After many failed attempts, spanning the course of eight years, just three hard rock crossings remained on a vital water supply line in Kota City, Rajasthan, India. Previous contractors had attempted to excavate the crossings in hard quartzite using hand mining and HDD, which were later abandoned due to low production rates. Simply hand mining the 11 m (36 ft) long by 4.5 m (15 ft) wide launch pit took four months, at rates of 200 to 300 mm (8 to 12 in) per day. In 2008, Contractor Vichitra Constructions Pvt. Ltd. was contracted to complete the difficult 50 m (164 ft) long crossings.
The 13 km (8 mi) pipeline, part of the government’s Rajasthan Urban Infrastructure Development Plan (RUIDP), was used to increase water supply and fight water contamination problems in the city. The finished system provides 24 million liters (6.3 million gallons) of water per day to about 70,000 people.
After researching various technologies, Vichitra purchased a 1.5 m diameter Robbins Small Boring Unit (SBU-A) with 11.5 inch disc cutters and a Robbins 60-1270 Auger Boring Machine (ABM). The technology was supplied by Robbins Tunneling and Trenchless Technology (India) Pvt Ltd, a local subsidiary based in New Delhi who also provided the contractor with technical support, crew members, and cutter rebuild services.
Much of the crossings consisted of quartzite rock (200-150 MPa / 29,000-36,000 psi), with some tracts of soil and mud.
Small Boring Units, available in diameters from 24 to 72 inches, are typically used on crossings up to 500 ft in length utilizing a standard Auger Boring Machine (ABM) and steel casing. During excavation, the SBU-A is welded to the lead casing while the ABM provides both torque and forward thrust to the cutting head. The circular cutterhead is fitted with single disc cutters to excavate hard rock, or a combination of single disc cutters, two-row tungsten carbide insert cutters, and carbide bits in mixed ground. Disc cutters penetrate the rock face, creating a “crush zone” through which fractures propagate. Material between adjacent crush zones then falls from the face. Muck scrapers scoop the muck into openings on the cutterhead. Muck is then transferred through a full-face auger for removal.
Three rail bores were completed by autumn 2008 in abrasive, hard rock. The crossings below a railroad were excavated in two 50-m (164-ft) long passes from either side of the tracks. The first pass, with advance rates of 1.5-m (5-ft) per hour, broke through into a center pit located between the two sets of tracks. A third 14-m (46-ft) long bore was added underneath a roadway after difficulties with open-cut operations in hard rock.