ca 3687 oedometer test.pdf

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Department of Civil and Architectural Engineering

CA 3687 Soil Mechanics

Laboratory Report

Experiment 6 : Oedometer Test (1D Consolidation Test)

Programme : Bachelor of Engineering (Honours) in Civil and Structural Engineering

Group : 8

Date Submitted : 21/03/2014

Members : Au Yuk Kit (53307091)

Chan Chun Tong Tony (53238775)

Wong Po Ying (53420604)

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CONTENT

Page

1 INTRODUCTION 3

2 OBJECTIVE 3

3 THEORY 3

4 APPARATUS 6

5 TESTING PROCEDURES 7

7 RESULT AND CALCULATION 13

8 DISCUSSION 17

9 CONCLUSION 19

10 REFERENCES 20

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INTRODUCTION

The standard oedometer consolidation test for saturated clays is carried out by

applying a sequence of vertical loads to a laterally confined specimen having a height

of about one quarter of its diameter. The vertical compression displacement under the

vertical load is observed over a period of time, usually up to 24 hours. Since no lateral

deformation is in this test, one-dimensional consolidation parameters can be derived.

OBJECTIVE

To determine consolidation characteristics (coefficient of volume compressibility, mv,

and coefficient of consolidation, cv) of soils with low permeability.

THEORY

The one-dimensional consolidation test procedure was first suggested by Terzaghi.

The test is performed in an oedometer.

The soil sample is placed inside a metal ring with two porous stones each at the top

and the bottom of the sample. The samples are usually 63.5mm in diameter and

25.4mm thick. Load is applied on the sample through a lever arm and compression is

measured by a micrometer dial gauge. The sample is kept underwater during the test.

Usually each load is kept for 24 hours.

After that, conventionally, the load is doubled, thus doubling the pressure on the

sample, while measurement of the compression continues. At the end of the test, the

dry weight of the test sample is determined.

The general shape of the plot of deformation of the sample versus time for a given

load increment is shown in Figure 1. The plot shows three distinct stages that may be

described as follows:

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Figure 1. : Time-deformation Plot During Consolidation for Given Load

Increment (Source: Das 1979)

The general shape of the plot of deformation of the sample versus time for a given

load increment is shown in Figure 1. The plot shows three distinct stages that may be

described as follows:

Stage I: Initial compression, which is mostly due to preloading.

Stage II: Primary consolidation during which, due to expulsion of pore water pressure,

is gradually transferred into effective stress.

Stage III: Secondary consolidation after complete dissipation of excess pore water

pressure - some deformation of the sample is caused by plastic readjustment

of soil fabric.

The aim of the consolidation test is to determine two important consolidation

parameters for the clay sample:

Figure 2 .: Clay Sample

1. The coefficient of volume compressibility, mv (in m2 / MN) is given by the

formula

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Where

H1 is the height of the specimen at the beginning of the stage (in mm).

H2 is the height of the specimen at the end (in mm).

p1 is the pressure applied to the specimen for the previous loading stage (in

kPa).

p2 is the pressure applied to the specimen for the considered loading stage (in

kPa).

2. The coefficient of consolidation, cv (in m2/year).

The coefficient of consolidation, cv, may be determined by finding the time required

for

90% consolidation of the sample (U = 0.9).

For the case in the oedometer test which is the condition of double drainage:

When U =0.9

Tv =0.848

Since 𝑇 𝑐𝑣𝑡

ℎ2

∴ 𝑐 𝑇90

𝑡90ℎ

0.848ℎ2

𝑡90

𝑐 0.848

ℎ

1000 2 60 4 365 . 5

𝑡90 𝑦𝑒𝑎𝑟

𝑐 0.446ℎ2


𝐶 0.


Where

Tv is time factor

t is time elapsed since the start of the consolidation (in min.)

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h is length of the drainage path (in mm)

H is the thickness of the clay sample at time t (in mm)

cv is the coefficient of consolidation (m2/year).

