New Techniques

Advancements in TBM, micro-tunnelling and NATM technologies

The tunnelling sector has been gaining traction in the past few years. A large number of infrastructure projects are being implemented across sectors such as railways, roads and highways, water supply and sewerage, and urban mass transit. This has led to a surge in tunnelling activity in the country. The sector has received greater attention due to increased congestion in urban areas, which renders little or no scope for on-ground construction. New technologies and techniques are also being adopted to ensure better execution of the tunnelling projects.

Indian Infrastructure takes a look at the advancements in tunnelling technologies…

Tunnel boring machines

Tunnel boring machines (TBMs) have emerged as the preferred tunnelling technique in congested urban areas. TBMs have been predominantly deployed for the construction of tunnels in the metro rail sector (41 per cent). Metro is followed by the irrigation, hydro and water supply sectors at 22 per cent, 15 per cent and 15 per cent respectively. Based on the projects tracked by India Infrastructure Research, tunnels spanning a length of around 656 km have either been awarded, are under construction or have been constructed using TBMs (as of April 2021). In terms of numbers, a total of 167 TBMs have been deployed/planned to be deployed for the construction of underground metro tunnels across key metro projects in Delhi, Mumbai, Chennai, Kolkata, Jaipur, Pune, Ahmedabad and Lucknow (north-south corridor).

Some new technologies being deployed for TBMs are the TUnIS Ring Sequencing System, wherein the optimum ring sequence for tunnel advances is automatically calculated. In navigating tunnelling activities, it is ensured that tunnels that are even several kilometre in length will be driven to their destination with the minutest accuracy. Another new system being used is the Automatic Tailskin Clearance Measurement System, which enhances the ring build process by measuring the ring position relative to the tail skin axis with fine accuracy. Keeping in line with the digitalisation initiatives, the data management system deployed by TBM is a web-based process to handle complex infrastructural building projects. It ensures efficient control of TBMs and many other operating processes within tunnelling projects. One of the major challenges faced by TBM contractors is with regard to geotechnical and hydrogeological uncertainties. Despite proper soil investigations and other preliminary investigations, actual soil conditions are unpredictable. With the number of TBMs being planned for deployment in the upcoming projects, shortage of skilled manpower might worsen the situation. TBM contractors also face problems in mobilising/demobilising TBM equipment and machinery, especially while transferring the same equipment to project sites located in different parts of the country.

New Austrian Tunnelling Method

The New Austrian Tunnelling Method (NATM) has predominantly been deployed for the construction of tunnels in the railway sector. Based on the projects tracked by India Infrastructure Research, 132 tunnels spanning a length of about 412 km have either been awarded, are under construction or have been constructed using the NATM (as of April 2021). The maximum deployment of the NATM technique has been witnessed in the Himalayan region. State-wise, Jammu & Kashmir has the lion’s share of 38.88 per cent in the total number of completed, ongoing and awarded tunnels using the NATM technique of tunnelling. Jammu & Kashmir is distantly followed by Maharashtra (19.35 per cent) and Delhi (10.05 per cent). Over the past few years, the NATM tunnelling has gained traction in India. The railways account for the majority share in the deployment of the NATM technique of tunneling. The railway sector is distantly followed by the hydropower sector wherein 23 tunnels spanning a length of over 78 km have been completed/ongoing/awarded using the NATM technique of tunneling. Other than these, the metro sector has also deployed the NATM for a considerable tunnel length. The NATM involves the deployment of articulated 10 tonne loader and 35 tonne dump truck, tunnel excavators, shotcrete robots, MIA pumps (used for rock bolts), Casagrande and drill jumbo. The drill jumbo is a multipurpose equipment. It is used for drilling blast holes, fore poles, rock bolts (holes), probe holes and weep holes.

With the pipeline of metro projects planned for implementation, the use of NATM is expected to increase further in the times ahead. the NATM is a preferred method of tunnelling in uncertain geological and hydrogeological conditions. It is also appropriate for the construction of approach roads of adequate geometry, caverns and mines. The NATM, which is based on deformation monitoring data to evaluate the quantity and quality of the support system, has also proved to be an appropriate tool for tunnelling in the young Himalayas.

Like other tunnelling techniques, the NATM faces multiple issues. In particular, the NATM relies significantly on available soil data. In fact, most of the NATM tunnel collapses are linked to insufficient data and unexpected ground conditions. Further, for the NATM to be successful, interruptions should be reduced in excavation and support work.  NATM requires constant quality assurance and quality control (QA/QC), as there is no prefabricated support. Besides, it requires a lot of manual work, therefore, labour requirements are higher.

Micro-tunnelling

Micro-tunnelling, a trenchless construction method, is used to install pipelines beneath highways, railroads, runways, harbours, rivers and environmentally sensitive areas. This technique has been particularly deployed in the construction of sewerage tunnels. Based on the projects tracked by India Infrastructure Research, tunnels spanning a length of around 60 km have been constructed using the micro-tunnelling technique (as of April 2021). Delhi has the maximum share in the total length of tunnels constructed/being constructed that have deployed the micro-tunnelling technique. The trenchless technology includes the laying of underground utilities such as pipes and cables, and ducts using various installation techniques such as horizontal directional drilling, auger boring micro-tunnelling and direct pipe technology. The method requires considering the geotechnical and soil investigation reports and the loads applicable on the ground.

Emerging as a new trend in India, micro-tunnelling is being extensively used to lay water supply pipelines and sewers in congested areas. It is especially used for projects that 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. Mumbai was the first city to experiment with micro-tunnelling for the World Bank-funded Mumbai sewage disposal project. Other cities such as Delhi and Kolkata have also used this technology.

Micro-tunnelling also faces geotechnical and hydrogeological uncertainties. While the micro-tunnelling method offers highly accurate installations of pipeline at any depth and under various difficult ground conditions, one of the main disadvantages of this method is the high initial cost of micro-tunnel boring machines. Besides, micro-tunnelling is not appropriate for low strength pipes due to the jacking procedures involved. Obstructions in the soil profile may cause problems during the cutting procedure, thus hampering the tunnelling process.

The road ahead

The industry has adopted modern technology for tunnel construction. There is greater room for the adoption of international standards in tunnel design and construction given that it offers more efficiency. However, the tunnelling industry relies heavily on other countries for advanced tunnelling machinery. Going forward, there is a need to switch to mechanised ways and deploy smart techniques in order to ensure safety of workers. The use of high-end technologies in tunnelling will lead to improvement in operational efficiency for all the stakeholders involved.

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