Stability Analysis for the Back Slope of Power House Bhasmey HEP-Case Study Author- Hemlata Gupta 04th Dec 2018 09/10/2018 Numerical Modelling of slope stabilization using software

Regional Site & Geology CONTENTS Introduction Chapter 1 Chapter 2 Regional Site & Geology Chapter 3 Nature of Problem Chapter 4 Reasons for Slope failure 09/10/2018 Numerical Modelling of slope stabilization using software

Investigation Chapter 5 Conclusion Chapter 6 CONTENTS 09/10/2018 Numerical Modelling of slope stabilization using software

LOCATION . Numerical Modelling of slope stabilization using software 09/10/2018 Numerical Modelling of slope stabilization using software

INTRODUCTION-General Layout A surface Power House 1 nos. of 3.4m dia. Pressure Shafts. 13 m dia. 91m high Surge Shaft. 5m dia. 4.6 Km long Circular Head race tunnel. 35m high barrage. PROJECT HIGHLIGHT 09/10/2018 Numerical Modelling of slope stabilization using software

INTRODUCTION-General Layout POWERHOUSE GA 09/10/2018 Numerical Modelling of slope stabilization using software

Regional and Site Geology Phillytes & Quartzite- Lithological units Phyllites Chloritic phyllites Phyllitic quartzite Quartzite and Carnonaceous The project area exposes predominantly low grade metamorphic rocks comprising of phyllites and quartzites. Bhasmey powerhouse slope is a combination of complex geology comprising of phyllite with intermittent bands of quartzites and river borne material. Shear seams with which was having the property to hold water are also present in this slope. . 09/10/2018 Numerical Modelling of slope stabilization using software

Nature of Problem Bhasmey HEP was started in 2011.Initial works like stripping, excavation and diversion being already started in Bhasmey HEP during the year but was halted in between these works. Even after initiation of already mentioned works during 2010 - 2011 the project was put on hold for 5 years and was restarted in 2015. Subsequent excavation of 20m down into Power House pit de-stressing at rocks was evident in the form of cracks on old slope and shotcrete faced walls also in horizontal benches near surface drain. There was a clear cut separation between 100mm thick shotcrete. Wide cracks were visible all over the slope surface and slope collapsed after 7 months of strict initial warnings in the same way as explained above precisely due to de-stressing of self-drilling rock anchors broken rock anchors and separation of shotcrete etc. It was a clear indication that there is a movement in the rock mass and due to this mentioned movement, rockmass slope measures like self-drilling anchors and shotcrete were stressed to an ultimate strength of the material due to heavy load resulting in slope surface collapse on one of the weak zones at corner of PH slope. 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. PANAROMIC VIEW OF POWER HOUSE SLOPE 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. PANAROMIC VIEW POWER HOUSE SLOPE 09/10/2018 Numerical Modelling of slope stabilization using software

REASONS FOR SLOPE FAILURE Presence of seasonal nala and Heavy Monsoon This zone of subsidence with elevation difference (w.r.t surge shaft hill) of about 100m – 150m is a seasonal nalla (dry in most cases) but acts as a recharge zone as observed from the site. The drill hole results in concluding about the existence of these seasonal nala’s as they were fully charged in September (monsoon season) and remained dry in December. The nature of seasonal nalla was leading variation in water table drastically. Existence of Overburden Material Drill holes BH1 and BH2 reveals the overburden thickness of about 24m & 6m respectively. At the time of implementing cable anchor drill holes viz. no. 5, 7 & 14 being drilled at about El. 369 at a downward inclination of 150. All 03 holes encountered the zone of loose material with heavy water ingress after the depth of 30m. . 09/10/2018 Numerical Modelling of slope stabilization using software

REASONS FOR SLOPE FAILURE Shear Seam For the assessment of reason behind the slope collapse, all the available data i.e. drill hole (DPR stage & Cable Anchor holes) and found Shear seams of thickness ranging between 0.50m to 20.0m was assessed through geological mapping. Based on site geological condition and available sub-surface data, a shear zone at a horizontal distance of about 30m from powerhouse hill slope acting as an impervious curtain (as confirmed through completed cable anchor holes) to retain the water behind it. This accumulated water exerts pore water pressure on the powerhouse hill slope from a long time during monsoon seasons. Pore water pressure thus, arises is likely a reason for sinking zone on the road from 2010 (as observed during site vist ) and subsequently responsible for the collapse of the slope. 09/10/2018 Numerical Modelling of slope stabilization using software

