An Experimental Investigation into the Effect of Heating on the Swelling of Rock Containing Anhydrite

Abstract In the temperature conditions normally associated with tunnels, anhydrite transforms to gypsum in the presence of water. The result is an increase in the volume of the rock, which in tunneling may cause major problems such as Heave of the tunnel floor or development of very high rock pressures upon the tunnel lining. In shallow tunnels, it can even cause uplift of entire lining blocks and the overlying rock, affecting the buildings above the tunnel. As anhydrite is the thermodynamically stable phase at temperatures higher than approximately 49 °C—rather than gypsum—one potential mitigation measure is to heat the rock (“artificial ground heating”). This paper presents and discusses experimental results from a testing campaign on artificially prepared specimens made from natural material. A single core sample of anhydritic rock was milled and homogenized, and each specimen was compacted under high pressure of 100 MPa. Long-term oedometer tests under varying temperatures and stresses were performed. The results show that Heating is an effective measure, even at temperatures below the 49 °C threshold. In particular, the higher the temperature, (i) the lower the swelling pressure that develops under full volumetric constraint, (ii) the lower the strain that develops under constant stress, and (iii) the lower the swelling rate. These effects become significant at temperatures of 40 °C (or 35 °C under a pressure of 1 MPa) and allow for the optimisation of artificial ground heating.