Tunneling in Swelling Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 753

23 Tunneling in Swelling Rock

23.1 Introduction

In Chapter 8 the fundamentals of tunneling in swelling rock were presented, including
a coupled model for stress-strain behavior, seepage flow, water uptake and swelling. In
addition, this model was validated in that chapter on the basis of the results of testing
and long-term monitoring. Thereby, special emphasis was given to the unleached Gyp-
sum Keuper, a rock mass containing clay and anhydrite layers exhibiting a particularly
high swelling potential that occurs for example in Southern Germany and Switzerland.
To allow a safe and economic design of tunnels located in this type of rock it is essential
to find out the reasons for damages and unforeseen heaving that have taken place in the
past and to avoid such occurrences in the future. For this purpose, analyses were carried
out with the aid of the coupled model presented in Chapter 8. The rock mechanical
parameters that these analyses are based on are derived in Chapter 8 and compiled in
Table 8.1 (see also Section 18.2).
As in other rock engineering structures, for the design of tunnels in swelling rock, the
boundary conditions must be considered. The results of the analyses, subsequently
dealt with, show that the influence of the elevation of the anhydrite surface with respect
to the tunnel cross-section represents an essential boundary condition (Section 23.2).
On this basis it can be explained why certain tunnels that were constructed in swelling
rock in the past were successfully constructed, whereas damages or large heaving up to
the ground surface occurred in some other cases.
In Section 23.3, the case history of a tunnel constructed in the unleached Gypsum Keu-
per in the late 1970s will be presented as an example of a tunnel successfully carried out
in swelling rock. The experience gained from this example led to recommendations for
the construction of tunnels in swelling rock.

23.2 Influence of the Elevation of the Anhydrite Surface on Swelling

Pressure and Heaving

23.2.1 Anhydrite Surface above the Tunnel Roof
Underneath the anhydrite surface, the unleached Gypsum Keuper is normally dry
although above the anhydrite surface water-bearing layers exist as, for example, the
leached Gypsum Keuper. This has been confirmed by experience (Wittke 1978) and