By Parikshit Mehra, Secretary, Tunnels and Safety, Government of Telangana
The Srisailam Left Bank Canal (SLBC) tunnel is one of India’s most ambitious irrigation tunnel projects, conceived to transfer water from the Srisailam dam reservoir to Hyderabad and Nalgonda district in Telangana. The project aims to provide a reliable source of drinking water to fluoride-affected villages while also supporting the region’s growing urban and industrial water demands. By enabling gravity-driven water conveyance, the tunnel is expected to replace the existing pumping-based system and substantially reduce long-term operational costs.
With a total length of 43.94 km, the tunnel represents one of the longest tunnel drives undertaken from two portals. Approximately 9.8 km of excavation remains to be completed. The alignment traverses beneath a reserve forest and tiger reserve, with an overburden of 300-500 metres, presenting significant engineering and environmental challenges. Restrictions on the construction of intermediate access adits and shafts further increased the complexity of execution, requiring excavation to be carried out solely from the inlet and outlet portals. The project’s shift to an observational tunnelling approach following a major geological challenge offers several insights and key takeaways for technical deployments and optimal construction strategies.
Key details, construction progress and methodology
The construction work on the SLBC tunnel commenced in 2006 based on walkover surveys and surface observations, without a detailed geotechnical exploration. Due to restrictions on the use of explosives, excavation was undertaken using tunnel boring machines (TBMs), with the approximately 44 km long tunnel being driven exclusively from the inlet and outlet portals.
The project had employed a double-shield TBM comprising a three-storey structure with seven gantries. The machine had a diameter of 10 metres, a shield length of 12.4 metres, a backup system extending 116 metres and a total weight of 1,500 tonnes. Tunnel support was provided through segmental lining consisting of seven plus one rings. Muck generated during excavation was removed through a conveyor system, while dedicated ducting ensured ventilation within the tunnel. The movement of personnel, equipment and materials was facilitated through a locomotive-based transportation system.
Significant progress was achieved from both ends of the tunnel, with excavation advancing to approximately 13.95 km from the inlet side and 20.43 km from the outlet side.
Overcoming challenges
In February 2025, the project encountered a major geological event that resulted in substantial inflows of debris and water into the tunnel. Water ingress increased rapidly from 3,500 litres per minute (lpm) to 10,000 lpm within a few hours, while muck accumulation reached 8 metres at the collapse location and 2 metres at a distance of 350 metres from the affected section. The incident marked a critical turning point in the project and necessitated a comprehensive reassessment of the construction strategy for the remaining tunnel works.
Following the geological challenge, a phased recovery and search operation was undertaken. The affected section was divided into operational zones, including no-go, marking and advanced zones, based on stability assessments and accessibility. Simultaneous activities were carried out to dewater the tunnel, channelise water ingress, remove muck, cut through damaged steel components, and extend ventilation and lighting systems to facilitate safe access. Following completion of the rescue efforts, attention shifted towards the remaining 9.8 km of tunnelling. The option of constructing an inclined shaft was examined. However, environmental restrictions associated with the tiger reserve and reserve forest, along with the lengthy approval process and construction timelines, rendered the approach impractical.
A revised strategy was therefore adopted. The damaged TBMs were removed and the project transitioned to an observational tunnelling approach. Since additional surface investigations within the reserve area were restricted, geological assessments were undertaken through helicopter-based surveys, walkover investigations and airborne geophysical studies conducted with support from specialised agencies. The revised construction methodology also required addressing challenges related to ventilation, muck disposal and workforce adaptation, particularly as excavation activities were being carried out up to 14 km and 20 km from the respective tunnel portals. Subsequent investigations provided critical insights into the geological conditions responsible for the collapse. The area was found to contain folded rock formations with a near-horizontal plunge, a feature that had not been adequately documented during the original project planning stage. Furthermore, the tunnel alignment intersected highly fractured and stressed rock masses associated with these folds. Prior grouting operations had temporarily stabilised the ground. However, once the TBM cutterhead advanced through the zone, the unsupported fractured rock mass-imposed loads exceeded the capacity of the segmental lining, resulting in collapse and large-scale ingress of rock material into the tunnel.
