Massey Bridge deal killer? Foundation bedrock below 600 – 700 meters of wet sand/mud

Gov BC image of bridge replacing George Massey Tunnel, from YouTubeThis rather shocking information is in a letter by Tom Morrison to the editor of the Delta Optimist, November 18, 2016. Could this information be the death knell for the bridge?


Bridge foundations can’t find solid ground
Delta Optimist

Re: Tunnel safety called out, Nov. 9

So the George Massey Tunnel is an accident black spot and difficult to get to when an accident happens. Everyone knows what happens when there is an accident on the Lions Gate and/or Second Narrows bridges.

OK, a fire is more dangerous in a tunnel than on a bridge – unless the fire damages the bridge structure. And, sadly, high bridges seem to be more of a focus for actual or threatened suicides than tunnels. You can argue that kind of safety back and forth.

There’s a bigger safety concern than that. In 2013, the B.C. Ministry of Transportation and Infrastructure contracted two geotechnical investigation boreholes, BH 13-01 and BH 13-02, on the north and south shores of the Fraser River, respectively, at the site of the proposed towers for a cable-stayed bridge.

The resulting geotechnical data report (Golder Associates, April 24, 2014) records these holes went to 335 metres (1,099 feet) depth in sand and silt – more than twice the height of the towers of the Alex Fraser Bridge – without tagging bedrock.

A 1995 Geological Survey of Canada paper (Britten, Harris, Hunter, Luternauer, The bedrock surface beneath the Fraser River delta in British Columbia based on seismic measurements, Figure 8.) estimates the depth to bedrock at the bridge site at 600 to 700 metres (1,970 to 2,300 feet). So the towers are going to be built on 2,000 feet of waterlogged sand and mud in an area assessed at moderate to high risk of liquefaction in the event of an earthquake (Geomap Vancouver).

It would be interesting to know: Can the towers be built in that location at all, or at reasonable cost? How? Have cable-stayed bridges been built on these foundation conditions before? Where? What happens in a major earthquake? Considering the foundation material, how much movement of the towers, due to settlement or earthquakes, can happen before the bridge breaks or becomes unsafe?

The ministry has refused to answer this question.

The alternative is to keep the existing tunnel and build a second tunnel on easily sampled and well understood foundation material, using long-proven technology. It would also be interesting to know the relative costs of these two alternatives and, in particular, how much taxpayer money has to be built into the bridge design to deal with poorly known foundation conditions.

Tom Morrison

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