When loose subsurface conditions stalled a jacked pipe tunnel boring machine approximately 25 feet below the Garden State Parkway in Clifton, NJ, immediate action was needed to stabilize the soils so that the TBM could be advanced without further ground loss. freezeWALL, Moretrench’s artificial ground freezing division, was key to the successful resolution of this potentially costly problem.
The majority of the tunnel alignment was through medium dense overburden. Rock was anticipated in the tunnel invert over the last to 30 to 40 feet. Groundwater was below the tunnel horizon. The TBM, a mixed face machine, had advanced to within 35 feet of the receiving pit when soil conditions abruptly changed from medium dense material to very loose silty sand embankment fill, with rock surface rising in the invert. The tunneling contractor could not control the running ground once the TBM slowed down on the rock below, resulting in development of a sinkhole on the shoulder of the highway embankment.
The Only Viable Option
The critical concern was immediate stabilization of the running embankment sands to avoid damage to the Parkway. With the TBM almost at the end of the alignment, the tunneling contractor’s jacking pressures were quickly approaching the jacking capacity of the system. Every day the TBM sat idle increased the likelihood that excessive jacking forces would be required to re-initiate movement. Ground freezing, which could be accomplished quickly regardless of the soil material, and without the need for detailed soils delineation, was the only viable solution in the time frame.
Design and Installation
In light of the urgency, liquid nitrogen was used to form the freeze. Working from within the receiving pit, freezeWALL used horizontal drilling techniques to install freeze pipes on a fan array and create a canopy of frozen ground above the TBM. With the TBMs jacking forces already severely limited, care was taken to keep the freeze away from the machine itself to avoid locking it in place. Holes were therefore surveyed as they were installed. Any loose ground between the freeze and the TBM could slough in, leaving a protective "umbrella" of frozen soil.
The freeze pipe array was installed in just four days. The natural moisture content of the soil above the water table was sufficient to form the freeze, which was completed in less than two days. Mining of the tunnel continued immediately thereafter.
Contact Moretrench, one of the key sinkhole companies, for services and sinkhole repair cost.