The 2.3-mile extension to the Route 7 expressway is designed to bypass its previous terminus at a busy Brookfield retail center, known as the Four Corners, and thus alleviate long-term severe traffic congestion. Construction of the new roadway required cutting into the slope of an existing hill, on top of which is a cemetery dating from the 1700s. Design and installation of a permanent retention system to support the steep cut was coupled with the need to preserve the cemetery site.
The owner’s original retention system design consisted of a rock-socketed, tied back soldier pile wall with pre- cast concrete facing. However, O & G Industries Inc. of Torrington, CT, which was awarded the bypass construction contract, resolved, in consultation with Moretrench, to pursue a value-engineered alternative. This consisted of a permanent soil nail wall with a sculpted facing that would simulate natural rock and be in keeping with this scenic area. Under direct contract with O&G Industries, a California-based engineering firm was retained to complete the design, which also included installation of mechanically stabilized fill atop the soil nail wall to address hill geometry and prevalent soil conditions. Moretrench was contracted to O & G to install the approximately 5,650 square foot soil nail wall. Subsurface conditions identified by Haley & Aldrich, Inc., of Glastonbury CT, the project’s Geotechnical Engineer, typically consisted of medium dense, uniform, fine to medium sands overlying marble rock. Since these soils exhibited very little cohesion, vertical soils nails (VNAILs) were installed by Moretrench prior to excavation. A VNAIL is a drilled and grouted, vertical or near vertical element designed to promote temporary arching in marginal ground, increasing the stand-up time of an excavated face sufficiently to allow conventional soil nailing.
VNAIL locations were spaced at 2.5-foot intervals along the proposed cut line. Wet rotary drilling methods were used to advance the slightly battered installation boreholes to top of rock. The boreholes were cleaned and tremie-grouted with 3,000 psi cement grout at w/c = 0.62, and Grade 75 threaded bars were placed to complete the installation.
Prior to production work, verification testing was conducted on several sacrificial nails to confirm design assumptions. Excavation and soil nailing followed in 5-foot vertical lifts. Since some nails would be required to penetrate into rock, both wet rotary and percussion drilling methods were used to advance boreholes to design length, which varied between 15 and 45 feet. Nails were typically installed at 15 degrees from horizontal on a vertical spacing of 5 feet and 5-foot offset lateral spacing. The design also incorporated drainage board installed prior to placement of a layer of galvanized wire mesh and 4-inch shotcrete facing.
Quality Control/Quality Assurance
During VNAIL installation, horizontal spacing, alignment and angle of installation of the boreholes were carefully checked to ensure that soil arching would develop as intended and that the VNAILs would be properly integrated with all other components of the soil nail wall system and consistent with final wall geometry. QA/QC protocols during production work included performance and proof load testing of soil nails at selected locations, as well as sampling and testing of grout and shotcrete panels.
The value-engineered soil nail alternative was successfully completed on schedule, and resulted in significant savings over the original design.