Modelling Tunnel Performance in Expansive Gypsum Claystone

ABSTRACT: Extreme swelling pressures (in excess of 5 MPa) were measured against the invert of a tunnel excavated by drill and blast method in a hard claystone containing variable proportions of anhydrite and gypsum dispersed in the clay matrix. When the invert was not present, floor heave reached displacements in excess of 60 cm in some cross sections. Displacements and pressures developed at a high rate (the maximum values mentioned were reached in a few months). Swelling was induced by gypsum crystallization in fissures and cracks. A crystallization model was included within the general purpose THM program CODE_BRIGHT. Discontinuities were simulated by means of an embedded discontinuity approach. Hydraulic characteristics of discontinuities change as their opening is modified, as a result of crystal growth induced strains. An evaporation front travels downwards and creates conditions for precipitation at increasing depths.). Calculated vertical swelling follows the patterns observed in the field by means of high precision extensometers.