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Case description

A tunneling contractor excavated a long sewer tunnel by dewatering the ground and using a TBM with steel ribs, filter fabric and wood lagging for temporary tunnel support. The contractor would occasionally encounter quicksand conditions that would breach the temporary tunnel support lining and would contact Peter White for emergency support services to regain control of ground conditions and allow tunneling operations to resume.

After completing each stage of tunneling, steel jump forms were used to install a thick concrete lining to complete the sewer tunnel, following which dewatering wellpoints were decommissioned and the external tunnel lining would become pressurized as the water table became re-established.

The contractor experienced numerous water leaks through the concrete lining and contacted Peter White for assistance to rectify these problems.

Solution

Quicksand inflows associated with TBM operations were usually controlled by drilling through the wood tunnel lagging and injecting chemical grout within the tail assembly of the TBM. Depending upon external soil conditions and the degree of ground disturbance encountered, the spacing and sequence of drill holes, as well as the volume of chemical grout injected at each location, were determined on site to stabilize the situation.

Water seepage associated with concrete construction joints required deep drilling of small-diameter holes around the circumference of the sewer pipe, followed by systematic injection of water-activated chemical grout to seal available apertures.

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