1. objective
The expansive clays increase in their volume when they come in contact with water owing to surface
properties of these clay types. The pressure which the expansive soil exerts
, if it is not allowed to swell or the volume change of the soil is arrested
, is known as Swelling Pressure of Soil. The swelling pressure is dependent
upon several factors namely (a) the type and amount of clay in the soil and
the nature of the clay mineral, (b) the initial water content and dry
density, (c) the nature of pore fluid, (d) the stress history of the soil
including the confining pressure and (e) drying and wetting cycles to which
the soils have been subjected to.
A soil with high swell pressure is considered “unsuitable” for use as embankment
fill material and in case the sub-soil is having high swell pressure then suitable
“ground improvement measures” may be needed before constructing embankment on such
soil. One of the methods for constructing embankments on expansive soils
is to lay a CNS (non-swelling clay) layer at the base of the embankment,
to isolate the sub-soil from the moisture (mainly rain water) coming in
vertically downwards direction. For any clay to be classified as “CNS
(non-swelling clay)”, it should not have swell pressure more than the
specified value (normally 0.5 kg/m2).
In laboratory, one dimensional swelling pressure test is conducted using
either fixed or the floating rings on both undisturbed and re-moulded soils
in the partially saturated condition. Two methods are employed, namely,
“consolidometer method” in which the volume change of the soil is permitted
and the corresponding pressure required to bring back the soil to its
original volume is measured and “constant volume method” in which the
volume change is prevented and the consequent pressure is measured.
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Fig. 1: Swell Pressure Test by Consolidometer
A device to hold the sample in a ring either fixed or floating with porous stones on each face
of the sample. A consolidometer shall also provide means for submerging the sample,
for applying a vertical load and for measuring the change in the thickness of the
specimen. The provision for fixing of the dial gauge shall be rigid and in no case
shall the dial gauge be fixed to a cantilevered arm. The dial gauges shall be fixed
in such a way to record the vertical expansion of the specimen.
(a) Specimen Diameter: 60mm.
(b) Specimen Thickness: Shall be at least 20mm in all cases. However, the thickness
shall not be less than 10 times the maximum diameter of the
grain in the soil specimen. The diameter to thickness ratio
shall be a minimum of 3.
(c) Ring: The ring shall be made of non-corrosive material.
(d) Porous Stones: The stones shall be of silicon carbide or aluminium oxide
and of medium grade. It shall have a high permeability compared
to that of the soil being tested. The diameter of the top stone
shall be 0.2 to 0.5mm less than the internal diameter of the
ring.
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Fig. 2: Swell Pressure Test by Constant Volume Method
The arrangement for Swell Pressure Test by Constant Volume Method consists of Loading unit
(5 tonnes), Proving ring (with high sensitivity, 200 kg capacity) along with all Apparatus as
used in Consolidometer method.
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3. reference
IS 2720(Part 41):1977 Methods of test for soils:
Measurement of swelling pressure of soils. Reaffirmed- December 2021.
4. procedure
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4.1 By Consolidometer Method:
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Preparation of Specimen from Undisturbed Soil Samples: Clean
and weigh the empty container. Cut-off the specimen either from
undisturbed tube sample or from block sample, the latter generally
being more representative of the field conditions. Remove about 30mm
height of soil from one end of the soil sample. Gradually insert the
consolidation ring in the sample by pressing with hands and carefully
removing the material around the ring. The soil specimen so cut shall
project as far as 10mm on either side of the ring. Then trim the specimen
smooth and flush with the top and bottom of the ring. Wipe the container
ring clear of any soil sticking to the outside and weigh again with the
soil. The whole process should be quick to ensure minimum loss of moisture
and if possible shall be carried out in the moisture room. The
representative specimens from the soil trimming shall be taken in moisture
content cans and their moisture content determined in accordance with
IS:2720 (Part-XI).
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Preparation of Specimen from Disturbed Soil Sample: The soil
sample shall be compacted to the desired (field) density and water content
in a standard compaction proctor mould. Samples of suitable sizes are cut
from it as given in Para (i) above.
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NOTE-1: Since swelling pressure of the soil is very much influenced by its
initial water content and dry density, it shall be ensured that in the case
of undisturbed soil samples, the specimen shall be collected from the field
for test during the driest season of the year, so that the swelling pressure
recorded shall be maximum. In case of remoulded soil samples, the initial
water content shall be at the shrinkage limit or field water content, so that
the swelling pressure recorded shall be maximum.
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NOTE-2: The desiccated soil obtained from the field coupled with smaller
thickness of consolidation ring make the undisturbed soil specimen always
in danger of being disturbed during trimming; hence great care shall be taken
to handle the specimen delicately with the least pressure applied to the soil.
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Saturate the porous stones by boiling in distilled water for at least
15 minutes and moisten all surfaces of the consolidometer which are to be
enclosed. Assemble the consolidometer with the soil specimen (in the ring)
and porous stones at top and bottom of the specimen, providing a wet filter
paper between the soil specimen and the porous stone. Then position the
loading block centrally on the top porous stone.
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Mount this assembly on the loading frame such that load applied
is transmitted to the soil specimen through the loading cap.
