TAG: GS 1: GEOGRAPHY
THE CONTEXT: An under-construction tunnel on the Yamunotri National Highway in Uttarakhand’s Uttarkashi district collapsed.
- The failure (the section that has collapsed) is located around 200-300 metres from the mouth of the tunnel.
- It could have happened due to a loose patch (of rock), which wasn’t visible during the construction.
- The patch might have consisted of fractured or fragile rock.
- Another reason could be the seepage of water through a loose patch.
- Water erodes loose rock particles over time, creating a void on the top of the tunnel, which can’t be seen.
WHAT ARE THE WAYS IN WHICH TUNNELS ARE EXCAVATED IN ROCK?
- There are essentially two ways:
- DBM involves drilling holes into the rock and loading them with explosives. When the explosives are detonated, the rock breaks apart.
- TBMs bore the rock from the front (using a rotating head) while supporting the excavating tunnel behind the machine by installing precast concrete segments.
- Building a tunnel with a TBM is more expensive than DBM, but much safer.
- Imported TBMs are used in India.
DOES THE METHOD OF EXCAVATION DEPEND ON THE TYPE OF TERRAIN?
- TBMs are ideal when the rock cover is up to 400 metres tall.
- A TBM can’t be used to drill through very tall mountains. Creating a void through a 1,000-2,000-metre-high mountain by using a TBM leads to rock burst.
- Underground tunnels for the Delhi Metro were dug using a TBM at shallow depth.
- DBM is usually used in places like Himalayas, including Jammu & Kashmir and Uttarakhand.
IS THE HIMALAYAN REGION TOO FRAGILE TO TUNNEL THROUGH?
- Geological context: 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.
- There are some patches where the rock is indeed too fragile for a tunnel. But at other places, the rock is very good.
KEY ASPECTS OF BUILDING A TUNNEL
- Investigation: The first and foremost thing is to thoroughly investigate the rock through which a tunnel is proposed to be made.
- It 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.
- 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.
- 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.
WHAT DOES IT TAKE TO CONSTRUCT A STABLE TUNNEL?
- After excavation and providing support for the tunnel, we need to monitor the spot. It’s essential to check how the rock is behaving at various places.
- Monitoring is done by instruments such as stress meters and deformations meters.
- Then, 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 stabilising rock excavations).
- Steel ribs or beams.
- 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.