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Evergreen Line tunnel boring safety concerns after series of sinkholes

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DH Vancouver Staff Jan 13, 2015 7:30 pm

The B.C. Ministry of Transportation and Infrastructure will be holding a special Community Information Meeting for local residents of Cecile Drive in Port Moody after a number of sinkholes occurred on the surface along tunnel boring route for the SkyTrain Evergreen Line.

The meeting will occur at the Evergreen Line Project Office (2900 Barnet Highway, Coquitlam) on Wednesday, January 14 from 7:00 p.m. to 8:30 p.m.

Since tunnel construction began in February 2014, the Evergreen Line project office says two small sinkholes have appeared at locations directly above where the tunnel boring machine is stopped for regular maintenance.

A sinkhole appeared in a parking lot at Chateau Place on October 27 and a second sinkhole opened up on January 2 in the middle of the road on Cecile Drive. The sinkholes were promptly filled by contractors and the public was not at risk as both sinkholes formed within fenced-off areas.

The tunnel boring machine runs through challenging soil conditions – an area that is mainly loose glacial till, a legacy of debris flows.

Normally, tunnel boring machines do not stop as a means to prevent sinkholes from happening, but unexpected soil conditions along the route required contractors to perform more stoppage and maintenance than originally anticipated. This increases the risk of the formation of air pockets which can cause cave-ins and surface sinkholes, especially where soil conditions are unstable.

Additional geotechnical drilling tests are being conducted to examine the soil stability along the two kilometre tunnelled route.

With two sinkhole incidents, property owners in the area have expressed concerns over the possibility of sinkholes forming under buildings that run directly under the completed tunnel route – from Barnett Highway to Cecile Drive, which is about 33 per cent of the total length of the tunnel.

For the remainder of its journey south of Cecile Drive, the tunnel boring machine will not be traversing under any private property: it will bore directly under Clarke Drive, right up to the south tunnel portal near Morrison Avenue.

The next scheduled machine maintenance location will be at Clarke Road near Seaview Drive. Officials say they will fence off the area as a precaution for sinkholes.

The Evergreen Line tunnel boring machine is 9.84 metres wide and 90 metres in length. It is significantly wider than the Canada Line’s tunnel boring machine so that both the outbound and inbound tracks can fit in the large tunnel; only a single pass of the tunnel boring machine is required to complete the underground segment of the $1.4-billion, 11 kilometre SkyTrain extension to Port Moody and Coquitlam.

 

Feature Image: Ministry of Transportation

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DH Vancouver Staff
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