Paula Wallis, Reporter

When Seattle's Beacon Hill light rail station opens next week, riders will be whisked down roughly 16 stories in one of four high-speed elevators to the deepest transit station platform in the United States. Although 165ft below street level, the artwork will have them looking into outer space. Stars twinkle on a midnight-blue porcelain ceiling, giant luminescent mobiles, including a purple starship, green jellyfish, and red dragon, hang above the platform, and a large video screen beams images from the Hubble Space Telescope.

Beacon Hill sinkhole

But even as Sound Transit puts the finishing touches on the July 18 grand opening celebrations of the 14-mile light rail starter line from downtown Seattle to Tukwila, it is reassuring nervous residents, worried by the recent discovery of a sink hole and six large voids behind the segmental lining of the bored TBM running tunnels on the Beacon Hill underground section.

The voids are the result of over excavation when the EPBM hit pockets of deep sand flows as it bored through mainly stable clays said Sound Transit spokesperson Bruce Gray.

The problem came to light on March 29th, some two years after the southbound tube was completed and about one year after excavation of the northbound tunnel finished. That March day a homeowner discovered a 21ft deep sinkhole in her front yard near the home's foundation.

Beacon Hill tunnel alignment

"Following the sinkhole we went back and looked at all of the data from the tunneling operation," said Gray. "In addition to the sink hole we saw six other locations where the data showed removal of an unusual amount of muck. Working with geotechnical consultant, Shannon and Wilson and contractor Obayashi. Sound Transit conducted test drillings and discovered all six voids pretty much where the tunneling data indicated they would be."

The voids were all located within the two blocks just east of the Beacon Hill station bounded West and East by 17th and 19th Avenues and Waite and Lander Streets, to the North and South.

"The six underground voids were between 20ft to 60ft below the surface and were found on both the north and southbound tubes," said Gray. "To fill them took anywhere from 200 to 400 cubic yards of controlled density fill (CDF). Some of them were pretty big spaces."

The geology in this section of the alignment was identified as an area of concern in the initial GBR (geo-technical) baseline report and caused Sound Transit to move the Beacon Hill station to the West to avoid sand flows during station excavation.

When asked how spikes in the data indicating over excavation were overlooked at the time, Gray pointed to problems with the weight control system on the project's continuous muck hauling conveyor.

"There were issues with the data recording the weight of the material and how much material was being hauled out. So there were some spots of spikes during excavation. Discussions with the contractor about what was going on at the time were chalked it up issues with the scales."

While the contract specifications didn't require the contractor to control the weight of tunnel muck generated by the Mitsubishi EPBM, Obayashi installed a belt scale system incorporating twin balances on the conveyor. A continuous conveyor was favored over a

As the opening of the rail line draws near, Sound Transit is reassuring residents it has identified all the voids and are filling them.

Beacon Hill station design

"We've look at the data along the entire alignment in Beacon Hill and this was the only area where spikes indicated haulage of more material than we ecessary," said Gray. "Teams have also gone back in and done some compaction grouting beneath the voids and the top of the running tunnels to make up for any spots that may have taken in less CDF initially. It will also firm up the ground between the voids and the tunnels in case of seismic activity. We're confident in the work to fill in the voids, because the amount of material put back in the ground adds up to the amount in those over-excavation spikes."

The work is estimated to cost upwards of $1 million. Whether the contractor pays, or the expense comes out of Sound Transit's contract contingency fund or its owner controlled insurance program, has yet to be determined.

"Looking back on it now, as we get ready to excavate the University Link were we have two to three miles of tunnels to build between downtown and the University of Washington, we're going to be extra vigilant in measuring the amount of material that's coming out," said Gray.

Meanwhile, Shannon and Wilson, engaged as Sound Transit's geotechnical instrumentation monitoring consultant on the project is still doing some monitoring work along the alignment and crews are finishing up some compaction grouting in time for next Saturday's grand opening.