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DISCUSSIONS AND CLOSURES

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Discussion of “Bearing Capacity of Shallow Foundations in Anisotropic Non-Hoek-Brown Rock Masses” by Mahendra Singh and K. Seshagiri Rao August 2005, Vol. 131, No. 8, pp. 1014–1023.

DOI: 10.1061/ ASCE 1090-0241 2005 131:8 1014

Brian C. Burman, M.ASCE1
1

Principal, Burman Consulting Pty. Ltd., 21 Telegraph Road, Pymble, Sydney NSW 2073, Australia. E-mail: brian.burman@optusnet. com.au

The discusser, having a long and abiding interest in the engineering of jointed rock masses and a particular interest in shallow foundations on rock, welcomes the paper by Singh and Rao for its explicit recognition of rock mass as a discontinuum that requires treatment as such. There are many appealing aspects of the authors’ bearing capacity analysis compared to more conventional approaches. However, the discusser is not able to accept the fourfold failure mode hypothesis, which is fundamental to their concept of bearing capacity for jointed masses. The authors have described failure modes associated with splitting, shearing, sliding, and rotation based on the results of Singh’s testing of a jointed block mass in uniaxial compression and published literature. The discusser does not have access to Singh’s data Singh 1997 but is familiar with Brown’s triaxial tests on block jointed models Brown 1970 and accepts the four failure modes under those test conditions. These failure modes would have wide acceptance throughout rock mechanics circles under general conditions. The point of difference here is that shallow foundations represent particular boundary conditions associated with a half space and, as a consequence, certain failure modes are inhibited. Just as with jointed rock slopes the more likely failure modes are slip by sliding along joints, shearing and toppling by rotation and failure by splitting is less likely; then the failure of shallow foundations is very unlikely to occur either by slipping or by rotation. The absence of a free face removes the possibility of kinematic mechanisms associated with those modes from developing along critical joint configurations, other than in special circumstances. The only practical failure modes for shallow foundations are splitting and/or shearing of intact material in general, and splitting alone in conditions of continuous jointing, as adopted by the authors. The discusser acknowledges that slip and rotation are basic deformation mechanisms within a block jointed mass but has concluded that, for conditions of shallow foundations, their effect, separately and in combination, is to bring about nonuniformity in load transmission between individual blocks within the jointed mass Burman and Hammett 1975 . The extent to which slip and rotational deformations between blocks can develop in a half space is limited, as compared to a slope configuration, by the absence of a free face. Yet even very restricted slip and rotational

movements of blocks within a jointed mass will lead to edge-toface and edge-to-edge contacts and will result in applied loads being transmitted though the mass by concentrated contact force trajectories. The general loading condition between blocks in a jointed mass is one of nonuniformity in contact stresses across joints, as a result of the mobility of individual blocks and their ability to slip and to rotate relative to one another. Nonuniformity is exacerbated by joint roughness and the other irregularities that occur with real joints. Geotechnical researchers have for years devoted significant resources and effort in attempts to measure rock strength under uniform compressive loadings. In the discusser’s view this is counterproductive because uniformity is essentially an academic construct: there is little of it in nature. When block jointed models are tested in uniform compression, in...
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