TBM of Sunkoshi Marin Diversion Multipurpose Project (SMDMP), Sindhuli, Nepal Encountered Adverse Geological Condition of Lesser Himalaya,TBM got Stuck Second Time. Problem Has Been Sorted Out.
After crossing the Main Boundary Thrust (MBT) at chainage 4+265m, on 31st March 2023, at Chainage 4+669 meter TBM of SMDMP encountered the adverse geological condition (Highly weathered Phyllite-different with Geotechnical Baseline Report) of the lesser Himalaya. The collapse occurred at the crown of the TBM face resulting in tremendous of muck without TBM advancement. Also, front and telescopic shield got squeezed within a short time. The geology of the front face as observed was deeply weathered phyllite with unstable and collapsing ground. The local collapse on the crown of the cutter face resulted in the formation of cavity. To resolve the issue, the Experts, TBM manufacturer Robbins and Tunnel Technical Team (TTT) recommended injecting Polyurethane (PU) chemical to fill the cavity at the crown and front face to consolidate the ground to prevent further collapse. After the PU injection, TBM was started but, the TBM couldn’t advance due to the ground squeezing. To overcome the friction on the surface of the front shield, telescopic shield and gripper shield lubrication was used. But again, the TBM couldn’t advance. Hence, the TBM Stuck condition was declared (on 5th April) and only one option left behind was By-pass tunnel construction to overcome the stuck of TBM. Subsequently, doing all necessary preparations, on 9th April construction of a bypass tunnel on both the right and left sides of TBM started to reduce friction force and release the TBM. TBM machine made free from squeezed condition and crown cavity was filled with chemical grouting and on 23rd April 2023 the TBM was taken out from stuck condition. After necessary preparation, TBM was started on 24th April 2023, and observed the smooth advancement and operation of TBM. After completing all the works like backfilling of bypass tunnel, PU Chemical/Silicate/Cement grouting backfilling etc., operation of TBM restarted on 26th April at 1 AM (Baisakh 13th, 2080 - morning at 1 AM) for excavation and now it has gone to the normal operation.
This is the second time stuck of TBM machine of SMDMP. We faced first problem at chainage 2+156 on 5th January, 2023. At that time also we sorted out the problem by constructing By-pass tunnel and consolidating loose mass of mud /sand stone, ingress of water by using Chemical PU/Silicate/Cement grouting.
I would like to acknowledge and thanks for valuable support/suggestions from national and international experts at the critical moment of the TBM stuck. Also, I would like to thank the experts and front liner workforces of COVEC (China), Consultant SMEC (Australia) and J/V partners, The Robbins Company and the entire SMDMP team and our Department/Ministry for their joint continuous effort and support to overcome this second time of TBM stuck condition efficiently.
Mitra Baral
Project Director, SMDMP