1. basics of soil mechanics

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Basic Soil MechanicsBasic Soil Mechanics:Basic soil properties, correlation between engineering parameters, geotechnicalinvestigation, bore log.

Pile foundation:Jacket main piles, skirt piles, driven piles, drilled and grouted piles, steel and concretepiles, axial capacity, point bearing and skin friction, factor of safety, lateral load onpiles, p-y, t-z and q-z curves, pile group effect, scour around piles, seabed subsidenceand design of piles against seabed movement, negative skin friction, cyclic degradation,main pile to jacket connections, skirt pile to jacket connections, API RP 2A provisions.

Pile Installation:Minimum pile wall thickness, pile handling stresses, static and dynamic stresses, pilestickup, stresses during stick up, wave and current loads, hammer selection, pile drivingstresses, wave equation analysis, pile driving fatigue, API RP 2A guidelines.

Pile Testing:

27 June 2009 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36

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Pile Testing:Working load test, ultimate load test, pile monitoring during driving, pile integritytesting, high strain dynamic testing, rebound method.Special foundations: Mud-mats: bearing capacity, sliding stability, over-turning stability,short term and long term settlements, factor of safety; Bucket foundation; Suctionanchors; Gravity foundation.

Basic Soil Mechanics

BASIC SOIL MECHANICS

Basic soil propertiesCorrelation between

engineering parametersGeotechnical investigationBore log



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Bore log

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GROUND FORMATION TYPES COHESIONLESS SOIL SAND GRAVEL SAND, GRAVEL

COHESIVE SOIL Clay, organic soil

ROCKY Sedimentary, granite etc



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CLAY STRUCTURES



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a) Dispersed, b) Flocculated, c) Book house, d) Turbostratic e) Example of a natural clay

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Soil type Term Field test

Sands, gravels

Loose

Dense

Can be excavated with a spade; 50 mm wooden peg can be easily driven Requires a pick for excavation; 50 mm wooden peg is hard to driveVisual examination; pick removes soil in lumps

BASIC CHARACTERS OF SOIL

Slightly cemented

Visual examination; pick removes soil in lumps which can be abraded

Silts Soft or looseFirm or dense

Easily moulded or crushed in the fingersCan be moulded or crushed by strong pressure in the fingers

Clays Very soft

SoftStiff

Exudes between the fingers when squeezed in the handMoulded by light finger pressureCannot be moulded by the fingers; can be indented by the thumb



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Very stiff Can be indented by the thumbnail

Organic, peats Firm Spongy Plastic

Fibres already compressed togetherVery compressible and open structure Can be moulded in the hand and smears the fingers


Homogeneous Interstratified

Deposit consists essentially of one soil typeAlternating layers of varying types or with bands or lenses of other materials (an interval scale for bedding spacing or

BASIC CHARACTERS OF SOIL

of other materials (an interval scale for bedding spacing or layer thickness can be used)

Homogeneous Weathered

A mixture of soil typesCoarse particles may be weakened and may show concentric layeringFine soils usually have crumb or columnar structure

Fissured (clays) Breaks into polyhedral fragments along fissures (interval scale for spacing of discontinuities may be used)

Intact (clays) No fissures



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Intact (clays) No fissuresFibrous (peats) Plant remains are recognizable and retain some strength Amorphous (peats)

Recognizable plant remains are absent

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Soil groups Subgroups and laboratory identification

GRAVEL and SAND may be qualified sandy GRAVEL and gravelly SAND, etc., where appropriate

Group symbol

Subgroup symbol

Fines (% less than 0.06 mm)

Liquid limit

GW GWof Slightly silty or clayey

British soil Classification System for Engineering Purposes

GWGGP

G-MG-F

G-CGM

GFGC

SW

GW

GPU GPg

GWM GPM

GWC GPCGML.etc

GCL, GCIGCH,GCVGCESW

0 to 5

5 to 15

15 to 35

CO

AR

SE

SO

ILS

35%

of

the

mat

eria

l is

fin

er

than

0.0

6 m

m

GR

AV

EL

S

Mo

re t

han

50%

of

coar

se m

ater

ial

is o

gra

vel

size

(co

arse

th

an 2

mm

)

