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Synchronized SBUs Bore Below Mississippi River: Multi-Tasking Auger Bores Move Muck For Force Main
Testing of the 395-foot crossing below Highway 67, the busiest roadway in the area, revealed a mixed face of silty, watery sand, clay and 10,000 psi UCS hard rock. “We opted to use SBU technology, even though other contractors said it could only be done with microtunneling. They thought this because of geology rather than crossing lengths,” said Keefe. Additionally, strict line and grade requirements were required to link up to the rest of the force main -- within six inches of line and two inches of grade, thus requiring a machine that was highly accurate. The company owned a Robbins 36-inch diameter SBU-A, and had leased several SBUs on at least a dozen projects over 15 years. “We were looking for the right project to use the SBU-M, and with the mixed ground, tight tolerances and longer length, we thought this would be the perfect opportunity,” said Keefe.
The contractor opted for a Robbins 72-inch diameter Motorized Small Boring Unit (SBU-M) with mixed ground cutterhead, used in conjunction with a 60-inch auger boring machine. The 72-inch machine, the largest SBU-M yet built, would install steel casing of the same diameter to allow room for three bundled force mains -- two 18-inch lines and one 24-inch line -- as the carrier pipe. “The ability to expedite the owner’s schedule by using the SBU-M and SBU-A simultaneously with a TBM was a big advantage for us,” continued Keefe.
Excavation with the 72-inch SBU-M began on Jan. 3, 2012, from a 25-foot deep launch pit. Crews encountered an unexpectedly long drive of hard clays about 15 feet into the bore, but were able to keep mining through the clays by adding water through the cutterhead. “We started with a vacuum assist for spoil removal, but when we got into the clay right away, we had better success when we stopped and switched out to the auger,” said Keefe.
“The main challenge while in the clays was in getting that softer ground material out without contaminating the cutting discs. We not only added water through the cutterhead, but also to the casing itself, up in front, to make sure the material kept coming through,” said Steve Lilo, operator/foreman of the SBU-M for L.J. Keefe.