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

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

3. Investigation Chapter 5 Conclusion Chapter 6 CONTENTS
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Numerical Modelling of slope stabilization using software

4. LOCATION . Numerical Modelling of slope stabilization using software
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Numerical Modelling of slope stabilization using software

5. 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
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Numerical Modelling of slope stabilization using software

6. INTRODUCTION-General Layout
POWERHOUSE GA
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Numerical Modelling of slope stabilization using software

7. 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
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Numerical Modelling of slope stabilization using software

8. 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 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.
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Numerical Modelling of slope stabilization using software

9. BHASMEY HEP…. PANAROMIC VIEW OF POWER HOUSE SLOPE
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Numerical Modelling of slope stabilization using software

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

11. 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 at a downward inclination of 150. All 03 holes encountered the zone of loose material with heavy water ingress after the depth of 30m. .
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Numerical Modelling of slope stabilization using software

12. 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.
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Numerical Modelling of slope stabilization using software

13. 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
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Numerical Modelling of slope stabilization using software

14. BHASMEY HEP…. POWER HOUSE SLOPE
Sand Pocket
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Numerical Modelling of slope stabilization using software

15. 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
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Numerical Modelling of slope stabilization using software

16. INVESTIGATION Sr. No. Rock Type RQD RMR Rock Class Year1
Phyllitic Schist and Quartzitic Phyllite
2.8% 47 IV
2011, from EL 2
Phyllitic - Quartzitic Phyllite
30-35
21-30
IV-V
2016 From EL
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 .
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Numerical Modelling of slope stabilization using software

17. 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
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Numerical Modelling of slope stabilization using software

18. 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
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Numerical Modelling of slope stabilization using software

19. Modelling & Analysis
Re- Modelling was done after failure to find out FOS from Road top EL. 420 to EL 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
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Numerical Modelling of slope stabilization using software

20. BHASMEY HEP…. POWER HOUSE SLOPE Numerical Model and Result
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Numerical Modelling of slope stabilization using software

21. BHASMEY HEP…. POWER HOUSE SLOPE
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Numerical Modelling of slope stabilization using software

22. BHASMEY HEP…. POWER HOUSE SLOPE
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Numerical Modelling of slope stabilization using software

23. 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
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Numerical Modelling of slope stabilization using software

24. THANKS FOR LISTENING
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Numerical Modelling of slope stabilization using software