LANDSLIDE RISK MANAGEMENT AT BC HYDRO
Experience with Landslide monitoring and drainage
Tom Stewart, January 2017 PRESENTATION OVERVIEW
1. BC Hydro - Overview
2. Landslide / Reservoir Slopes Program
3. Landslide Case Histories . Downie Slide . Dutchman’s Ridge . Revelstoke Dam
4. BC Hydro Experience – Lessons Learned
2 1. About BC Hydro
BC Hydro is a “Crown” (State-run) corporation, established in 1961, owned by the Province of British Columbia and regulated by the British Columbia Utilities Commission (BCUC), an independent body. . one of North America’s leading providers of clean, renewable energy . largest electric utility in British Columbia . approximately 1.8 million customers . serves about 95 per cent of the province’s population Capacity . 43 dam sites - 11,300 megawatts (MW) installed capacity . 97% of generation based on clean, renewable hydroelectricity . BC Hydro customers enjoy some of the world's lowest electricity rates Transmission . 18,600 kilometres of transmission lines . 57,278 kilometres of distribution lines
3 BC HYDRO DAMS – GEOGRAPHIC CONTEXT
1. Aberfeldie Dam 2. Alouette Dam 3. Bear Creek Dam 4. Buntzen Dam 5. Cheakamus Dam 6. Clayton Falls Dam 7. Clowhom Dam 8. Comox Dam 9. Coquitlam Dam (1902) (2001) 10. Coursier Dam (decommissioned) 11. Duncan Dam 12. Elko Dam 13. Elliott Dam 14. Elsie Dam 15. Falls River Dam 29. Ruskin Dam 16. Heber Diversion Dam 30. Salmon River Diversion Dam 17. Hugh Keenleyside Dam 31. Seton Dam 18. John Hart Dam 32. Seven Mile Dam 19. Jordan Diversion Dam 33. Spillimacheen Dam 20. Kootenay Canal Dam 34. Stave Falls Dam 21. La Joie Dam 35. Strathcona Dam 22. Ladore Dam 36. Sugar Lake Dam 23. Mica Dam 37. Terzaghi Dam 24. Peace Canyon Dam 38. W.A.C. Bennett Dam 25. Puntledge Diversion Dam 39. Wahleach Dam 26. Quinsam Diversion Dam 40. Walter Hardman Dam 27. Quinsam Storage Dam 41. Whatshan Dam 28. Revelstoke Dam (1985) 42. Wilsey Dam 43. Site C - current 4 2. BC Hydro – Landslide Program
Upper Peace Columbia 8 of 15 monitored landslides in this area
43 Dam sites
15 Monitored Landslides Kootenays
3000 km Reservoir Shoreline
Fraser Vancouver Valley- Island Bridge 5 3. Upper Columbia - Landslide Factors
Dutchman’s Ridge Large topographic relief (3500 m+) steep slopes (20 – 60o) Adverse geologic conditions (continuous structures: faults, foliation) Extensive precipitation 1500 – 2000 mm/yr
Downie Slide
6 3. Landslide - Drainage Case Histories
Dutchman’s Ridge
All on west side of Columbia valley All structurally controlled along foliation / shears
Downie Slide Precedent for large, rapid rock slides (2 in last 200 yrs)
Moderate seismic zone (pga ~ 0.3 g) Revelstoke Dam
7 DOWNIE SLIDE - OVERVIEW
Head scarp
• Identified 1950’s • Extensively investigated (1965-present) • 1.5 B m3 (9.5 km2 x 250m deep)
8 Downie Slide – Key Dam Safety Issues
1. Landslide generated wave - Dam over-topping – dam breach
2. Reservoir blockage
3. Dam flooding (Mica)
9
Figure 1: Key physiographic features of Downie Slide; bounding scarps and steep bulging toe Downie Slide – Revelstoke Dam
Dam Approval linked to Downie Stabilization Extensive Public Consultation
Figure 1: Key physiographic features of Downie Slide; bounding scarps and steep bulging toe and review process Drainage selected “to more than offset filling of reservoir” 10 Downie Slide – Geologic Section Detailed geologic model required for drainage design
Head scarp
• Identified 1950’s • Extensively drilled 1968-92 • 1.5 B m3 • Slowly moving (3 to 200 mm/yr) o Drilling - Monitoring installed 20-25 downslope dip • 1974-82 drainage Geology Mica schist/gneiss; continuous structures • 2008 drainage upgrades Groundwater Multiple water levels • Extensive ADAS Movement Slowly moving (3 to 200 mm/yr) • DS Issues: Overtopping wave / reservoir blockage / impact to reservoir users (on 2 main surfaces) 11 Downie Slide – Drainage Program
Head scarp
Drainage Adit
Adits - constructed using “Observational” approach (piezometers) Completed in three stages 1974-1977-1981 12 Downie Slide – Drainage Program
Head scarp
• Identified 1950’s • Extensively drilled 1968-92 • 1.5 B m3 • Slowly moving (3 to 200 mm/yr) Drainage Adit • 1974-82 drainage • 2008 Adits drainage primarily upgrades in slide mass • Extensive 2.45 kmADAS of adit • DS Issues:24,000 Overtopping m of drain wave holes / reservoir blockage / impact to reservoir users 13 Downie Slide – Drainage Adits
JHT