H is the average specimen thickness for the load increments (in mm)

𝐻1+𝐻2

In the standard oedometer consolidation test with double drainage the height H of the

specimen is equal to 2h.

APPARATUS

Testing Specimen

Oven Machine

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Vernier Calipers

Stop Watch, Palette Knife, Filter Paper

PROCEDURES

1. PREPARATION OF THE SAMPLE

a. The consolidation ring and steel plate were weighed separately to an

accuracy of 0.1 g

b. The height of the ring was measured to 0.05 mm at four equally spaced

points using the Vernier Caliper.

The average height was the initial height of the clay sample.

c. The internal diameter of the ring was measured 0.1 mm in two

perpendicular directions using the Vernier Caliper.

The mean diameter and the area were calculated to mm2.

d. Two pieces of filter papers were cut to fit the internal and external

diameter of the cutting ring.

e. Inside of the ring was lubricated with a thin smear of silicone grease or

petroleum jelly.

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f. A small amount of soil was extruded from the compaction mold using the

hydraulic jack.

g. The cutting ring, with the beveled sharp cutting edge downwards, was

pressed into the soil until the upper most rim of the ring was just below

the soil surface.

h. More soil was extruded so that the bottom of the ring was well clear of the

edge of the mould.

i. The excess soil on the top of the ring was trimmed off with the palette

knife.

j. The soil below the base of the consolidation cutting ring was cut off with

the spatula.

k. The steel plate was placed on the top surface and the specimen was gently

slid clear with the assist of a palate.

l. The ring containing soil sample was inverted and the upper surface of the

clay was trimmed off and leveled with the edge of the consolidation

cutting ring with the spatulas.

m. Voids were carefully filled with pieces of clay without compressing the

sample.

n. The steel plate, ring and the sample were weighed together to the nearest

0.1 gram.

2. PREPARATION AND ASSEMBLY OF CONSOLIDATION APPARATUS

a. A wet filter paper was put onto the porous disc at the base of the

consolidation cell.

The ring, containing the sample, was placed on the wet filter paper with

the beveled cutting edge facing upwards.

b. The top of the sample was covered with the second wet filter paper.

The collar of the consolidation cell was secured to the base by retaining

screws.

The consolidation ring and sample were held firmly together.

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c. The top porous stone and loading plate were placed on the top of the filter

paper.

3. ASSEMBLY IN LOAD FRAME

a. The consolidation cell was placed in position on the cell platform of the

oedometer.

b. The loading yoke of the oedometer was connected with the top platen of

the consolidation cell.

The counter balance weight of the beam was adjusted so that the beam

was slightly above the horizontal position.

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c. A very small positive downward load was given to the sample in the

consolidation ring by placing a 100g weight on the top pan of the weight

hanger.(seating load)

d. The beam ratio was set to 9:1.

e. The consolidation cell was filled with water at room temperature.

f. The compression dial gauge was clamped in position. Space was allowed

for swelling as well as compression of the sample. The initial dial gauge

reading was recorded.

g. The beam support jack was screwed up so that the beam is held fixed.

4. TESTING STEPS

A loading sequence is normally adopted in the consolidation test to give a

range of compression stresses suitable for the soil type and also for the

effective pressure which will occur in situ due to the overburden and the

proposed construction. The initial pressure should be large enough to ensure

that the sample in the consolidation cell does not swell.

A loading sequence of stages selected from the following range of pressures is

considered appropriate (see BS 1377, 1990, Part 5, p. 5 section 3.5.1.): 6, 12,

25, 100, 200, 400, 800, 1600, 3200 kPa. But the test is just done by using 100

kPa loading pressures only due to time limitation.

A typical test comprises four to six increments of loading, each held constant

for 24 hours and each applied stress being double that of the previous stage.

Unloading decrements are usually half the number of loading increments. For

the test, it is the fact that one increment is used only due to time limitation.

