Doublestar Drilling was awarded the design-build secant compression ring shoring & excavation scopes of work for three tunnel boring machine launch, access and retrieval pits in September 2017. The North Interceptor Sanitary Trunk Project is located in St. Albert, AB.

The circular secant compression ring pits ranged in diameters from 6m to 8.2m and to a maximum secant pile depth of 18.4m. The subsurface conditions were wet to saturated, ground water was as shallow as 0.7m below grade, and consisted of fill, clay silt and sand to silty clay.

There were 200ea secant piles installed, all were 880mm diameter and interlocked to provide decreased permeability of ground water into the shaft. The piles needed to be embedded up to 9m below the bottom of the excavation elevation to resist base heave, maintain base stability and provide support for the new manholes that were installed at each shaft location.

Segmental casing was used for the entire depth of every secant pile due to the ground water conditions and to maintain the designed interlock between each pile. The piles were installed by Doublestar’s Liebherr LB36 and Soilmec SR65 drill rigs.

Doublestar Drilling excavated each pit using a combination of drill rigs and small excavators. Once the shafts were excavated, we constructed a structural slab to support the new manholes that would be installed at each location once the tunneling work was complete.

As a result of the Doublestar Drilling’s success on the first shaft two more shafts were awarded to increase our scope of work from three shafts to five. The secant piling, excavation and structural slab work were successfully completed ahead of the tunnel boring machine work in July 2018.

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