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In the case of the lever loading system, the apparatus shall be properly
counter-balanced. If a jack with load measurements by platform scales is used
as the loading systems the tare weight with the empty consolidation apparatus,
excluding those parts which will be on top of the soil specimen, which rest
on the platform shall be determined before filling the ring with the soil
and this tare weight shall be added to the computed scale loads required to
give the desired pressures at the time of loading the soil specimen.
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Screw the holder with the dial gauge to record the progressive
vertical heave of the specimen under no load
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Place initial setting load of 50 gf/cm2 (this includes weight of
the porous stone and the loading pad) on the loading hanger and note
initial reading of the dial gauge.
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Connect the system to a water reservoir with the level of water
in the reservoir being at about the same level as the soil specimen and
allow water to flow in the sample. Then allow the soil to swell.
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Record the free swell readings shown by the dial gauge under
the seating load at different time intervals. The dial gauge readings
shall be taken till equilibrium is reached. This is ensured by making
a plot of swelling dial reading versus time in hours, which plot becomes
asymptotic with abscissa (time scale). The equilibrium swelling is normally
reached over a period of 6 to 7 days in general for all expansive soils.
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Then subject the swollen sample to consolidation under different
pressures. Record the compression dial readings till the dial
readings attain a steady state for each load applied over the specimen.
The consolidation loads shall be applied till the specimen attains its
original volume.
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4.2 By Constant Volume Method:
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Preparation of Specimen from Undisturbed Soil Samples as well as Preparation of
Specimen from Disturbed Soil Sample: Same as for “Consolidometer Method”.
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Keep the consolidation specimen ring with the specimen between two porous
stones saturated in boiling water providing a filter paper between the soil specimen
and the porous stone. The loading block shall then be positioned centrally on the top
of the porous stone.
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Then place this assembly on the platen of the loading unit. The load measuring proving ring tip
attached to the load frame shall be placed in contact with the consolidation cell
without any eccentricity. A direct strain measuring dial gauge shall be fitted to the
cell. Inundate the specimen with distilled water and allow it to swell.
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Note down the initial reading of the proving ring. The swelling of the specimen with
increasing volume shall be obtained in the strain measuring load gauge. To keep the
specimen at constant volume, the platen shall be so adjusted that the dial gauge always
show the original reading. This adjustment shall be done at every 1
mm of swell or earlier. The duration of test shall conform to the
requirements given in “Consolidometer method”. Then dismantle the
assembly and extract the soil specimen from the consolidation ring
to determine final moisture content in accordance with IS: 2720
(Part-IX).
5. observation and recording
Table 1 : Details of soil specimen
(By Consolidometer Method/ Constant Volume Method)
Description |
Test-1 |
Test-2 |
Description |
Test-1 |
Test-2 |
Weight of container ring+ wet specimen |
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Weight of container + wet soil |
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Weight of container |
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Weight of container + dry soil |
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Diameter of container |
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Weight of container |
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Initial thickness of soil sample |
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Weight of water |
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Wet density in g/ml |
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Weight of dry soil |
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Dry density in g/ml |
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Moisture content in % |
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Table 2 : Data Sheet for Swell - Compression Test (By Consolidometer Method)
Elapsed Time in Hours |
Swelling dial reading |
Elapsed Time in Hours |
Swelling dial reading |
0 |
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24 |
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0.5 |
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36 |
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1 |
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48 |
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2 |
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60 |
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4 |
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72 |
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8 |
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96 |
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12 |
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120 |
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Table 3 : Data Sheet for Swell - Compression Test (By Consolidometer Method)
Pressure Increment (kgf/cm2) |
Pressure Increment (kN/m2) |
Compression |
Change in thickness of Expanded Specimen |
0.0-0.05 |
0-5 |
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0.05-0.10 |
5-10 |
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0.10-0.25 |
10-25 |
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0.25-0.50 |
25-50 |
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0.50-1.00 |
50-100 |
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1.00-2.00 |
100-200 |
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2.00-4.00 |
200-400 |
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4.00-8.00 |
400-800 |
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8.00-16.00 |
800-1600 |
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6. Calculations and Report
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6.1 By Consolidometer Method
The observed swelling dial reading recorded in Table 1 shall be plotted
with elapsed time as abscissa and swelling dial reading as ordinates on
natural scale. A smooth curve shall be drawn joining these points. If the
curve so drawn becomes asymptotic with the abscissa, the swelling has reached
its maximum and swelling phase shall be stopped, and the consolidation phase
shall be started. The compression readings shall be tabulated as in Table 3 and
a plot of change in thickness of expanded specimen as ordinates and consolidation
pressure applied as abscissa in semi-logarithmic scale shall be made. The swelling
pressure exerted by the soil specimen under zero swelling condition shall be
obtained by interpolation and expressed in kN/m2 (kgf/cm2).
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6.1 By Constant Volume Method
The difference between the final and initial dial readings of the proving ring gives
total load in terms of division which when multiplied by the calibration factor gives
the total load. This when divided by the cross-sectional area of the soil specimen
gives the swell pressure expressed in kN/m2 (kgf/cm2).
Table 4 : Swell Prssure Data (By Constant Volume Method)
Date |
Time |
Strain Dial Gauge Reading Before Adjustment |
Proving Ring Reading |
Differences |
Load in kg |
Swell Pressure in kg/cm2 |
Remarks |
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