Renovations to the St. Lucie County Courthouse in Fort Pierce, FL included extension of an underground tunnel and relocation of an existing elevator shaft used to bring prisoners in for hearings. Moretrench was one of two geotechnical contractors approached by the general contractor, Boran Craig Barber Engel Construction (BCBE), to conduct a site visit and give recommendations on how to provide support of excavation for tunnel extension and installation of the new elevator shaft inside the operational building.
The soil borings report prepared by A.M. Engineering of Port St. Lucie showed approximately six feet of fill overlaying fine sands. Moretrench proposed a permeation grouting program, which could be readily accomplished within the tight work area and would increase the strength of the granular soils to allow workers to safely install the concrete masonry unit (CMU) walls for the tunnel extension and elevator shaft from within the excavation. Permeation grouting is also a low-impact technique that would minimize disturbance to ongoing court business. Based on this approach, Moretrench was awarded the contract for the work.
Given the limited access and tight working conditions, Moretrench had anticipated jetting the Tube à Manchette (TAM) grout pipes in place. However, permeation grouting had also been used previously for soil strengthening prior to excavation for the existing shaft. Difficulty in initial jetting for the new shaft indicated that in some areas the previously placed grout had travelled further than expected, rendering some of the current target soils too dense for jetting. TAM pipe installation was therefore accomplished using limited access, low-headroom drilling. Moretrench also needed to work around a number of utilities crossing the target soils and operate with a heightened level of spoil control within the confines of the building.
The new shaft was to be located 10 feet from the existing shaft and would extend to a depth of 13.5 feet below working grade, resulting in approximately 50 linear feet of treated soil. To achieve the required soil stabilization, Moretrench utilized a 35 percent sodium silicate grout mixture with an organic setting agent.
One-inch diameter TAM pipes, with sleeved ports at 15 inches on center, were installed within 3-inch cased boreholes drilled to two feet below excavation subgrade. Grout pipe locations were determined relative to the limits of the excavation and field-fitted where necessary to avoid utility conflicts. Also to avoid utility conflicts, some casings were battered. The annulus between the casings and the TAM pipes was filled with a brittle sheathing grout and the casing withdrawn from the borehole. Initial high injection pressure fractured the sheathing grout to initiate grout flow, and permeation grouting was then performed in a primary-secondary sequence until a pre-determined volume was reached at each location.
During the grouting program, daily grout logs were analyzed to ensure that proper coverage had been achieved. Excavation began one week after completion of the injection program to allow for grout curing. Moretrench returned to the site as each 5-foot level was exposed to ensure wall stability to subgrade.