Tunnel- Planning, Design &Construction with Mitigation Plan and Risk Assessment
Tunnels and Their Importance
History and Importance of Tunnels
Tunnels have been used for transportation for centuries in countries like India, China, and Egypt. Excavation records indicate that some tunnels are over 4,000 years old. The first known tunnel, built in 530 B.C., was constructed on the island of Samos for water supply purposes (ASCE).
The Romans extensively utilized tunneling techniques for various purposes, including mines, water supply, sewage, drainage, and military operations. Similarly, in Australia, a network of deep tunnels exists beneath the streets of Melbourne, constructed centuries ago to facilitate steam transport and telegram communication.
Purpose of Road Tunnels
Road tunnels serve as feasible alternatives to cross water bodies or navigate through physical barriers such as mountains and existing highways. Additionally, tunnels are constructed in specific locations to preserve ecological and environmental features (FHWA).
Definition of a Tunnel
According to the International Tunnel Association (ITA), a tunnel is defined as an artificial subterranean passage that is open at both ends.
Famous and Longest Tunnels in the World
The most popular tunnel and the longest undersea tunnel is 50.46 kilometers railway tunnel that connects Britain with Europe, and it is also the most visited one.
Tunnels in Japan
We all know that Japan is a small country. As per the report, there are more than 11,000 locations in Japan where more than 4,500 km of tunnels are built as a part of the highway system. This is due to the fact that it has very adverse natural ground conditions among the world, resulting in the need for the maximum use of underground space (Tunnel Activity 2020 Overview).
Longest Road Tunnels
The longest road tunnel in the world is in Switzerland. It is known as St. Gotthard, and its length is 16.918 km.
In Asia, the longest tunnel is the Zojila Tunnel in India, which is under construction and is likely to be completed by the end of 2026. Interestingly, this tunnel is bidirectional.
Tunneling in India
In India, tunneling work is being carried out in rail, highway, and hydropower projects. The design of the tunnel is preceded by planning, which encompasses an interdisciplinary approach involving:
- Geology
- Geotechnical engineering
- Methodology of machine tunneling to be used
- Design of formwork
To get better performance and quality output, a suitable contract system should be adopted, including prevalent international bidding contract systems such as FIDIC (Guidelines for the Design of Tunnel – ITA) for engaging competent agencies as contractors.
For accuracy in determining geological and hydrological mappings and ground profile, the adequacy of site exploration is the most essential requirement.
Silkyara Bend-Barkot Tunnel Collapse in Uttarakhand
On 12th November, 2023, a section of the Silkyara Bend-Barkot tunnel in Uttarakhand, India collapsed. The region is prone to natural calamities such as landslides, earthquakes, and flash floods. This zone is a part of the Himalayan ecology and contains highly fragile rocks.
Environmental Concerns
Environmentalists are of the view that rapid construction in the region is causing land subsidence, and this is one of the contributing causes of the tunnel collapse.
Geological Mismatch
The German-Australian engineering consultancy firm hired by the Indian firm engaged in this tunnel construction had stated that the geological condition was not matching with those in the tendered document of the contract from the very start of the tunnel driving work.
Safety and Risk Mitigation
Regular Safety Audits, as envisaged by the Indian Roads Congress, and adhering to the parameters suggested by international bodies will certainly help in early assessment of forthcoming risks. Accordingly, a mitigation plan can be formulated and implemented.
Key Geological Factors Affecting Tunnel Construction
The most important geological factors affecting tunnel construction, which must be considered to avoid risks during the construction phase, include:
- Quality & cover of rock
- Geometry of the geological structures
- Groundwater pressure
- Permeability of the rock mass
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Importance of Site Exploration in Tunnel Construction
Optimum numbers of site exploration is essential to obtain accurate geological and hydrological mappings and determine the correct ground profile. There may be differences in approaches based on ground conditions and the tunneling method proposed. The approach mainly depends on the type of rock, whether it is soft, medium, or hard rock.
Ground Support in Different Rock Conditions
- In soft ground, immediate support is necessary to protect the surface. This is provided by a stiff lining, such as pressurized slurry for face support.
- In hard rock, only a thin lining may be sufficient to protect the surface.
If the ground condition varies along the tunnel axis, then ground strengthening methods such as injection, anchoring, draining, and freezing should be applied as suggested by experts.
Geological Considerations for Tunnel Design
Depending on the layer thickness of the rock mass, its fractures with strike & dip orientations, its color, texture, and mineral composition, the seismic velocity observed, and the quality of the rock mass, the ground at a location can be divided into geological units, each with distinct characteristics.
Role of Geological Experts in Safety Audits
For a correct appraisal, it is crucial that a Geological Expert is included in the Safety Audit Team to address geological complexities that may lead to uncertainties in geotechnical investigations.
