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Thermohydromechanical behaviour of soils and soil structure interfaces 251

Figure 5.37 Modes of soil structure interface failure. Redrawn after Tsubakihara, Y., Kishida, H.,
Nishiyama, T., 1993. Friction between cohesive soils and steel. Soils Found. 33 (2), 145 156.

are performed under CNS conditions, the volumetric dilation of the interface induces
an increase in the normal effective stress and an increase in the available shear strength
(cf. Fig. 5.36A). Simultaneously, the normal stiffness acts as a partial restraint for the
free dilation of the interface and the samples tested under CNS conditions consistently
dilate less than the corresponding samples under CNL conditions (cf. Fig. 5.36B).

5.7.5 Shearing and sliding of soil structure interfaces

Different deformation mechanisms occur at soil structure interfaces: shearing of the
soil and sliding between the soil particles and the interface (involving a relative dis-
placement between the two materials). Depending on the structure roughness, the
two previous mechanisms can contribute in different ways to the formation of three
failure modes of soil structure interfaces (cf. Fig. 5.37): in mode 1, that is when the
interface surface is rough, shear failure occurs in the soil; in mode 2, that is when the
interface surface is smooth, full sliding occurs at the interface; in mode 3, shear failure
and sliding displacement occur simultaneously.
Results corroborating the previous failure modes have been presented, for example
by Uesugi et al. (1988), Tsubakihara et al. (1993) and De Jong et al. (2003). In the
case of smooth interfaces, during the first phase of shearing a reorientation of the soil
particles takes place so that they tend to turn parallel to the interface. In this phase, the
sliding displacement is small and the soil shearing governs the interface deformation.
Then, after a transitional phase and when all the particles are oriented parallel to the
interface, sliding occurs with limited soil deformation (constant volume conditions).