The Indian tunnelling industry is under-going a massive transformation. Mechanised techniques are gradually replacing traditional, non-mechanised methods of tunnel construction. Advanced, technology-driven, efficient and cost-effective methods of tunnelling are being deployed in almost all the infrastructure sectors in the country.
The appropriate technique
Determining the right technique for tunnel construction depends upon geological studies and investigations. In fact, the different methods being used for tunnelling depend on factors such as geology of the location, soil conditions, contracting terms and conditions, tunnel length, etc. The geological studies and investigations include factors such as the quality of the rock that needs to be excavated, in-situ stress, field stress including overburden and other applied stresses, groundwater conditions, etc. Moreover, the geotechnical investigation analyses factors such as regional tectonics, palaeo stress history, etc.
Unlike most other engineering structures, deep tunnels are constructed in natural materials (rock, soil, etc.). They are excavated in the rock mass, which needs to be analysed for the stability of the tunnel. In order to analyse the stability of an opening, the correct appreciation of a rock mass is essential. The necessary supports can be designed once the rock mass has been properly appreciated.
Rock mechanics is the theoretical and applied science of the mechanical behaviour of rocks. It is that branch of mechanics which is concerned with the response of the rock to the force field of its physical environment. The determination of rock mass deformation modulus is an important exercise in rock mechanics. The presence of discontinuities is recognised as an important factor influencing the deformability of rock masses. Discontinuities include joints/fractures, shear zones, folds, faults, bedding and interfaces. Of these, joints form the most significant discontinuities and occur with some degree of clustering around the preferred orientations associated with the formation mechanisms.
However, it is pertinent to note that given the geological complexities, tunnelling is a risky exercise, as an incorrect choice of technique could lead to serious cost and time overruns. Therefore, serious attention needs to be paid during this stage of investigation.
Apart from geotechnical investigations, cost influences the choice between mechanised and non-mechanised tunnelling. Project components such as material, equipment, transport and personnel tend to vary significantly in terms of the cost involved across various techniques of tunnel construction. In general, mechanised tunnelling involves higher equipment and transportation costs in comparison to the conventional tunnelling techniques. However, mechanised tunnelling offers speedy completion of projects, thereby allowing faster inflow of revenues.
Tunnelling methods and techniques
Tunnelling methods have evolved over the years. The most common methods of tunnelling are conventional, drill and blast method (DBM), tunnel boring machines (TBMs) and the New Austrian Tunneling Method (NATM). With regard to the different methods and techniques of tunnel excavation, conventional technologies dominate the tunnelling industry. At present, DBM is the most commonly deployed technique across all infrastructure sectors. Mechanised methods of tunnelling such as TBMs are gaining prominence with an increase in tunnelling activity in congested urban spaces.
Non-mechanised tunnelling techniques are mostly deployed in hilly terrains of the Himalayas and the Western Ghats, while mechanised techniques like TBM and NATM are gaining prominence in congested urban spaces, primarily for metro and urban water supply projects.
Sector-wise, NATM has been slowly gaining traction. The technique is increasingly being used to construct road and railway tunnels. The increase in its use has been partly because of the breakthroughs in shotcrete technology in recent years. Other than providing significant savings in support material costs and personnel and a reduction in project timelines, NATM exhibits great resistance to geological pressures. It prevents disintegration of the rock mass and uses the rock mass as far as possible to endure the additional stresses resulting from excavation.
Unlike the conventional methods where the tunnel is immediately supported without allowing it to deform naturally, NATM allows the deformation of the rock mass before stabilising the tunnel, thereby reducing the amount of additional support materials required. In the world of tunnelling, this method is seen as a set of principles rather than a technical method. Some of the common underlying principles of this technique are:
- The ground around the tunnel is perceived not just as a load, but as a load-bearing element of support.
- The type and quantity of support elements required are systematically adjusted in combination with the time-dependent development of ground reactions as a result of tunnel excavation.
- The ground reactions, taking the form of lining deformations and lining pressures, are measured and the stability of the excavation is confirmed by frequent monitoring.
- Depending on the project conditions (shallow soft ground tunnel, deep rock tunnel, etc.) and the results of geotechnical measurements, the requirements for a specific support are determined.
Tunnelling in the young Himalayan mountains faces challenges due to the existing geological structure. Of all the available tunnelling methods, NATM is the most suitable in such varying geology. One of the most important projects in which NATM was used is the 8.8 km long Rohtang tunnel. Besides, the recently completed Chenani-Nashri tunnel in Jammu & Kashmir, said to be the country’s longest tunnel at 10.89 km, was also constructed using the NATM technique.
Besides these techniques, other methods such as the DRESS (drainage, reinforcement, excavation, support and solution) methodology; the P5 system (plug, probing, pressure relief, protection of roof, and pre-grouting and support); the ground freezing technique; and the pre-grouting technique are being used in cases where the terrain is rocky and water ingress high. In addition, micro-tunnelling is emerging as a new trend in the country, prominently being used to lay water supply pipelines and sewers in congested areas. The use of this technology has gained momentum in the past two decades. Also known as trenchless or pipe jacking technology, micro-tunnelling is especially used for projects which require tunnels under roads with high traffic volumes, railways, rivers, etc. It is used for laying large diameter gravity sewers in cities where open cut installation is difficult, for the installation of product pipelines in areas where the soil condition does not allow for horizontal directional drilling, and for long individual crossings across rivers. This method is particularly used for constructing tunnels with diameters ranging from 600 mm to 3,000 mm.
The way forward
Tunnelling activities in the Himalayas face diverse geological problems such as difficult terrain conditions, thrust zones, shear zones, in-situ stresses, ingress of water or gases, geothermal gradient, high level of seismicity, etc. Developing effective technologies to tackle these problems will help in reducing time and cost overruns. The key factors that need to be taken into consideration prior to the commencement of tunnelling works include adverse environmental effects of DBM tunnelling, high capital costs of specialised equipment like TBMs, unclear contract documents resulting in a mismatch of the tunnelling method and ground conditions, inadequate investigation of ground and soil conditions (which could lead to the failure of a tunnelling technique), unexpected geological complexities and safety risks and not selection of the most appropriate method/technique for tunnel construction.
Regarding tunnelling methods, DBM is likely to remain the most dominant technique of tunnel construction in the country, especially in sectors such as hydropower and railways, where most of the tunnels are being built in hilly terrains. Mechanised techniques of tunnelling (such as NATM) are gradually gaining traction as a cost-effective alternative to non-mechanised techniques in certain areas. In the times ahead, the overall share of such mechanised techniques of tunnelling is likely to increase across different infrastructure sectors like metro rail, water supply and sewerage where tunnelling is undertaken in densely populated areas.
Based on a presentation by Lt. Col. Parikshit Mehr, Joint Director, Tunnels, Border Roads Organisation, at a recent India Infrastructure conference