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.
2. Apparatus Required
Fig. 1: Swell Pressure Test by Consolidometer
(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 aluminum 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.
Fig. 2: Swell Pressure Test by Constant
Volume Method
3. Reference
IS 2720(Part 41):1977 Methods of test for soils: Measurement of swelling pressure
of soils. Reaffirmed- December 2016.
4. Procedure
4.1 By Consolidometer Method:
i. Preparation of Specimen from Undisturbed Soil Samples: Clean and weigh the empty contain Cut-off the specimen either from undisturbed tube sample or from block sample, the latte
generally being more representative of the field conditions. Remove about 30mm height of so
from one end of the soil sample. Gradually insert the consolidation ring in the sample b
pressing with hands and carefully removing the material around the ring. The soil specimen s
cut shall project as far as 10mm on either side of the ring. Then trim the specimen smooth an
flush with the top and bottom of the ring. Wipe the container ring clear of any soil sticking t
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. Th
representative specimens from the soil trimming shall be taken in moisture content cans an
their moisture content determined in accordance with IS:2720 (Part-XI).
ii. 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
suitable sizes are cut from it as given in Para (i) above.
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, s
that the swelling pressure recorded shall be maximum. In case of remoulded soil sample
the initial water content shall be at the shrinkage limit or field water content, so that the swelling pressure recorded shall be maximum.
NOTE-2: The desiccated soil obtained from the field coupled with smaller thickness
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.
iii. Saturate the porous stones by boiling in distilled water for at least 15 minutes and moisten a
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
wet filter paper between the soil specimen and the porous stone. Then position the loading block centrally on the top porous stone.
iv. Mount this assembly on the loading frame such that load applied is transmitted to the so
specimen through the loading cap.
v. In the case of the lever loading system, the apparatus shall be properly counter-balanced. If
jack with load measurements by platform scales is used as the loading systems the tare weigh
with the empty consolidation apparatus, excluding those parts which will be on top of the so
specimen, which rest on the platform shall be determined before filling the ring with the soil an
this tare weight shall be added to the computed scale loads required to give the desire
pressures at the time of loading the soil specimen.
vi. Screw the holder with the dial gauge to record the progressive vertical heave of the specimen under no load
vii. 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. 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.
viii. 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 t
swell.
ix. 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 b
making a plot of swelling dial reading versus time in hours, which plot becomes asymptotic wit
abscissa (time scale). The equilibrium swelling is normally reached over a period of 6 to 7 day
in general for all expansive soils.
x. 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 origin
volume.
4.2 By Constant Volume Method:
i. Preparation of Specimen from Undisturbed Soil Samples as well as Preparation of Specimen from Disturbed Soil Sample: Same as for “Consolidometer Method”.
ii. Keep the consolidation specimen ring with the specimen between two porous stones saturate
in boiling water providing a filter paper between the soil specimen and the porous stone. Th
loading block shall then be positioned centrally on the top of the porous stone.
iii. Then place this assembly on the platen of the loading unit. The load measuring proving ring to attached to the load frame shall be placed in contact with the consolidation cell without an
eccentricity. A direct strain measuring dial gauge shall be fitted to the cell. Inundate the specimen with distilled water and allow it to swell.
iv. 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 assemble and extract the soil specimen from the consolidation ring to determine final moisture content
accordance with IS: 2720 (Part-IX).
5. Observation And Recording
Table 1 : Details of soil specimen (By Consolidometer Method/ Constant Volume Method)
Table 2 : Data Sheet for Swell - Compression Test (By Consolidometer Method)
Table 3 : Data Sheet for Swell - Compression Test (By Consolidometer Method)
6. Calculations And Report
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).
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 Pressure Data (By Constant Volume Method)
7. When to do swelling pressure:-
Before conducting swelling pressure we conduct free swell index and if the swell index is coming more than 50 % than swelling pressure test of soil is conducted to know the actual pressure of soil.
This actually gives idea about its expansive nature of soil and we need to do remedial measures for its to overcome the expansiveness of soil to make it stabilized.
8. Articles and Video References:-
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