Hydropower Tunnel design The main design principle of unlined tunnels and shafts for hydropower plants is to place them in suitable rock mass with sufficient confinement. The placement is selected to avoid hydraulic jacking, which could lead to water leakages. Local sections with unstable rock mass, including swelling and/or friable materials are mapped and provided with sufficient support. In such tunnels, hydraulic jacking needs to be prevented by ensuring that the maximum water pressure in the tunnel does not exceed the minimum principal in-situ stress in the rock mass.
This process is done by the Geologists. Rock classification is the most important for the tunnel excavation, as all other works are directly dependent on it. Classification of rocks depends upon the Q Barton system. In this system the value of Q decides the type of rock.
|Q-Value||TYPES OF ROCK|
|0.1<Q<0.5||VERY POOR ROCK|
After the process of rock classification drilling is done. Drilling is done so that blasting process can be further proceeded. The type of rock decides the length of the rock drill. Approx. 60 to 70 drill holes are made in a D-shaped faced tunnel of diameter 5m depending upon the type of rock. The diameter of each drill is of 42mm so that the explosive capsule of 40mm can be inserted easily. Drilling can be done by various machines:
- JACK HAMMER
- ROCKET BOOMER (DRILL JUMBO)
- ROC (ROCK ON CRAWLER).
DRILLING LENGTH FOR DIFFERENT ROCK CONDITIONS
- VERY GOOD/GOOD ROCK – 3.5 to 4.5m
- FAIR ROCK – 2.5 to 3m
- POOR ROCK – 2 to 2.5m
- VERY POOR ROCK – 2 to 2.5m
Components used in blasting procedure:
Explosive- Nitra mixture
Product name- Power gel 801
Detonators are used to trigger an explosive device. Detonators can be chemically, mechanically or electrically initiated, the latter two are most common. In this project two types of detonators are used:-
1) NED (Non electric detonator)
2) ED (Electric detonator)
Non electric detonator
A non-electric detonator is a shock tube detonator designed to initiate the explosions, generally for the purpose of demolition of buildings and for use in blasting of rock in mines and tunnel construction.
Electric detonator provide the precise control necessary to produce accurate and consistent blasting results in a variety of blasting applications in mining. It explodes instantaneously when electricity is turned on. It can explodes at minimum charge of 0.5 V. During blasting at least two ED should be provided so that if one missed another one will explodes. For hard rocks more ED are used.
Defuming is the process in which the tunnel is kept clear from any human contact just after blasting so that proper settlement of rock particles can take place. Defuming ensures that the environment inside the tunnel is suitable again for further work. Defuming time generally varies from 1 hour to 2 hours.
MUCKING AND CHIPPING
Mucking is the process which is done after the blasting and de-fuming. Due to the blasting lots of loose rocks, stones and sand or clay is present which is needed tobe removed for further working of tunnel works and for clearance of path formoving machines. Thus, mucking is done for removing of such material from thesite. Mucking is done by excavator and dumper.
- In drop shaft, pressure shaft and surge shaft mucking is done by the Gantry machine.
It is defined as a mixture of cement, aggregate, water and accelerators in correctproportions, with maximum size of aggregates less than 10 mm projected at highvelocity with spray nozzle on to a surface to form a layer of pneumatically applied concrete on that surface. Shotcrete can be either wet or dry mix. It is used for protection and supporting rock surfaces after excavation and rock bolting, to fill the cavities caused by over break or weathering. Material for shotcrete shall comprise cement, aggregates, steel fiber, water and admixtures. The water cement ration for shotcrete in place should be between 0.32 and 0.45.
A rock bolt is a long anchor bolt, for stabilization of rock excavations and bound the crack rock together. It transfers load from the unstable exterior, to the confined (and much stronger) interior of the rock mass. The work to be done consists of providing and fixing 25mm, 32mm, 36mm and 40mm diameter high yield strength deformed steel Rock bolts in holes of minimum diameter equal to one and half times the diameter of the Rock bolts drilled in all formations of rock/concrete including necessary anchoring arrangements.
RIBS AND LAGGING
Mainly, the ribs are provided for “very poor rocks” for tunnel construction. The ribs provide support to the above lying rock mass which get loosened after blasting. If the rock mass is very poor in nature, it faces considerable amount of fractures in it and the equilibria of mass is disturbed .Due to that, the need for support arises which shall be fulfilled by provision of rib.
Components of rib:
- The crown
- Two columns
- Steel plate
- Angle plates
- Nuts and bolts
After the erection of ribs, it also became necessary to provide support at the vacant space left between two consecutive ribs. The purpose was served by providing laggings. Laggings are nothing but small reinforced blocks of uniform size and thickness, held between two consecutive ribs through hooks. The lagging slabs are pre-cast elements made of M20A20 grade concrete with reinforcement provided of 8mm dia. and 10mm dia. bars. The lagging slabs have hooks on ends which helps in connecting it with ribs.
After excavation is complete, the tunnel is lined with concrete to achieve a finished diameter of 4.90m and to a modified horse shoe shape. The concrete lining work is executed by gantries (collapsible formworks) and by pumping the concrete. After concrete lining the area between concrete surface and excavated surface suitably grouted to ensure monolithic structures. After completion of all above works, ADITs will be plugged with concrete.
Concrete lining is 300mm thick of M20 Grade.