Synchronized SBUs Bore Below Mississippi River: Multi-Tasking Auger Bores Move Muck For Force Main

July 2012, Vol. 67, No. 7
The 72-inch diameter Robbins Motorized SBU used in Clinton, IA, was the largest diameter machine of this type ever made.

Using added water, crews were able to maintain advance rates of 20 feet per day in the clays. Ultimately the machine excavated 200 to 240 feet of clays, and then hit solid rock for the final 150 to 170-feet of the run. Rates increased while in the rock, to about 30 feet per day in one, nine to 10 hour shift.

One of the biggest advantages of the Robbins SBU-M, as mentioned by the contractor, is in its ability for real-time alignment control through articulation cylinders and laser guidance. “We were within ¾-inch to an inch of line and grade the entire time and there was very little drift. I’m used to steering articulated machines, because we use a lot of TBMs. In the softer ground, the weight of the casing helps with control. Keeping on line and grade was easy, particularly when we got into the rock,” said Lilo.

The SBU-M broke through into a receiving pit on Feb. 14. “The SBU-M finished within its tolerances, with almost no deviation whatsoever, even going through the mixed face ground,” said Keefe.

Undercutting the Mississippi
At the same time as the highway bore, two SBU-A operations were ongoing 30 to 35 feet beneath a tributary of the Mississippi River. The 60-inch and 42-inch SBU-As, launched in November and December of 2011, utilized two 60-inch diameter ABMs to excavate hard rock over 10,000 psi.

“We had some challenges with the first crossing, because we had some tight tolerances to match the invert on the receiving side -- it had to be a 0.16 slope. We had to do quite a bit of work to correct grade as we were tunneling across. We pulled the casing two or three times to make sure alignment was accurate, and were able to match it to within an inch and a half to two inches of design elevation,” said Keefe.

Scott Dixon, operator/foreman of the SBU-A for L.J. Keefe, elaborated on the line and grade process: “We were in hard rock with a lot of fractures in it, but it was manageable. Line was very good, but the grade we had to play around with. The 60-inch went fairly well, but with the 42-inch SBU-A we fought a little. We pulled back the casing and adjusted it. We did have a water level earlier on, but we lost it in about 100 feet because of the hard rock.”

Despite the challenges, Dixon was able to average 20 feet per day with the machines, and as much as 40 feet per day on the 60-inch bore in nine to 10 hour shifts.