Field studies show that although the principal agents of stress generation in the crust (overburden and plate motion) often give rise to regionally uniform stress fields, a variety of factors conspire to generate stress heterogeneities on all scales. Fractures are particularly important in this regard and the manner in which they disturb the stress field within a fractured rock mass questions the significance of stress determinations made at points within such a rock. In addition these field studies indicate the way in which fracture networks in rocks are built up by the superposition of different stress regimes over geological time. They confirm that fractures interact and that as a result the networks have a very specific topology which exerts considerable control of the rock properties and which therefore must be considered when attempting to numerically model and predict the properties of a fractured rock mass. When the scale of the fracture network is much smaller than the rock volume of interest, then if the appropriate fracture network geometry and fracture properties are used, ‘bulk’ properties can be determined which adequately describe the behaviour of the rock mass. In other situations the properties are likely to be controlled by a local feature such as a specific shear zone or fracture zone within the network.


Geological studies of the evolution of fracture networks by the superposition of several fracture sets reveal an interaction between the fractures of the same set and also between the fractures of different sets. This interaction results in a significant modification of the stress distribution within the fractured rock mass and in the formation of a fracture network with a very specific topology. In this paper the relationship between stress and fractures in the crust is considered in an attempt is made to obtain a better understanding of the geometry of natural fracture networks and the properties these may impart to the rocks that contain them.


The state of stress in a tectonically relaxed part of the crust is primarily the result of the overburden Fig. 1a. In a uniform rock type of constant density the vertical stress increases linearly with depth according to;

  • (Equation full paper)

where σ v is the maximum compressive stress, the rock density, g the acceleration due to gravity and z the depth. If the rock at depth is prevented from expanding laterally by the confining effect of the surrounding rock then a horizontal stress is induced to prevent this expansion. The magnitude of the induced stress is determined by the Poisson's ratio (v), a measure of the compressibility of the rock (see e.g. Price & Cosgrove 1990);
  • (Equation full paper)

where m, Poison's number, is 1/v. This horizontal stress also increases with depth, Fig. 1a. It follows from eqs. 1 and 2 and Fig. 1a that the differential stress (σ v – σ h = σ1 - σ3) increases with depth.

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