The city of Richmond, Indiana is a growing community of 50,000 people. To meet future projections, the city created a plan to double the current capacity of its sewer system via a 6.4 km (4 mi) long extension. The new gravity sewer would also eliminate the need for costly pump stations.
In December 2006, the Richmond Sanitary District awarded a USD $4.7 million contract to general contractor Brackney, Inc for construction of the Chester Boulevard Interceptor Sewer—a 3.2 km (2 mi) stretch of the new pipeline to provide service to a commercial district and nearby hospital. Subsequently, Midwest Mole, Inc. was subcontracted by Brackney to excavate four hard rock crossings underneath a river and historic walking trails. Midwest Mole opted to use a 1.2 m (48 inch) diameter SBU-A for the two shortest bores (55m / 180 ft in length), and a 1.4 m (54 inch) Single Shield Rockhead for the two longest bores of 120 m (400 ft) each.
The crossings were located in shale and limestone rock up to 70 MPa (10,200 psi) UCS. All of the crossings were in competent ground with little to no water inflows.
Rockheads are the most efficient technology for hard rock or mixed ground conditions from 25 to over 175 MPa (4,000 to over 25,000 psi) UCS that are above the water table. Midwest Mole decided on a Robbins Rockhead for the longest bores due to the length of the crossings and the strict line and grade requirements. Both crossings required accurate excavation at a grade of 0.25%–an easy feat for the Rockhead, which is continuously steered via an operator’s console inside the machine’s rear shield.
The cutterhead on the 1.4 m (54 inch) machine was fitted with 6.5″ diameter patented Single Disc Cutters for optimal boring in solid rock. The machine is owned by Midwest Mole and had been used on six previous projects since 2005. After boring over 1,200 m (3,900 ft), the machine was sent in to the Robbins shop for its first refurbishment and change of cutters prior to the set of gravity sewer crossings.
Midwest Mole excavated the first 120 m (400 ft) long crossing in March 2007. The Rockhead was welded to the lead 1.4 m (54 in) diameter steel casing and launched from the bore pit using a pipe jacking system. The machine averaged 6 to 8 m (20 to 26 ft) per 10-hour shift, finishing both on time and within grade requirements.The second crossing excavation began in May 2007, with the machine accomplishing even better excavation rates of up to 9 m (30 ft) per shift.
Both SBU-A crossings were finished with similar results. Each of the 55 m (180 ft) crossings, on a 0.42% grade, required continuous monitoring of the machine’s heading. Stabilizer pads, located in each quadrant of the machine, were used to stabilize the machine and allow for steering during the first 6 to 8 m (20 to 26 ft) of the bore. Crews adjusted the machine’s heading by changing the height of the stabilizer pads using a hydraulic cylinder. After the first 6 to 8 m (20 to 26 ft), monitoring alignment required the use of a dutch level. If the alignment had drifted the auger was then pulled and reset on the correct heading. The machine averaged up to 6 m (20 ft) per 10-hour shift on both crossings.