When multiple geotechnical considerations need to be addressed on a single site, Moretrench’s ability to marshal diverse services from within its own organization offers a distinct advantage, both in cost and scheduling. Such was the case for the construction of a new apartment building in East Rutherford, NJ.
Moretrench was initially contracted to provide temporary earth retention for the site. However, evaluation of the soils report showed groundwater at five feet above the proposed subgrade and the presence of low concentrations of Tri Chloro Ethelyne, indicating the need for both dewatering and groundwater treatment in addition to the earth retention. Moretrench offered General Contractor Millennium Homes, of Livingston, NJ, a design/build option that would address all three aspects under one contract.
Site soils consisted of predominately medium-dense to dense silty sand, with existing elevation varying from +56 feet to +63 feet. Proposed subgrade was at +36 feet. Moretrench’s soldier beam and lagging earth retention design resulted in installation of the retention system for a cut of 16 to 25 feet. Tieback anchors, ranging in design load from 64 to 105 kips, were incorporated where the depth of cut dictated the need for additional lateral support.
INSTALLATION IN THE DRY
Piles varying in length from 20 to 36 feet were driven at 8 to 10 feet on center around the perimeter of the excavation, and the earth retention system was installed to approximately elevation +43, 2 feet above the groundwater table. The 1,100 linear foot perimeter wellpoint system was then installed around the perimeter of the site in front of the proposed soldier beam wall to draw groundwater down below the design subgrade.
Wellpoints were jetted in place at each soldier beam location. A carbon filtration system with bag filters was installed to treat the contaminated groundwater before it was discharged into the storm sewer system. With the dewatering and groundwater treatment system in place and fully operational, the earth retention was resumed. Where required, the temporary tiebacks, with lengths ranging from 30 to 50 feet were installed at 30 degrees from horizontal and proof tested to 133 percent of design load. At one location, where drilling for tieback installation was not permitted beneath a building adjacent to the excavation, lateral support was provided by raker braces with concrete heelblocks.
Approximately 6,700 square feet of soldier beam and lagging earth support was installed for the project. During the work, the amount of groundwater generated for treatment (70 gpm) proved to be greater than originally anticipated. However, the filtration system was able to be immediately upgraded with minimal disruption to the dewatering or earth retention operations.