Abstract:
During excavation of a 9m wide x 6.5m high tunnel along one of the busiest streets in Canada, a sizeable sinkhole developed. The sinkhole was approximately 28m wide x 40m long and occurred within an infilled valley extending to the proposed tunnel alignment at 30 metres below ground surface. Tunneling through the infilled valley presents several challenges, and conventional tunnelling techniques may not be effective without ground stabilization. The sinkhole was immediately backfilled with approximately 3000 m3 of low-strength concrete. However, ground stabilization was required beneath the backfilled materials. Jet grouting and compaction grouting were performed to surgically treat and stabilize the disturbed soils by increasing the density, reducing the settlement and improving the permeability of in-situ soils within the target zone. Prior to installing the ground improvement elements, probe holes were advanced to get a general understanding of the disturbed subsurface ground condition. Jet grout columns were installed to create a jet grout headwall (5.5m x 15m in plan x 11.5m high) and compaction grout holes were installed in a grid pattern east of the headwall/tunnel face (13m x 15m in plan). Additionally, compaction grout holes were installed close to the face of the existing tunnel to mitigate loss of jet grout backflow into the existing tunnel. Ultimately, the grouting and ground stabilization program reduced settlement and improved the subsurface ground condition for tunneling to proceed. This paper provides a detailed case history of this project by reviewing the project setting, subsurface conditions, design considerations, construction challenges and approach for both jet grouting and compaction grouting and quality control program implemented by the specialty geotechnical contractor.