Abstract

Swelling in sulphate bearing rock causes damage to structures, but particularly to tunnels with large heave or large swelling pressures. Swelling pressures determined in laboratory tests are substantially larger than in-situ swelling pressures that are still large. Swelling of sulphates is a process that involves flow of water and dissolution of anhydrite. Once solutions exceed the saturation limit, gypsum precipitates and crystallizes in the rock mass. This leads to heave and swelling pressures on structures but also on pressures within the rock mass around the underground opening. Discontinuities will close and the transport by dissolution and precipitation of gypsum will be reduced, until, with a sufficiently high pressure the swelling process ceases. The average swelling pressures are substantially lower than local crystallization pressures. Lining stresses have been measured with flat jacks or in other cases with measurement of strains in the concrete. Measurements with contacts stress cells generally indicate larger stresses.

1 Introduction

Swelling of sulfates in rock has caused many problems in tunnels (Steiner & Metzger, 1988; Wichter, 1991; Steiner, 1993; Alonso et al., 2013) although substantial research and studies have been carried out, many open questions and many contradictions and differences exist in understanding the process of sulfate swelling in-situ.

Laboratory tests on small scale samples indicate extremely high swelling pressures that appear not realistic. The transformation of sulphates and thus swelling passes over the fluid phase and is important in understanding the process, together with the rock structure. Anhydrite and other sulphates dissolve and when the solution is oversaturated gypsum precipitates. The formation of fissures around an underground opening due to excavation is a key factor.

With sufficient resistance of the tunnel liner, mainly in the invert, the rock mass around the tunnel is compressed, the fissures as flow path will be closed and the process of dissolution and precipitation will ultimately with sufficient counter stress cease.

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