Hydro Tunnels in Himalayas – A Challenge for Man and Machine

Channeling a mighty Himalayan River into a tunnel for generating hydro-electricity is more than an engineering adventure for not only does it draw upon heroic determination to bore through strata fraught with geological surprises but it also demands the sharpest minds around to throw up innovate solutions to get the job done.

Inch by inch carving out a tunnel through kilometers of solidified Himalayan rocks that has metamorphosed over a million year period from sedimentary and volcanic rocks, demand highly skilled engineering and a motivated workforce to take up such a daunting task.

For the Rampur hydropower project in North India, a 15.177 Km long and 10.5 meter dia. tunnel will channelize waters without constructing a separate dam from outflow of the upstream 1500 MW Nathpa Jhakri plant on Satluj River to operate a 412 MW generation capacity plant. This is one such engineering feat that will harness clean energy for the benefit of mankind.

Pitched against the mountain, Jagmohan Rawat, a 35 year old worker on the project sums up the heroic efforts demanded to execute such large tunnels by saying “for days, months and years together working with solid rocks does harden you but an underling excitement always persists that this tunnel will someday see light at the other end.”.

Not deterred by working deep inside mountains, having lived through falling rocks, not surprised by encountering hot (upto ± 66º temperature at Wadhal point in Jhakri project) or cold water flows deep inside rock formations, getting past dusty and claustrophobic space that is freshly created by blasting rocks Jagmohan only says, “This job is kind of addictive. As the tunnel progresses, the desire to see it to completion grows in you.”

With Kumar Lama - a Nepali, Kanju – a Bengali, Gudda Kumar - a Bihari and Surinder Kumar - a native from Jharkhand as teammates, working in day shift on a critical section of the tunnel, Jagmohan, who is a native from Uttrakhand says “when one is working deep inside a mountain, one may not seeing daylight for a fortnight and one does temporarily loses the very concept of time.”

Working conditions do test the grit of man and machine for hot and humid temperatures hover at or above 48- 50 degree Celsius. Constantly working larger blowers for cooling and supplying fresh air into the working area are a must. To contain dust levels, a regular water supply at all drilling points has to be ensured and should there be a power or water supply failure, the tunnel can be one nasty place to be in.

“One is left gasping for fresh air,” says Kumar Lama, who has hardened working in the tunnel for two years. “And then all the while there is danger about sudden rock fall lurking around. Safety standards adopted are high and not one worker has even been injured while I have been around,” he says.

February 8, 2011 was a momentous day for tunnel construction. SJVNs top brass converged into one section of the tunnel in order to perform the last blast ceremony between two reaches of the tunnel. This controlled explosion successfully breached one last rock layer and successfully joined an upstream tunnel section of Kajo Adit with a downstream one of Left Bank. Managers, Engineers and Workers together rejoiced the moment.

“It was a day to celebrate,” the project head recalls. A continuous stretch of 7.7 Kms of tunnel had been achieved. Much hard work in investigation, designing and precision in execution was involved to get there, he said.

Belying perceptions about heavy explosives used to blast through the hardened rock, modern hydro-tunneling techniques rely on highly controlled synchronized blasting that has maximum impact and releases with minimum sound and vibrations into the surroundings.

The project head explains, “not conventional but powder laden explosives are laid in co-centric circles into the targeted area that explode in a sync, one after another when triggered. The sound and the vibration of the explosion released into the surroundings does not cause any damage to even those buildings that are in close vicinity. “This fact has been corroborated by scientific studies carried-out by accredited institutions of international repute,” says the project head.

