Lecture 3.0 Compressibility of Soil

8/13/2019 Lecture 3.0 Compressibility of Soil

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Dr. J. BERLIN P. JUANZON CE, MBA, MSCM

CE 152

Geotechnical Engineering

2

JB JUANZON


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JB JUANZON

CONTENT

SETTLEMENT

CONSOLIDATION TIME RATE OF CONSOLIDATION


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Leaning tower of Pisa is one of the most famous buildings in the world. It

has been built before hundreds years ago.

Even today the great mass

continues to sink very slowly. It is a

question of about 1 mm. every year.

Since nobody can state with

mathematical security that thissinking will continue in the future at

the present yearly rate, without its

ceasing, remedies by means of

adequate measures, based onscientific studies and projects, are

under consideration. In the

meantime supervision with

instruments of very high precision is

continuously being carried out.


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JB JUANZON

SETTLEMENT Definition

The total vertical deformation at the surfaceresulting from :

External Load

Dewatering Settlement Components

Immediate Settlement ; Se

Primary Consolidation Settlement ; Sc Secondary Settlement (Creep) ; Ss

sce SSSS


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JB JUANZON

SETTLEMENTPurpose

Study the settlement behaviorDetermine the settlement value and time

Study the settlement influence to the

structure stability


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JB JUANZON

SETTLEMENT INFLUENCE


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IMMEDIATE SETTLEMENT

Defined as settlement which occurred directlyafter the application of a load, without a change inthe moisture content.

Caused by soil elasticity behavior

The magnitude of the contact settlement willdepend on the flexibility of the foundation and thetype of material on which it is resting.

For clay, the immediate settlement generally verysmall comparing to the consolidation settlement,therefore this immediate settlement mostlyignored.

Usually considered at sand or sandy soil.


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General Equation

Se= Elastic Settlement

Cs= shape and foundation rigidity factor

q = P/B2(net vertical pressure applied)

Es= Elasticity modulus of soil

B = Foundation width

Ip= Influence factor

= Poissons ratio of soil

EsqBCS se

21 Eqn 1.0

Eqn 2.0 pe IEsqBS

21


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Problem 1.0

Estimate the immediate settlement of a column footing 1.5m in diameter that is constructed in an saturated claylayer, given that the total load carried by the column

footing is 150kn, Es = 7,000kn/m2

and = 0.25. Assume thefooting to be rigid.

JB JUANZON


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JB JUANZON

CONSOLIDATION When the stress on a saturated clay layer in the field is

increased, the pore water pressure in the clay willincrease.

Because the coefficients of permeability of clays are verylow, it will take some time for the excess pore waterpressure to dissipate and the stress increase to betransferred to the soil skeleton gradually.

Consolidation is the process of dissipation of excess porewater pressure in a row of time.

Note:Dissipation of pore water pressure occurs simultaneously with the squeezing out ofthe pore water. Therefore the consolidation time depend on:

The distance of pore water to be squeezed out

The coefficient of permeability of soft soil


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JB JUANZON

CONSOLIDATION Consolidation Type

Normal consolidation

Pre-consolidation pressure (Pc) just equals the

existing effective vertical overburden pressure (Po) Over consolidation

If the soil whose pre-consolidation pressure (Pc) isgreater than the existing overburden pressure


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CONSOLIDATION

Normal Consolidation

oc pp OR 1p

p

o

c o

oc

oc

p

ppH

e

CcS

log..

1

Where:

Sc = primary consolidation settlement

Cc = Compression Index

H = thickness of clay layer

eo= in situ void ratio

Po = average effective stress at mid-height of clay layer orgeostatic pressure

P = average increase of effective stress on clay layer


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CONSOLIDATION

oc pp OR 1o

c

p

p

po+ p < pc

o

oc

oc

p

ppH

e

CsS

log..1

po< pc< po+pc

oc

oo

cc

oc

pppH

eCc

ppH

eCsS

log..

1log..

1

Over Consolidation

Cs = swell index (ranges from 1/5 to 1/10 of Cc


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DETERMINATION OF CONSOLIDATION

PROPERTIES


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PROPERTIES

Procedures :

1. Determine the point O on thee-lop p curve that has thesharpest curvature (that is,the smallest radius of

curvature)2. Draw a horizontal line OA

3. Draw a line OB that is tangentto the e-log p curve at O

4. Draw a line OC that bisectsthe angle AOB

5. Produce the straight lineportion of the e-log p curvebackward to intersect OC. Thisis point D. The pressure thatcorresponds to the point p isthe preconsolidation pressure,pc.


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PROPERTIES

1

2

21

p

plog

eeCc


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PROPERTIES

3

4

43

p

plog

eeCs


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CONSOLIDATION SETTLEMENT

Other equation

p.H.mS cvc

Where :

mv= Compression Index

Hc = Thickness of soft soil layer

p = The stress increment due to theexternal

load

o

vv

'1

'2

21v

e1

am

pp

eea


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SECONDARY CONSOLIDATION (CREEP)

p

pc

p

st

ttlog.H.

e1CS

Where :ep= void ratio at the end of primary consolidationtp= time at the end of primary consolidationt = time incrementt2= t

p+t

p

2

t

t

log

eC

Is the additional settlement that occurs at a constant value of effetive

stress after excess pore water pressure has been dissipated. The process

typically continues slowly for a long period of time. Secondary settlement is

usually small compared to primary consolidation settlement.


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SECONDARY CONSOLIDATION (CREEP)

PROBLEM 2 0


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PROBLEM 2.0A 2.1m layer of clay is buried beneath a 3.0m stratum of verycompact granular soil. Compact sand underlies the clay. The layer

of granular soil is composed of material having a unit weightof20.46kN/m3. The clay unit weight is 16.52 kN/m3. A laboratorycompression test indicates a compression index of 0.40 and anatural void ratio of 1.3. A planned building loading will cause a26.38kPa stress increase at the middle of the clay layer.

1. what amount of primary compression occurs in the clay forthe indicated conditions?

2. How much primary compression of the clay layer wouldresult if the groundwater table was the ground surface.

3. How much clay layer compression would occur if the claywas an over consolicated material, the past maximum pressurewas 95.94kPa and th Cs value was 0.10? Assume a deep watertable

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Lecture 3.0 Compressibility of Soil