This team should be entrusted with conducting audits during the planning, design, and construction stages of the project.
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Stress Field Changes During Tunnel Excavation
As tunnel excavation progresses, the stress field pattern changes. Therefore, during the design stage, the interactive behavior between the ground and lining must be considered. Sometimes, swelling due to groundwater inflow may cause pressure to increase on the lining.
Thus, it is advisable to ensure prompt monitoring during excavation, and this monitoring should continue at intervals even after construction. This aspect must be incorporated into the design to check safety and ground response during tunnel construction.
Geotechnical Report Components
The geotechnical report primarily consists of two parts:
- Geotechnical Data Report (GDR) – Presents factual data, including:
- Geotechnical investigation report
- Landslide survey report
- Soil report
- Foundation report
- Geotechnical Design Memorandum (GDM) – Based on the geotechnical report, it interprets geotechnical information and helps in understanding the assumptions in the design procedure.
Geometric Design Considerations for Tunnels
Apart from highway design standards, tunnel geometric design must account for:
- Fire life safety standard parameters
- Proper ventilation
- Adequate lighting
- Safe traffic movement
- Timely fire detection and smooth communication features
The cross-section of a tunnel should be designed to accommodate:
- Shoulders & sidewalks
- Drainage systems
- Ventilation & lighting
- Tunnel utilities
- Water supply lines for firefighting
- CCTV cameras for surveillance
- Communication equipment
- Requisite travel lanes
(Source: Technical Manual for Design and Construction of Road Tunnels – Civil Element: FHWA)
Learning from Past Tunnel Fire and Catastrophic Accidents
For better performance and quality delivery, we should learn from earlier cases of fire inside tunnels across the world and other catastrophic accidents that have led to the loss of lives and significant damage to national resources.
For example, catastrophic tunnel fires, such as:
- The fire between Britain and France
- The fire in the Mont Blanc road tunnel in France (connecting France and Italy) on 24th March 1999
- The fire in Gotthard Tunnel on 24th October 2001 caused by a truck carrying inflammable materials
These incidents were very serious, causing the death of several users and resulting in substantial financial losses to the concerned countries. (Proceedings of International Symposium on Catastrophic Tunnel Fires)
Tunnel Fire Incidents in India
India has also faced several tunnel crash incidents that led to fire and loss of lives. For example:
- A fire broke out in the Baijnathpur-Lohgad Road tunnel in 1981, caused by a truck carrying inflammable materials, leading to several deaths.
- In 1987, a head-on collision between a public transport bus and a truck inside the Mhow-Indore tunnel resulted in numerous fatalities.
- An explosion of a truck carrying LPG cylinders inside the Jammu-Srinagar Highway tunnel caused the deaths of several people.
There have been several other incidents like these, leading to both human casualties and losses to the national exchequer.
Importance of Safe and Efficient Transportation Through Tunnels
It is quite clear that the issue of safe and efficient transportation through tunnels is essential nowadays due to their locations, which are often under densely inhabited areas, rivers, seas, or mountains at significant depths.
Fires and Their Consequences
Fires due to vehicular traffic inside tunnels may lead to serious consequences, as the confined and restricted space aggravates the situation, causing the fire to spread rapidly. The speed of the fire and its size depend on:
- Fire load
- Condition of the affected vehicle
- Airflow characteristics inside the tunnel
- Tunnel design with respect to fire safety parameters
Risks Due to Lack of Ventilation and Flammable Materials
- Lack of sufficient ventilation and the use of flammable materials in the tunnel structure can lead to the accumulation of heat, which is very dangerous.
- The operational efforts depend on mechanized ventilation and the method used to manage it.
It has been observed that thick cross-sections of the tunnel structure and rock mass inside may affect the rapid heat dissipation in the event of a fire. Therefore, during the design phase, the consequences of heat generated in case of fire and the fire load of structural materials should be examined in detail (Road Tunnel Manual by PIARC).
Managing Uncertainties and Risks in Tunnel Construction
In conclusion, uncertainties and risks are always apparent in tunnel construction, and these may affect the functional utilities, productivity, and the ecology of the surroundings. To avoid these uncertainties, mitigation efforts are essential at the design, planning, construction, and post-construction phases of the project (Procedures for Risk Analysis and Management in Tunneling Projects – ITESE 2019).
At the time of contract negotiation, i.e., before the finalization of the Contract Agreement, risk assessment and its sharing pattern between the client and contractor must be finalized to avoid any future uncertainties.
✍️ Author Details
📌 C S VERMA
🏅 F.I.E., Ex-General Manager, National Highway Authority of India
📧 Email: csverma62@gmail.com
📞 Mobile: +91-7319885740 - Geotechnical Data Report (GDR) – Presents factual data, including:
