ABSTRACT: In this paper, the DDA method with a new hydro-mechanical algorithm is used to study the effect of rock discontinuities on uplift and seepage in concrete gravity dam foundations. This paper presents an alternative method of predicting uplift and seepage at the base of concrete gravity dams. A sensitivity analysis was carried out to study the importance of several parameters on dam stability such as the orientation, spacing, and location of discontinuities. The effectiveness of drains in reducing uplift and increasing dam stability is also investigated. The study shows that joint water flow and adverse geological conditions could result in unusual uplift at the base of concrete gravity dams, well in excess of what is predicted with the classical linear or bi-linear pressure assumption. Drains can be very effective in reducing that uplift if their location and diameter are chosen properly. It is shown that, in general, the DDA program with the hydromechanical algorithm can be used as a practical tool in the design of gravity dams built on fractured rock masses.
INTRODUCTION The Discontinuous Deformation Analysis (DDA) method is a recently developed technique that can be classified as a discrete element method. Shi (1988) first proposed the DDA method in his doctoral thesis; computer programs based on the method were developed and some applications were presented in the thesis as well as in more recent papers. Various modifications to the original DDA formulation have been published in the rock mechanics literature over the past ten years. For instance, Lin (1995) improved the original DDA program of Shi (1988) by including four major extensions: improvement of block interface, calculation of stress distributions within blocks using sub-blocks, block fracturing, and viscoelastic behavior. Although various extensions of the DDA method have been proposed in the literature, its application in rock engineering is still limited. For instance, until recently the method could not be used to model water-block interaction which is of particular concern when analyzing the interaction of engineering structuresuch as dams and tunnels with fractured rock masses. Recently, the authors have been able to incorporate a fully coupled hydro-mechanical algorithm in the DDA method. With that algorithm, the authors were able to analyze the effect of water on tunnel stability (Kim et al. 1998).
Current practice in the design of gravity dams is to assume that the uplift pressure at the b?se of a dam without drains varies linearly from full reservoir pressure at its heel to tailwater pressure at its toe. In the presence of drains, the pressure variation is assumed to be bi-linear. These approximations are based on the assumption that the rock mass behaves like a porous continuum with respect to seepage irrespective of the foundation geology; an assumption that is not always valid if the rock mass contains discrete fractures or geological features. Since the distribution of pressure in a jointed rock mass is controlled almost solely by the properties of the joints, the orientation, spacing and location of discontinuities must influence the uplift pressure in concrete gravity dam foundations.