Swelling Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 181

8 Swelling Rock

8.1 Introduction

The penetration of water into sedimentary rocks containing clay minerals and anhy-
drite leads to a volume increase as a result of absorption of water and/or chemical
transformation (e.g. Wittke 1990). Such rock types exist within the Keuper formation
and are named “Gypsum Keuper” which occurs in Southern Germany, Switzerland
and other European countries.

Figure 8.1 (upper) shows a photograph of the temporary face, taken during excavation
of the Hasenberg tunnel for the urban railway in Stuttgart located within the Gypsum
Keuper and a corresponding structural model. The Gypsum Keuper in the area of
Stuttgart is composed of thin to massive layers of sulfate rock which alternate with
claystone and siltstone layers. Occasionally, dolomitic marl stones are intercalated. The
sulfate component is predominantly finely distributed in the rock mass, but also appears
in the form of massive layers and of nodules. The rock mass contains horizontal to
sub-horizontal bedding-parallel discontinuities that are persistent and widely closed.
Perpendicular to the bedding, two orthogonal joint sets exist. The steeply dipping joints
are usually very infrequent, non-persistent and closed or partly healed with fibrous an-
hydrite and gypsum (Fig. 8.1, lower).
Figure 8.2 shows a typical profile of the Gypsum Keuper in the area of Stuttgart. Under-
neath the so-called “anhydrite surface” A, the sulfate rock predominantly consists of an-
hydrite. Above the anhydrite surface, the anhydrite, as a consequence of water access, has
been transformed completely into gypsum. Above the sulfate rock denoted as “unleached
Gypsum Keuper”, the leached Gypsum Keuper is located in which the gypsum has been
dissolved and transported away with the groundwater. The boundary between the leached
and the unleached Gypsum Keuper is referred to as the “leaching horizon” Y.
The transformation of anhydrite (CaSO4) into gypsum (CaSO 4

2H 2 O) occurs when wa-
ter penetrates into the rock. This process is described by the following reaction equation:

⋅

CaSO 4 + 2 H 2 O → CaSO 4 ⋅ 2H 2 O

The volume balance shows that a complete transformation of anhydrite into gypsum
leads to a volume increase of some 60% if swelling is unconfined (Fig. 8.3, upper).
Unconfined swelling of the unsupported invert of old railway tunnels located in un-
leached Gypsum Keuper has caused large amounts of heaving as in the Kappeles-
berg tunnel (Fig. 8.4) and the Schanz tunnel. To maintain operation, the height of
the tracks in these tunnels has had to be lowered several times. Finally, also damages