Bearing capacity of shallow and deep foundations on rock mass : analytical and experimental investigations

Abstract

It is well known that, except for potential installation problems, most rock masses are excellent and trouble-free foundation materials. A significant economy can be therefore realized in design if the foundation can be constructed on or in the surface or near-surface rock. Nevertheless, the construction of challenging structures with usually high loads and special requirements on rock foundations, have made it necessary to determine the bearing capacity of rock media accurately. It is within this framework that the present research was carried out. This dissertation provided analytical solutions for bearing capacity problems of both shallow and deep foundations resting on or in a rock mass that obeys the Hoek and Brown failure criterion. Considering the strip footing problem, improvements were brought by incorporating a non-associated flow rule in the plane strain upper bound limit analysis technique and analysing its effects on failure mechanisms and bearing capacity values. By assuming a rigid strip footing resting on a rock mass obeying the generalized Hoek and Brown failure criterion, the upper bound estimate for the bearing capacity was derived by preserving exactly the original nonlinear criterion. The multi tangential method was used to handle non-linear strength criterion issues. Furthermore, a sufficiently significant but conveniently simple formulation of a non-constant dilatancy angle model was derived. Closed-form expressions of the maximum resistance work rate were developed in any virtual velocity field. For practical purposes in rock engineering, tables and charts allowing easy and accurate estimates of the bearing capacity factor 0 were provided. The effect of the flow rule on 0 was carefully examined using different dilatancy parameters. To investigate the efficiency of the proposed solution, a series of static penetration tests of small-scale model footings (strip and circular) laying on a highly porous calcarenite rock mass was conducted. A reduction factor defined as the ratio of the measured acceptable bearing capacity to ultimate theoretical one was introduced. This factor reflects by how much should the theoretical bearing factor reduced to get an acceptable value of bearing capacity. It was suggested to estimate values as a function of the rigidity index and the friction angle of the rock mass. As for the bearing capacity of the pile foundation, an analytical calculation procedure was proposed to predict the shaft resistance for piles drilled to some depth into a rock layer. The developed solution requires a numerical integration over the embedment depth. Thus, for practical purposes, design charts, as well as an approximate solution, were provided allowing easy and accurate estimates of the average shear resistance. Results obtained from developed solutions were in excellent agreement when compared to published analytical results and very close to finite element analysis results. Moreover, this analysis procedure offered reasonable results when compared to field test measurements. Furthermore, and always within the framework of pile foundation, an analytical solution based on the cavity expansion theory was proposed for computing the bearing capacity at the tip of a pile embedded in rock mass obeying the H-B failure criterion. The pile end bearing capacity was evaluated by assuming that the pressure exerted at the boundaries of a pressure bulb immediately beneath the pile tip is equal to the limit pressure required to expand a spherical cavity. Besides, a relationship was derived to predict the pile load-settlement response. To demonstrate the applicability of the presented solution, the results of the present work were compared to those of 91 field tests from technical literature. Despite some limitations, it was found that the computed end bearing resistance was in good agreement with the measured field test results.(FSA - Sciences de l'ingénieur) -- UCL, 202