The 41 workers trapped in a tunnel near the Himalayan town of Uttarkashi since November 12 are safe. But the accident is a wake-up call about the perils of undertaking massive construction projects in the fragile Himalayan region, which is under threat from intense human activity and the vagaries of the climate crisis.
India, of course, is no novice in the field of mountain tunnelling. It has done – and is doing – tunnels in Kashmir and Ladakh and other parts of the country. These are capital-intensive projects because tunnelling is the most sensitive, complicated and expensive process in construction. Tunnelling helps to reduce travel time and plays a strategic role by enabling quicker passage of defence armaments and military troops. But, with the growing urgency to speed up construction, the manifold risks and challenges involved in the construction of tunnels are often ignored. The current construction mishap in Silkyara, Uttarkashi in the Char Dham highway project is a glaring reminder of the need for careful planning and execution of tunnels and the overall need for sensitivity when attempting to tunnel through the Himalayas.
Boring and building a tunnel through a mountain is perilous as it is. It is as fraught as removing a column from a high-rise building. No doubt, there are advanced technologies like Tunnel Seismic Prediction (TSP) and Ground Penetrating Radar (GPR) which give a reasonable idea about the prevailing geomorphological conditions, but natural geological surprises can never be ruled out. The Himalaya is a young mountain and tunnelling here is particularly prone to experience squeezing rock conditions, the inrush of water, roof falls or chimney formations and gas explosions. These problems occur because of the presence of shear zones, fault zones, thrust zones, difficult terrain conditions and folded rock sequences.
In order to prevent failures, extensive and thorough planning and surveying needs to go into tunnelling projects. It is, therefore, important to understand the causes of tunnel failures and the remedial measures that need to be adopted to prevent them.
The most common cause is the ab initio poor designing of the tunnel project. Many times, failures occur in the tunnels due to improper soil investigation and connected inadequate design. Along the different stages in tunnel construction, there is a need for proper ground investigation and high-quality design. The design has to be complemented with uncompromising proof-checking by experienced and competent experts. Once the design is finalised, supervision and deployment of suitably qualified engineers to understand, appreciate and execute the design scrupulously is critical.
Substandard construction, poor workmanship and lack of quality control are other reasons for the failure of tunnels. In the Heathrow Express tunnel collapse case of 1994, basic systems of construction working had gone unchecked leading the inquiry commission to conclude: “The collapses could have been prevented, but a cultural mindset focused attention on the apparent economies and the need for production rather than the particular risks.” Therefore, a robust quality control system should be followed and monitoring has to be carried out continuously and meticulously and even the slightest defect remedied promptly.
Another cause is the wrapping and twisting of the tunnel support system. The bearing capacity of ground soil near the portal decreases due to the continuous ingress of water to the bottom of the tunnel portal. This water causes the concrete lining to settle into the ground and, therefore, leads to the wrapping and twisting of the tunnel support system. Proper designing of the drainage system is, therefore, crucial.
If swelling clay soil is observed above the crown of the tunnel, it will be amenable to hydrothermal pressure, which can cause an increase in the volume of soil particles, resulting in heaving at the base of the pavement surface. To reduce the passive flow of material on the soil particles, rock bolts or anchor bolts are usually recommended when the clay soil swells and cannot be avoided.
Sometimes due to heavy rainfall, the ground near the tunnel portal can collapse because of excessive water pressure. The deployment of an umbrella technique, which holds the material surrounding the periphery of the tunnel, reduces the risk of such collapse.
If fault and fracture zones are not suitably anticipated during the blasting, they can have serious consequences. In the course of blasting, the existing faults and fractures get loosened, and the loosened rock mass creates an excessive shear force on the periphery of the existing tunnel which can cause shear failure and cracks into the concrete lining. If the fault or fractures exceed the permissible limit, then grouting with chemical stabilisation is necessary. Near the fault or fracture regions, the strength of the tunnel support system should be increased by reducing its spacing. The blasting system should be well-designed, incorporating all the functional parameters such as stemming length, burden, bench spacing and control of fly rock.
Sometimes cavities may be generated in the surrounding rock mass after or during the execution of the blasting process. If the assessment of cavities in the alignment of the tunnel is not worked out before the advancement of each and every trail, then it can lead to the failure of the tunnel near the advancement portal. Therefore, trail holes should be made before each and every advancement in the tunnel section and if any cavity is observed, it should be filled with bentonite slurry.
Highways and, in particular, tunnels in the Himalayan region need the highest level of attention to detail and cannot be left to the contractor’s whims. Geotechnical investigations, correct design, suitable support system, quality control and continuous monitoring during and after the excavation, are crucial to the success of tunnel construction. Protocols and SOPs for future maintenance and repair are also critical even if they entail temporary closure.
The implementing agency, whether the Border Roads Organisation or the National Highways & Infrastructure Development Corporation Limited (NHIDCL), needs to be more than a mere contracting agencies and should emerge as a specialist organisations for sustainable engineering in mountain highways and tunnels. They should be tasked to build suitable skill sets and resources to be able to effectively supervise contractors and deliver failsafe and lasting tunnel projects.