Bscs Classification System Chart

A BS cone penetrometer test was carried out on a sample of clay with the following results:

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

The results from the plastic limit test were:

Test no. Mass of tin Mass of Mass of

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

The plot of cone penetration to moisture content is shown in Fig. 1.6. The liquid limit is the moisture content corresponding to 20 mm penetration, i.e. wL = 55%. The plastic limit is determined thus:

The plasticity index is the difference between Wl and wp, i.e. Ip = 55 - 19 = 36%

Moisture content (%)

Moisture content (%)

1.5 Common types of soil

Soils are usually a mixture, e.g. silty clay, sandy silt, etc. Local names are often used for soil types that occur within a particular region, e.g. London clay, etc. Boulder clay is an unstratified and irregular mixture of boulders, cobbles, gravel, sand, silt and clay of glacial origin. In spite of its name boulder clay is not a pure clay. Moraines are gravel and sand deposits of glacial origin. Loam is a soft deposit consisting of a mixture of sand, silt and clay in approximately equal quantities.

'Fill' is soil excavated from a 'borrow' area which is used for filling hollows or for the construction of earthfill structures, such as dams or embankments.

1.6 Soil classification and description

1.6.1 Soil classification systems

Soil classification systems have been in use for a very long time, the first being created some 4000 years ago by a Chinese engineer. 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 form of the system proposed by Casagrande in 1947 is the basis of the soil classification system used in Britain.

The British Soil Classification System (BSCS)

The British Standard BS5930 (1981), Code of practice for site investigations, gives a full description of the BSCS and the reader is advised to obtain sight of a copy.

The system divides soil into two main categories. If at least 35 per cent of a soil can pass through a 63 fim sieve then it is a fine soil. Conversely, if the amount of soil that can pass through the 63 ¡xm sieve is less than 35 per cent then it is a coarse soil. Each category is divided into groups, depending upon the grading of the soil particles not passing the 63 /¿m sieve and upon the plasticity characteristics of the soil particles passing the 425 /¿m sieve.

A summary of the BSCS is shown in Table 1.1 and its associated plasticity chart in Fig. 1.7.

To use the plasticity chart it is necessary to plot a point whose coordinates are the liquid limit and the plasticity index of the soil to be identified. The soil is classified by observing the position of the point relative to the sloping straight line drawn across the diagram.

This line, known as the A-line, is an empirical boundary between inorganic clays, whose points lie above the line, and organic silts and clays whose points lie below. The A-line goes through the base line at IP = 0, WL = 20 per cent so that its equation is:

Table 1.1 British Soil Classification System for Engineering Purposes (after BS 5930: 1981).

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