Lab Hint- Consolidation - Oedometer Test

8/11/2019 Lab Hint- Consolidation - Oedometer Test

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Specimen: 19 mm thk x 75 mm dia Plan Area: 4417.9 mm2

Final thickness of soil (Hs) found by oven specimen when test end.

{Hs Hf

or Hs

}

Change ofdial gauge

reading

Stress=

R =

Table 4sFinal

reading x

0.002

R =

Table 4s

reading x

0.002

Hf= 19

mm -

R x

0.002

Hs =(see the

above)

Hv=Hf-Hs

e Hv

Hs

Initial readin

R = Final readin

R reading = Final Initial

H

s

water

soil

H

f

Hi = 19

mm

Hf Hs Hv

e

Reduced thickness (mm) =

Dial gauge reading x 0.002


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1. Compression Index, Cc

The compression index, Cc, is the slope of the straight line portion (latter part

of the loading curve) of the e-log p curve, or

where e1 and e2are the void ratios at the end of consolidation under stresses p1

andp2respectively

2. preconsolidation pressure,pc

The preconsolidation pressure,pc, is the maximum past effective overburden

pressure to which the soil specimen has been subjected. It can be determined

by using the following simple graphical procedure (Figure 8b):

1. Determine the point 0 on the e-log p curve that has the sharpest

curveature.2. Draw a horizontal line OA.

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

4. Draw a line OC that bisects the angleA OB.

Produce the straight line portion of the e-log p curve to intersect OC atD. The

1

2

21

12

21

logloglog

p

p

ee

pp

eeCc


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pressure at pointD is the preconsolidation pressure,pc.

3. Swelling Index, Cs

The swelling index, Cs, is the slope of the unloadingportion of the e-log p

curve.

In most cases the values of Cs is 1/4 to 1/5 of Cc.

4. Determination of Coefficient of Consolidation Cv

a.The Root Time Method

Graph of settlement thickness (mm) vs root time t (min)

The theoretical curve is linear up to about 60% consolidation and at 90%

consolidation the abscissa (AC) is 1.15 timesthe abscissa (AB) of the

projection of the linear part of the curve.

By trial and error or draw a reference line (1.15 time)

Find thet90

Determine cvusing the following procedure:

Coefficient of Consolidation cv= 0.848H2/t90

3

4

43

logp

p

eeC

s


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4. Determination of Coefficient of Consolidation Cv

b.The log time method

Graph of the dial gauge readings in the oedometer test against the logarithm of

time in minutes.

The theoretical curve consists of three parts: an initial curve which approximatesclosely to a parabolic relationship, a part which is linear and an a final curve to

which the horizontal axis is an asymptote at U = 1.0 or 100%. (a100 )

1 In the experimental curve the point corresponding to U= 0 can be determined

by using the fact that the initial part of the curve represents an approximately

parabolic relationship between compression and time. Two points were on the

curve are selected (A andB in Figure 7) for which the values of t are in the

ratio of 4:1, and the vertical distance between them is measured. An equal

distance set off above the first point fixes the point (as) corresponding to

U= 0. The point corresponding to U= 0 will not generally correspond to the

point (a0) representing the initial dial gauge reading, the difference being due

mainly to the compression of small quantities of air in the soil, the degree of

saturation being marginally below 100%. This compression is called the initial

compression.

2 The point corresponding to U=50% can be located midway between theas

anda100 points and the corresponding timet50obtained

Determine cvusing the following procedure:

Coefficient of Consolidation cv= 0.196H2/t50

Lab Hint- Consolidation - Oedometer Test

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