With centre C and radius CD draw the circle establishing the points A and B on the x-axis.

By scaling, OD = resultant stress = 10.6kN/m2. With protractor, 4> = 19°; 6 = 55°.

Note From the diagram we see that

It is possible to make a vertical cut in silts and clays and for this cut to remain standing, unsupported, for some time. This cannot be done with a dry sand which, on removal of the cutting implement, will slump until its slope is equal to an angle known as the angle of repose. In silts and clays, therefore, some other factor must contribute to shear strength. This factor is called cohesion and results from the mutual attraction existing between fine particles that tends to hold them together in a solid mass without the application of external forces. In terms of the Mohr diagram this means that the strength envelope for the soil, for undrained conditions, no longer goes through the origin but intercepts the shear stress axis (see Fig. 3.9). The value of the intercept, to the same scale as <7n, gives a measure of the unit cohesion available and is given the symbols c or cu.

3.5 Coulomb's law of soil shear strength

It can be seen that the shear resistance offered by a particular soil is made of the two components of friction and cohesion. Frictional resistance does not have a constant value but varies with the value of normal stress acting on the shear plane whereas cohesive resistance has a constant value which is independent of the value of <rn. In 1776 Coulomb suggested that the equation of the strength envelope of a soil could be expressed by straight line equation:

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