Abstract

Rock is an inhomogeneous material consisting of intact rock mass and a certain quantity of discontinuities, e.g., fractures and stratification. Depending on the amount and direction of the discontinuities the user can choose to model the rock mass with different constitutive models. For instance, if there are few discontinuities and those discontinuities are of high quality, meaning that the discontinuities have little effect on the strength and stiffness of the entire rock mass, the Hoek‑Brown model is commonly used. On the other side of the rock mass spectrum, if there are many discontinuities and the direction of the discontinuities is randomly distributed, then again, typically the Hoek-Brown model is used to model the rock mass including discontinuities as if it is a homogeneous material.

But what if the direction of the discontinuities is well aligned in one or more directions, would it still be possible to use the Hoek-Brown model? In principle no, because the Hoek-Brown model assumes isotropic behaviour of the rock mass; an assumption that is no longer valid when discontinuities are aligned rather than scattered in random directions in the rock mass. To overcome this problem PLAXIS has introduced the Jointed Rock model that has 2 important features in order to handle aligned discontinuities:

1. The model distinguishes an elastic stiffness parallel to the main discontinuity direction ( E_t ) and a stiffness perpendicular to the main discontinuity direction ( E_n ). Hence, the model has anisotropic elasticity.
2. The model can handle up to 3 discontinuity directions, each of them with its own specific strength parameters in terms of cohesion and friction angle.

It should be noted, however, that the Jointed Rock model is still a continuum model. Hence, the effect of the discontinuities is smeared out rather than modelling the individual discontinuities. This approach works very well if the distance in between the discontinuities is not too large compared to the size of both the rock mass and the project we are trying to model. The important question here would then be: what would be the point (with respect to the ratio of inter‑discontinuity distance and project size) where it's better to model discrete discontinuities rather than using the Jointed Rock model?

In the example presented here, we will look at the difference in results using discrete discontinuities versus the Jointed Rock model for a tunnel problem. Additionally, we will have a look if the Jointed Rock model behaves similar to discrete discontinuities in case the distance between the discontinuities is very small.

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