Rehabilitation of underground structures located within congested urban environments is inherently challenging. The New York City Metropolitan Transit Authority (NYCMTA) consistently encounters such challenges when upgrading its facilities. The work is further complicated by the requirement for deep excavation adjacent to aged buildings. At the Rockwell Place vent shaft rehabilitation site, where construction-induced settlement had already occurred, void grouting, fracture grouting and provision for compensation grouting were employed to mitigate future problems.
Subsurface soils consisted of 15 feet of miscellaneous sandy fill overlying a 5 to 10 foot stratum of till. Beneath this were fine, medium dense non-cohesive sands. Support of deep excavation by the installation of a soil mix wall was underway when the contractor encountered boulders in the till. Pre-drilling through these obstructions in order to advance the wall induced excessive settlement of an old, 9-story masonry structure founded on shallow spread footings. Soil mixing operations were temporarily halted while the general contractor sought proposals to mitigate further settlement. Moretrench was awarded the contract with an engineered remedial approach that involved three grouting phases.
TAM pipes were installed between the affected building and the adjacent excavation zone, extending to the bottom of the proposed soil mix wall. A cement grout void filling program was then conducted which would also identify and treat loose or voidaceous soil zones. Next, a pre-treatment phase of fracture grouting was performed to tighten up the soil mass in preparation for compensation grouting should additional settlement occur during the balance of the work. Several rounds of fracture grouting were performed in both the fill and the lower sand zone until grouting pressures had increased sufficiently and/or slight heave was observed, indicating that the required pre-treatment had been achieved.
Moretrench’s ground treatment, together with the contractor’s subsequent revised construction methods, mitigated further settlement potential and eliminated the need for compensation grouting.