Permeability and Seepage Flow - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 127

6 Permeability and Seepage Flow

6.1 Introduction

Permeability and seepage flow in a rock mass play an important role in tunneling, dam
construction and for the stabilization of rock slopes.
The excavation of a drained tunnel below the groundwater table results in a seepage flow
directed towards the tunnel and consequently leads to a lowering of the water level. The
water seeping into the tunnel must be drained to the portal. In addition, hydrostatic and
hydrodynamic water pressure on the rock mass may cause stability problems at the tem-
porary face, such as sliding and falling of rock wedges. This often results in difficulties
in tunneling. Furthermore, a drawdown of the water table causes a loss of uplift in the
area between the new phreatic surface and the groundwater level which existed before
tunneling. Thus, subsidence at the ground surface and under adjacent buildings occurs,
the magnitude of which depends on the deformability of the ground. Furthermore, for
environmental reasons, lowering of the groundwater table is often inadmissible.
In dam construction, seepage around the abutments and underneath the foundation
of the dam causes leakage, and this has an effect on the stability. A safe and economic
design of sealing and drainage measures requires both a good knowledge of the rock
mass permeability and the capability to carry out adequate analyses.
In rock slopes, seepage forces can have also a considerable impact on the stability.
The permeability of the majority of intact rocks is very low. Thus, it can normally be
assumed that seepage flow through jointed rock takes only place through the discon-
tinuities. The permeability of the rock mass then is determined by the permeability of
the discontinuities k D (Fig. 6.1, left), and we talk about a “joint aquifer” (Fig. 6.2).
Adequate models of the permeability and of seepage flow through these types of rock
can be established on the basis of structural models (Section 2.7.3).

Figure 6.1 Rock mass permeability models