Silkyara Tunnel Structural Collapse: Unraveling Causes & Ensuring Safety

Silkyara Tunnel: To Enhance Connectivity for Char Dham Pilgrimage in Uttarakhand

In March 2018, the Ministry of Road Transport & Highways initiated the construction of a 4.531 km two-lane tunnel at Silkyara, connecting Gangotri and Yamunotri through the Radi pass in Uttarakhand. The project, managed by the National Highways & Infrastructure Development Corporation Limited (NHIDCL), received a sanctioned budget of Rs 1383 crore. NHIDCL entered into a contract agreement under the EPC mode. with Navyuga Engineering Company Limited for the Tunnel project, amounting to Rs. 853.79 crore.

The Silkyara tunnel, a key element of the Centre's Char Dham project, aims to provide all-weather connectivity to four prominent Hindu shrines in Uttarakhand: Badrinath, Kedarnath, Gangotri, and Yamunotri. It will benefit pilgrims by reducing the snow-affected length of NH-134 (Dharasu-Barkot -Yamunotri road) from 25.6 km to just 4.531 km. This reduction translates to a travel time of 5 minutes, a mere one-tenth of the current 50-minute journey.

Dr. Rakesh Kumar Khali, Renowned Tunnel & Underground Expert

In my opinion, it’s a clear case of negligence and a casual approach towards the execution of the job in this prestigious project, where many experienced people were involved. In tunnelling, when we observe any squeezing condition or convergence while executing through NATM, and when buckling and twisting of lattice girders are observed, we must take instrument readings and check the actual squeezing by proper interpretation of the data, and then forward it to the design team for necessary corrective action. In most cases, it is observed in shear zone areas. Here, reprofiling and rectification started in the squeezing zone for replacing the lattice girder without stopping the activities at the face, and in this process, a huge rock stratum got detached, resulting in the trapping of people inside. This incident gained publicity because people were trapped inside and were alive; otherwise, it would have been treated as a simple tunnel collapse, which is common in Himalayan geology. Besides these issues, inadequate investigations were conducted before starting the work, and the L1 contractor was selected without assessing their execution capability and track record. For obtaining the EPC contract, people are quoting very low prices compared to the estimated cost, compromising with the quality to save money and somehow complete the project in a very tight financial condition.

Preventive Measures:

1. Tunnelling is inherently against nature, so proper monitoring and interpretation of data are required to assist designers in taking corrective steps once squeezing and profile variations are observed.
2. The provision of an escape tunnel or adit must be included in all future tunnels in the Himalayas, as seen in prestigious projects like the Chanani Nasri Project in J&K.
3. Tunnel faces longer than 2 km from one end should be avoided. Although NATM is the most advanced technique globally and gaining momentum in India for soft ground tunnelling, fear should drive safety considerations. Therefore, we should adopt pipe roofing technology, which may be more costly but significantly safer for advancing in Himalayan geology, especially in shear zone areas. Cost saving should not be a reason to overlook its implementation.

A Closer Look at the Factors Behind the Incident

On November 12, a section of the tunnel, specifically between 205 and 260 meters from the Silkyara side, collapsed, trapping 41 workers beyond the 260-meter mark for 17 days. Preliminary reports indicate that the collapse of the Silkyara tunnel occurred after reprofiling work conducted on November 10 in the section extending from 260m to 265m into the tunnel. Subsequently, on November 12, breaking work commenced in the segment ranging from 205m to 250m into the tunnel, where the reprofiling work had been previously completed.

While authorities emphasize the need for a thorough investigation, various theories suggest a possible link to a landslide in the fragile Himalayan region. Some experts also point to the potential consequences of rapid development in this ecologically sensitive area. A geological report submitted to the Ministry of Road Transport and Highways (MoRTH) before commencing the Silkyara-Barkot tunnel project indicated potential challenges. The report noted weaker rocks on the Silkyara side compared to the Barkot side, highlighting the risk of "slip circle failure," a type of landslide detrimental to tunnel construction.

The report, based on exploratory drilling at Silkyara, Barkot, and Radi Top, aimed to gather sub-surface information about the region's geology. Dr. Naveen Juyal, a Himalayan region geologist, pointed out the report's limitations, emphasizing that determining rock types from only three drillings is insufficient. The report acknowledged the absence of “very good” quality rock, with only 20% categorized as good, and the remaining as fair, poor, and very poor, indicating geological instability in the area. Dr. Juyal clarified that he wasn't dismissing the feasibility of the tunnel in the region but questioned the construction methodology.

Dr Manoj Verman Tunnelling & Rock Engineering Expert

The Silkyara tunnel collapse, while still under investigation, appears to have resulted from a combination of factors. Key among these could be shortcomings in geological, hydrological, and geotechnical investigations. In the complex and dynamic geological conditions of the Himalayas, thorough and accurate site investigations are crucial. Any oversight or underestimation of the geological challenges can lead to critical structural vulnerabilities.

