

4. The portion “ BC” is cal led

plastic phase of the soil

deformation and the

pressure at “C” is known as

“ Limiting Pressure”

Injected volume (cm3)

P r e s s u r

e ( k P a

)

1. Menard’s pressure modulus, Em = 2.66 Gm

2 G is called Menard’s shear modulus for curve between V and V

CPT Correlations With Engineering Properties



2. Gm is called Menard’s shear modulus for curve between V0 and Vf

3. The Young’s modulus E= Em/α (where α called Rheological parameter)

and depends on soil type and [Em/(p l-σh)]

Type of Soil [Em/(p l-σh)] α NC clay 9 to 16 0.67

OC clay > 16 1.00

Silt-NC 8 to 14 0.5

Silt-OC > 14 0.67

Sand 7 to 12 0.33

MPM Correlations With Engineering Properties



d c

m

des

hl bcvultimate

l u

F B B

B d B

E

q H

p kq

pC

=

=

σ

0

0

1

29

9

where

σ1v = effective horizontal stress in kPa σh = horizontal soil stress at rest

kbc = bearing capacity factor q ultimate= ultimate bearing capacity

ΔH= settlement of shallow foundationqdes= net design bearing pressure

B0= a reference dimension=0.6 m B= width or diameter of the foundation ≥B0

α= rhelogical factor λc, λd, = shape factors = f (L/B of foundation)

Em= Menards shear modulus Fd= depth factors (varoes between 1 to 1.2)

Problems with MPM

Di t b d t th



Disturbance caused to thesides of the BH can vitiate

the results

Expansion of soil due torelease of in-situ pressures

Diameter of BH being toolarge or too small when

compared to the un-inflatedprobe may cause some

errors.

Plate Load Test (PLT)- Procedure1.Selection of location for test



Se ect o o ocat o o test

The test is conducted at proposed foundation level

In case GWT is within the depth equals to the width of plate,the test shall be conducted at GWT level / 1m below thefoundation level (to avoid unrealist ic stiff behaviour of soil)

In case of GWT is higher than test level, it shall be lowered

to the test level and maintained by pumping through a sump,

away from the test plate In case of soils like non-plastic silt and fine sand which

cannot be drained by pumping from the sump, the test level

shall also be at water table level.

In case of foundations on silt and fine sand, this testprovides direct measure of “compressibility/settlement” of

soils and “ Bearing Capacity” of shallow foundations

Plate Load Test (PLT)



Plate Load Test (PLT)- Procedure2. Test Pit Size



The pits, usually at the foundation level (with 5 mm sand

layer), having width equal to five times the test plate

It shall have a leveled and cleaned bottom at test level,protected against disturbance or changes in naturalformation

3. Size and shape of the plates

Except in case of road problems and circular footing;square plates may be adopted.

For clayey and silty soils, for loose to medium dense sandysoils with N < 15, a 450 mm square plate shall be used.

In the case of dense sandy, gravelly soils (15 < N < 30 )three plates of sizes 300 mm to 750 mm shall be used.

The side of the plate shall be at least four times maximum

size of the soil particles

Plate Load Test (PLT)- Procedure4. Test arrangement and Loading

Th l di l f h ll b d b i bl



The loading platform shall be supported by suitable means

at least 2.5 m from the test area with a height of 1 m or

more above the bottom of the pit to provide sufficientworking space

A minimum seating pressure of 70 g/cm2 shall be applied

and removed before starting the load test

The two supports of the reference beam or datum rod shallbe placed over firm ground, fixed with minimum two dial

gauges resting at diametrically opposite ends of the plates.The dial gauges shall be so arranged that settlement is

measured continuously without any resetting in between

Apply the load to soil in cumulative equal increments up to1 kg/cm2 or one-fifth of the estimated ultimate bearing

capacity, whichever is less.

Plate Load Test (PLT)- Procedure5. Settlement and observation

S ttl t h ld b b d f h i t f l d



Settlements should be observed for each increment of load

after an interval of 1, 2.25, 4, 6.25, 9, 16 and 25 min and

thereafter at hourly intervals to the nearest 0.02 mm. For soils other than clayey soils, each load increment shall

be kept for not less than one hour or up to a time when the

rate of settlement gets appreciably reduced to a value of0.02 mm/min

In case of clayey soils the ‘time settlement’ curve shall be

plotted at each load stage and load shall be increased tothe next stage either when the curve indicates that the

settlement has exceeded 70 to 80 percent of the probable

ultimate settlement at that stage or at the end of 24 hourperiod

The next increment of load shall then be applied and theobservations are repeated

Plate Load Test (PLT)- Procedure5. Settlement and observation cont…



The test shall be continued till, a settlement of 25 mm

under normal circumstances or 50 mm in special cases

such as dense gravel, gravel and sand mixture, isobtained or ti ll failure occurs, whichever is earlier.

Alternatively where settlement does not reach 25 mm, thetest should be continued to at least two times the

estimated design pressure. If needed, reboundobservations may be taken while releasing the load.







