Abstract:

Swelling anhydritic rocks consist of a clay matrix with distributed anhy-drite. They belong to the most problematic rocks in tunnelling and have caused severe structural damage and operational problems in a number of Swiss tunnels. The swell-ing is usually attributed to the formation of gypsum crystals from calcium and sul-phate ions that are dissolved in the pore water. It is a pronounced time-dependent pro-cess which in nature may take decades to finish. Important factors for the evolution of the swelling process over time are the kinetics of anhydrite dissolution and gypsum precipitation and transport processes such as seepage flow and diffusion. The present paper focuses on the kinetics of the chemical reactions by considering a system with-out transport (closed system). The paper starts with a literature review on the kinetic constants of the system an-hydrite – gypsum – water and presents estimations of these parameters by analyzing existing experimental results. After establishing the range of the values of the kinetic constants, we study the time evolution of anhydrite dissolution and gypsum precipita-tion separately, investigating the effect of the initial size and shape of anhydrite and gypsum particles by means of parametric studies. The computational results indicate the possible orders of magnitude of the duration of the dissolution and precipitation processes and show how important the specific surface of the involved rock minerals is in this respect.

1 INTRODUCTION

Swelling rocks experience a volume increase when absorbing water. In tunnelling, the swelling leads to a heave of the tunnel floor or, in the presence of an invert lining, to the development of a considerable rock load (the so-called swelling pressure). Anagnostou et al. (2010) gave an overview of the theoretical models for the swelling problem and of recent or ongoing investigations at different scales (micro- to mega- scale).

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