%

Slightly silty or clayey GRAVEL

Slightly GRAVEL Clayey GRAVEL

Very stilty GRAVEL

Very clayey GRAVEL

Slightly silty or



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SSP

S-MS-F

S-C

SPu SPg

SWM SPM

SWC SPC

0 to 5

5 to 15

CL

ess

than

3

SA

ND

S

Mo

re t

han

50

of

coar

se

mat

eria

l is

of

san

d s

ize

(mo

re t

han

2

mm

)

g y yclayey SAND

Silty SAND

Clayey SAND

Basic Soil MechanicsVery Silty SAND

Very clayey SAND

SMSFSC

SML, etc

SCLSCISCHSCVSCE

15 to 35

ll G G

SA

ND

S M

ore

than

50%

of

coar

se m

ater

ial i

s of

san

d si

ze

(mor

e th

an 2

mm

)

Gravelly SILTGravelly CLAYEY

Sandy SILTSandy CLAY

SILT (M-SOIL)

MGFGCG

MSFS

CSM

MLG, etc

CLGCIGCHGCVG CEGMLS, etc

CLS, etcML, etc

<3535 to 5050 to 7070 to 90>90

FIN

E S

OIL

S5

% o

f th

e m

ate

ria

l is

fin

er

tha

n 0

.06

mm

Gra

velly

or

sand

y S

ILT

S a

nd C

LAY

S

35%

to 6

5% fi

nes



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( )

CLAY

MFC

ML, etc

CLCICHCVCE

<3535 to 5050 to 7070 to 90>90

Mo

re t

ha

n 3

SIL

TS

AN

D

CLA

YS

65%

to

100%

fine

s

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Main Terms Qualifying Terms

GRAVEL G Well graded W

SAND S Poorly graded uniform

PPu

gap-graded PgFINE SOIL, FINES F Of low plasticity (wL < 35) L

SILT (M-SOIL) M Of intermediate plasticity (wL 35 - 50) I

CLAY C Of high plasticity (wL 50 - 70)Of very high plasticity (wL 70 – 90)Of extremely high plasticity (wL > 90)Of upper plasticity range (wL > 35)

HV EU



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Organic (may be a suffix to any group) OPEAT Pt


Name Group

symbols

Laboratory criteria Fines (%) Grading Plasticity Notes

Coarse grained (more than

Gravel (more than 50% of

Well graded GWGravels,

0 – 5 CU > 41 < CZ <

Dual symbols if 5 12%

UNIFIED SOIL CLASSIFICATION SYSTEM

(more than 50% larger than 63% um BS or No. 200 US sieve size

50% of coarse fraction of gravel size)

sandy gravels, with little or no fines

Poorly graded GPgravels, sandygravels, withlittle or no fines

Silty gravels, GM

0 – 5

>12

3

Not satisfying GW require-ments

Below A-line

5 – 12%fines. Dual symbols if above A-line and 4 < IP < 7



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Silty gravels, GMSilty sand gravels

Clayey gravels GCclayey sandygravels

>12

>12

Below A line or Ip<4Above A-line and IP > 7

Contd…

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Name Group

symbols

Laboratory criteria Fines (%) Grading Plasticity Notes

Coarse grained

Sands (more than 50% of

Well graded GWsands,

0 – 5 CU > 61 < CZ < 3

Dual symbols if


(more than 50% larger than 63% um BS or No. 200 US sieve size

coarse fraction of gravel size)

sa ds,Gravely sands, with little or no fines

Poorly graded GPsands, gravellysands, withlittle or no fines

Silt sands SM

0 – 5

>12

C 3

Not satisfying SW require-ments

Belo A

5 – 12%fines. Dual symbols if above A-line and 4 < IP < 7



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Silty sands SM

Clayey sands SC

>12

>12

Below A-line or Ip<4Above A-line and IP

> 7

Contd…


Name Group symbols

Laboratory criteria

Fine grained (more than 50% smaller than 63 μm BS or No.200 US sieve size)

Silts and clays (liquid limit less than 50)

Inorganic silts, silty MLor clayey fine sands,With slight plasticityInorganic clays, silty CL

Use plasticity chart



Silts and clays (liquid limit less than 50)

Inorganic clays, silty CLclays, sandy clays of low plasticityOrganic silts and OL organic silty clays of low plasticity Inorganic silts of high MH plasticity In organic clays of CH plasticity Organic clays of high OH





U l ti it h t



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Highly organic soils

Organic clays of high OH plasticity

Peat and other highly Pt organic soils


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Basic Soil MechanicsPLASTICITY CHART : UNIFIED SYSTEM



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BS Sieve Particle size* Percentage smaller

Soil A Soil B Soil C Soil D63 mm 100 100

20 mm 64 76

6.3 mm 39 100 65

2 mm 24 98 59600 μm 12 90 54212 μm 5 9 47 10063 μm 0 3 34 95

0.020 mm 23 69



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0.006 mm 14 460.002 mm 7 31

* From sedimentation test

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BS Sieve Particle size Percentage smaller

