| The site conditions and the performance of nine station chambers for the Washington, D.C., Metro were investigated and evaluated. Available strain and displacement instrumentation results were evaluated and additional data was collected in the caverns.;Lining loads, and lining and rock displacements were greatest in the chambers in the more heavily sheared ground. Rock loads were typically 25 to 35 ft (8 to 10 m). When installation of the full lining was delayed, rock movements were larger (1 to 3 in.; 25 to 75 mm) and propagated along the planar shear surfaces to the ground surface, resulting in settlements as large as 2.4 in. (61 mm).;The chambers in the less heavily sheared rock had much smaller lining loads and rock displacements {less than 12 ft (4 m) rock load and 0.1 in. (2.5 mm) of rock movement}. Lining moments and thrusts were estimated from the strain measurements and were found to be approximately 1/6 the ultimate capacity of the linings, which had additional capacity due to their increased thickness resulting from overbreak.;The chambers were typically 50 ft (15 m) high by 60 ft (18 m) wide and were supported with rock bolts and a structural shotcrete-steel rib lining. They were constructed in foliated gneisses and schists, at depths of 60 to 90 ft (18 to 30 m), with 30 to 60 ft (9 to 18 m) or rock cover over the crown. Rock in many of the caverns was very blocky and seamy and contained shear zones parallel to the steeply dipping, N-S foliation. In such rock, joints and shears were planar and continuous, and sufficient joint sets were present to form large, discrete blocks of rock requiring support in the crown and sidewalls of the chambers. Sidewall stability problems developed in several cases on the East wall of the chambers, where the foliation shears dipped into the excavation. The length of the bolts required to stabilize the walls was significantly greater than bolt lengths normally required on rock walls. |
