A general and efficient formulation of fractures and boundary conditions in the finite element method

The need to assess quantitatively the safety of waste repositories in deep geological media has fostered the development of efficient numerical models of groundwater flow and contaminant transport in fractured media. These models usually account for water flow through fracture zones embedded in a 3D rock matrix continuum. The first formulation of fractures in groundwater flow finite element models was proposed by Kiraly, and later revisited and generalized by Perrochet. From a mathematical viewpoint, fractures can be considered as in-dimensional manifolds in an n-dimensional Euclidean space (mless than or equal ton). The key step of this formulation ties in an expression relating the hypersurface element dS(m) to the infinitesimal local co-ordinates dzeta(i) (i = 1,...,m). Here we present a novel proof for this relation using a different approach to that of Perrochet, and explore the efficiency and accuracy of the formulation. It is shown that the aforementioned relation leads to a general and compact formulation which is not only applicable to elements of any dimension (e.g. 1D, 2D and 3D elements in a 3D domain), but also overcomes the cumbersome and case-specific calculations of traditional approaches. This formulation has been implemented in a versatile finite clement program for modelling groundwater flow, solute transport and heat transport in porous and fractured media. The efficiency and accuracy of the proposed formulation has been analysed using a synthetic case dealing with flow and solute transport through a 2D fractured rock block. The proposed formulation, in which fractures are discretized by means of 1D elements is more efficient and accurate than the traditional finite element formulation of discretizing fractures by means of 2D elements. The capability of the proposed formulation to cope with complex systems is illustrated with a case study of groundwater flow induced by the construction of the access tunnel to ail underground research laboratory in Aspo (Sweden). The numerical model is able to reproduce the observed records of water levels in boreholes and flow rates into the tunnel. Although the proposed formulation has been implemented and tested within the framework of groundwater flow and solute transport in fractured porous media, it should be of interest for other boundary value problems. Copyright (C) 2002 John Wiley Sons, Ltd.