REASONS FOR SLOPE FAILURE Deep excavation without support- After resumption of work excavation started after EL. 420m to expedite the work Contractor did excavation without providing immediate support. Drainage Holes ;- It was envisaged that existing Drainage hole length provided was insufficient. No Instrumentation Deep excavation without support- After resumption of work excavation started after EL. 420m to expedite the work Contractor did excavation without providing immediate support. Drainage Holes 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. POWER HOUSE SLOPE Sand Pocket 09/10/2018 Numerical Modelling of slope stabilization using software

INVESTIGATION Exploratory Drilling 5 exploratory drill holes from 50 to 140m was assessed. The depth of overburden encountered ranged from 0 m to 33 m. Grey, fine grained, soft, thinly laminated Quartzitic Phyllite . The geological section reveals that the slopes at powerhouse mainly comprises of Phyllite and Quartzitic Phyllite overlain by overburden material. The rockmass available at these slopes is categorized as Very Poor rock mass as it is having a RMR value of 36. Shear seam is also intersected through this bore holes. It comprises of clay gouge and Grey, fine grained, soft, broken and fractured Quartzitic Phyllite During the explorations rock and soil samples are collected from drill holes and in-situ test such as SPT and Permeability test are performed 09/10/2018 Numerical Modelling of slope stabilization using software

INVESTIGATION Sr. No. Rock Type RQD RMR Rock Class Year 1 Phyllitic Schist and Quartzitic Phyllite 2.8% 47 IV 2011, from EL.420-406 2 Phyllitic - Quartzitic Phyllite 30-35 21-30 IV-V 2016 From EL 416-355 In all the above stages the encountered rock mass is of poor to very poor rock class. This heavy support measures are required to stabilize the powerhouse slopes Sub Surface Exploration Drill holes PH 01 and PH02 are drilled at powerhouse location to explore the sub-surface geological condition. Surface Geological Mapping & RMR Classification . 09/10/2018 Numerical Modelling of slope stabilization using software

INVESTIGATION-Lab Testing Description Type of sample Bulk Density (g/cc) (wet) Density (g/cc) (dry) Water Absorption (%) Poisson Ratio Triaxial Test Modulus of Elasticity C Cohesion (kg/cm2) Angle of internal friction (degrees) Rock Sample Core 2.86 2.75 4.02 0.15 42 350 Description Standard Proctor Type of Test Triaxial Test Max. Dry Density (g/cc) Optimum Moisure Content (%) C Cohesion (kg/cm2) Angle of internal friction (degrees) Soil Sample 1.85 13.80 UU 0.60 7 09/10/2018 Numerical Modelling of slope stabilization using software

INPUT PARAMETERS After exploration of different literature and doing back calculation sensitivity analysis for various C and PHI value we adopted the values for rock mass C = 0.9 Kg/ cm2 Phi = 25 deg 09/10/2018 Numerical Modelling of slope stabilization using software

Modelling & Analysis Re- Modelling was done after failure to find out FOS from Road top EL. 420 to EL. 366 . Slope is modelled as rock mass with shear parameters Cohesion and friction angle mentioned above. PHASE2 software has the ability to calculate a Strength Reduction Factor (SRF). In joint set analysis no joint was prone for failure therefore joint was not modelled and rock mass properties was given 2-3 sections have been identified and supports provided as per critical section where max. SRF was obtained. Design Loads LOAD COMBINATION Self weight Earthquake loads Surcharge Loads . S. No Load Combinations FOS 1 Static Condition 1.4 2 Static condition with seismic effect in X and Y direction 1.1 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. POWER HOUSE SLOPE Numerical Model and Result 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. POWER HOUSE SLOPE 09/10/2018 Numerical Modelling of slope stabilization using software

BHASMEY HEP…. POWER HOUSE SLOPE 09/10/2018 Numerical Modelling of slope stabilization using software

CONCLUSION Understand the behaviour of slope Detailed Investigation Choosing the input parameters with Intelligence Economic Design to the extent possible Design and construction should compatible 09/10/2018 Numerical Modelling of slope stabilization using software

THANKS FOR LISTENING 09/10/2018 Numerical Modelling of slope stabilization using software