Geological Complexities: Executing tunnel construction projects in difficult terrains

Executing tunnel construction projects in difficult terrains

With the rising focus on infrastructure development, tunnelling activity in the country has also witnessed a significant uptick in the past few years. Several tunnel projects are being undertaken in the rugged terrains of the Himalayan and peninsular regions. In fact, a considerable proportion of the hydropower potential is concentrated in the Himalayan region and hydro projects require extensive tunnelling. Projects that are currently being undertaken include the Parbati hydroelectric project, Stage II, in Himachal Pradesh and the PakalDul hydroelectric project in Jammu & Kashmir. Besides, various tunnelling projects are also being undertaken in the road sector (Shimla bypass project, Rohtang bypass tunnel, etc.) and railway sector (Jiribam-Imphal rail project, Bhairabi-Sairang rail line, etc.) to provide seamless connectivity in the regions. However, adverse geological conditions in the Himalayan region and the Western Ghats create challenges in the implementation of tunnel projects.

Key issues and challenges

The Himalayan mountains with their complex geological structure create some of the most challenging ground conditions in the world. Geological discontinuities and surprises in the form of joints, faults, folds, shear zones, ingress of water and gases, vulnerable slopes, high level of seismicity, etc., are the major issues faced by engineers during the excavation of tunnels in the region. Besides, the high overburden stress from the mountains can also cause problems. In addition, the inaccessible nature of some project locations and climatic extremes that include heavy snowfall, flash floods and landslides lead to difficult working conditions. These conditions often result in contractors altering previously approved tunnel designs, leading to time and cost overruns.

Potential solutions

As several tunnelling projects need to be executed in tough geological conditions, it is important to ensure that these projects are systematically planned to ensure smooth and timely implementation. Adequate geological and geotechnical investigations need to be carried out to reduce uncertainties and optimise construction activities. Besides, the use of modern survey tools such as aerial surveys, photogrammetry-based surveys and GPS-based systems that have high accuracy levels can help in obtaining pertinent data and anticipating geological conditions. Further, construction materials such as grouting materials and concrete need to be procured well in time to avoid project delays. Also, adequate provisions need to be made to take care of contingencies that may arise in case geological surprises are encountered. Moreover, new construction techniques and methodologies such as the P5 system (plug, probing, pressure relief, protection of roof, and pre-grouting and support) and ground freezing can also support tunnelling projects in challenging conditions. The adoption of digital solutions such as data loggers and navigation systems too can help in identifying potential risks.

Notable projects

  • Rohtang tunnel project: The 8.8 km Rohtang tunnel is being constructed in the PirPanjal range of Himachal Pradesh. Upon completion in September 2020, the tunnel will become the world’s longest highway tunnel above 10,000 feet. The project, which started in 2010, has faced several challenges owing to the high overburden and tough geological conditions. Construction could only be undertaken from two portals (north and south), of which the north portal was not accessible for more than half the year. Further, the project was delayed by almost five years due to heavy water ingress from the Seri Nullah fault zone. The issue of water inflow in the tunnel was addressed by completing crucial re-profiling and heading works in the fault zone. Moreover, the design was also modified midway to provide a deep invert for all-round circular ground support.
  • Udhampur-Srinagar-Baramulla rail link (USBRL) project: One of the most challenging projects being undertaken by the railways, the 272 km USBRL aims to provide a safe, alternative and reliable transportation system in Jammu & Kashmir. The project involves the development of 38 tunnels with a total length of 119.21 km. The Katra-Banihal section alone involves the construction of 27 tunnels. Various difficulties encountered during tunnel construction under the project were resolved by taking appropriate remedial measures. Drainage holes and pressure relief holes have been provided at several locations inside the tunnel where medium to high ingress of groundwater was found. Steel fibre reinforced shotcrete has been sprayed wherever necessary. Further, geotechnical instruments such as tape extensometers and load cells have been deployed to measure deformations.

In sum

Going forward, with the focus on infrastructure development, tunnelling activity in the Himalayan and peninsular regions is expected to increase. However, there is a need to resolve issues such as inadequate investigations, contractual problems and lack of skilled manpower to ensure timely implementation of projects.