Beneath India’s largest tiger sanctuary, the Nagarjuna Sagar National Park, tunnel boring machines are orchestrating one of the largest water transfer schemes in India. A Robbins Double Shield TBM is boring tunnel No. 2 of the Pula Subbaiah Veligonda project for Coastal Projects Ltd. (CPL), of the CPL/ Hindustan Construction Company (HCC) JV.
On the Krishna River, on the right bank of the Srisailam Canal, lies the future inlet site for the Pula Subbaiah Veligonda Project. Once complete, the system will draw 1.2 billion cubic meters (317.0 billion gallons) of flood water annually from the foreshore of the Srisailam reservoir. Two parallel, 19.2 km (11.9 mi) long tunnels will transfer water via a network of five canals to over 1,600 square kilometers (395,368 acres) of farmland in the three districts of Prakasam, Nellore, and Kadapa. Up to 243 cubic meters per second (64,193 gallons per second) of water will travel through the bored tunnels to a feeder canal.
In October 2007, a USD $180 Million contract was awarded to Coastal Projects Pvt. Ltd (CPPL). In November, CPPL signed a contract for a 10.0 m (32.8 ft) diameter Robbins Double Shield TBM and continuous conveyor system. In addition to the machine and conveyor, spares and key operating personnel were sent to the jobsite to excavate tunnel No. 2 starting from the outlet end.
The Veligonda tunnel No. 2 is located in sedimentary rock on the western margin of the Cuddapah Basin, where a number of faults and folds make for complex geology. Rock includes quartzite with interbedded shale (60%) and shale with limestone and phyllite (40%) ranging from 90 to 225 MPa (13,000 to 33,000 psi) UCS. Two major faults are present along with some ground water.
The Double Shield machine utilizes sixty-seven 20-inch diameter back-loading cutters to combat the tough ground conditions. Specially designed drive motors allow the machine to run at a higher than normal RPM, compensating for low penetration rates in the hard rock. In squeezing ground, the cutterhead is also capable of vertical movement allowing for overboring. The machine also has a probe drill which allows for verification of geology 30 m (98 ft) ahead of the TBM. The drill is capable of 360º rotation and can alternatively serve as a grout consolidation drill. Large 40 kW (54 hp) dewatering pumps located on the back-up system have been specially designed to pump any water away from the tunnel face. As the TBM bores, it erects 300 mm (12 inch) thick concrete segments in a 6+1 arrangement, making the final tunnel diameter 9.2 m (30 ft).
Muck haulage requires one of the most extensive conveyor systems ever used in India. The continuous steel cable belt, the longest single flight ever provided by Robbins, will eventually extend 19.2 km (11.9 mi) and requires four main drives and three booster drives.
The machine was assembled in just four months using Onsite First Time Assembly (OFTA). OFTA is a process that allows machine components to be initially assembled at the jobsite, rather than in a manufacturing facility, typically providing savings in terms of man-hours and shipping costs. Assembly went well despite harsh local temperatures, which can climb to 45˚C (113˚F) daily. In addition, some components could only be installed at night due to thermal expansion in the midday heat.
The Robbins TBM was launched into difficult ground at rates of up to 330 m (1,080 ft) per month, by adopting an extensive program of probe drilling and pre-grouting. Multiple drill holes were bored 30 m (100 ft) ahead prior to every machine push, and grout was then injected at depths of 25 to 30 m (80 to 100 ft). The Robbins TBM then advanced into an unforeseen area of disturbed geology and was inundated with flowing material. The machine became stuck, despite the probe drilling program. It was freed by excavation of a bypass tunnel and continues to excavate in difficult geology.
Updates of this project will be posted as boring continues.