Soil E Soil F Soil G Soil H

63 mm 10020 mm 94

6 3 69 1006.3 mm 69 1002 mm 32 98

600 μm 13 88 100212 μm 2 67 95 10063 μm 37 73 99

0.020 mm 22 46 880 006 mm 11 25 71



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0.006 mm 11 25 710.002 mm 4 13 58

Liquid limit

Plastic limit

Non-plastic 32

24

78

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Basic Soil MechanicsPHASE DIAGRAMS



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PLASTICITY OF FINE GRAINED SOILS

PLP WWIThe water content range is defined as the plasticity index

Limit PlasticW

Limit LiquidW

P

L

P

L I

wwI

The water content range is defined as the Liquidity index



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Limit Plasticw

contentWater w

P

PI


The degree of saturation is the ratio of the volume of water to the total volume of void space

Saturation ratio

waterof VolumeV

saturation of DegreeS

w

r

v

wr V

VS



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space voidof volumeTotalVv

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Basic Soil MechanicsThe void ratio is the ratio of the volume of voids to the volume of solids

ratio voide

s

v

V

Ve

solids of VolumeV

space voidof volumeTotalV

s

v

The porosity is the ratio of the volume of voids to the total Volume of the soil

V

Vn v



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soiltheofVolumeV

voidsof olumeV

Porosity n

v

V


The void ratio and the porosity are inter-related as follows:

ne

e

en

n

1

1

The specific volume (v) is the total volume of soil whichcontains unit volume of solids



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contains unit volume of solidsv = 1 + e

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air voidsorcontentAirA

V

VA a

The air content or air voids is the ratio of the volume of air to the total volume of the soil

soiltheofVolumeV

air of VolumeV

air voidsor content Air A

a

The bulk density of a soil is the ratio of the total mass to the total volume

V

M



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VolumeV

Mass Total

densityBulk

M


The specific gravity of the soil particles is given by

w

s

ws

ss V

MG

density Particleρ

particles soil ofGravity G

s

s

The Saturation Ratio is given by

wGS



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waterof VolumewGs

e

wGS s

r

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In the case of a fully saturated soil, Sr = 1, hencee = wGs

wGeA s

The air content can be expressed as

eA

1

s wG

)1(

A = n(1- Sr)

The bulk density of a soil can be expressed as



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we

1

wrs

e

eSG

1

or


For a fully saturated soil (Sr = 1)

ws

sat e

eG

1

ws

d e

G

1

For a completely dry soil (Sr = 0)

The unit weight of a soil is the ratio of the total weight (a force) to the total volume



24V

Mg

V

W

(a force) to the total volume

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rs

ws

eSGe

wG

1

)1(

Apply in the case of unit weights

Where a soil in-situ is fully saturated the solid soil particles (volume 1 unit, weight Gs w) are subjected to upthrust (

wrs

e

1

sws GG , 1

w ). Hence the buoyant unit weight is given by



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wsat

w

eiee

,

:..11

Basic Soil MechanicsPARTICLE SIZE DISTRIBUTION CURVES



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DETERMINATION OF LIQUID LIMIT



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Planning Equipment

SOIL INVESTIGATION

Field testing Sampling

Bore Holes Location and Numbers

Field Tests



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Field Tests Laboratory Tests

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Basic Soil MechanicsSCHEMATIC OF WASH BORING

OPERATIONSPHOTOGRAPH OF WASH

BORING OPERATIONS



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Field tests Standard Penetration Test (SPT)

SOIL TESTING METHODS

Field Vane Shear test Cone Penetration Test

Laboratory Tests Direct Shear Test Tri-axial Shear Test



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Tri axial Shear Test Unconfined Compressive Strength test

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50mm External Diameter 35mm internal Diameter Split Barrel Sampler – 650mm length

SPT Test Parameters

length 65 Kg Hammer is dropped from a height of

760mm Number of Blow for 300mm penetration is

called SPT “N” Value It indicates relative strength of soil E i i l l ti b t th



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Empirical correlation between other parameters available.