Additionally, construction malpractices, if present, could have contributed to the incident. This includes potential deviations from the designed construction methodology, inadequate quality control, and insufficient monitoring of the tunnel's structural integrity during construction. It's also essential to consider the role of natural factors, such as unexpected hydrological conditions or seismic activities, which are prevalent in the Himalayan region.

Preventive Measures

To prevent future tunnel incidents, a multi-pronged approach is necessary:

• Enhanced Geotechnical Investigations: Implementing more rigorous and technologically advanced geotechnical and hydrological studies to accurately assess site conditions.
• Adoption of Advanced Technologies: Utilizing advanced tunnelling technologies, equipped with real-time monitoring systems, can enhance precision and safety.
• Robust Design and Planning: Ensuring that tunnel designs are adaptable to the variable Himalayan geology and that contingency plans are in place for unexpected geological scenarios.
• Quality Control and Best Practices: Strict adherence to international best practices in tunnel construction, regular quality audits, and continuous training of the workforce to handle the unique challenges of Himalayan tunnelling.
• Regular Monitoring and Maintenance: Establishing a routine for thorough monitoring of the tunnel's structural health during and after construction, using techniques like Distributed Fibre Optic Sensing (DFOS).

In conclusion, the Silkyara tunnel incident serves as a stark reminder of the challenges in Himalayan tunnelling. It underscores the need for a holistic approach encompassing advanced technology, rigorous methodologies, and stringent adherence to safety and quality standards.

A recent report by TOI reveals that Uttarkashi's Silkyara Tunnel witnessed approximately 19-20 minor to medium-level collapses in the past five years during its construction. Anshu Manish Khalkho, a senior official from NHIDCL, explained to the publication that the geological characteristics of the region are a contributing factor to the recurring collapses.

Khalkho acknowledged that incidents of collapses are not uncommon in tunnel construction projects, but unfortunate circumstances led to workers being trapped in this instance. According to him, such collapses, also referred to as 'cavities,' occurred on both the Silkyara and Barkot ends of the 4.5 km-long tunnel. Notably, there were more collapses on the Barkot side than on the Silkyara side.


Khalkho specified that a segment measuring 160 to 260 meters inside the tunnel's mouth from the Silkyara end was identified as a 'red zone' or a 'shear zone' due to the presence of brittle rocks.

Reports highlight that the construction plan included an escape tunnel, which was never built. International tunneling expert Arnold Dix acknowledged the absence of an escape tunnel, stating it is typically placed at the end due to the expectation that tunnels would not collapse. However, he added a disclaimer about considering geological conditions.

In response to these concerns and queries, the government clarified that a separation wall was incorporated within the tunnel. It featured egress openings at regular intervals, designed to serve as an escape route in the event of an emergency. Union Road Transport and Highways minister Nitin Gadkari said, "In the Silkyara bi-directional tunnel on Dharasu-Yamunotri highway (NH-134), a provision of separation wall has been provided at the centre of the carriageway along with egress openings at an average interval of 565 metres for vehicular crossover and at an average interval of 300 metres for pedestrian cross passage for escape purposes during the emergencies."

Senior geologist (consultant) Dinesh Sati has expressed the view that inadequate tunneling practices could have played a role in the failure of the Silkyara tunnel. He highlighted that the tunnel alignment runs parallel to a formation strike, indicating a shear contact between the underlying shales/siltstones/fine-grained sandstone sequence and the overlying metabasics/amphibolites sequence, occurring approximately 300m into the tunnel.

Sati, who conducted the preliminary geology and geotechnical feasibility investigations for the tunnel, pointed out that in tunnels aligned in such a manner, the crown area is particularly susceptible to failures. He suggested that the "reckless retrofitting" of the tunnel during the lining work at approximately 265-270m into the tunnel might have exerted pressure on the already vulnerable sheared mass, potentially leading to the failure observed.

Dr. Dinesh Sati Consulting Geologist

It was the re-profiling work on the 12th morning of November 2023, wherein the deflected primary tunnel supports (the lattice girders) were supposed to be replaced by the new sets. The removal of one such deflected lattice girder at CH-265 initiated the collapse.

The Design Consultant (DC) appointed by the Contractor (under EPC mode) also did the detailed geological/geotechnical mapping for producing a Geotechnical Interpretation Report (GIR) before the design and start of construction. Unfortunately, with the collected field data, DC could not find a zone of probable shearing or poor rock mass anywhere around the zone of tunnel collapse. On the other hand, a 40m thick zone of possible shearing along two contrasting lithologies was already inferred between CH-300 and CH-240 in the Pre-Feasibility Report (PFR).

