Swelling Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 201

Figure 8.17 Results of swelling tests on unleached Gypsum Keuper

8.7 Gypsum Keuper in its Natural Condition

It has been found by exploration that a thin low permeability layer is located directly
underneath the anhydrite surface. This is probably the reason for the observation that
the unleached Gypsum Keuper is normally encountered in dry condition. This, at a
first glance, is surprising because in most cases, above the unleached Gypsum Keuper,
water-bearing formations have existed for several thousand years.
To explain this phenomenon a simple analysis was carried out with the aid of the cou-
pled model using the columnar computation section represented in Fig. 8.18 (Wittke &
Wittke-Gattermann 2010). For the unleached Gypsum Keuper the parameters specified
in Table 8.1 are taken as a basis for this analysis. Above the leaching horizon Y, a
water-bearing overlying rock mass is assumed so that water can seep into the unleached
Gypsum Keuper in a vertical direction (Fig. 8.18, left).
As a result, swelling is initiated, which then closes the discontinuities within a zone of
decimeters to meters of thickness underneath the anhydrite surface A. Consequently
the permeability of this zone decreases significantly after a time period of 100 years,
and seepage flow through this tight zone is practically inhibited (Fig. 8.18, right). Below
this zone the rock mass consequently remains in dry condition as observed in nature.
From the result of this analysis, it can be concluded that without interventions such as the
excavation of a tunnel no swelling of the Gypum Keuper takes place over long periods.