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Capitol Hill at risk of sandy ‘voids’ too: How Sound Transit says they’ll deal with them

What does the appearance of holes in the ground related to the boring of the light rail tunnels through Beacon Hill mean for Capitol Hill’s light rail future? Similar to the experiences related to business mitigation and ‘transit oriented development,’ we’re lucky we didn’t go first.

“I would not say plans have changed,” Sound Transit spokesperson Bruce Gray told CHS today. “We looked at lessons learned in Beacon Hill as we created the bid documents for the UW to Capitol Hill route.” Gray said the result was hiring a contractor for the job who could provide machinery that will better deal with the conditions causing the Beacon Hill holes.

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“We finished our first tunnel on Beacon Hill, three years ago,” Gray said. “We haven’t seen any surface issues since the tunnels besides these voids. We’ve learned a lot on Beacon Hill. We have another 15 months before we dig on Capitol Hill.”

Sound Transit says that the seven ‘voids’ discovered on — or in, really — Beacon Hill are the result of the displacement of pockets of sandy soil by the tunnel boring process. They are not technically sinkholes which are large on top and narrow as you go deeper. These holes, on the other hand, have small openings and expand as you go deeper until rounding out in a base. “It’s like being inside of a tea kettle,” Gray said the engineers have told him.

Sound Transit documents detailing the geology and soils of the planned University Link light rail line say the same type of structures exist deep underneath our feet here on Capitol Hill.

Glacial lacustrine deposits were encountered beneath the outwash deposits to the depth of the borings (130 to 160 feet). For alternatives to Capitol Hill, greater thicknesses of pre-Vashon non-glacial fluvial deposits were observed in borings drilled by the PSTC Design Team (2005), which appear to thicken from upwards of 60 to 80 feet of granular soils near the Montlake Cut to more than 130 feet near E Roy Street.

Sound Transit has dug 67 different test bores across the Hill, some down to about 400 feet near the water tower at Volunteer Park. The results confirm the comparable geology — layers of clay and sand throughout most of the Hill.

So, now you know there are soft pockets of sandy soil surrounded by hard-packed clay throughout the Hill. What did you think was down there, anyhow? The PDF attached to this post doesn’t have the entire 140+ page write-up including the geology study from Sound Transit but it does have the maps from the documentation. If you want the big PDF, let me know and I’ll find a way to share it.

Frontier-Kemper

Gray says the company that won the contract to bore the tunnel from UW to Capitol Hill will use a machine capable of better handling these kinds of conditions. The Traylor/Frontier-Kemper tunnel boring machine is reportedly capable of exerting high pressure as it digs thus holding loose soil in place as the tunnel rings are put in place. The machines also inject grout as they dig to further stabilize the soil.

Still, Gray says, Sound Transit is concerned about running into the same problems on Capitol Hill. “We’ll be extra vigilant. We’re going to be keeping a real close eye on all the materials that are coming out from that machine.”

In the meantime, Sound Transit is filling the Beacon Hill holes, some as large as 21-feet deep, and trying to calm down concerned residents in that neighborhood. Gray said Sound Transit plans to have contractors available at tonight’s public meeting on Capitol Hill to answer questions about the situation. “We are learning a lot,” Gray said. “But every job is different.”

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John Niles
14 years ago

Public Interest Transportation Forum has published Sound Transit’s geotechnical report on the conditions to be faced in the University Link tunneling at http://www.bettertransport.info/pitf/resourcelinks.htm . This is a 14 megabyte document. We obtained this document through the Public Disclosure Act process in summer 2006.