With the growing number and increasing complexity of infrastructure projects in the country, the demand for tunnel development has gained momentum in the past few years. With projects involving longer tunnel lengths and given varying geological challenges, innovative technologies are being inducted.
Excavation using tunnel boring machines (TBMs) has been quite successful in constructing metro, water supply and sewerage tunnels. However, the high cost of TBMs and the difficulty in mobilising the machines to job sites, especially in the Himalayan region, remain big deterrents. Flooding and mucking have sometimes resulted in TBMs getting buried, leading to time and cost escalation. Hydropower projects such as Parbati II in Himachal Pradesh and Dulhasti in Jammu & Kashmir are cases in point. Microtunnelling is another advanced method which is finding increasing deployment in congested urban areas.
TBMs: Experience so far
In the tunnelling space, TBMs have found applications with the development of metro projects in the country. Sector-wise, TBMs are the most widely utilised for tunnels in urban rail and water supply and sewerage projects in congested urban areas. This technique has been successful in projects such as the Delhi metro and the Srisailam Left Bank Canal tunnel scheme. That said, a major deterrent in its deployment has been the high cost associated with it. Besides, there are complexities involved in mobilising machines to job sites, especially in hilly terrains, limiting its use in such areas.
Current scenario for TBMs
Based on the projects tracked by India Infrastructure Research, tunnels spanning a length of around 488 km have either been awarded, are under construction or have been constructed using TBMs. The method has been used extensively in the metro rail sector (43 per cent), followed by the irrigation (21 per cent) and water supply (20 per cent) sectors. With regard to the irrigation sector, TBMs are slowly replacing the drill and blast method of construction that was used earlier. So far, tunnels spanning a length of more than 100 km are either being constructed or are under construction in the irrigation space. In the water supply and sewerage sector too, the use of TBMs for tunnelling started only recently, primarily in urban areas. Heavy movement of traffic, space constraints and congestion have been the main reasons behind the use of this technology.
Key trends in TBM deployment
- Most TBMs have been used in the metro sector: TBMs have predominantly been deployed in the metro sector vis-à-vis other sectors, with around 90 per cent of the metro tunnels completed and under construction using this technique (in combination with other techniques). Given the current pipeline of upcoming projects in the metro rail sector, a tunnel length of over 38 km has been planned for construction using this technique (projects which have been approved, planned and are at the bidding stage). While TBMs are getting popular in other sectors such as irrigation and water supply, the use of this technology is likely to dominate the metro sector going forward.
- Imported TBMs are less expensive than domestically manufactured ones: There exists a significant difference between the cost of imported TBMs and domestically manufactured/assembled ones. The main reason for this is the taxation structure. If a TBM is imported, it is 100 per cent duty-free, whereas if it is manufactured domestically and sold to customers, full duties and taxes are applicable. This has therefore defeated the main purpose of the Make in India mandate and has resulted in the establishment of hardly any TBM manufacturing/assembly units inthe country. Herrenknecht is the only foreign company that has established a manufacturing facility in India. Its state-of-the-art TBM assembly and cutting tools facility,spread over 10 acres, is loacted in Chennai.
- Greater reliance on China for import of TBM technology: The share of various countries exporting tunnelling machinery to India has changed substantially over the past five years. In 2017-18, China accounted for a share of 58.47 per cent as compared to 73.96 per cent in 2013-14 in the total imported tunnelling machinery. Although China’s share has declined, the country still continues to dominate the segment. Germany, on the other hand, has emerged as a new market for the import of tunnelling equipment in India with its share increasing from only 6.96 per cent in 2013-14 to 28.31 per cent in 2017-18.
- Resale/Buyback arrangements in the TBM market: The TBM market has seen an increasing trend of contractors entering into buyback arrangements with suppliers. Since the type of TBM deployed for a particular project is subject to geological and soil conditions, TBMs once used are either refurbished to cater to different projects or sold back to the manufacturer for a fraction of the purchase price. In many cases, contractors have sold their TBM equipment to other contractors in the market for an agreed-upon price.
