Soil Behavior: Dynamic Soil-Structure Interactions - Dynamic Behavior of Concrete and Seismic Engineering - page 128

4.1.2. Experimental description of soil behavior

Both the dynamic response calculations of a soil profile and soil-structure
interaction problems usually consider seismic motion are caused by a shear wave
propagating vertically from the substratum. Under such conditions, a soil element
taken from the soil layer is subjected to the stress cycles represented in Figure 4.1.

%u

T ' 0 T ' 0 T ' 0

%h

K 0 T ' 0

K 0 T ' 0 K 0 T ' 0

Figure 4.1. Idealized loading sequence

Initially, in the case of a horizontal soil profile, the element is balanced under the
real vertical stress V' v and the real horizontal stress K 0 V' v , where K 0 is the thrust
coefficient of the ground at rest. The passing shear wave is discerned by the
application of a shear stress W(t) on the horizontal sides, and then on the vertical
sides of the soil element (to preserve equilibrium conditions). Under the effect of the
stress, the sample is subjected to a simple shear strain, which, for an elastic behavior
material, involves a volume variation equal to zero. The shear strain, also called
distortion, is described in Figure 4.2.

'
'

u

Ȗ [4.1]

h

If in a laboratory we reproduce symmetric and constant amplitude strain cycles
similar to those in Figure 4.1, we obtain the curves shown in Figure 4.2. The latter
shows that in the plane (W, J), soil behavior is characterized by a hysteresis loop, the
area and inclination of which is determined by the amplitude of the strain. The wider
the latter, the more important loop area is, and the less horizontally inclined it
becomes. In addition, it shows that experimentally, the shape of the hysteresis loop
is unaffected by the loading rate.