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.

The Robbins SBU-A is a type of hard rock and mixed ground cutting head mounted with disc cutters and used in conjunction with an auger boring machine (ABM). The SBU-A, available in diameters from 24 to 72 inches, consists of a circular cutterhead mounted with single disc cutters capable of excavating rock from 4,000 to over 25,000 psi. For excavation, the SBU-A is welded to the lead steel casing. The ABM provides both torque and thrust to the cutting head, while a full-face auger is used for spoil removal.

In hard rock, disc cutters penetrate the rock face and create a “crush zone” through which fractures propagate. Material is then chipped from the face into openings in the cutterhead called muck buckets, which transfer the material to the auger. In mixed geology operation of the machine is the same, though the cutterhead is fitted with a combination of tungsten carbide insert cutters, disc cutters and drag bits to excavate the ground.

The Robbins SBU-M operates similarly to the SBU-A. It is designed as a manned entry, hard rock boring machine for use on both long utility installations (300 feet and over, depending on geology) and crossings with specific line and grade requirements.
At machine launch, the SBU-M is bolted to the lead casing and a small invert auger inside the casing provides spoil removal. An in-shield motor and drive train provide torque to the cutterhead, while forward thrust is provided by an ABM or pipe jacking unit through the casing.

The machine can be steered throughout the drive from an operator’s console inside the rear shield. Control of line and grade is made possible through articulation cylinders and adjustable stabilizer pads, while a laser targeting system provides accurate monitoring of the machine’s position.