- Rental of TBM equipment trending in India: Renting TBMs is another trend in the market. Contractors, depending on the pipeline of projects, geology and cost-effectiveness of refurbishing the existing machinery, have been renting equipment, as many projects involve the use of different machines for short periods of time. However, the majority of large companies prefer buying TBM equipment. Besides renting, many TBM manufacturers have started offering machinery along with their services on a contractual basis. Some of the companies that have forayed into the organised rental sector are Quippo (owned by Srei Infrastructure), Gmmco (CK Birla Group) and the Sany Group. Also, Herrenknecht provides services for the installation of TBMs, rental equipment, etc.
Technical advances in TBM technology
With technical advancements, the efficiency of TBMs has been increasing significantly. Navigation, ring sequencing and better monitoring have been made possible by equipping TBMs with advanced technological systems.
- TUnIS Ring Sequencing System for predictive ring sequencing: To execute tunnel projects with segmental lining most efficiently, many parameters need to be considered for the ring build process, including the tail skin clearance of the last ring, the orientation of the tail and front shield of the TBM as well as the predicted future TBM position. The TUnIS Ring Sequencing System automatically calculates the optimum ring sequence for tunnel advances that include segmental lining. Based on the ring position of the previously installed ring, the system allows for predictive ring sequencing taking into account the current TBM position.
- SLuM Ultra for optimal future ring sequence: To determine the optimal future ring sequence, it is necessary to observe the position of the last built ring. SLuM Ultra enhances the ring build process by measuring the ring position relative to the tail skin axis with millimetre accuracy.
- RCMS for overcoming potential 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 the potential risks caused by deformations at the earliest possibile time.
- SDS for better monitoring: A modular production and logistics management system, the Segment Documentation System (SDS), comprehensively controls, monitors and documents all major processes within the production and storage of segments. It also captures their position and installation data in the tunnel on a real-time basis. With its modules, SDS.Production, SDS.Storage and SDS.TBM, the system comprehensively controls, monitors and documents all the major processes in the production and storage of segments and captures their position and installation data in the tunnel.
Microtunneling technology
Microtunnelling, a type of trenchless technology, is new to the country and is finding increasing use in the water supply and sewerage sector. It is a pit-launched trenchless technique and uses laser-guided controls. It is used to lay water supply pipelines and sewers in congested areas. It is being put to use 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. It is suitable for constructing tunnels with diameters ranging from 600 mm to 3,000 mm. Even though microtunnelling is a bit more expensive than horizontal directional drilling (HDD), its accuracy, reliability and lower maintenance cost of pipelines makes it a better choice.
According to India Infrastructure Research, tunnels spanning a length of around 60 km have been constructed using this technique. Lately, new advances are being witnessed in microtunnelling. Herrenknecht has introduced the Direct PIPE technology. This has opened up new possibilities for installing pipelines in a more efficient way. This method combines the advantages of microtunnelling and the HDD technology. In a one-step solution, a prefabricated pipeline can be installed and the required borehole excavated simultaneously. This allows speedy and highly economical installation of pipelines with lengths of more than 1,500 metres. The footprint of the equipment at the launch and reception points is very small in the Direct PIPE method. Therefore, the technology can be deployed in densely populated residential areas.
Conclusion
With new technological advances, both TBM and microtunnelling technologies hold promise for the times ahead. However, much will depend on the choice of technology which in turn would be determined by the data available on soil conditions as well as the type of project to be constructed. For TBMs, mobilising the equipment is one of the biggest challenges faced by contractors, especially while transferring them to project sites located across the country. Geotechnical and hydrogeological uncertainties are other
challenges faced by operators. Better topographic analysis and investigations will prove beneficial for tunnelling projects. Another investment that is needed is in skilling manpower to work with these technologies.