Richard Jewell

A study of the mechanism of flexural toppling failure of rock slopes

SummaryThe mechanism of flexural toppling failure of jointed rock slopes has been investigated through a series of centrifuge experiments conducted on models manufactured from two types of materials (brittle: a sand-gypsum mixture; and ductile: fibre-cement sheeting). The basal failure plane observed in the centrifuge models, has been found to emanate from the toe of the slope, and orient at an angle of 12 to 20° upward from the normal to the discontinuities. A theoretical model based on a limiting equilibrium approach (Aydan and Kawamoto, 1992) has been adopted to analyse the centrifuge test data. After calibration, the model was found to accurately predict the failure load for all the tests reported in this study. Using this model, a set of charts has been prepared to assist with the analysis of slopes susceptible to flexural toppling.

A measuring scheme for determining in situ stresses and moduli at large scale

This paper proposes and analyses a new measuring arrangement for the cylindrical jack method of determining in situ stresses and elastic moduli. The new methods measuring scheme is free of the shortcomings associated with Dean and Beattys variant, and its proposed system will facilitate in situ stress and moduli determination over a large scale. Its main innovations are its proposed arrangement of measuring points, and the adoption of modern displacement measurement transducers with a sufficient resolution level. The paper presents mathematical analyses of the Dean-Beatty measuring scheme for the cylindrical jack method and of the proposed new measuring scheme. An algorithm is described, which uses the least squares method to solve the system using the proposed scheme, and reconstruct the shear modulus, the stresses, and Poissons ratio. A numerical example is given of the reconstruction of the stresses and moduli. The proposed measuring scheme allows the simultaneous reconstruction of the stress components and elastic moduli of a rock mass, at a scale considerably larger than with conventional methods such as overcoring, although it can only be applied near the surface of an excavation.

An approach to large-scale field stress determination

The construction of stable structures in rock masses requires knowledge of the in situ stresses at the scale of excavations. However, the measurements obtained by the conventional overcoring technique are related to a small scale (centimetres). To extrapolate them to the scales of interest to rock mechanics (from meters to kilometres) requires a large number of individual stress measurements, followed by statistical analysis to avoid a considerable scatter of the measured values. In this paper, a method is proposed based on (a) large-scale surface stress and modulus measurements using the cylindrical jack method complemented by a special measuring scheme and then (b) back analysis for a given excavation shape. The method allows the simultaneous reconstruction of the stress components at the scale of excavation. A numerical simulation for a cylindrical excavation in an isotropic rock mass demonstrates the high accuracy and robustness of the method. The presence of a fractured zone surrounding the excavation can hamper the stress reconstruction, hence special measures should be taken to conduct the measurements in competent rock.