Failing Tunnels from Changed Conditions?

Construction of a large sewer in Louisville, Kentucky required five tunnels under two railroad lines, a roadway, several high-pressure gas transmission pipelines, and a creek. The 4.3-m tunnels were to be built 8.2-13 m below grade in alluvium and lacustrine deposits. The 8-10-m-thick alluvium consisted primarily of sensitive silt and silty clay layers, whereas the lacustrine soils were silty clays of moderate plasticity. During hand mining of the liner-plate tunnels, the headings squeezed and the contractor resorted to poling, ground freezing and, finally, a tunnel boring machine. Three tunnels were completed before the contractor filed a claim of "changed conditions." A tunneling expert hired by the contractor inspected one of the tunnels and asked for samples from new borings. The 75-mm Shelby tube samples obtained at the site were shipped 400 km to the expert's laboratories, then were extruded and tested by graduate students. The expert reviewed the test results and said that the undrained shear strengths, as low as 9.6 kPa, were inadequate for hand mining, and much lower than strengths indicated in pre-construction documents. The expert noted contortions in thin bands of silt and clay, adjacent to uncontorted layers, and concluded that natural processes had caused the contortions, and low strengths. The authors were retained by the tunnel owner to investigate the claims of changed conditions and "naturally" caused contortions. Knowing the sensitive nature of the soils at the construction site, the authors worked with an expert driller to obtain best-quality 75-mm Shelby tube samples. These samples were extruded carefully and tested by the authors in their nearby laboratory. The measured strengths were equal to or higher than those reported in pre-construction documents. The source of the contortions also was discovered. Records of adjacent tunneling operations by another contractor also were used to explain the cause of the distress.