Smith's Elements of Soil Mechanics. Ian Smith

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СКАЧАТЬ valid, else a third penetration test is done. The average penetration is noted, and a water content determination is carried out on the soil.The procedure is repeated at least four times with increasing water contents. The amount of water used throughout should be such that the penetrations obtained lie within a range of 15–25 mm.Fig. 1.6 Fall cone apparatus.To obtain the liquid limit, the variation of cone penetration is plotted against water content and the best straight line is drawn through the experimental points. The liquid limit is taken to be the water content corresponding to a cone penetration of 20 mm, expressed as a whole number.

      2 Casagrande apparatusAlthough still used worldwide, the Casagrande test has now been largely superseded by the fall cone method because the latter achieves more repeatable results. In the UK, the fall cone test is adopted as the first‐choice test method.

       Plastic limit test

      About 20 g of soil prepared as in the liquid limit test is used. The soil is mixed on the glass plate with just enough water to make it sufficiently plastic for rolling into a ball, which is then rolled out between the hand and the glass to form a thread. The soil is judged to be at its plastic limit when it just begins to crack at a thread diameter of 3 mm. At this stage, several pieces of the thread are taken for water content determination. The test is repeated until two water content tins contain an adequate mass of soil threads – approximately 10–15 g in each tin.

It is interesting to note that in some countries, the fall cone penetrometer is used to determine both w_L and w_P. The apparatus uses a 30° angle cone of mass 76 g. The procedure is the same as the fall cone described above, only this time w_L is taken at a penetration of 17 mm and w_P is taken as the water content at a penetration of 2 mm.

Example 1.4 Consistency limits tests

A cone penetrometer test was carried out on a sample of the clay from Example 1.3, with the following results:

Cone penetration (mm)	16.1	17.6	19.3	21.3	22.6
Water content (%)	50.0	52.1	54.1	57.0	58.2

      The results from the plastic limit test were:

Test no.	Mass of tin (g)	Mass of wet soil + tin (g)	Mass of dry soil + tin (g)
1	8.1	20.7	18.7
2	8.4	19.6	17.8

      Determine the liquid limit, plastic limit and the plasticity index of the soil.

       Solution:

The plot of cone penetration to water content is shown in Fig. 1.7. The liquid limit is the water content corresponding to 20 mm penetration, i.e. w_L = 55%.

Fig. 1.7 Example 1.4.

      The plastic limit is determined thus:

      The plasticity index is the difference between w_L and w_P i.e.

      1.5.6 Activity of a clay

      In addition to their use in soil classification, the w_L and w_P values of a plastic soil also give an indication of the types and amount of the clay minerals present in the soil.

      It has been found that, for a given soil, the plasticity index increases in proportion to the percentage of clay particles in the soil. Indeed, if a group of soils is examined and their I_P values are plotted against their clay percentages, a straight line, passing through the origin, is obtained.

      If a soil sample is taken and its clay percentage artificially varied, a relationship between I_P and clay percentage can be obtained. Each soil will have its own straight line because, although in two differing soils the percentages of clay may be the same, they will contain different minerals.

The relationship between montmorillonite, illite, kaolinite and the plasticity index is shown in Fig. 1.8. The plot of London clay is also shown on the figure and, from its position, it is seen that the mineral content of this soil is predominantly illite. London Clay has a clay fraction of about 46% and consists of illite (70%), kaolinite (20%) and montmorillonite (10%). The remaining fraction of 54% consists of silt (quartz, feldspar and mica: 44%) and sand (quartz and feldspar: 10%). In Fig. 1.8, the slope of the line is the ratio:

Fig. 1.8 Relationship between I_P and clay percentage.

      Modified from Skempton (1953).

      Skempton (1953) defined this ratio as the activity of the clay. Clays with large activities are called active clays and exhibit plastic properties over a wide range of water content values.

1.6 Soil classification and description

      1.6.1 Soil classification

      Soil classification systems have been in use for a very long time with the first recorded use being in China over 4000 years ago. In 1896 a soil classification system was proposed by the Bureau of Soils, United States Department of Agriculture in which the various soil types were classified purely on particle size and it is interesting to note that the limiting sizes used are more or less the same as those in use today. Further improved systems allowed for the plasticity characteristics of soil, and a modified СКАЧАТЬ