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Wet, Cold Can’t Stop Metro St. Louis Microtunneling Project
“The work pit was approximately 40-feet long and 15 feet wide interlocked with sheet pile with steel beam walers. The come-out pit was shored with a trench box and pushed in sheet pile.”
The second tunnel of approximately 600 linear feet was about a half mile from the first tunnel.
“Again, an interlocked sheet pile with walers was used for the work pit which also required a continually-operated dewatering system,” Heisler said. “The come-out-pit was a 12-foot diameter drilled casing with the lower 18-feet in solid limestone. No dewatering was required for the excavation.”
Drives were made with an Akkerman SL44 microtunneling machine with 400 tons of thrust capacity.
“The guidance system was laser mounted outside the tunnel at the back of the pit and projected to a target mounted on the front of the machine,” Heisler said. “It was monitored and steered from a control station on ground level positioned next to the pit.”
Joints of 48-inch steel pipe were 20-feet long and hoisted into position and welded to the previously-installed pipe. A 20-foot Derrick Modular Separation System, placed about 25 feet from the work pit, processed slurry which discharged into a settling pond adjacent to the machine.
Wet and cold
Heisler said the 434-foot tunnel was through very wet, silty sand. The 600-foot tunnel began in very wet, silty sand conditions but the last 100 feet was in solid limestone.
“Wet and cold conditions were a factor throughout the project,” Heisler said. “The project was scheduled to start in August 2009, but was delayed until December due to permits and easements. Freezing weather added a degree of difficulty that would not have been encountered in warmer conditions. Extreme cold affected the mud slurry entering and leaving the separation plant.”
Construction of the tunnels was completed in February 2010.
J & J Boring began operations as a horizontal boring subcontractor in 1983. To meet changing customer needs, the company has evolved to include pipe jacking, horizontal tunneling and liner plate tunneling.
J & J Boring has completed more than 1,000 projects, most for water, sanitary and storm sewer systems. Typically the company works as a subcontractor, but has the capability to turnkey boring and tunneling projects. J & J Boring is a certified WBE, DBE contractor.