The Srisailam Left Bank Canal (SLBC) tunnel, a crucial part of the Alimineti Madhava Reddy Project in Telangana, India, experienced a catastrophic collapse on February 22, 2025. This unfortunate incident trapped eight workers—Manoj Kumar (project engineer), Sri Niwas (site engineer), Sandeep Sahu, Jagta Xess, Santosh Sahu, Anuj Sahu (all daily wagers), Sunny Singh, and Gurpreet Singh—inside the tunnel, halting the project and raising concerns about tunnel safety in underground construction.
Design & Construction
The SLBC tunnel, an integral part of the Alimineti Madhava Reddy Project, aims to draw 30 TMC ft of water from the Krishna River at the Srisailam reservoir to irrigate 400,000 acres in the Nalgonda and Khammam districts. As the world’s longest irrigation tunnel, spanning 44 kilometers from Srisailam to Devarakonda, it stands as a testament to engineering ambition. The tunnel traverses the Nallamala Hills in Nagarkurnool district, a region known for its challenging geological formations.
Initially, the project envisioned utilizing two Tunnel Boring Machines (TBMs) working from opposite ends, with the expectation of them meeting midway to mark the completion of the tunneling works. However, the second TBM, located at the outlet end near Teldevarapally village in Nalgonda district, has been non-operational for the past few years, awaiting a critical spare part—a giant bearing—needed for repairs1. This collapse not only destroyed the first TBM but also placed the onus of completing the remaining 10 kilometers of the tunnel solely on the second TBM, once it is repaired. The tunnel excavation was being carried out by the US firm Robbins Inc, and contractor JP Associates.
Construction of the SLBC tunnel primarily employed TBMs, specifically designed for excavating tunnels in diverse geological conditions. TBMs are massive, complex machines with a rotating cutter head that breaks down rock and soil. The excavated material is then removed through a conveyor belt system6. In addition to TBMs, the Drill and Blast Method (DBM) is another technique used for tunnel excavation in India, particularly in mountainous regions with hard rock formations. DBM involves drilling holes into the rock, loading them with explosives, and detonating them to break the rock apart. This method is often used in the Himalayas, Jammu & Kashmir, and Uttarakhand.
It is important to note that tunnel construction in India adheres to the Indian Standard Codes IS 15026 (2002) and IS 4756 (1978), which provide guidelines for tunnel design, lighting, and ventilation. These codes ensure that safety standards are maintained throughout the construction process.
TBM: Technical Specifications
The TBM used in the SLBC tunnel played a critical role in the project. While the specific details of this particular TBM are not available, TBMs are generally categorized based on their diameter, the type of ground they are designed to excavate, and their support mechanisms. The TBM employed in the SLBC tunnel was a 200-meter-long machine weighing approximately 1,500 tonnes, assembled on-site due to its size.
TBMs utilize various types of cutters, including disc cutters, roller cutters, and drag bits, depending on the geological conditions. Disc cutters are commonly used for hard rock excavation, while roller cutters are more suitable for softer ground. Drag bits are employed for breaking up very hard rock. The selection of cutters is crucial for efficient and safe tunnel boring.
Modern TBMs are equipped with an array of sensors and monitoring systems that provide real-time data on ground conditions, machine performance, and potential hazards. This data is vital for ensuring safe and efficient tunnel excavation. However, it’s crucial to acknowledge that even with advanced technology, geological uncertainties and unforeseen events can still lead to collapses.
In this incident, the TBM itself suffered damage, hindering rescue efforts and adding to the complexity of the situation. The damaged TBM is currently being cut up and dismantled using Ultra Thermo plasma cutters and other welding machines to facilitate debris removal and create access for rescuers.
Possible Causes of Collapse
The collapse of a section of the SLBC tunnel was triggered by a sudden influx of water and slush, pushing back the TBM and causing a portion of the tunnel to cave in. Several factors could have contributed to this incident:
- Weak Geological Conditions: The tunnel passes through the Nallamala Hills, characterized by fragile rock formations and fault lines, making it susceptible to collapses. The presence of a shear zone, an area with weakened geological formations due to prolonged exposure to water seepage, likely played a significant role in the collapse. Shear zones are particularly vulnerable to failure under stress.
- Water Seepage and Leakage: Reports indicate that maintenance work was being carried out to fix a leak before the collapse, suggesting pre-existing structural weaknesses due to water seepage. Continuous exposure to high water pressure could have weakened the rock structure over time, making it more susceptible to collapse under the pressure exerted by the TBM.
- Structural Instability: The caving in of a 3-meter section of the tunnel roof points towards potential weaknesses in the concrete reinforcement and erosion of support structures. This could be due to inadequate design, material defects, or insufficient maintenance.
- Human and Technical Factors: Inadequate real-time monitoring to detect early warning signs like vibrations or minor cracks could have exacerbated the risk of collapse. Human error in operating the TBM or assessing ground conditions could have also contributed to the incident.
It is likely that a combination of these factors, rather than a single cause, led to the tunnel collapse. The weak geological conditions, coupled with water seepage and potential structural instability, created a vulnerable environment. The lack of adequate monitoring and potential human error might have further contributed to the unfortunate event.
–Rashmi Kumari
