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The Transbay Tube, currently under construction, is a 3.6 mile sunken tube railroad tunnel under San Francisco Bay between San Francisco and Oakland. The Tube traverses a stratum of nonuniform silty clay with pockets of sand, shell, and organic matter, known locally as San Francisco Bay Mud.
Earthquake resistant design requirements involved investigation of dynamic soil properties in order to: (1) evaluate potential loss of soil shear strength during an earthquake and (2) determine deformations and loads imposed on the Tube by ground deformations during earthquake.
The investigation included an underwater field seismic survey, installation of an earthquake sensing and recording system in the Bay sediments, and dynamic testing of undisturbed soil samples. The dynamic tests were triaxial compression tests under alternating strains and simple shear tests under confining pressure with alternating strains. Soil properties used in analysis were derived from experimental stress-strain curves of a hysteretic type and consisted of shear moduli and viscous damping coefficients. The soil properties established in this manner were used to estimate free-field responses and soil-structure interaction.
It was concluded that in these soils significant loss of strength will not occur under earthquake strains and durations. Stresses imposed on the Tube by ground deformation are not of significant magnitude. Differential movements between the Tube and the ventilation buildings at either end required provision of a special universal sliding joint.
earthquakes, soil dynamics, tunnels, dynamic tests, clay soils, soil mechanics, tests evaluation
Associate, Parsons, Brinckerhoff, Quade & Douglas, Inc., New York, N. Y.
Chief soils engineer, Parsons, Brinckerhoff, Quade & Douglas, Inc., New York, N. Y.