Disc cutter technology is still far from the standard method in India,where many contractors utilize hand mining for all hard rock trenchlesscrossings. Hand mining is very often exceedingly slow, with typicaladvance rates of about 300 mm (12 in.) per day. At the same time, the Indian government has in recent years issued a national prohibition on the use of drill and blast for trenchless crossings.
The limitations of the underground excavation technology have led contractors to consider other methods. In 2008, Vichitra Constructions Pvt Ltd. of New Delhi came across hard rock cutting heads with disc cutters and decided to give it a try. The technology offered daily advances 20 to 30 times those of hand mining — a very important factor considering the challenging project ahead of them.
After eight years and multiple failed attempts by previous companies, Vichitra had been contracted to excavate three crossings in extremely hard quartzite. “We were looking for a completely different method, and found that disc cutter technology would likely work best in the very hard, abrasive rock we had,” said Mr. Sudhir Agrawal, Vichitra executive director. The contractor successfully utilized a Robbins Small Boring Unit (SBU-A) and ABM on the project, marking India’s first-ever use of disc cutter boring attachments. Vichitra has not stopped there, and is currently excavating a series of tunnels with a specialized type of Small Boring Unit, known as the Robbins Rockhead, in the state of Rajasthan.
Many towns and cities within the state of Rajasthan are operating with aged and over-utilized infrastructure. The Rajasthan government is addressing these problems with the Rajasthan Urban Infrastructure Development Project (RUIDP), a large scale water supply, waste management, and urban improvement scheme.
Part of the RUIDP involves a new water line in Kota City, one of the state’s largest municipalities. Situated on the banks of the River Chambal, Kota is a primarily industrial city with a largely contaminated and inadequate water supply. The project required a new pipeline, as well as 15 pumping stations, traveling 13 km (eight miles) from Kota Barrage dam on the River Chambal to the Govindapura area of Kota.
The system adds 24 million liters (6.3 million gals) per day to the current water supply and services about 70,000 people.
Work on Kota Water Supply Project 16 started in 2000, utilizing cut-and-cover methods to install 700-mm (28-in.) diameter steel pipe from the river into the city. Numerous attempts to complete a 100-m (328-ft) rail crossing were unsuccessful. By 2007, most of the line had been excavated by cut and cover, though the Indian government had recently mandated that all rail crossings had to be excavated by trenchless technology with casings. Vichitra was contracted to complete the final crossing beneath heavily trafficked rail lines, which could not be open cut.
Finding the Optimal Technology
In 2000 when work began on the crossing, crews hand-mined an 11-m (36-ft) long by 4.5-m (15-ft) wide launch pit — a process that took four months at rates of 200 to 300 mm (8 to 12 in) per day in extremely hard quartzite up to 200 MPa (29,000 psi) UCS. The first attempt at excavation utilized a small pipe jacking system and hand mining, which was abandoned after several months of low production rates. Subsequent attempts utilized HDDs with rock cutting heads, but both times the drills failed in the hard rock.
In 2007 after researching various methodologies, current contractor Vichitra purchased a 1.5-m (60-in) diameter Robbins Small Boring Unit (SBU-A) with 292-mm (11.5-in) 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.
How it Works: Small Boring Unit (SBU-A)
Small Boring Units, available in diameters from 600 mm to 1.8 m (24 to 72 in), are typically used on crossings up to 150 m (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/or 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. Spoils are removed using a full-face auger inside the casing.
Both rail bores were completed by Autumn 2008 in abrasive, hard rock. The 100 m (328 ft) crossing was excavated in two 50 m (164 ft) long passes from either side of the tracks. During the first bore, the machine holed through into a center pit between the two tracks. Advance rates were up to 1.5 m (5 ft) per hour. “We have shown the railway authorities that crossings can be effectively excavated with Robbins SBUs. Owners are now becoming more familiar with the technology and confident with its capabilities,” said Mr. Sudhir. A third 14-m (46-ft) long bore was added underneath a roadway after difficulties with open-cut operations in the rock.
Current Project: Mt. Abu
The contractor’s latest project has taken SBU technology to greater lengths. In late July 2009, Vichitra launched a 1.8-m (72-in) diameter Robbins Double Shield Rockhead (SBU-RHDS) — the first time a Rockhead has been used in India. The machine, a type of self-propelled, manned entry boring unit utilizing disc cutters, is excavating three hard rock crossings.
The crossings, of 290 m (950 ft), 238 m (780 ft), and 375 m (1,230 ft) in length, will form part of a new sewer line in Rajasthan’s resort town of Mt. Abu, located in an oasis and protected wildlife area. “The original contract specified trenchless technology, but was not specific to a method. We decided against microtunnelling, as it has a very long setup time, and slurry disposal is very costly—particularly in the sensitive environment of Mt. Abu,” said Mr. Sudhir.
The contractor opted for the Robbins Rockhead due to a number of factors. “The Rockhead is the only machine that can tunnel hard rock on line and grade at a substantial speed with minimum setup time. It also doesn’t pose the environmental hazard that slurry removal does,” said Mr. Sudhir.
As the machine excavates, an internal drive motor provides torque, while forward thrust is derived by pushing off the tunnel walls using a gripper system. Articulation cylinders allow the Rockhead to be steered throughout the drive from an operator’s console in the rear shield, achieving increased line and grade accuracy. A laser targeting system provides continuous monitoring of the machine’s position. Spoils are removed using a belt conveyor.
Geology on the first crossing consists of quartzite up to 250 MPa (36,300 psi) UCS with loose sections of soil and mud. To tackle the conditions, the cutterhead is utilizing 317 and 355 mm (12.5 and 14 in) diameter single disc cutters. The machine is being used with a pipe jacking system that is pushing casing for the first 15 to 20 m (50 to 65 ft), a section that consists of loose soils. After this section is complete, liner plates may be installed for any further unstable ground.
Future Potential in India
The Robbins Co., through its Indian subsidiary, hopes with the two Rajasthan projects to familiarize contractors with the efficacy of SBUs compared to hand mining, as well as mechanized underground construction in general. Robbins India is building an inventory of machines, and currently offers a range of support including field service, spare parts, and cutters.
“Our goal is to provide effective solutions, consisting of SBUs and ABMs, for contractors facing hard rock strata above the water table. We also want to establish the machines as a reliable alternative to slurry microtunnelling in hard rock,” said Shyam Chengalath, managing director of Robbins India. “SBUs are ideal in the central and southern regions of India, where the Deccan Plateau geology consists mainly of granite and basalts.”
Mr. Sudhir expressed a similar opinion: “SBUs have definite potential in the western and southern parts of India, which have rocky terrain. SBUs are convenient for trenchless crossings in rock, because they are the least costly method and have the shortest setup time.”
Vichitra plans to re-use all of their Robbins machines, including the SBU-A, ABM and Rockhead, on various projects in the future.
Desiree Willis is a technical writer for The Robbins Co.