CHAPTER_7-consolidation_new.ppt

7/27/2019 CHAPTER_7-consolidation_new.ppt

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CONSOLIDATION



In saturated cohesive soils the effect of

loading is to squeeze out porewater; this

process is called consolidation.



When a saturated clay is loaded externally,

saturated clay

GL

the water is squeezed out of the clay over a

long time (due to low permeability of the

clay).



4

This leads to settlements occurring over a

long time,

which could be several years.

time

s e t t l e m e n t



5

Granular soils are freely drained, and thus

the settlement is instantaneous.

time

s e t t l e m e n t





here’s a spring (soil skeleton) housed

in a sealed chamber filled with waterthe valve in the seal is closed

an extra load (pressure) is applied to

the water takes on this extra pressuspring (soil skelton) feels nothin

valve opens, water drains out

spring (soil skeleton) starts tocompress as it takes on the extra load

finally, the soil skeleton has taken

on all the extra load

drainage stops



From this analogy, the process of

consolidation involves three main features :

A) Flow of water

B) Compression

C) Displacement pressure



Coefficient of compressibility

Strain per unit volume of added stress. Rates

depend on the compressibility of the soil

reinforcement and change of stress.

Where, ∆P – Added stress

∆H – Change in thickness

H – thick of sample

Mv = ∆H x 1

H ∆P



Coefficient of permeability

Where, e1 – initial of void ratioe2 @ ef – Final void ratio after stress applied

Δe = e1 – e2

Mv = e1 – e2 x 1

1 + e1 ∆P



Coefficient of consolidation

Where,k – coefficient of permeability

γw– unit weight of water

Mv – coefficient of compressibility

Cv

= kγwMv



Time Factor

Used to calculate the period of time, t

required to settle the land.

Where ; Cv - coefficient of consolidation

t - settlement timesd – length of drainage path

= ½ length if 2 way drainage

= length if 1 way drainage

Tv = Cvt

d²



Degree of consolidation, Uv

Where;∆Ht – consolidation settlement at time t

∆Hc – final consolidation settlement

Uv = ∆Ht

∆Hc



Refers to the vertical downwarddisplacement at the base of a

foundation or other structure due to

ground movement.





Increment of load

Topcap

porous stonesample

water confiningring

settlementdial gauge

Find and briefly explain about the oedometer

test (including procedure, picture and data

from oedometer test)



Graph pressure against the thickness of

sample.

Example :The following readings were obtained from an

oedometer test on a specimen of saturated clay.

The load being held constant for 24 hours before

the addition of the next increment.

Applied stress(kN/m2)

0 50 100 200 400 800 0

Thickness(mm)

25.0 24.6 24.4 24.2 23.9 23.7 24.2



At the end of the last load period the load was

removed and the sample allowed to expandfor 24hr, at the end of which time its

thickness was 17.92 mm and its water

content found to be 31.8%. The specific

gravity of the soil was 2.66.

a) Plot the graph of pressure against

thickness, determine the coefficient of

volume compressibilty, Mv for a stress 400kN/m2.

Graph and result

http://localhost/var/www/apps/conversion/tmp/scratch_2/graf%20ex%201%20consolidation.xls

http://localhost/var/www/apps/conversion/tmp/scratch_2/graf%20ex%201%20consolidation.xls



Graph pressure against the thickness of

sample.

Exercise :The following readings were obtained from an

oedometer test on a specimen of saturated clay.

The load being held constant for 24 hours before

the addition of the next increment.

Applied stress(kN/m2)

0 50 100 200 400 800 0

Thickness(mm)

20.0 19.65 19.52 19.35 19.15 18.95 19.25



At the end of the last load period the load was

removed and the sample allowed to expandfor 24hr, at the end of which time its

thickness was 17.92 mm and its water

content found to be 25%. The specific

gravity of the soil was 2.65.

a) Plot the graph of pressure against

thickness, determine the coefficient of

volume compressibilty, Mv for a stress rangebetween 250-350 kN/m2.



Example :

A standard consolidation test carried out on a soil

sample initially 20 mm thick, drained on top andbottom provide the following results over a

pressure range of 100 – 200 kN/m2. After 24

hours the thickness of sample was reduced to

17.69 mm.

Time (minutes) 0.25 1.00 2.25 4 9 16 25

Thickness ofsample (mm)

19.84 19.66 19.52 19.43 19.29 19.13 18.99



a) Plot the graph of compression (thickness of sample)

against square root of time (Taylor method) and show

which part of the curve represents consolidation

settlement.

b) Estimate the coefficient of consolidation, Cv for this

sample.

c) If the coefficient of compressibility, Mv was 0.000046

kN/m2, estimate the coefficient of permemability, k

Time (minutes) 36 49 64 81 100 121 144

Thickness of

sample (mm)

18.85 18.67 18.57 18.42 18.30 18.20 18.10

Time (minutes) 169 196

Thickness of

sample (mm)

18.04 17.99

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CHAPTER_7-consolidation_new.ppt