Of the 114km between Siliguri and Gangtok, 90km of the NH10 lies in the mountains and of this approximately 65km is along the banks of the Teesta where a number of dams have been constructed. Now IRCON is also building a tunnel for a railway which when finished approx next year end will link Sevoke with Rangpo in Sikkim.
NH10 is the lifeline for the state of Sikkim and Kalimpong and also a strategically important road and supply route for the Indian Army.
I have reproduced a press clip & photo from the STH blog dated 26Sep2020 below:
Rainfall in Kalimpong on 23Sep2023 was 58.4mm
Annual rainfall in the Teesta Valley is approx 2500-3000mm. The entire region is in Seismic Zone IV.
Why the failure occurred?
I think there are 2 factors at play. First, the Darjiling-Sikkim Himalaya is riddled with out-of-sequence faults that have been active recently. The Seti Jhora practically runs along the reactivated Ramgarh thrust fault core. Such fault cores consist of powdery crushed material that have very little strength. The roads that run over such zones have much lower strength and load carrying capacity. If you saturate such zones with water chances are that they will liquify and cave in. September end would be a time where the monsoon has been around for a while and has saturated these zones particularly when a reservoir is also underneath it. Such a cave-in also happened in the Birrik fault core last year on the side of the road. No reservoir was involved there. Just the fault core material and rain.
- Prof Malay Mukul, Dept of Earth Sciences, IIT Mumbai