Sound Transit’s U230 project was designed to connect Seattle — Tacoma International Airport with the University of Washington by expanding an existing section of rail with twin tunnels. Malcolm Drilling’s scope included construction of three soil improvement break-in/break-out blocks to assist the Tunnel Boring Machines (TBM), by provided an impermeable strengthened structure for the TBM to mine at critical juncture points.
Construction of the first ground improvement zone occurred at Capitol Hill Station’s two Southbound tunnels. This single block of treated ground was 80 feet by 40 feet and 50 feet deep. This work was followed by a ground improvement zone of similar proportions at the opposite end of the station to receive the TBM drives advancing from the University of Washington. The final treatment zone was located at the Pine Street Station in downtown Seattle. The complex geology of Seattle’s Capitol Hill posed as one of the project’s greatest challenges. City street right-of-way constraints were also a significant issue. The construction requirements were maintained with column diameters ranging from 8 to 12 feet, drill depths up to 86 feet, and drilling inclinations of up to 26 degrees off vertical.