Method:
Mix 70gms of the sample with drinkable water to the consistency of a thick paste. Smooth
the sample into the curved dish to a maximum depth of 10mm (Fig 4.8). Separate the
mixture into two equal portions by drawings the grooving tool across the diameter of the
dish. This should leave a V-shaped groove, with a gap at the bottom 2mm wide, and slopes
to the groove at an angle of 600 from the horizontal (Fig 4.9). If you don't have a suitable
grooving tool, carefully use a knife blade to make as accurate a groove as possible.
Hold the dish firmly in one hand with the groove in the soil pointing away from you. Then
tap the dish slightly with a horizontal motion against the heel of the other hand 10 times.
The dish should be moved a distance of about 30-40mm to strike against the other hand,
which should remain still (fig 4.10). If it takes more than 10 taps to make the groove flow
together, the soil is too dry: add a little water. If it flows together with less than 1 0 taps, it
is too wet: add more dry soil. When the moisture content of the soil has been adjusted so
that the two portions of soil come into contact at the bottom of the groove along a
continuous distance of 13mm after 10 taps (Fig 4.11), the soil is at the liquid limit.
Once you think you have the mixture about right, grease the inside of your mould. Silicon
grease is best, but a small amount of cooking oil or old engine oil will do. Now press the soil
mixture into the mould box making sure there is no air left in the mixture and that the mould
is completely full.
The mould is then left for the soil to dry. If drying is in the shade, this can take about 10
days. Drying in the sun will take about 5 days. It is a good idea to put two moulds containing
the same soil, one in the sun and one in the shade to compare results.
HOW TO UNDERSTAND THE RESULTS: If the soil shrinks a lot and "hogs" up (see photo on
following page), then there is a lot of clay in it. A soil that shrinks less and cracks across the
width of the mould (photo) is high in sand and low in clay and silt.
If necessary, push the portions of soil in the mould back together and measure their total
length. Subtract this length from the length of the mould to get the shrinkage.
If OPC is used as the stabilise, then the more clay that there is in the soil the more OPC will
be needed. The table below shows how to know the amount of OPC needed from the shrinkage
tests.
Measured shrinkage
Under 15mm
15 to 30mm
30 to 45mm
45 to 60mm
Cement to soil ratio
1 part cement to 18 parts soil (5.56%)
1 part cement to 16 parts soil (6.25%)
1 part cement to 14 parts soil (7.14%)
1 part cement to 12 parts soil (8.33%)
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