Around Moretrench: When Dewatering Can’t be Done

May 4, 2015

When unanticipated subsurface conditions are encountered on the job site, Moretrench has the in-house resources and expertise  to develop and implement viable alternatives for a wide range of underground construction challenges.  And when this capability is coupled with the ability to respond rapidly when the call comes in, general contractors and owners can look to getting the project back on track quickly and efficiently.  Such was the case for a recent microtunneling project in Camden, NJ.

The alignment of a  48-inch diameter, 340-foot long utility pipe lay in very close proximity to Newton Creek.  Dewatering to below excavation subgrade is the most straightforward and economical approach to ensuring dry/stable working conditions,  but dewatering was not permitted due to the presence of naturally occurring heavy metals in the groundwater.   Efforts had been made to drive the steel sheet piling excavation support  into the underlying extremely dense (100+ blows per foot) sands sufficiently  below excavation subgrade to minimize groundwater infiltration to quantities that could be handled by sumping.  However, this ultimately proved unsuccessful.  With the project now on the fast track,  the tunneling subcontractor  brought in Moretrench to develop an effective solution that could be implemented quickly.  For this project, the solution that was deemed the best option, based on project objectives and the prevailing site conditions,  was the installation of jet grouted bottom seals.

Continuity of the jet grout was verified by coring.

Continuity of the jet grout was verified by coring.

With time of the essence, mobilization was underway within two weeks of the initial phone call. Working 10-12 hour shifts, six days a week, Moretrench installed 42, 6-ft diameter, 12-ft thick  bottom seals columns within the launch pit excavation support and 43, 6-ft diameter, 8-ft deep columns at the receiving pit.  The contract also included jet grout stabilization of the soils behind the microtunnel break-in and break out portals, and jet grouting  to seal the corners of the steel sheetpiles, which were not interlocked.

During bottom seal installation, similar groundwater concerns arose for the auxiliary transition cofferdams that would be used to bring the 48-inch pipe within the reach of conventional trenching installation. This prompted the owner to add these areas to Moretrench’s scope of work. Despite this additional commitment, and working in  harsh winter weather conditions with temperatures dipping as low as low as 0°F, the entire expanded contract was successfully completed in  two months, allowing excavation to proceed soon thereafter.