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A long-standing problem in rock mechanics is the estimation of rock mass strength. The term “rock mass” denotes a large volume of fractured rock in which yield of intact material and discontinuities (joints) must both occur for overall failure to take place. The difficulty in characterising a rock mass arises from the impossibility of testing directly (to failure) a large extent of rock. Because the proportion and configuration of the discontinuities (relative to the proportion of intact rock) determine the strength of the composite material, there is a pronounced size effect, such that large volumes appear weaker than small volumes. It is therefore important to consider the size effect in the design of large structures in rock.
Recently, a numerical approach, called synthetic rock mass (SRM), has been developed and applied in several projects. The SRM is a bonded-particle assembly representing brittle rock that contains multiple joints, each consisting of a planar array of bonds that obey a special model, the smooth joint model (SJM). The SJM allows slip and cracking at particle contacts, while respecting the given joint orientation rather than local contact orientations. Overall failure of an SRM element depends on both fracture of intact material (bond breaks) and yield of joint segments.
Results are presented from a series of numerical experiments on 3D elements of various sizes. For the first time, we are able to quantify the variation of rock mass strength as a function of size. The results are discussed in relation to empirical methods commonly used in design.
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