The stage consolidation test performed was for a stress of 100 kPa.

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a. The value of mass(in kg) needed on the weight hanger pan to produce

100kPa stress on the specimen (σ’vc) was determined as follow:

𝑐

𝑎

𝑎

𝑐

𝑎

Where σ’vc is the vertical stress applied to the specimen (kPa).

m is the mass or equivalent mass, supported by the specimen

(kg).

a is the lever arm ratio (9:1).

A is the area of the specimen in .

b. With the screw jack support in supporting position, the weight hanger was

loaded with the necessary weights. The dual gauge was set to zero and the

seating weight was removed.

c. The stop watch was checked to be working correctly. The started date and

time were recorded and the stop watch was activated. The beam support

jack was lowered at the same time to allow the consolidation to begin.

d. Readings of the compression gauge was taken at the following time

sequence (minutes): 0.25, 0.5, 1, 2, 4, 9, 16, 25, 36 and 49. A final reading

was taken at approximately 24 hours after starting the test.

e. A graph of compression dial gauge readings versus √ 𝑒 was plotted

using the recorded sample compression data.

After 24 hours, when the consolidation will be virtually complete, the

sample was unloaded and the following data was recorded:

Mass of consolidation ring + sample, wet; and

Mass of consolidation ring + sample, dry (dried to constant weight in an

oven at 105℃)

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From this data the final moisture content and void ratio of the sample

were determined.

f. The value of √ 0 was determined from the graph of compression vs.

√ 𝑒 by:

1. The best fitted straight line to the early portion of the curve was drawn

(usually within the first 50% of compression) and extended to intersect

the ordinate of zero time. The corrected zero point d0 was represented

by the point.

2. A straight line through the d0 which all points has abscissae 1.15 times

as great as those on the best fitted line was drawn. The 90%

compression point, d90, was given by the intersection of this line with

the experiment curve. t90 was read off from the experiment curve

according to the d90.

3. The value of the coefficient of volume compressibility, mv (m2/MN),

was determined from the settlement data for the loading.

4. The value of the coefficient of consolidation, cv (m2/yr.), was

determined.

g. The record of data obtained from a full consolidation test with several

stages of loading and unloading. A graph of void ratio versus log10 applied

pressures could be plotted.

For the single stage test, only settlement versus √ 𝑒 was plotted in

order to t90 by Taylor’s curve fitting method. For the determination of t90

and cv for each stage over several stages, separated graph of settlement

versus √ 𝑒 would be plotted.

Figure 3. : Typical Example of the Expression of Square Root Time

Method

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RESULT AND CALCULATION

Dimensions

Initial

Specimen

Overall

Change

Final

Specimen

Specimen

Preparation

Method

Diameter (mm) 75 0 75

Re-moulded Area (mm2) 4417.86 0 4417.86

Height (mm) 19.62 0.98 18.64

Volume (cm3) 86.68 4.33 82.35

Weighting Initial Specimen Final Specimen

Wet soil + ring + tray (g) 532.33 300.91

Dry soil + ring + tray (g) 504.21 275.01

Ring + tray (g) 346.57 117.37

Wet soil (g) 185.76 183.54

Dry Soil (g) 157.64 157.64

Water mass (g) 28.12 25.9

Moisture Content (%) 17.84 16.43

Density 3 2.14 2.23

Dry Density 3 1.82 1.91

Void Ratio 0.48 0.46

Degree of Saturation (%) 100 100

Height of Solids (mm) 19.62 18.64

Soil Description : Compacted Decomposite Granite

Machine No. : 2 Specimen diameter : 75 mm Height : 20 mm

Cell No. : 2 Lever Ratio : 9 : 1 Area : 4418 mm2

Loading

Increment No. / Started 1 14/03/2014 1342

Load (kg) / Pressure (kPa) 5 / 100

Mean Daily Temperature 25℃

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Elapsed Time

(min)

√ Gauge Reading x

0.01

Cumulative

Compression (mm)

0.25 0.5 59.5 0.595

0.5 0.707 62.4 0.624

1 1 66.3 0.663

2 1.414 71.6 0.716

4 2 77.3 0.773

9 3 85.3 0.853

16 4 90.6 0.906

25 5 93.2 0.932

36 6 94.5 0.945

49 7 95.1 0.951

1440 37.947 98.0 0.980

Glossaries:

Negative sign means the soil is in compression.

