Experimental studies to address strengths of limestone cores, sulfate heave problems in limestone cores and tunnel lining

Sulfate induced heave is a common scenario in tunnels located on sedimentary rocks containing Pyrites. Pyrite oxidation produces Gypsum which is a major source of natural sulfates. DART NC-1B tunnel is located in Dallas, north Texas. Dallas is located on sedimentary rocks with high Pyrite content and has a history of sulfate induced heave problems in several projects. In spring 2005 & 2006, the shotcrete liner in DART NC-1B tunnel showed cracking associated with water leaks. Considering the local geology and the history of the tunnel, researchers at The University of Texas at Arlington (UTA) suspected probable occurrence of sulfate induced heave and consequent formation of Ettringite. To confirm the formation of ettringite researchers at The University of Texas at Arlington designed an experimental program to address the quality of limestone behind the tunnel lining and also to study their microstructure. In this research a comprehensive experimental program was designed to study the affects of distress on strength and structure of the limestone. A series of engineering tests including Unconfined Compression Strength tests (UCS), Indirect Tensile Strength tests (IDT or ITS) and Unconsolidated Undrained Triaxial tests (UU) were conducted to address the affects of distress on strength and deformation characteristics of limestone. Mineralogical studies including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Micro Analysis were conducted to confirm the presence of Ettringite and also to address the micro level structural changes in distressed regions. Chemical studies were conducted to determine soluble sulfate levels in limestone cores.; All the engineering tests conducted indicated perceptible reduction in the strength and deformation characteristics of limestone cores degraded by sulfate induced heave. Mineralogical studies indicated confirmed presence of ettringite in the limestone cores. Chemical studies showed high sulfate levels in severely distressed regions. Together the experimental program designed in this research served its purpose.