There are many types of commercial software packages available that can be used to model a dam structure and its underlying jointed rock foundation. In this paper, results are compared between a continuum model (FLAC) and a discontinuum model (UDEC), in order to evaluate which code gives the most realistic prediction. The results of this research are compared with the existing conventional analytical methodology which is based on limit equilibrium theory. It is concluded that both codes give similar results to the conventional theoretical approach as long as the rock foundation blocks are not rotated by the stress regime created by the dam and reservoir. However, this similarity was found to depend on the joint strength. As the joint strength is reduced, UDEC was found to provide a better representation of the behaviour of a jointed system and arguably a more realistic prediction of the stress distribution under the dam.

1. Introduction

The construction of a safe and economic dam requires a detailed understanding of the geotechnical environment in the area surrounding the dam location. Most historical dam failures are related to deficiencies in the dam foundation due to the presence of jointing in the bed rock.

There are various publications relating to dam incidents, such as ICOLD (1974, 1983, 1995) and Douglas (2002). According to these, about thirty percent of incidents occurred due to some deficiency of the rock foundations. For example the Austain Dam (also known as Byless Dam) failed because of sliding between sandstone and shale layers (Martt et al., 2005) and the Malpasset Dam failure was a result of open joints upstream of the dam and an inactive fault downstream (Jansen, 1988).

Continuum software packages have been extensively used to analyse and design dams on rock mass, although the rock mass is discrete and its behaviour depends on the joints and the intact rock. One of these packages is FLAC (Fast Lagrangian Analysis of Continua) (Itasca, 2008), which is a 2D explicit finite difference program. Recently the discontinuum approach has gained popularity in geomechanics, especially in mining, tunnelling, and slope stability problems. UDEC (Universal Distinct Element Code) (Itasca, 2011) is a discontinuum software code available for analysing geotechnical problems (Cundall, 1980). UDEC is a 2D distinct element, explicit finite difference program that treats a medium as a collection of discontinuous shapes that interact with each other in space. It is argued that the most powerful tool available to study the rock mass under a dam is the distinct element method because it is capable of modelling the stresses between rock blocks and flow through discontinuities (joints and faults) within the rock mass (Gimenes and Fernández, 2006; Bretas et al., 2013). It should be noted, however, that FLAC can also model discontinuities using interfaces, but this takes up considerable computational time so its use is limited to a few joints within the rock mass. Also, the flow cannot be modelled through the joint.

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