ISLAMABAD: As many as 11 postulated root causes of the blockage of tailrace tunnel (TRT) in the Neelum Jhelum Hydropower project happened on July 4, 2022, which have been identified, discloses the interim report prepared by an Independent Panel of Experts (IPoE).
The tailrace tunnel (TRT) of 969 MW Neelum Jhelum Hydropower Project (NJHPP) experienced damage and a blockage that forced shut down of the complete plant on July 6, 2022. An Independent Panel of Experts (IPoE) was formed to investigate the possible causes of the damage and to validate the best workable engineering short-term and long-term solutions. The IPoE visited the NJHEP from Sept 22 to Sept 25, 2022.
The IPoE has pointed out that collapse of the TRT between ST 0+250 and ST 0+293 was generated by the combination of root causes that include increased external pressures — Ground, presence of unidentified erodible or swelling seam, hydraulic conditions, insufficient ground support, absence of concrete lining, faulty construction procedures and incorrect ground support installation, long term decompression of the rock mass, increased external pressures — water, altered ground conditions, and unexpected seismic load. The panel has recommended the installation of a full reinforced concrete lining in the TRT and installation of instrumentation and monitoring plan that allows proper long-term monitoring of the TRT.
The IPoE comprises Cane Cekerevac from Switzerland, who is chairman of the panel and tunnels and rock mechanics expert, Enrique Cifres from Spain, dam safety and monitoring expert, Carlos Jaramillo from (USA), tunneling and rock mechanics expert, Luis Pedro Gonçalves from Portugal, geology and tunneling expert, and Aslam Rasheed from Pakistan, a hydraulics expert.
The IPoE will prepare several reports. This report is the Interim Report — Part 1, and Part 2 looking at the potential TRT recovery methods and long-term safety measures. The interim report also came up with recommendations to Neelum Jehlum Hydropower Project’s top management to avoid such collapses in the tunnel.
According to the IPoE interim report submitted on November 13 with higher authorities in Islamabad, also available with The News, NJC (Neelum Jhelum Company) reported that in addition to the Main Collapse Blockage, there are about 40 other locations that require remediation: These include major rock wedge failures at 4 locations, moderate damages at approximately 17 locations and minor damages at 17 locations.
Earlier, the tunnel was inspected in 2019 after a year of operation. Several issues were identified and corrected. Shotcrete spalling was one of the most common. Spalling in this case is most probably due to an excessive rock load acting on the support. The IPoE evaluation of the information available, and based on their experience, points to the series of incidents resulting in the collapse that includes the external ground pressures on the ground support increased due to the presence of unidentified swelling seams in the mudstones, or redistribution of stresses around the excavation, returning to the new stress distribution with high in-situ stresses in the sandstones. These higher loads encountered a ground support that was insufficient to resist the loads, or a ground support not properly installed. And the overloaded ground support system resulted in cracking of the shotcrete, spalling, and/or deformation of the shotcrete where the welded wire mesh provided sufficient ductility to accommodate the deformation. However, these spalled, cracked and deformed areas allowed flowing water to enter in contact with weak and potentially erodible formations, some of which had been decompressed and presented open fractures. A reinforced concrete lining, in addition to the ground support, could arguably control the damage at this point.
The flowing water removed sufficient material to disturb the arch action, which is the base of the ground support, prompting smaller rock falls, until a more resistant rock unit lost support and collapsed. The IPoE also opines that the larger, heavier and more resistant blocks encroached on the flow area, making a partial obstacle for water flow through the constricted tunnel, increasing water pressure on already damaged tunnel section and increasing the external water pressure. This induces the acceleration of the failure process, causing more removal of material, and increased upstream pressures to drive the flowing water through a constricted tunnel, until complete shutdown by the collapsed rock mass.
The collapse also involved at least 10,000 to 15,000 m3 of rock in situ, which filled completely 50 m of the tunnel and partially approximately 250 m, plus all the material that was transported by the flow before the complete shutdown.
A review of the geological in-situ conditions showed the existence of several areas with characteristics (geometrical wedge conditions and joint /rock mass properties) very similar to the collapsed zones. Based on the review of all existing project documents, the site visit and experience of the IPoE also came with a set of recommendations stressing for the installation of a full reinforced concrete lining in the TRT and the installation of the instrumentation and monitoring plan that allows a proper long-term monitoring of the TRT. The panel has also recommended to proceed to install proper ventilation and lighting to protect the health of the workers in the tunnel. It has also recommended to proceed to stabilize areas identified during the inspection, as recommended in the October inspection report. It also urged to proceed to develop and implement the tunnel recovery method using fore poling and backfilling.