No abstract

The behaviour of four drystone masonry retaining walls of different geometry has been modelled numerically using the discrete element code UDEC, and the results have been compared with the ®eld trials carried out by Burgoyne in Ireland in 1834. By using appropriate soil and wall mass densities, strengths and stiffnesses, it was possible to reproduce in the numerical analyses the ®eld behaviour observed by Burgoyne. Reasonably close agreement was obtained between the horizontal components of earth pressures calculated in the numerical analyses and using the earth pressure coef®cients given by Caquot and Kerisel. Basal stress distributions calculated using the condition of equilibrium of the wall were also broadly consistent with those resulting from the numerical analyses. The results also con®rm both the in¯uence of the geometry of a drystone masonry retaining wall on its performance and ultimate stability, and the soundness of Burgoyne's engineering judgement in specifying his programme of ®eld tests.

This paper describes the results of a series of numerical plane strain test simulations on a particulate material, carried out using the three-dimensional particle flow code PFC-3D. Samples comprised about 10 000 non-spherical particles, each formed by strongly bonding two spheres together. The simulations demonstrate the ability of such a model to capture the essential macro-features of soil behaviour as observed in laboratory tests, including the dependence of peak strengths on the initial void ratio relative to the critical. The development of strain localisations or shear bands associated with the use of rough loading platens, and the sensitivity of the model to the initial sample porosity, particle shape factor and interparticle friction angle, were also investigated.

In this paper, the factors controlling the deformation of drystone retaining walls are investigated by means of discrete element analyses. It is shown that toppling failure of unweathered drystone retaining walls is likely to occur in a brittle manner, with wall crest deflections not exceeding 1% of the backfill height until the factor of safety (based on soil strength) falls below 1·05. A compressible sub-base and weathering of the blocks will both tend to reduce the backfill height at failure to below that indicated by a limit equilibrium analysis. Bulging failure is more likely to be associated with a deterioration in block joint stiffness due to weathering than a compressible sub-base, although the latter will decrease the reduction in joint stiffness needed to cause bulging failure. Bulging is much less brittle than toppling, and the proximity to failure of bulging walls could in some circumstances be assessed on the basis of the size of the bulge.

The fracturing of cap rocks of very low permeability—in particular shales—by the intrusion of hydrocarbons under various in situ stresses has been considered. The conditions are discussed for the domination of the migration mode by the formation of either hydrocarbon dykes or sills, or a combination of both. Application of concepts of pro-elasticity and fracture mechanics to the process of dyke formation indicates the key roles of capillary pressure, premigration water pressure and in situ rock stresses, and provides conditions for maintaining open dykelets, for the change in width and spacing of growing fractures, and their closure, and explains differences in the behaviour of oil- and gas-filled dykelets. In discussing the situations where the state of in situ stresses favours the development of bedding-parallel intrusion fractures, various mechanisms are suggested which may allow interconnecting dykelets to form and feed hydrocarbon sills at stratigraphically higher levels.