From inception, the slopes from surge shaft to powerhouse is mostly overburden (colluvium/ slide debris) dominated with sporadic rock outcrops. The hill mass in the entire area is mostly cultivated with habitations. The Thotney Khola is a major perennial nalla having significant discharge, which crosses the road alignment and also the penstock alignment between AB-12 & AB-13. From all the available information, it is evident that the hill mass is mostly charged with water due to these perennial and seasonal drainages as well as heavy rainfalls.

During the construction of the valve house area and the adjacent Adit-2 portal area, the natural slopes has been destabilised through construction of additional roads (Photo 7-1) without provision of slope protection measures like toe supports and drainage arrangements. In addition, the seepage of HRT through Adit-2 is also allowed to free flow through the affected area causing removal of fines and as a result, subsidence of overburden dominated hill mass. The effect of the instability in the form of subsidence and downward movement has also affected the penstock area, mainly from AB-1 to AB-5. Similarly, the road alignment downstream of the valve house area has also not been provided with proper slope protection measures. This continuous erosional phenomenon and poor road planning led the failure of slopes in the nearby areas of valve house and the penstock alignment.

Based on the above observations the following recommendations are made by Consultant.

• The destabilisation/ movements are observed till AB-5, but, it is suggested to divert the seepage from Adit-2 away from the penstock alignment to protect the further destabilisation of the slopes in the form of removal of fines from the overburden dominated slope along penstock from AB-5 till AB-12.
• During the site visit, although the slope seems to be stabilised and has taken natural slope, but, it is recommended to monitor the complete slope for any further development of the existing cracks.
• It is also advised to monitor the valve house area for any new crack/ slumping in the dislocated mass. Since, the area receives heavy rainfall and the hill mass is having high ground water table, the existing cracks should be protected from the surface runoffs.
• Other than the above mentioned the following is recommended.
  • Topographic survey of the surge shaft- valve house area and the adjacent disturbed zone.
  • Shift entire valve house including transitions and flowmeter sensors towards downstream.
  • Shift portal of valve house at the existing location of upstream wall of valve house.
  • Excavate the tunnel length between proposed and existing portal location though steel rib support and Forepoling / Pipe roofing.
  • Valve house Transformer and DG set can be shifted to appropriate location.
• It is recommended not to load the crown portion of the destabilized back slope and side slopes of the valve house and provide geogrid and grass seed in the back slope area of valve house for its stabilization.