The blue line is the curve come from experiment

The black solid line is the best fit line.

The red solid line is the line of 1.15 times to best fit line.

From the graph,

t90 is equal to 3.8 of square root of time in minute. Therefore, it takes 14.44 minutes to

attain 90% of ultimate settlement.

S90 is the gauge reading of 90% consolidation, which is equivalent to 0.882 mm.

-120

-100

-80

-60

-40

-20

0

0 5 10 15 20 25 30 35 40

Gauge R

eadin

g x

0.0

1

Time-Consolidation Curve Using Sqaure Root Time Method

Time-…

(3.8,88.2)

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CALCULATION FOR MASS (M) OR EQUIVALENT MASS (IN KG)

SUPPORTED BY THE SPECIMEN

𝑜

𝑎

𝑎

𝑜

𝑎

Where σ 𝑜 is the vertical stress applied to the specimen (kPa)

m is the mass or equivalent mass, supported by the specimen (kg)

a is the lever arm ratio (9:1)

A is the area of the specimen in mm2

m π(

752

)

m 5. Kg

Therefore, 5 kg should be applied on it in order to achieve vertical stress of 100kPa.

EVALUATION FOR COEFFICIENT OF VOLUME COMPRESSIBILITY

The value of coefficient of volume compressibility, mv (in m2/MN) is given by the

formula

(

)(

)

Where H1 is the height of the specimen at the beginning of the test (in mm)

H2 is the height of the specimen at the end of test (in mm)

p1 is the pressure applied to the specimen for the previous loading stage (in kPa)

p2 is the pressure applied to the specimen for the loading stage being considered

(in kPa)

Assume initial apply stress (p1) is 0 kPa and increase stress (p2) is 100kPa with the

equivalent mass of 5 kg

H1 – H2 = 0.98 mm where H1 = 19.62 mm and H2 = 18.64 mm

( .

.62)(

)

.5

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COEFFICIENT OF CONSOLIDATION CV (m2/year)

The coefficient of consolidation, cv , may be determined by finding the time required

for 90% consolidation of the soil sample (U = 0.9)

The test is in double drainage as initial assumption.

The value is calculation by using Terzaghi’s Analysis.

When U = 0.9

Tv = 0.848

Since

𝑇 𝐶

ℎ

𝑐 𝑇 0

0ℎ . 4 ×

ℎ

0

𝑐 . 4 × (

ℎ

)

×6 × 24 × 365.25

0

𝑦𝑒𝑎𝑟

𝑐 .446ℎ

0

𝑦𝑒𝑎𝑟

𝑐 . 2

0

𝑦𝑒𝑎𝑟

Where Tv is the time factor.

t is the time elapsed since the start of the consolidation (in min).

h is length of the drainage path (H is the thickness of the clay sample at time t)

(in mm)

cv is the coefficient of consolidation (m2/year)

is the average specimen thickness for the load increments (in mm)

i.e. 𝐻1+𝐻2

, H1 is initial height, H2 is final height.

In the standard oedometer consolidation test with double drainage the height H of the

specimen is equal to 2h.

t90 = (3.8)2 = 14.44 minutes

+

2

.62 + .64

2 . 3

𝑐 . 2

4.44× . 3 2. 4 𝑦𝑒𝑎𝑟

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DISCUSSION

HISTORY OF OEDOMETER TEST

Consolidation experiments were first carried out in 1910 by Frontard. A thin sample

was cut and placed in a metal container with a porous base. This sample was then

loaded through increments and allowed equilibrium to be achieved after each loading.

