ENGINEERING GEOLOGIC DESIGN AND CONSTRUCTION CHALLENGES OF CENTRAL LINK LIGHT RAIL IN TUKWILA AND SEATAC, WASHINGTON

This paper discusses the challenges of characterizing subsurface conditions for engineering design, remedial work during construction, and approaches to site investigation that might be used on future projects. The southern 10.7 kilometers of the Sound Transit Central Link Light Rail traverses extremely variable geologic conditions. A steeply dipping and/or eroded bedrock surface comprised of sandstones, siltstones, volaniclastics, and andesite interflows was encountered on the north end of the alignment. Marine and alluvial deposits of the Duwamish embayment mantle the bedrock and vary in thickness up to 91 meters. Glacial till and advance outwash deposits characterize the south end of the alignment, with transitional beds of lacustrine and recessional outwash deposits in the central portion of the alignment. Groundwater aquifers were encountered in these transitional areas, with flowing artesian groundwater up to 10 meters above ground level.

The variable geologic conditions required a variety of foundation solutions including spread footings on soil, spread footings on rock with anchor tiedowns, drilled shafts, and driven piles. Stone column ground improvements were required at the Duwamish River crossing. At-grade portions of the alignment included soldier pile, soil nail, and mechanically stabilized earth walls. Dewatering with deep-drilled wells was used to lower piezometric levels before drilled shaft installation.

Subsurface conditions were explored with a combination of mud rotary and hollow-stem auger borings, to depths ranging from about 3 to 91 meters below existing grades. More than 300 borings were advanced along the alignment, 25 of which were completed as wells. Three additional borings had vibrating wires installed to monitor groundwater levels. Thirteen cone penetration test probes were performed that included measurements of shear wave velocity and pore pressure dissipation. We performed down-hole measurement of shear wave velocity by geophysical measurement at three typical locations. Although the subsurface investigation program was extensive, unanticipated conditions still occurred.