Despite meticulous planning and exhaustive surveys, nasty geological surprises and unforeseen can derail schedules by years and even threaten the very viability of such large projects. Encountering cold or even hot water flows 180 L/sec – 350 L/sec other than slowing down progress, involves employing special chemicals to dry up patches of freshly exposed rock surfaces to channelize the water seepage area before proceeding ahead. In one section of a tunnel of the upstream 1500 MW Natpha Jhakri project, encountering hot water flows raised the temperatures inside the tunnel to a unbearable level of over 65 degree Celsius. A special ice factory had to be set up and ice blocks were used as shields to bring down temperatures so that men could work. The Rampur project too did encounter hot water flow in a tunnel section but having gained from experience the problem was surmounted. However, the all important tunnel was beset with a different set of obstacle.

Whereas geological survey had projected a ratio of 65:35 of hard (good) and soft (bad) quality rock on the tunnel path, engineers at work encountered layers of bad rock concentrated in one section (20:80) good and bad rocks leaving them struggling for solutions as targets began to slip. Tunneling in hard rock is relatively easier than in soft ones due to the fact that a freshly cut out cavity in a hard rock is more self supportive than one in soft rocks which needs additional support systems to hold the tunnel in place. To bring the project back on rails, curtailing the length of one adit( tunnel) where rock was very bad, introducing the technology of lattice girders where more flexible support system is used in place of conventional steel ribs were the solutions adopted to ensure progress and for better stability..

Introduction of an additional adit (tunnel) provided two more faces from work could be resumed and brought down delay time schedule by 2 years to position the project for commissioning by September 2013. While the additional access adit (tunnel )was beginning to show results, encountering a major cavity deep inside the mountain in this reach put the engineers and projects planners through another difficult test.

Explaining the geology of such cavity formations, a project geologist monitoring the rock structures along the tunnel route says, “Tectonic movements often cause one rock layer to fold upon another. During the process, sometimes a layer of crumpled rock gets trapped between two hard layers. When tunneling through such sections loose rocks fall through and the tunnel fails to hold.”.

The obstacle posed an engineering challenge as available heavy machinery could not be deployed and controlled blasting was of no help.

When two attempts to push the tunnel through this section failed consultants were roped in to formulate a strategy to fill the cavity with rocks and concrete so as to enable the tunnel formation to hold, so that the tunnel progress did not lag. Because of laying the tunnel through the cavity, another challenging decision was taken to bye-pass the cavity so that commissioning schedule of September 2013 could be adhered to. Falling back on a time tested multi-drift method for tunnel execution, work resumed and it took about 15 months to clear the 25 meter stretch of tunnel construction in this cavity troubled spot. Elsewhere, where good quality hard rock was available, work progressed at a brisk pace of tunneling through over 150- 175 meters of rock stretches in a month.

 Where work on the main feeding tunnel slowly overcame obstacles encountered, excavation of 40 meter diameter and 150 meter vertical tunnel called surge shaft was relatively an easier task and was completed ahead of schedule. Being part of the team that started work on the vertical tunnel, Rati Ram - a 52 year old worker from Sirmaur in Himachal Pradesh, gleams with pride to be there to have completed the task. Recollecting his initial days on work he says, “Descending into the pilot tunnel was considered to be fraught with risks. The camaraderie with fellow workers grew stronger as we went deeper into the tunnel making it a lifetime of experience.” Vikram, a 25 year old fellow worker from Sirmaur confesses, “When we began to descend deeper into the tunnel it did get scary at first but the safety measures undertaken soon restored confidence.”

 Water after flowing through 15.177 Km in the main tunnel also called Head Race Tunnel; at the surge shaft tunnel gets diverted into three small tunnels called penstock tunnels. The three penstock tunnels are further bifurcated to drive 6 turbines at the over ground generation station in Bayal village that will have a built up capacity to generate 1770 million units of clean energy in a dependable year.

 A planet sensitive to vagaries of climate change mandates usage of clean energy so as to reduce the impact on global warming. Hydropower happens to be one of the biggest generators of clean energy and run of the river projects like the Rampur project is to harness just that.

 A tribute to the genius of man, tunneling remains the most challenging and critical component of executing a run of the river hydropower plant in the Himalayan region.