Tunnel Access Shafts for Sewer Network

GeoTerre Limited commissioned Gavin and Doherty Geosolutions (GDG) to undertake a finite element analysis to investigate potential mechanisms of failure associated with the base stability of a deep shaft being excavated as part of the Toronto Sewer project.

Scope of Work:

A confined granular aquifer underlies a large portion of Toronto city. Due to the water pressures  and  highly permeable nature of the soils within this zone, the aquifer can pose a challenge for deep excavations in the city.  This project involved a major upgrade of the Toronto sewer network that required several such excavations to construct the deep access shafts needed to accommodate the horizontal tunnel construction. Due to the challenging ground conditions and the close proximity of the base of the access shaft to the aquifer, there were significant concerns regarding the stability of the excavation against base heave during both the temporary and permanent works.

GDG was commissioned to review the shaft design (including the temporary works) and to develop a full time dependent finite element modelling of the construction works. The shaft construction included a secant pile wall through upper granular deposits, with the lower reaches of the shaft supported by a structural shotcrete lining. The shaft was 6m in diameter and 15m deep. In the long-term condition, a concrete base plug was  constructed to resist uplift, however the time required for the final excavation step and for the base plug to harden was deemed a significant risk.

GDG performed detailed analysis of each phase of the construction works, including excavation, piling, shoring and even construction delays. The analysis showed that some small movements would occur but the shaft would remain stable for up to 30 days after construction, providing sufficient time for the base slab to be poured.

A simple monitoring strategy was also suggested by GDG which involved surveying the base movements between working shifts to validate the construction model against the actual conditions on site.