The load that can be supported by a foundation or a machine 
depends primarily on the resistance of the soil to shearing deformation. 
This shear resistance can be measured by various apparatus; the most com- 
mon method is by compression tests in which an axial load is applied to the 
specimen and increased until failure occurs. Failure takes place by shear on 
one or more inclined planes, and it is possible to compute the normal pressure 
and the shearing stress on such a plane at the instant of failure (Bishop and 
Henkel, 1957). 
A series of shear tests on identical samples of soil, each subjected to 
a different normal pressure, can be plotted in a diagram showing the normal 
stress, p, aS a function of the shearing stress, s, at the instant of failure. The 
results for most soils approach a straight line as shown in Figure 29. The 
equation of the line shown in Figure 29 is: 
s = c + ptand (8) 
The value c, called the cohesion, is equal to the shearing resistance on the 
failure plane where the normal pressure on that plane is zero. The angle 
is Known as the angle of internal friction. 
Experience has shown that natural masses of soft clay, if loaded 
during testing so rapidly that little drainage can occur, usually behave as if 
o = O, in which case the above equation becomes 
S$ = 6 (9) 
Under such conditions, shear strength is a function of the cohesion only. Soft 
clays on the ocean bottom would behave in this manner (Peck, Hanson, and 
Thornburn, 1953). 
Stresses acting within a submerged mass of soil that are transmitted 
from grain to grain of the solid constituents are called intergranular pressures. 
Those that act within the water that fills the voids in the soil are called pore- 
water pressures. Only the intergranular pressures can produce frictional 
resistance in the soil. It can be shown that the intergranular pressure, p, 
on any horizontal plane in the submerged soil mass is equal to the depth, 
z, of the plane below the soil surface multiplied by the difference in unit 
weight of the saturated soil, V,,,, and the unit weight of water, V, (Peck, 
Hanson, and Thornburn, 1953). 
p= 2. = WV (10) 
an) 
So 