Load settlement curve obtained from PLT



Applied Pressure (kPa)

S e t

t l e m e n

t ( m m

)

Determination of SBC and settlement from PLT The safe bearing pressure, q bearing, for medium and dense

sands could be read from the above figure corresponding to



sands could be read from the above figure, corresponding toa settlement ( Sp), which shall be estimated using below Eq.1

In case of clays consolidation settlement, which constitutesmost part of the total settlement, cannot be predicted throughplate load test. Hence plate load test is of not much relevance

in clayey soils. Thus the following equation may be used topredict the allowable bearing pressure

1.3.0

3.02

Eq B B

B BSS

f p

p f

p f

=

whereSf = settlement of foundation in m

Sp = sett lement of plate in mBf = width of the footing

Bp = width of the plate

=

p

f

p

f

B

B

S

S

Applied Pressure (kPa)

Bearing pressure of cohesion less soils from PLT



qbearing

S e t

t l e m e n

t ( m m

)

Sp as per Eq. 1

Ultimate bearing pressure from PLT In case of granular soils the bearing capacity increases with

size of the foundation (plate), IS 1888 recommends to conduct



size of the foundation (plate), IS 1888 recommends to conductthree load tests using test plate of three different sizes and

bearing capacity values extrapolated to real size of footing

If above procedure is not practicable, then use Eq. 2 and 3

)(2. granular Eq B

Bqq

p

f upuf

=

S B C

( k P a )

Size cm

)(3. cohesive Eqqq upuf

Limitation of Plate Load test Plate load test provide immediate settlement, however in

case of clays primary and secondary/creep (organic clays)



case of clays, primary and secondary/creep (organic clays)

settlements are signif icant

Non-homogeneity of the soil may errors due to size effect, asshown below

Self study from IS 1888, IS 1809 part I and Gopal Ranjan Text

Corrections to plate load test Data



Book pp. 506

Geophysical Methods1.Seismic Refraction Method

Based on the fact that seismic waves exhibits varied



velocities in different types of geomaterials (soil and rocks)

Waves are refracted when they cross the boundary between

two layers

The method enables to identify the nature of the soil layerand approximate depth of strata

Induce shock waves by detonation of small charge orimpact on plate with sledge hammer

The radiating wave are recorded by geophones, installed atknown distance

Seismic Refraction Method



Some waves travel along ground surface called “ Direct/Primary waves”

Some waves may reflect quickly and can be captured by thegeophones placed near to the source

Other waves travel down and get refracted if pass through the

stratum having different seismic velocity

D

Seismic Refraction Method

C



A

B

Seismic Refraction Method If the underlying layer is denser, the refracted waves travels fast

As the distance between source and geophone increases, the



refracted waves reaches the geophone earlier than the primary

wave (represented by BC) If the source geophone is placed at a distance less than “xc”

the direct wave reaches the geophone earlier than refractedwave and visa-versa

As the distance between source and geophone increases, the

refracted waves reaches the geophone earlier (represented by AB) than the primary wave

If the seismic velocities of the upper and lower stratum are V1and V2 and then depth of upper layer “ D” equals to…….

2

1

12

12

2

=

V V

V V X D c

Seismic wave velocities of different materials

Type of Soil

Density

(kN/m3)Porosity Velocity



Type of Soil (kN/m )(%) (m/sec)

Sandy ? ? 180 to 365

Sandy clay ? ? 365 to 580

Gravel ? ? 470 to 790

Shale ? ? 790 to 3350

Granite ? ? 3050 to 100

This method is quite reliable and fast in establishing the profilesof dif ferent strata

However this method cannot be used to identify the exact typeof the strata. For this purpose boring and sampling is essential

Geophysical Methods1.Electrical Resistivity Method

The method is based on measured change of mean resistivity



of the material

Test is carried by driving four metal spikes (electrodes) intothe ground along straight line at equal or specified distance

The current flow through outer electrodes produce an

electrical field

The potential difference between inner two electrodes is

measured, apparent resistivity of the soil is calculated, whichis weighted average of true resist ivity up to a depth “ a” .

The soil close to the surface being heavily weight than soil at

greater depths

If a stratum of low resistivity overlies stratum of highresistivity the is current is forced to flow closer to the ground

surface, resulting in higher voltage drop and high apparentresistivity





Electrical Resistivity Different Materials

Type of SoilResistivity

(kOhm-cm)



(kOhm cm)

Clay and saturated sil t 0 to 10

Sandy clay and wet sil ty sand 10 to 25

Clay sand and saturated sand 25 to 50

Sand 50 to 150

Gravel 150 to 500

Sound rock 150 t0 4000

Electrical Resistivity Method Current Flow Profile

I



Electrical Resistivity Method-Potential Profi le



Electrical Resistivity Method1.Method of Sounding

Self study from Gopal Ranjan Text Book pp. 694



Electrical Resistivity Method1.Method of Profiling

Self study from Gopal Ranjan Text Book pp. 694



SITE INVESTIGATION AND SUBSOIL EXPLORATION



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CE3350 Soil Eigxploration

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