Basic Soil MechanicsSCHEMATIC DIAGRAMMS OF THE THREE COMMONLY USED HAMMERS



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Description Very loose Loose Medium Dense Very dense

Relative density D 0 0 15 0 35 0 65 0 85

Empirical values for ø, Dr, and unit weight of granular soils based on the SPT at 6 m depth and normally consolidated

SPT N70: finemediumcoarse

1 – 2 2 – 33 – 6

3 – 64 – 75 – 9

7 – 158 – 2010 – 25

16 – 3021 – 4026 – 45

?> 40> 45

ø: finemediumcoarse

26 – 2827 – 2828 – 30

28 – 3030 – 3230 – 34

30 – 3432 – 3633 – 40

33 – 3836 – 4240 – 50

< 50

λwet, pcf

(kN/m³)

0-100†

(11 – 16) 90 – 115(14 – 18)

110 – 130(17 – 20)

110 – 140(17 – 22)

130 – 150 (20 – 23)

Relative density Dr 0 0.15 0.35 0.65 0.85



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( )

† Excavated soil or material dumped from a truck will weigh 11 to 14 kN/m³ and must be quite dense to weigh much over 21 kN/m³. No existing soil has a Dr = 0.00 nor a value of 1.00 – common ranges are from 0.3 to 0.7


N Value Classification Id (%) (NI)60

DENSITY INDEX OF SANDS

N Value Classification Id (%) (NI)60

0 – 4 Very loose 0 – 15 0 – 3 4 – 10 Loose 15 – 35 3 – 8 10 – 30 Medium dense 35 – 65 8 – 25 30 – 50 Dense 65 – 85 25 – 42

> 50 Very dense 85 – 100 42 – 58



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> 50 Very dense 85 100 42 58

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Consistency ,70N Remarks

CONSISTENCY OF SATURATED COHESIVE SOILS†

Very soft

Soft

Medium

Stiff

Very stiff

0 – 2

3 – 5

6 – 9

10 – 16

17 – 30

NC

easi

ng

OC

RY

oung

cl

ay

Age

d /

men

ted

Squishes between fingers when squeezed

Very easily deformed by squeezing

Hard to deform by hand squeezing

Very hard to deform by hand



35

Hard > 30Incr

e O Ace

m

Nearly impossible to deform by hand

† Blow counts and OCR division are for a guide in clay “exceptions to the rule” are very common


EFFECTIVE OVERBURDEN PRESSURE



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Basic Soil MechanicsCORRECTION OF MEASURED VALUES OF STANDARD

PENETRATION RESISTANCE



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Basic Soil MechanicsMECHANICAL (OR DUTCH) CONE, OPERATIONS

SEQUENCE, AND TIP RESISTANCE DATA



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VANE SHEAR TESTS

Two types available Field Vane Shear Laboratory Vane Shear Laboratory Vane Shear

Principle is same but only the size of equipment different

Method based on shearing a circular soil surface and relating it with the torque applied

Gives the undrained shear strength of soil



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Gives the undrained shear strength of soil, mostly clay based soils.


VANE SHEAR



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VANE SHEAR TEST

32 dhd

lengthvaneh

width vaneoveralld

failureat Torque

62

32

t

dhdcT u



41

length vaneh


SHEAR TEST



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DIRECT SHEAR APPARATUS



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Basic Soil MechanicsPRINCIPAL LINE DETAILS OF TRIAXIAL CELL



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THE MOHR-COULOMB FAILURE CRITERION

The shear strength of a soil at a point on a particular plane was originally expressed by Coulomb as a linear function of the normal stress on the plane at the same

StressNormal

StrengthShear

tan

f

f

ff c

function of the normal stress on the plane at the same point



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ParametersStrength Shear c, f



In accordance with Terzaghi,s fundamental concept that shear stress in a soil can be resisted only by the skeleton of solid particles, shear strength is expressed as a

in ParametersStrength Shear ,

tan,,

,,,

c

c ff

of solid particles, shear strength is expressed as a function of effective normal stress:



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stress effective of terms

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expressedbealsocan soilaofstrength shear The


2sin)(2

1

failure.at and stresses

principalminor andmajor effective of in terms

,3

,1

,3

,1

f



47

2cos)(2

1)(

2

1 ,3

,1

,3

,1

, f


σ΄1

σ΄1τ

Stress Conditions at Failure

σ΄1

σ΄3 σ΄3θ

τ1

τ1



48

σ΄σ΄3 σ΄1σ΄1

2θ

1

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major ebetween th angle al theoretic theis

245

hatapparent t isIt

failure.ofplanetheand plane principal

,O



49

2


:Now obtained. be alsocan parameters

strength shear theand failureat stresses

principal effective ebetween th iprelationsh The

Therefore,

)(21

cot

)(21

sin,3

,1

,,

,3

,1

,

c



50

)2

45tan(2)2

45(tan,

,,

2,3

,1

oo c

1. basics of soil mechanics

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