Swelling Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 186

to TOT layers. Absorption of water, which is associated with a widening of the basic
inter-lamellar spaces, is known as hydration or “inter-crystalline swelling” (Jasmund
& Lagaly 1993 and other textbooks on clay mineralogy). However, inter-crystalline
swelling in argillaceous rocks, because of their natural water content, has general-
ly taken place in nature and thus is not relevant in tunneling (Kiehl 1991a, Wittke-
Gattermann 1998).
A further access of water into the inter-lamellar spaces and a corresponding swelling
takes place if there is a gradient of ion concentration between the inter-lamellar pore
water and the groundwater. This kind of water absorption may be described by a dif-
fusion process and can be explained by the theory of diffuse double layers (Madsen
1976) and is referred to as “osmotic swelling”. Osmotic swelling to a large extent is a
reversible process. In tunneling, osmotic swelling is initiated by an unloading, which
produces negative pore pressure (suction) and hence is the driving force for capillary
water uptake.

Swelling of rocks containing clay minerals and anhydrite
Figure 8.6 shows the swelling mechanisms taking place in the unleached Gypsum
Keuper containing anhydrite.

The clay matrix in which the anhydrite is finely distributed in the form of thin layers
consists of a considerable portion of corrensite, an expandable mixed-layer mineral
containing chlorites, smectites and vermiculites. When water is absorbed by the clay
matrix, osmotic swelling of corrensite leads to a volume increase V C (Fig. 8.6).

The absorbed water dissolves the anhydrite layers at their surfaces. Then SO 4 ions can
penetrate into the clay matrix and the discontinuities. Since under normal conditions
the solubility of gypsum is smaller than that of anhydrite, on supersaturation of sulfate,
gypsum precipitates from the solution, forming gypsum crystals. This leads to a further
volume increase

Δ

V C (Fig. 8.6). Thus, the volume increase due to the transfor-
mation of anhydrite into gypsum is large compared with the volume increase due to
osmotic swelling of the corrensite (BMV 1975).

Δ V G >> Δ

The transformation of anhydrite into gypsum is supported by the presence of clay. In
the laboratory it has been observed that samples of pure anhydrite in contact with wa-
ter only swell at their external surfaces. The largest swelling pressures were measured in
specimens with a clay content of 10 - 15% (Madsen & Nüesch 1991).
The amount and rate of swelling of unleached Gypsum Keuper depends on a number
of influencing factors such as the anhydrite content in the rock, diagenesis and meta-
morphosis of the rock, the structure and permeability of the rock mass, the rock mass
in-situ stress, sulfate content of the groundwater and the temperature (Rauh 2009).
The rate of transformation of anhydrite into gypsum also increases with increasing
speciic surface of the anhydrite. Thus, rock containing finely distributed anhydrite
swells faster than coarse-grained, crystalline or massive anhydrite rock (Rauh 2009).