Tunnelling Problems in Granites: Dr. B. R. Ambedkar Pranahitha Chevella Sujala Sravanthi, State

K. Ravindra Nath Aarvee Associates Architects Engineers & Consultants

Abstract

DR. B.R. Ambedkar Sujala Sravanthi is a major multistage lift irrigation project .contemplated to divert 160 tmc of waters from Pranahitha River for irrigating 667206.5 hectares of lands in higher reaches of northern part of Telangana state, spread over in Godavari as well as Krishna river basins.

This mammoth project involves lifting in 22 stages for a total height of 1757 m (From RL152.0 m at Pranahitha Barrage to 645.0 m at Chevella Tank, near ) over a distance of 1055 km, and it is divided into 7 main links and being executed as 28 inter linked packages.

Main Water conductor system under these Packages comprises of 18 main tunnels with dia varying from 5.0-11.0m and lengths from 4.0 to 18.0 km. with cumulative length of 206 m. Some of the Packages have twin tunnel components and some other only single tunnels, depending on discharge requirements. These tunnels are under in different stages of excavation by drill and blast method

The entire project area is located in Peninsular Gneissic terrain ,except in northern part of Godavari river, where Gondwana Super Group of rocks (Packages 1,2,4 &5)and western marginal portion, where Deccan Traps are exposed (Package 25).

Peninsular Gneissic complex comprises granites, granite gneisses and migmatites which are at places intruded by dolerite and gabbro dykes and pegmatite veins. At places charnockites, pyroxenites, granultes belonging to Karimnagar Granulite belt, which are partly migmatized, are exposed within this vast granitic terrain

In Packages 6, 11, 21 and 23, tunneling through these rock types is examined. In general granitic media at these Packages falls in fair to Good category. On the basis of which support system has been designed and being provided. However, in other wise ideal tunnel media, roof collapses, cavity formation at places leading into chimneys have occurred in package 6, 7 and 21. Cavities of different magnitude have occurred in general along shear zones under water seepage conditions. And in locations where tunnels are located just below 40 m depth, some of the cavities have even propagated up to G.L resulting in ground subsidence

In Granites erratic weathering is observed even up to maximum depths of 25 m which has affected tunneling conditions due to deeply weathered joints, seams and pegmatite and low rock cover conditions. Some of the features specific to granitic terrain, witnessed at these projects are highlighted in this paper