“Given the right ground conditions, such as a rock job or harder ground, it’s the go-to weapon,” said Claude H. Nix Construction vice president and COO Jon Nix.
The Utah-based contractor honed its use of the rock/mixed ground cutting attachments at a phosphate refinery in Wyoming — perhaps not the most obvious site for a trenchless construction success story. The plant, owned by The Simplot Co. of Rock Springs, Wyo., needed water cooling lines installed for phosphate production. The trajectory of the lines, meant to cool down byproducts of the plant’s production, required crossing under State Highway 430. The contract did not specify the method, so C.H. Nix, in joint venture partnership with JASCO Inc., acted as a surveyor on the project to determine the best way forward.
“The original alignment was supposed to be for five 36-in. (900-mm) casings below the highway. When we dug a test hole, we hit a solid sandstone layer below clay, about 11 ft (3.4 m) down,” said Nix.
The contractor already owned a Robbins Auger Boring Machine (ABM) and given the rocky conditions, contacted Robbins Small Boring Unit (SBU) product manager Kenny Clever to discuss the possibilities.
“Kenny [Clever] suggested a 54-in. (1.4-m) Motorized SBU (SBU-M) and did some CAD drawings to see how many 24-in. (600-mm) water pipes could fit in the larger casing. We reduced the scope to two crossings, which cut down on pit construction costs and project duration from about 180 days to 60 to 80 days,” said Nix.
The contractor made this proposal to project owner Simplot and it was approved, resulting in $250,000 in savings.
Getting to Work
Robbins Field Service assisted the crew in launching the mixed ground SBU-M, which was specially designed for mixed face conditions consisting of rock and clay. The mixed ground cutterhead was mounted with both carbide bits and disc cutters, while large muck openings allowed muck to flow into an invert auger running through the casing to the ABM.
The machine was launched in a 45-ft (14-m) wide by 50-ft (15-m) long bore pit at 20 ft (6 m) deep and welded to a 20-ft (6 m) length of permalok casing. The custom casing, specified by Simplot, included a special epoxy coating on the inside. During excavation, spoils were removed via an invert auger attached to the ABM, transferring spoils out a door in the ABM’s master pusher. Line and grade were monitored using a laser targeting system, and adjustments were made using hydraulic articulation cylinders. The crew made slight steering changes from the operator’s console inside the machine’s rear shield.
The first 300-ft (90-m) long crossing below the highway was completed in about six weeks, despite difficult ground, including sticky clay that required a water injection system to be installed behind the machine. While the auger could transport the clay during machine operations, when boring stopped for the day, layers of clay would dry onto the auger, creating a buildup over time. The water line added enough liquid to make the clay the consistency of a slurry, easing transport through the invert auger. Hole through was within line and grade requirements — since the crossings were just 5 ft (1.5 m) apart, line was the more important of the two. Robbins Field Service members were a big part of proper training that led to the success, said Nix. “They were instrumental — we have a lot of experience with Akkerman TBMs, but running an SBU is different from a soft ground TBM. They helped us out immensely.”
The second crossing, done without a learning curve, was completed in just three weeks, with no cutter changes required during either drive. Nix stated that he would definitely use the machine on future projects given the right ground conditions: “This was an awesome piece of equipment. I was very impressed with the construction quality of this machine, and if I came across another job that required this technology I would use it again.”
Desiree Willis is a technical writer for The Robbins Co., based in Kent, Wash.