CONSISTENCY LIMITS OF SOILS.
( IS : 2720 – PART – 5 )
INTRODUCTION:
The physical properties of fine-grained soils, especially of clay differ much at different water contents. Clay may be almost in liquid state, or it may show plastic behavior or may be very stiff depending on the moisture content. Plasticity is a property of outstanding importance for clayey soils, which may be explained as the ability to undergo changes in shape without rupture.
Liquid limit: defined as the minimum water content at which the soil will flow under the application of a very small shearing force.
Plastic limit: defined as the minimum moisture content at which the soil remains in a plastic state.
Plasticity Index (PI): is defined as the numerical difference between the liquid limit and plastic limits. PI thus indicates the range of moisture content over which the soil is in a plastic condition.
Consistency limits and the plasticity index vary for different soil types. Hence these properties are generally used in the identification and classification of soils.
Liquid limit test:(Mechanical liquid limit device)
Object:
Determination of the liquid limit of soil by mechanical liquid limit device.
Apparatus:
i) Mechanical liquid limit device consists of a cup and arrangement for raising and dropping through a specified height and standard grooving tools.
ii) Balance of 200 g capacity and sensitive to 0.01 g.
iii) Oven to maintain 1050 to 1100C.
Procedure:
About 120 g of dry pulverized soil sample passing 425 micron IS sieve is weighed, and mixed thoroughly with distilled water in the evaporating dish to form a uniform thick paste. The liquid limit device is adjusted to have a free fall of cup through 10 mm. A portion of the paste is placed in the cup above the lowest spot, and squeezed down with the spatula to have a horizontal surface. The specimen is trimmed by firm strokes of spatula in such a way that the maximum depth of soil sample in the cup is 10 mm. The soil in the cup is divided along the diameter through the center line of the cam followed by firm strokes of the grooving tool so as to get a clean sharp groove. Grooving tool (b) may be used for all soils, where as grooving tool (a) may be used only in clayey soils free from sand particles or fibrous materials.
The crank is rotated at the rate of two revolutions per second (either by hand or electrically operation) so that the cup is lifted and dropped. This is continued till the two halves of the soil cake come into contact at the bottom of the groove along a distance of about 10 mm, and the number of blows given is recorded. A representative soil is taken, placed in the moisture container, lid placed over it and weighed. The container is dried in oven and the dry weight determined the next day for finding the moisture content of the soil. The operations are repeated for at least three more trials with slightly increased moisture contents each time, noting the number of blows so that there are at least four uniformly distributed readings of number of blows between 10 and 40 blows.
Calculations:
Taking the number of blows in the log scale on the X-axis, and the water content in arithmetic scale on the Y-axis plots the flow curve. The flow curve is straight line drawn on this semi-logarithmic plot, as nearly as possible through three or more plotted points. The moisture content corresponding to 25 blows is read from this curve, rounding off to the nearest whole number and is reported as the liquid limit (LL or wl) of the soil.
The slope of the straight-line flow curve is the flow index. It may be calculated from the following formula:
w1 – w2 w10 – w100 Where, w10 = water content at 10 blows.
log n2 – log n1 log 100 – log 10
Liquid limit test: (Cone penetration)
Object:
To determine the liquid limit of the soil by cone penetration apparatus.
Apparatus:
i) Cone penetration apparatus confirming to IS: 11196-1985 (cone angle 300+/-0.50) and weight of assemble is 80 +/- 0.5 g including all).
ii) Balance of 200 g capacity and sensitive to 0.01 g.
iii) Oven to maintain 1050 to 1100C.
Procedure:
About 150 g of dry pulverized soil sample passing 425 micron IS sieve is weighed, and mixed thoroughly with distilled water in the evaporating dish to form a uniform thick paste. The soil paste shall then be transferred to the cylindrical mould of the cone penetrometer apparatus and leveled up to the top of the cup. The penetrometer shall be so adjusted that the cone point just touches the surface of the soil paste in the trough. The scale of the penetrometer shall then be adjusted to zero and the vertical rod released so that the cone is allowed to penetrate into the soil paste under its weight. The weight should be 80 +/- 0.5 g and the penetration shall be noted after 5 seconds from the release of the cone.
If the difference in penetration lies between 14 and 28 mm the test is repeated with suitable adjustments to moisture either by addition of more water or exposure of the spread paste on a glass plate for reduction in moisture content. The test shall be repeated at least to have four sets of values of penetration in the range 14 and 28 mm. The exact moisture content of each trial shall be determined.
Calculations:
A graph representing water content on the Y-axis and the cone penetration value on the X-axis shall be prepared. The best fitting straight line is then drawn. The moisture content corresponding to cone penetration of 20 mm shall be taken as the liquid limit of the soil and shall be expressed to the nearest first decimal place.
Liquid Limit Graph (Mechanical Device Method):
Liquid Limit Graph (Cone Penetration Method):
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