To prevent the degree of saturation of clay drops, the experiment was done in a high

humidity indoor.

In 1919, Karl von Terzaghi, father of Soil Mechanics, began his research about

consolidation at Robert College in Istanbul. Through these experiments, Terzaghi

started to derive his theory of consolidation. Consequently his result of research was

published in 1923, which named Terzaghi’s Consolidation Theory. He also develop

modern soil mechanics with his theories of lateral earth pressures, bearing capacity, and

stability.

The Massachusetts Institute of Technology played a key role in early phase of

consolidation development. Terzaghi and Arthur Casagrande, American Civil

Engineer, spent time at M.I.T., Terzaghi from 1925 to 1929 and Casagrande from 1926

to 1932.

Figure 5 & 6. : Karl von Tezaghi (Left), Arthur Casagrande (Right)

At that time, the testing methods and apparatuses for consolidation testing were

improved. Donald Taylor was in-charged for the research involved it at MIT in the

1940s. Also, other experts were participated in it. After a series of testing, the

technique was further enhanced and the method is used until nowadays.

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Figure 7. : Initial Model for Oedometer Test Frame (Bishop)

Figure 8. : Testing Arrangement of Oedometer Test Nowadays

SOURCE OF ERRORS

There is no doubt that error is a part of experiment, no perfect in the world. The possible

errors are listed and described briefly below.

1. Unsaturated Clay - The clay might not be fully saturated because there are bubbles

exist in the voids between the soil particles. That may cause

contradiction from initial assumptions, compressible pore water

and the amendment of applied stress effect.

2. Friction Effect - Friction between the soil and the consolidation ring reduces the mean

stress of the soil because a layer of Vaseline is painted on the inner

surface of the ring.

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3. Initial Compression Error - Compression of apparatus (porous stones, filter paper),

compression of gas bubbles (not instantaneous), expansion

of the ring, compression of water and solids might cause

the real initial compression.

4. Time Scale Error - Since the duration of the consolidation procedures were so long,

disruption of the experimental setup was inevitable. Also, due

to the small scale of the readings, It may cause large distribution

with respect to the original values.

5. Human Error – The soil in assembled manually so that the internal soil void may

much large than theoretical. Also, the soil may not saturate because some water may

evaporated to the surroundings. Moreover, reading from dual gauges may exist difference.

These factors affect the value of settlement.

RECOMMENDATIONS

To optimize the experiment, it is suggested that the following, to accurate the testing

result.

1. Using wide thin specimens and by reducing friction effect between soil and ring.

2. Use a correction factor for soil, attempt to ensure saturated soil.

3. Connect the instruments to computer to obtain the accurate reading (i.e.: Settlement at

different time)

CONCLUSION

From the experiment, the coefficient of volume compressibility is 0.50 m2/MN

and the coefficient of consolidation is 2.84 m2/year. Actually, the test should be done

by adding different value of pressure. Because of allowed time, one value of stress

was done of the oedometer test, only. In reality, it is required to complete with specific

phase with different applied stress.

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REFERENCES

1. Soil Mechanics & Foundations (2010), 3rd Edition, Muni Budhu, John Wiley

2. Soil Mechanics – Concepts and Applications (2004), 2nd Edition, Wilie Powie,

Spon Press

3. Solving Problems in Soil Mechanics (1993), 2nd Edition, B.H.C. Sutton, Longman

Scientific and Technical

4. Explanation of Consolidation and its Calculation,

http://environment.uwe.ac.uk/geocal/SoilMech/consol/soilcons.htm

5. Oedometer Test, http://en.wikipedia.org/wiki/Oedometer_test

http://environment.uwe.ac.uk/geocal/SoilMech/consol/soilcons.htm

http://en.wikipedia.org/wiki/Oedometer_test

ca 3687 oedometer test.pdf

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