Sound Transit spent $1.7 million to fill nine underground voids that appeared after construction of the twin light-rail tunnels through Beacon Hill, according to final costs outlined at the agency’s Sept. 10 board meeting.
The work was performed on an emergency basis after changes in two contracts that were formally ratified by Sound Transit’s board of directors on Sept. 10. The board approved increases in its contract with Shannon and Wilson, Inc. by $1.7 million for geotechnical services related to the work. The board also approved increasing a second contract with Hatch Mott MacDonald/Jacobs Engineering by $125,000. The money was paid out of surplus and contingency funds.
The voids were discovered in March after a resident noticed a 21-foot-deep hole in her yard at the corner of 18th Avenue South and South Lander Street, about 135 feet above one of the light rail tunnels. Engineers discovered a second void on her property, and seven more voids deeper underground following the tunnel line for about 350 feet.
Crews filled the voids, which resulted from shifts in layers of sand caused by the tunnel-boring machine, with a quick-setting, cement-based filler. Between April and August, a team of engineers and geologists drilled 38 holes to probe for other loose pockets of soil and pumped about 2,550 cubic yards of fill material, known as controlled density fill, according to Sound Transit.
Other areas of loose soil around the voids were “compacted” by pumping
cement grout through a pattern of drill holes near the locations, according to Sound Transit. In June and July, crews drilled 61 holes and pumped in approximately 293 cubic yards of grout.
According to Sound Transit documents, “these loosened zones or layers required stabilization to avoid any potential further ground displacements including during any future earthquakes.”
To read the materials, check out this link on Sound Transit’s Web site.
The 14-mile Link light rail line, which opened July 18, passes through the one-mile side-by-side tunnels and the underground Beacon Hill station between Sodo and the Rainier Valley.
Sound Transit is soon to begin construction on the U-Link segment, a 3.15-mile underground line that will run beneath Capitol Hill and to the University of Washington.