Following the transition from TBM excavation to an observational tunnelling approach, the remaining 9.8 km of the SLBC tunnel alignment was mapped and characterised. The revised methodology relies on continuous instrumentation, deformation monitoring and geological observations to identify ground instability. This enables the timely implementation of support measures. In line with this, observational tunnelling has facilitated continuous geological assessment and three-dimensional monitoring, improving the management of complex ground conditions.
The inlet side continues to experience occasional rock burst activity due to laminated rock formations under approximately 300 metres of overburden, while the outlet side comprises largely massive and stable rock with minimal deformation. Excavation is being carried out through controlled drill-and-blast methods, supplemented by spot bolting and shotcrete support wherever required. The project has already achieved approximately 250 metres of excavation under the revised approach as of June 1, 2026, with tunnelling progressing simultaneously from both ends.
Further, to expedite project completion, the final lining for the remaining stretch is proposed to be executed using spray concrete rather than conventional gantry-based lining systems. Since final lining will be installed only after ground deformations have stabilised, the design approach excludes rock load considerations at that stage, reducing the required lining thickness from nearly 500 mm to 275 mm. This approach is expected to improve constructability while maintaining structural adequacy. Besides, as the tunnel is intended for water conveyance, achieving suitable hydraulic performance remains a key consideration. Efforts have therefore been focused on obtaining smooth tunnel surfaces and appropriate roughness characteristics through trial applications and optimisation of lining techniques.
Deployment of innovative techniques and solutions
The SLBC tunnel demonstrates the growing importance of technology-driven construction practices in delivering large-scale underground infrastructure. As tunnel projects become larger and more complex, better investigations, modern monitoring systems and improved design approaches have become increasingly important. Close collaboration between project stakeholders and industry experts can help reduce risks, control costs and ensure timely project delivery.
For the SLBC tunnel project, various technological solutions have been evaluated to enhance surface quality and operational efficiency. The experience gained from the project brings out the importance of robust project conception, detailed investigations and comprehensive geological characterisation during the planning stage. The absence of adequate exploration and intermediate access arrangements significantly increased construction risks and constrained response options following the tunnel incident.
The project also highlights the need for a greater focus on rock mechanics principles, excavation and support sequencing, deformation control and timely installation of support systems. Continuous deformation monitoring and support optimisation not only improve safety but also enhance project economy. Furthermore, the adoption of advanced geomechanical modelling techniques, in place of an exclusive reliance on empirical rock classification systems, can provide a more accurate understanding of rock mass behaviour and support requirements.
Tracing future pathways
Based on the lessons from the construction of the SLBC tunnelling project, going forward, future tunnelling projects require stronger technical expertise across clients, contractors and consultants, along with more flexible contractual frameworks. Improved dispute resolution mechanisms and technical review processes can help manage geological uncertainties more effectively and reduce project delays. Alongside, standardisation of tunnel design, particularly for road tunnels, can improve efficiency across the project life cycle. Uniform cross-sections, drawings and formwork systems will play a significant role in reducing design complexity, streamlining construction processes and facilitating faster project execution. Similarly, the incorporation of geotechnical baseline statements in contracts can provide clearer allocation of geological risks by defining thresholds for variations arising from unforeseen ground conditions.
In addition, greater emphasis should be placed on geological data sharing and the creation of central repositories containing geological records, face logs and investigation data from tunnel projects. Improved access to such information can enhance project planning and support informed decision-making. Moreover, appropriate contractual frameworks are essential for managing the financial and operational risks associated with tunnelling projects. Mechanisms for addressing delays, disruptions, unforeseen geological conditions and high indirect costs can improve project sustainability and ensure continuity of construction activities.
Additionally, large infrastructure projects can also benefit from dedicated project authorities or specialist corporations empowered to make timely decisions and access expert technical guidance.
Such institutional arrangements can improve responsiveness, reduce administrative delays and facilitate more efficient project management.
