Rock mass collapse floods Tarbela hydropower work areas during tunnel excavation

Construction has been suspended at Pakistan's Tarbela Fifth Extension Hydropower Project after a weak rock mass collapsed during tunnel excavation, allowing water from the intake pit to surge into nearby underground works and the under-construction powerhouse.

The incident occurred during excavation of the second bench of a connecting tunnel associated with the T-5 project. According to Pakistan's Water and Power Development Authority, the rock failure opened a path for water from the intake pit to enter the tunnel and powerhouse area, prompting an immediate emergency response and evacuation of workers.

All personnel were moved to safety and no fatalities were reported. One worker was reportedly swept into the water during the flooding but was successfully rescued by emergency teams.

Initial reports described the event in different terms, including a breach of a protective embankment. Wapda, however, stated that the immediate trigger was the collapse of a weak rock mass during excavation and rejected reports that the main T-5 tunnel itself had collapsed.

The authority also said the main tunnel, tailrace culvert, switchyard and under-construction powerhouse structure had not sustained reported structural damage. Water from the intake pit was subsequently drained, stopping further inflow into the downstream construction areas, while dewatering of the powerhouse began.

Engineers and project officials are now assessing the tunnel, powerhouse and surrounding works to determine whether any local damage occurred and what remedial measures will be required before construction can resume. A formal investigation is also expected to examine whether geological weakness, excavation conditions, temporary protection measures or another technical factor contributed to the event.

The incident highlights the risk of sudden hydraulic inflow during underground excavation close to large reservoirs and intake structures. A local rock failure can rapidly create a connection between a pressurised or impounded water source and an excavation, producing high flow rates before isolation or drainage measures can take effect.

Authorities also issued precautionary warnings downstream after additional water was released from Tarbela Dam, with residents advised to avoid the Indus River for swimming, boating or other activities. Officials stressed that there was no indication of a dam failure or threat of major downstream inundation.

The event is the latest setback for the 1,530 MW Tarbela Fifth Extension Project, which will add three 510 MW generating units and increase Tarbela's installed capacity from 4,888 MW to 6,418 MW.

The project has already faced delays and cost escalation. Earlier in 2026, an inquiry into a separate cofferdam collapse attributed responsibility across the contractor, consultant and employer and identified unauthorised post-contract design changes among the contributing issues.

Following the latest incident, construction will remain suspended while the affected areas are dewatered, inspected and assessed. The key engineering task will now be to establish the exact failure mechanism and confirm that the excavation and water-control systems are sufficiently secure before workers return underground.