“Soil conditions were extremely soft, located under the water table, and required the use of a 300-foot-long, 110-ton tunnel boring machine (TBM) known as an earth pressure balance machine (EPBM) designed for these precise conditions,” Moran said.
As the EPBM goes to work, it mines and pushes the soil and muck through a screw conveyor system that transports the residue away from the machine.
This Harbor Deepening Program was the first in New York City to use an EPBM.
“The WSP team used innovative data collection and interpreting techniques to understand and control the performance of the TBM,” said Kevin Moon, resident engineer for the project. “We monitored the earth pressures to make sure there was no over-excavation and monitored the level of wear to the rippers – the ‘teeth’ of the machine – to ensure the machine was operating properly.”
The starting point in Staten Island and the end point in Brooklyn were both located in densely populated urban areas. The tunnel passed 80 feet below the Belt Parkway, a six-lane highway along the South Brooklyn shoreline where the ground conditions were particularly challenging.
“Most of the ground was extremely soft, almost like soup; especially the fine-to-medium sand under the Belt Parkway, which proved to be very challenging to excavate,” Moon said. “Our geotechnical experts developed a sophisticated construction management plan to monitor potential settlement and observe the performance of the TBM around the clock.”
A four-shift inspection schedule provided the project with a team responsible for monitoring the muck volume, tunnel face pressure, ground settlement points and groundwater table.