Interlocking grains contribute a large proportion of the strength in densely packed granular soils and shear failure occurs by overcoming the frictional resistance at the grain contacts .Conversely, interlocking has little or no effect on the strength of very loosely packed coarse grained soils in which the mobility of the grains is greater (see Borowicka ). Figure 2.1shows that dense sand has a high peak strength and that ,when it is subjected to shear stress, it expands up to the point of failure ,after which a slight decrease in volume may occur .Loose sand ,on the other hand ,compacts under shearing stress and its residual strength may be similar to that of dense sand ,and tends to remain so .hence a constant void ratio is obtained ,that is ,the critical volume condition which has a critical angle of friction and a critical void ratio .These are independent of initial density being a function of the normal effective stress at which shearing occurs .for a discussion of the shear strength of sands (see Rowe and Frossard) .