Remedial Base Grouting of Caissons under Artesian Conditions-Paper

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Forest Hill Station, part of Toronto’s Eglinton Crosstown LRT, was constructed using a top-down method with king piles and secant-pile shoring. During installation of king pile S110, approximately 25 m³ of additional concrete—about 63% above the theoretical volume—was required. Follow-up investigations found reduced soil density near the pile, indicating that drilling without stabilizing fluid had disturbed the ground and caused soil loss.<br /><br />Because access from the surface was obstructed by utilities, businesses, and the station roof slab, remedial permeation grouting was carried out from an adjacent tunnel. The work involved installing Multiple Port Sleeve Pipes (MPSPs) through the pile bases and into the soil below. Drilling and grouting took place under artesian conditions, requiring a closed drilling system and a blowout preventer to control water and soil migration.<br /><br />Cementitious grout was injected through individual ports at specified pressures, with injection flow and pressure monitored in real time. The project team used drilling logs, grout records, and surveys to check installation details and treatment progress. Grouting was completed over approximately three weeks: 7.9 m³ of grout was injected through 41 ports, and 37 ports achieved pressure refusal or were considered closed. Some ports could not be reopened or regrouted, and grout efflux occurred at two locations during connected injection attempts.<br /><br />No structural movement was detected during the work. Although site constraints prevented post-treatment SPT or CPT testing, the client’s geotechnical consultant reviewed the grouting data, and modeling and movement monitoring were used to assess treatment effectiveness. The case demonstrates that permeation grouting can provide a practical ground-stabilization and verification method in restricted, artesian conditions while minimizing disruption to ongoing construction.

Keywords

Forest Hill Station

Eglinton Crosstown LRT

top-down construction

king piles

secant-pile shoring

permeation grouting

Multiple Port Sleeve Pipes

artesian conditions

ground stabilization

Toronto construction