There was no mention of PFR in the GIR but somehow it kept the PFR-suggested tunnel axis and portals in place! The contractor found the zone of shearing only during excavation (January - October 2019) where several cavities were formed and the size of some of these reached as deep as 25m or so in the tunnel crown between CH-160 and CH-260, which they flagged as the Red Zone. Incidentally, the predicted zone of shearing of the undersigned when plotted on the tunnel axis falls between CH-200 and CH-300 which matches very closely with the excavation data. It suggests that DC and contractor completely overlooked the earlier investigations and started tunneling.

We all know that the excavated rock profile takes little time to settle, and it largely depends on the prevailing ground conditions (rock mass, bedrock structures, span of the tunnel, load, hydrology, etc), and the construction method wherein well-timed placing of suitable primary support is most important. The excavated tunnel reach between CH-80 to CH-300 was left unlined for the past 4 years because the stresses never ceased in the identified Red Zone at least for two consecutive years of construction (2019-2021).

This suggests that the designed support was not adequate to withstand the operational stresses and it directly questions the appreciation of ground conditions and construction methodology that led to a compromise of the safety of the constructed structure as well as the workers who pass through that reach. The start of re-profiling without taking care of the immediate Red Zone sounds as if a surgeon operating on a patient without knowing his/her medical history and surely such construction mismanagement is a violation of safety.

If the Authority’s Engineers (AE) having the mandate of reviewing design drawings and overall construction supervision in the project had been more alert, an evacuation plan (in case of tunnel collapse) would have been in place. The passage of escape was merged with the carriageway by putting a separation wall in the middle, making the collective excavation width of the tunnel (at SPL) almost 16m. I don’t understand why the AE has not looked into the design feasibility in such a poor rock mass (Class IV)!

Preventive Measures

In my opinion, a Geotechnical Baseline Report (GBR) should be made mandatory for getting approval for tunnel and underground constructions in India as this report not only allocates much of the risk involved with the work, it also serves as the basis for bid preparation and is used extensively in resolving disputes during construction. The GBR describes the conditions expected to be encountered during tunnel construction including descriptions and summaries of the following:

1. the tunnel and other underground works
2. the geological and geotechnical information
3. the predicted ground and groundwater conditions on the tunnel alignments, and
4. the effect of ground conditions on the underground works.

Here, I wish to say that most of the projects on EPC contracts in the country, where the DC is appointed by the contractor itself, are facing similar problems, especially when they are awarded on significantly low bids. I sincerely wish the projects would not be awarded below the bid cost estimated/proposed by the client. Accepting lower costs than the bid amount suggests that the client itself is not sure of the Detailed Project Report (DPR) or GIR findings and in such a case the money spent on the DPR/GIR production is useless, hence such a report should be abolished. Also, when the client is ignorant about such underground works it cannot entrust its AE for a better output. In the present case, it is evident that both the client and its AE did not have the level of expertise required for tunnel projects, else this mishap would have been averted.

Call for Enhanced Safety Measures

Following the tunnel's collapse, National Highways Authority of India (NHAI) had announced that it would undertake a safety audit of all 29 under-construction tunnels across the country to ensure their safety and adherence to the highest quality standards.


MoRTH has set up a five-member expert panel to investigate the structural collapse of the Silkyara tunnel and provide a report on the causes. The panel is expected to recommend amendments to standard operating procedures for tunnel collapse scenarios and propose safety measures during construction, with a deadline of 45 days for these recommendations.


The terms of reference for the expert team include an on-site inspection, a review of the project and rescue operation records, scrutiny of the Detailed Project Report (DPR) and detailed design reports submitted by the project contractor. The team will also assess geological mapping, geotechnical and geophysical investigations carried out during the DPR and execution phases. Site conditions, including collapse zones, failures, hydrogeology, and ground type, will be studied. Furthermore, the entire project implementation system by the contractor, consultant, and NHIDCL, including quality control, will be subject to evaluation.

Dr. Manoj Verman, Tunnelling & Rock Engineering Expert, underscores the collapse of the Silkyara tunnel as a compelling reminder of the inherent risks associated with tunneling through the challenging Himalayan terrains – and one of the world's most formidable tunnelling mediums due to their fragile nature.


He says, “It's understood that tunnelling through these majestic mountains is fraught with challenges. Therefore, it's imperative that every phase of a tunnel project, from its conception to commissioning, is approached with an extraordinary level of preparedness and vigilance. However, the grim reality often falls short of these stringent standards. At every stage – from planning, investigation, design, construction, to monitoring – gaps in safety measures and oversight are glaringly evident. It's high time we address these safety lapses head-on and ensure that the marvels of engineering do not come at the cost of human lives."

(Sources: PIB, HT & TOI)