Forensic investigations to evaluate sulfate-induced heave attack on a tunnel shotcrete liner

This paper presents the results of a comprehensive research study to determine the potential causes for an inordinate distress developed on a shotcrete liner material of a tunnel located near Dallas, TX. This tunnel was originally founded on a limestone material. Distress locations were identified where possible delamination of shotcrete layer and moisture leaks were either suspected or noticed. As a part of the research, rock cores and white powderlike substance behind the liner were collected around the distressed locations, and these cores and powder material were subjected to chemical, mineralogical, and engineering tests to understand the potential causes of this distress. Mineralogical tests, in particular, X-ray powder diffraction analysis on a powder material and gel-like substances collected on the liner, showed the presence of anhydrite, gypsum, and ettringite traces. High amounts of sulfate measurements in chemical and energy dispersive X-ray spectroscopy studies also showed that both gypsum and ettringite formations were possible in and around the limestone material. Upon hydration, mineral expansion of ettringite and anhydrite led to heaving and subsequent cracking of the adjacent shotcrete layer. Engineering characterization tests including unconfined compression strength (UCS), indirect tensile strength (ITS), and triaxial tests on rock cores embedded with a powder type sulfate material revealed that low strength cores were obtained near high distress zones and high strength cores were collected at low distress zones. The UCS values ranged from 6.2 (high distress) to 13.8 MPa (low distress) whereas the ITS values of the cores varied from 0.5 to 1.1 MPa for the same distress locations. This indicates the potential loss of strength of these rock materials from the presence of gypsum material in them. Possible methods to mitigate this heaving problem behind the liner are also discussed