A tunnelling technique that is seeing increasing acceptance in the country is microtunnelling. Emerging as a new trend, it 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, and for railways, rivers and other environmentally sensitive areas.
At a recent conference organised by India Infrastructure, Yogesh M. Killedar, assistant engineer, Municipal Corporation of Greater Mumbai (MCGM), discussed the microtunnelling and pipe jacking methods for the construction of sewerage tunnels, noteworthy initiatives taken and the difficulties faced in sewerage projects.
Microtunnelling and pipe jacking methods
Microtunnelling is a process of excavating tunnels for installing underground pipelines using laser-guided remote-controlled mini shields. It involves the installation of pipelines of up to 250 mm in diameter without digging up the ground surface. Pipe jacking involves lining a tunnel bore formed by a shield or other means by pushing especially designed jacking pipes (reinforced concrete or other pipes) into the tunnel bore from one shaft (jacking shaft) to another (receiving shaft). Both the techniques are performed simultaneously.
A microtunnelling and pipe jacking system comprises five parts:
- Microtunnel boring machines (MTBMs) (shields): It is a mechanised, steerable mini boring machine equipped with a suitable cutter head in the front to excavate smaller diameter tunnels under controlled conditions in which the pressure at the tunnel face and the groundwater pressure are continuously balanced as the shield excavates and moves forward. The operation and steering of the shield are remotely controlled with the aid of a laser and/or CCTV camera.
- Automated spoil removal system: Discharge pumps transport the slurry through the circuit to the separation plant and up to the vibrating screen, where the coarse material is screened out and deposited in a container. The screened suspension is then transported by pumps into cyclones where the fine material is separated from the suspension.
- Jacking system for pushing the jacking pipe: The jacking system comprises high thrust hydraulic jacks mounted on a jacking frame capable of exerting the required jacking force against a purpose-built thrust wall to push the pipes and the shield forward through the ground. The jacking force is transferred evenly through the entire diameter of the jacked pipe.
- Guidance system to guide tunnel excavation: The guidance system comprises a laser beam device or a theodolite with a laser beam attached. The device is installed on the jacking shaft and the beam is set to the desired level, gradient and alignment. Some machines have photosensitive cells on the target panel located at the rear of the shield that converts the laser position into digital data. The data is then electronically transmitted to the operator’s control panel.
- Remote control system to operate shield and other equipment: It measures the important data and monitors and records it. Faults are displayed on the screen monitor.
Case study: Sewer tunnel project using microtunnelling
One of the important projects executed by the MCGM is a 4.94 km long sewer line using the microtunnelling technique. The key issue faced during project implementation was the unstabilised soil condition (marshy land). Though there are a number of ways of stabilising soil (grouting, soil cement mixing, etc.), the MCGM undertook lime stabilisation of the unstable project area. It improved the soil properties in terms of consolidation and shear strength of the treated soil without the need of preloading. Further, the lateral expansion of the lime columns absorbed water from the soft soil, thus making it stable.
Pain points and the way forward
Delays in obtaining no-objection certificates from different organisations (traffic police, the Mumbai Metropolitan Region Development Authority, the Public Works Department, the Ministry of Environment, Forest and Climate Change, etc.) is one of the key reasons behind the MCGM facing delays in project execution. Further, restriction on the use of heavy machinery in areas with high population density, encroachments in the project alignment and the presence of underground utilities adds to project delays. Just like other tunnelling techniques, microtunnelling too faces issues arising from geotechnical and hydrogeological uncertainties.
As is the case with other infrastructure sectors, the issues faced with respect to sewerage tunnel construction have been aggravated by the COVID-19 outbreak. Changing guidelines, non-availability of workforce and raw materials due to lack of transportation facilities and the mass exodus of migrant labour to their native places are some of the challenges that have been thrown up by the pandemic.
Nevertheless, there are huge opportunities for microtunnelling, particularly in the development of water and sewerage tunnels. The method is expected to gain prominence in areas where open-cut tunnelling is not feasible.