Moretrench’s ability to be proactive when unanticipated conditions are encountered during the course of the work has been instrumental in minimizing delays on a number of projects. The West End Residences development in Boston, MA is a case in point. Moretrench was initially retained by A.A. Will Corporation of Stoughton, MA to install upper level tiebacks to support earth retention systems at two building sites that, together, encompassed two city blocks. When extremely weathered and friable rock was encountered during excavation at the larger site (Building A/B), Moretrench was quickly able to mobilize and accomplish a supplemental stabilization program.
Building A/B Site
While the sites were only approximately 150 feet apart, their subsurface profiles were markedly different, dictating different excavation support systems. At the Building A/B site, approximately 15 feet of fill and natural sandy soils overlaid what was anticipated to be competent sedimentary argillite. Given the high rock profile, a drilled soldier pile and lagging system was selected to retain the overburden soils. Tiebacks ranging in capacity from 89 to 225 kips and toe ties on the order of 15 to 60 kips were used to externally support the soldier piles.
However, as drilling and blasting to subgrade proceeded below the tip of the soldier piles it became evident that the exposed rock was significantly more weathered and fractured than previously thought and resembled cornflakes. Moretrench worked with A.A.Will’s engineer to develop a cost-effective rock bolt solution that could be rapidly put into effect.
Rock stabilization is typically accomplished by drilling and grouting rock bolts in place on a 15-foot grid pattern and covering the rock face with wire mesh secured to the rock bolts. However, given the extremely poor quality of the rock, the approach here was more akin to soil nailing, with rock bolts installed on a nominal 5-foot by 5-foot grid pattern, reinforcing mesh placed and a 3-inch thick shotcrete facing applied to complete the retention system. More than 17,000 square feet of rock face, up to 30 feet high in places, was stabilized for the project.
Site soils beneath the Building C footprint, situated close to the Charles River, consisted of up to 20 feet of granular fill overlying clay, with glacial till over argillite encountered at approximately 40 feet below existing grade. Groundwater was at 11 feet above the proposed new subgrade. Given the high groundwater table, a braced sheetpile cofferdam was selected for the approximately 24,000 cubic yards of excavation. While upper level bracing consisted of cross lot and corner struts, lower level bracing was provided by tiebacks and rakers. Moretrench installed 25 tiebacks ranging in capacity from 99 to 175 kips for the project, some of which involved carefully coordinated drilling beneath existing, pile-supported buildings. At several column locations, tiedown anchors were installed to resist seismic forces. Moretrench also provided tiedown anchors for supplemental support of driven piles installed to support the general contractor’s tower crane.