UTTARAKSHI TUNNEL

2. Causes of the Tunnel Collapse

There are two main possible causes of the tunnel collapse

• A loose patch of rock: The collapsed section of the tunnel is located around 200-300 meters from the mouth of the tunnel. It is possible that there was a loose patch of rock in this area that was not visible during construction. This patch could have consisted of fractured or fragile rock, which would have made it weak.
• Water seepage: Another possibility is that water seeped through a loose patch of rock, eroding the rock particles over time and creating a void on the top of the tunnel. This void would not have been visible during construction.

3. Methods of Tunnel Excavation

There are two main methods of tunnel excavation

• The drill and blast method (DBM) involves drilling holes into the rock and loading them with explosives. When the explosives are detonated, the rock breaks apart.
• Tunnel-boring machines (TBMs) are more expensive than DBM, but it is also much safer. TBMs bore the rock from the front using a rotating head and supported the excavated tunnel behind the machine by installing precast concrete segments.

4. Choice of Excavation Method

• The choice of excavation method depends on the type of terrain. TBMs are not suitable for drilling through very tall mountains.
• Creating a void through a 1,000-meter-tall mountain using a TBM can lead to a rock burst, which occurs when a part of the rock suddenly falls due to high stress.
• TBMs are ideal when the rock cover is up to 400 meters tall. Underground tunnels for the Delhi Metro were dug using a TBM at shallow depth.
• On the other hand, in places like the Himalayas, including Jammu & Kashmir and Uttarakhand, DBM is usually used.

5. Challenges of Tunneling in the Himalayas

• The Himalayas are still young (they were formed between 40 million and 50 million years ago) and they are still growing due to the collision between the Indian tectonic plate and the Eurasian tectonic plate. This makes them a challenging environment for tunnelling.
• There are some patches where the rock is indeed too fragile for a tunnel. However, in other places, the rock is very good.
• Manoj Garnayak a former project director at L&T and an expert in underground construction has worked in the Himalayan region and built tunnels without facing major issues. He has witnessed very small failures, which were rectified immediately by fixing the crown of the tunnel. So, even though in some places the rock is fractured or fragile, there are technical solutions to fix it.

6. Key Aspects of Tunnel Building

• The first and foremost thing when building a tunnel is to thoroughly investigate the rock through which the tunnel is proposed to be made.
• This is done by sending seismic refraction waves through the rock to check which patches are fragile or solid.
• In India, engineers dig a borehole into the rock to extract a core sample and send it for petrographic analysis (microscopic examination to determine the mineral content, grain size, texture and other features that have a bearing on the mechanical behaviour of the sample).
• Such investigations help determine if the rock can take the load of the overburden when a tunnel is created.
• If the rock layer and its strength are good, it takes the entire load of the overburden by redistribution of the stresses, and it remains stable.

7. Steps to Construct a Stable Tunnel

• Even after excavation and providing support for the tunnel, it is essential to monitor the spot to check how the rock is behaving at various places. Monitoring is done by instruments such as stress meters and deformations meters.
• The supports provided to the tunnel need to be tested for adequacy. Supports can be of various types, including shotcrete (sprayed concrete that works as a membrane to prevent parts of the rock from falling), rock bolt (a long anchor bolt for stabilizing rock excavations), steel ribs or beams, and tunnel pipe umbrella, built by using steel tubes that can hold really fragile rocks.
• It is also important that an independent specialist geologist visits the tunnel for examination and to check for probable failures. They will also determine the rock's stand-up time the duration for which a rock can remain stable without any support. Support is given to the rock within its stand-up time.
• Most importantly, in India, more time needs to be spent on studies before starting the construction. Currently, the design and construction of a tunnel project are done simultaneously.