Brightwater TBM Rescue

Lake Forest Park, Washington


The Project

An 18-foot diameter tunnel boring machine became stuck 300 feet below a residential neighborhood and two miles into a four mile long tunnel construction project. The high ground water pressure at the 300-foot depth made repair from inside the TBM impractical. An innovative but somewhat risky solution requiring a second, smaller diameter TBM, coming from the opposite direction, to be driven into the stuck TBM’s empty shell was the only feasible way to complete the project.


The Challenge

Disturbed glacial sand and clays and high groundwater pressure surrounding the stuck TBM had to be stabilized. The stabilization and “sealing” around and under the stuck TBM had to be completed from a residential street 300 feet above with minimal disruption and noise. Soil stabilization needed to protect workers inside the stuck TBM while they dismantled the conveyors, bulkhead walls, etc., leaving only the steel shell in the ground under 5 bars of water pressure.


The Solution

SoilFreeze created a continuous, veri􀃶able, water-tight structural mass of frozen soil around and below the front section of the TBM. Specially designed “zone” freeze pipes were installed that concentrated the freeze in the lower 50 foot “zone” on top, sides and in front of the TBM, which used less power. Single phase “residential power” could therfore be used instead of noisy diesel generators. To freeze the soil directly below the TBM, a few short freeze pipes were installed from inside the tunnel.


The Results

The soils around the stuck TBM were completely frozen creating structural support and sealing out groundwater throughout the two month TBM dismantling process. The contractor was able to drive the smaller 16-foot diameter rescue TBM though the stabilized (frozen) soil to successfully complete the tunnel.

 

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