Increased infrastructure activity across sectors has paved the way for more tunnel construction in the country. Positive breakthroughs have been witnessed in the past few years with important tunnels such as the Zojila tunnel, the Chenani-Nashri road tunnel, the Banihal-Qazigund rail tunnel, the Kashang hydro tunnel, tunnels for the Teesta 3 and Kishanganga hydro projects, and several tunnels completed for metro projects in Delhi, Kolkata, Chennai and Lucknow. There are various techniques being used for tunnelling construction. While conventional methods dominate the hydro segment, mechanised methods such as the tunnel boring machine (TBM) and New Austrian tunnelling method (NATM) are gaining traction in the metro and railway sectors.
Tunnelling techniques
The drill and blast method (DBM) is the most often used technique for tunnelling across sectors and is usually deployed in less congested areas. This technology has also been preferred in hilly areas such as the Himalayas and the Western Ghats where it is difficult to deploy TBMs. While DBM is the most prevalent in the hydropower sector, others such as the railway, metro rail and water supply sectors also account for significant tunnel length being constructed using this technique.
TBMs have predominantly been deployed in the metro sector vis-à-vis other sectors. With regard to the irrigation sector, TBMs are slowly replacing the DBM tunnelling technique which was used earlier. In the water supply and sewerage sector too, the use of TBMs for tunnelling started only recently, specifically in the urban areas. Factors such as heavy movement of traffic, space constraints and congestion are responsible for the use of this method.
In recent years, NATM has been gaining prominence, especially in the railway and metro sectors. While the hydropower sector accounts for the largest share (49 per cent) of the total tunnel length constructed (completed projects) using NATM, the railway sector accounts for the largest share (75 per cent) of the total tunnel length currently under construction.
Micro tunnelling technology is new to the country and presents a huge opportunity, particularly in the water supply and sewerage sector. Tunnels spanning a length of around 60 km have been constructed using this technique.
Advancements in tunnelling equipment and techniques
There is demand for high-tech equipment as geological complexities are the biggest challenge in tunnelling. Soil and rock investigation, analysis of ground behaviour during tunnelling and assessment of risks are important considerations. Contractors are also experimenting with new techniques and methodologies such as the P5 system and ground freezing for the more challenging projects, and are using new and innovative materials such as geosynthetics, geomembranes, steel anchors and self-drilling rock bolts. Meanwhile, navigation systems, computerised jumbos and advanced drilling systems are being deployed for better monitoring and identifying potential risks.
Navigation systems deployed for precision
The navigation systems deployed in TBMs help in providing a continuous reference point for the position of the TBM. In the navigation system, a beam is picked up by targets at the back of the TBM. This, in turn, helps the TBM driver by showing the location and altitude (tilt or skew) on a real-time basis as compared to the planned, pre-programmed position. These navigation systems make the continuous determination of the TBM’s position much easier. The shield driver is provided with all the necessary data and information at every moment, for steering the TBM along the tunnel axis. Further, to cope with the curved alignment of the tunnels and to balance out the refraction effects in hot tunnels (tunnelling in areas with high temperatures), prisms are installed on the sides of the tunnel to deflect the beam.
RCMS for overcoming deformation risks
During a tunnelling project with segmental lining, a number of different forces, either natural or advance-related, act on the tunnel and the rings installed. The ring convergence measurement system (RCMS) detects potential risks caused by deformations at the earliest possible time. The various advantages offered by RCMS include:
- Real-time monitoring of ring convergence with continuous data collection for immediate intervention
- Making the system independent of line of sight in TBM backup area
- Assurance of quality and safety of the tunnel building
- Minimisation of damages and cost in ring building by early recognition of deformations
- Minimal work effort because of automatic data acquisition.
Upgraded software
Advanced modelling software has improved the way blast patterns are analysed. A case in point is the 2D Bench software. It provides cost-effective solutions for blast design and blast analysis. Further, the simultaneous use of 2D Bench with Surpac (a geology and mine planning software) provides additional benefits such as 3D modelling. It also allows blast designs to be created using topographic information and pit designs.
Advanced drilling systems for monitoring
One of the most important advancements that have increased the efficiency of the DBM is with respect to drill equipment. The controller area network (CANbus control) provides onboard monitoring and maintenance assessment. In addition, modern drills are equipped with various capabilities including rock recognition software, auto drilling, GPS hole location, etc., enabling efficient tunnelling. Rock recognition software allows geologists to examine and explore the features of rocks and minerals without a petrographic microscope, an expensive technology. Geology Toolkit is one such android application which makes it easier to examine a thin section of rock to understand the overall characteristics of the rocks. Meanwhile, ANFO Hopper is a device which fits on to a front-end loader or interchangeable tool carrier. The device is able to safely carry ammonium nitrate/fuel oil (ANFO) explosives, thereby greatly reducing the risk of injury from repetitive manual handling of explosives.
Innovative explosives
There are various factors which need to be considered while selecting the kind of explosives required for blasting during tunnel construction. These include the blast hole diameter, hardness of rocks, water presence, ground conditions and proximity to vibration-sensitive receivers. The various kinds of explosives available to better match the rock type and the explosives used for efficient tunnelling are ANFO, heavy ANFO and emulsion.
Remote blast monitoring
Remote blast monitoring services have been developed for blast vibration assessment and air overpressure measurement. The other advantages offered are immediate access to the event data, eliminating the need to travel to and from the monitoring location and being less time consuming, among others.
Growing demand for computerised jumbos
Cost-effective and technologically advanced DBM equipment such as computerised jumbos, designed to undertake tunnelling with greater precision, have been gaining prominence in recent years. This equipment is capable of minimising losses by eliminating the chance of over-excavating tunnels, which then require costly materials to fill the space.
Issues
The choice of tunnelling techniques is heavily dependent on the available data on soil conditions. Any discrepancy in the data can lead to a collapse of the tunnel. In particular, the NATM relies significantly on the available soil data. In fact, most of the tunnel collapses while using the NATM are linked to insufficient data and unexpected ground conditions. One of the major challenges faced by contractors is ensuring the safety of workers during drilling and blasting. Compared to other tunnelling techniques, DBM involves higher risk to construction workers. Other challenges in drilling and blasting include higher environmental impact, greater noise and vibration levels, increased contamination, rehabilitation of settlements, disturbance to wildlife, etc.
Mobilising/Demobilising of TBM equipment and machinery is one of the biggest challenges faced by contractors, especially while transferring these to project sites located across the country. This is primarily due to the sheer size of the equipment. Geotechnical and hydrogeological uncertainties are other challenges faced by TBM operators. Though tunnelling with TBMs in the Himalayan regions has not been particularly successful, numerous problems associated with TBMs can be avoided if sufficient information is available in advance. In this regard, adequate investigation of ground and soil conditions will go a long way in avoiding problems associated with TBMs. Further, advanced geophysical methods like topographic analysis and radar, though expensive, can be cost effective in the long run, particularly in the Himalayas.
Ishita Gupta