Frequent landslides, the Kedarnath tragedy, a sinking Joshimath and the most recent Silkyara tunnel collapse are among the disasters that have kept the spotlight on the Himalayan region and the debate about the dangers of construction in this fragile mountain terrain. The Himalayas are young and still under the mountain forming process or orogeny, resulting in stability issues and complex geological conditions. A situation which in this case is also aggravated by earthquake occurrences as it is an active seismic zone. These have led to the question repeatedly being asked, “how safe is it” to construct here.
It is a situation and a question that India, Pakistan, Nepal and Bhutan, countries located around these mountains, are constantly confronting. However, balanced with the need for roadways, tunnels, hydropower projects to harness the continuous water supply from perennial snow-fed rivers, there is no let-up in construction here.
Drawing attention to the hazards involved is the latest event, Silkyara. The Silkyara Bend–Barkot tunnel was being constructed as part of the Char Dham project, which is intended to connect the Char Dham pilgrimage sites with two-lane, all-weather paved roads. The tunnel located on the Yamunotri end of NH134, connects Dharasu with Yamunotri.
“The construction of this tunnel will provide all weather connectivity to Yamunotri, one of the dham on the Chardham Yatra, encouraging regional socio-economic development, trade and tourism within the country,” said a Press Information Bureau release announcing clearance of the Silkyara Bend-Barkot tunnel by the Cabinet Committee on Economic Affairs chaired by the Prime Minister in February 2018. The tunnel was approved as part of the Chardham Mahamarg Paryojana, the PIB release said. The little over 4.5km long tunnel shortens the route by about 20 kilometres and travel time by about an hour. On November 12 while work was in progress a section of the Silkyara Bend–Barkot tunnel collapsed trapping 41 construction workers inside. As everybody now knows, after a rescue operation that lasted 17 days and drew global attention, all 41 workers were safely rescued.
Multiple questions on tunnel cave-in
The tunnel cave-in has raised multiple questions about the reasons for the collapse. There have been many media reports on the collapse and often a recapitulation of earlier events. But most of them have made vague and generalised statements about the Himalayas being a ‘young’ and ‘fragile’ mountain chain and the terrain being ‘unsuitable’ for large infrastructure projects. Many media reports have also blamed ‘tectonic faults’ for the collapse.
However, these are generalisations and such reports do not provide a good understanding of the geological setup which is extremely important in comprehending the reasons for such mishaps. An understanding of geology would offer an insight into what went wrong and help prevent such occurrences in future. So, in cases like the Silkyara-Barkot tunnel, a better approach is to look at the local geological picture rather than making general remarks about the fragility of the Himalayan-mountain chain, which doesn’t really convey anything significant.
The tunnel passes through the Garhwal Himalaya, west of Uttarkashi. This region is characterised by multiple superimposed thrust faults. The most important among them is North Almora Thrust, which is known for neotectonic activity and earthquakes. The North Almora Thrust itself is displaced by two steeply inclined, near vertical tear faults known as Koteshwar and Barkot Faults which are close to Barkot.
The broad lithology (general type of rocks) of this area consists mainly of metamorphic rocks such as phyllites and metavolcanic rocks. These rocks are known as the Chandpur Group in stratigraphy which is the branch of geology concerned with the order and relative position of strata and their relationship to the geological timescale. According to geologists, these metamorphic rocks are tightly folded, cut by faults at places, foliated and the foliation has a steep dip (38 to 550). It is not known for sure, whether the tunnel cuts across a major fault, but it probably passes through the tightly folded and deformed metamorphic rocks.
A geological report before the commencement of the Silkyara tunnel project has shown that the proposed tunnel could encounter weak rocks and adequate support structure would be needed to prop up the weak rocks. According to the report, the “rock type to be encountered along the diversion tunnels would be 20% good (Class 2), 50% fair (Class 3), 15% poor (Class 4) and 15% very poor (Class 4)”. The report has also highlighted possibility of wedge failure or rotational failure in the tunnel. When rotational failure occurs, the failed surface will begin to move outwards and downwards. It occurs when rotation by a slip surface causes the slope surface to curve.
Need for geological mapping
This draws attention to the need and extreme importance of detailed geological investigations in such situations. A detailed geological mapping followed by several exploratory drill holes and detailed core-logs help in deciphering the lithology and structure of the location and safety/precautionary measures, if any, can be put in place. This is followed by RMR (Rock Mass Rating) classification of the rock types which assigns ratings to the rock mass based on properties such as the number of discontinuities (such as fractures) in the rock, spacing between discontinuities, nature of the fractures, groundwater, and rock quality designation (RQD) index. RMR classifications help in understanding the possible behaviour of the rock and then engineers can decide the kind of construction and precautionary measures that need to be put in place.
Speaking about the tunnel project, in an interview to a newspaper, Prof. Navin Juyal, a prominent and respected geologist, has said “First of all if the 4.5 km road tunnel project in the sensitive Himalayas region was taken up based on this geological report, it is insufficient. One cannot know the type of rocks by just three exploratory drillings. Further, the report clearly says there was no very good quality rock in the area where the tunnel was being built. Only 20% of rock is of good quality, the rest is fair and poor and very poor. The report admits that the area was not geologically stable.”
This statement clearly points out and highlights the need for a meticulous geological survey, besides geological investigations, before undertaking any construction project in terra infirma like the Himalayan-mountain chain. A number of such disasters could have been avoided if local geology was understood or warnings from experts had been heeded.
(The author is a noted geologist. Views are personal. By special arrangement with The Billion Press)