Blocky Material Research Articles

Load distribution and redistribution in discontinua

Deformation in a blocky (jointed) material is, in many cases, dependent on the mechanisms of slip and rotation. Once slip occurs it is shown using photoelastic blocky models, that the load redistributes in what appears to be a regular and repetitive pattern. The contact area between particles, be they rotund or blocky, appears to determine whether moment transfer has a marked effect on the load transmitting characteristics between the particles. This is the basic difference between the load transmitting characteristics of a rotund granular material (sand) and a blocky material (rock). Moment transfer between blocks readily induces stress gradients within the intact material making up the blocks with a consequent development of tensile stresses. This readily creates an environment where the appropriate blocks may rupture. Whether this rupture, which causes a redistribution of interblock loads, will continue appears to be a function of the energetics related to the joint sets and failure characteristics of the intact material. Besides particle fracture there is the problem of overall stability of the deforming discontinuous body. That is, a stage is reached where, after a number of slips and particle failures, the blocky mass as a whole will collapse. This aspect is considered in terms of a discontinuous structure such as a voussoir arch.

Deformational Response in discontinua

Deformation in a discontinuum, be it rotund or blocky, is related not only to material deformation but also mechanisms such as slip and rotation. Because of this, deformation is inextricably related to the frictional characteristics of the surfaces of slip. In turn, this slip has a profound influence on the load transmitting characteristics of the slip surfaces. These load transmitting characteristics effect the stress distribution within the intact material and therefore control the subsequent deformational response and modes of failure. In the first part of this paper, the minimum energy concepts, as applied to a granular material, are shown to be applicable to a blocky material representing a rock. From this it is shown, as for granular material, that there is a relationship between the principal stresses given in terms of the joint friction angle. The magnitude of this angle is however not only dependent on the smooth joint friction angle and roughness but also on the rotation of the blocks brought about by slip. This rotational effect on load transmission in a granular material (sand) is minor while in a blocky material (rock) the effect on load transmission is most important. It is shown that in a jointed material it is not possible to define conclusively the mass deformational response when inhomogeneous stresses are induced by joint slip and or block rotation. Therefore the use of any continuum analysis using conventionally determined design parameters is suspect.