Conventional drill and blast; “horseshoe” shape; extensively supported 14 Downie Slide - Groundwater Response
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Y1 Legend Y2 Legend DHS49P1A DHS25P1 DHS49P1 DOWNIE SLIDE
STANDPIPE PIEZOMETERS (MANUAL) North Knob Area S25 P1 and P2
TWSPZ77 January 27, 2017
1. Significant drainage effects – up to 100 m pressure reductions 2. Met initial objective but increasing pressure trends observed 3. Additional drainage required (2007-08) 15 Downie Slide - Groundwater Response
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S42 Sensor tip elevations: A 2440 2440 P5 2286.6 n
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Y1 Legend Y2 Legend DHS42P5 DHS42P5A DHS42P4 DHS42P4A DOWNIE SLIDE DHS42P3 DHS42P3A DHS42P2 DHS42P2A STANDPIPE PIEZOMETERS DHS42P1 DHS42P1A South Scarp area near toe S42 P1 to P5 TWSPZ18 January 17, 2017
1. Most piezometers encountered “perched” water levels 2. Water typically perched above the 2 active zones 3. Piezometers below slide base could indicate “dry” conditions 16 Downie Slide - Displacement
3 (2 mm/yr) Slope displacement response to drainage, measured across lower slide plane:
1. Slight reduction following drainage (5 > 3 mm/yr)
2. Increase during reservoir filling 1 (10 mm/yr) 2 3. Relatively consistent post reservoir fill ( 2 – 3 mm/yr)
17 Downie Slide – Summary
1. Groundwater levels reduced to below pre-reservoir levels
2. Displacement rates are similar or less than pre-reservoir conditions 3. Stability increased by ~ 7% (FOS) 4. Water License Requirement – must maintain system
18 BC HYDRO LANDSLIDES – Mica Dam
LCS DR
Mica Dam - 250 m high earthfill dam
The most upstream of 14 dams on Columbia River
Major upstream landslides
19 Dutchman’s Ridge Potential Slide
• Identified mid-1960’s (airphotos)
• 1969 drilling
• 30 M m3 • Physical wave model studies (50 m+ waves)
• Dam construction revised to accommodate slide waves
• Decision not
to drain 20 Dutchman’s Ridge - Displacement
1. Pre-reservoir – negligible displacement detected
2. Reservoir filling (190 m) – initiated movements of 25 mm/yr 3 3. Slope displacement acceleration resulted in decision to further investigate 2
1
Slope displacement response to reservoir filling (pre-drainage) 21 Dutchman’s Ridge – Investigations
Geological Model (Updated): • Well defined continuous basal sliding surface • Depth of sliding – 75 to 150 m Slide Base • Movements of up to 200mm occurred during reservoir filling • 115 M m3 (~4x original)
Drainage Adit
Analyses concluded: • Raising of reservoir reduced stability by ~ 10% • Large, first time slide possible
• Remediation preferred – drainage option selected 22 Dutchman’s Ridge Slide - Reservoir • 1969 estimate of slide volume 30 M m3
• Post - Outline of currently active slide reservoir re- evaluation (1982)
• Slide size reassessed 115 M m3
• Additional analyses (larger wave) Drainage Adit • Decision to install drainage system 23 Relevance of Dutchman’s Ridge – Aknes
Similarities: • Size • Geology • Setting Aknes • Landslide wave hazard
Dutchman’s Ridge
British Columbia 24 Dutchman’s Ridge – Drainage Design
• Extensive monitoring system installed (350 measurement ports): To identify where water pressures could be drained To evaluate effectiveness of drainage system 25 Dutchman’s Ridge – Drilling Observations
Piezo profiling during drilling
Slide Base Open hole measurements could be dry below slide
Most drill holes showed “perched” water levels Testing allowed optimum design of piezo installation 26 Dutchman’s Ridge – Piezo Testing
Piezometric testing during drilling Custom “through-the-drill-string” set up Reliable – no issues with getting stuck Data immediately available
27 Dutchman’s Ridge – Drainage Layout
982 m of Adit installed (1986-88) ; 20,000 m of drainholes 28 Dutchman’ Ridge – Groundwater TH
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8633MP37 8633MP35 8633MP33 MICA RESERVOIR SLOPES 8629MP31 8633MP29 Dutchman's Ridge 8633MP27 Multi-Point Piezometers 86-33A Ports 27-37 (2 of3) PBS DMRPZ27 January 18, 2017 1. Large portions of potential slide fully drained. 2. Up to 100 m pressure drawdown – lasting effect 29 Dutchman’s Ridge - Displacement
Inclinometer – Across Slide Base
Pre-drainage Post-drainage 0.5 mm/yr ~15 mm/yr
Slope displacement response to drainage - rate reduced by > order of magnitude30 Dutchman’s Ridge – Adit Conditions
Flow rates have reduced but pressures have remained “fully drained” 31 Dutchman’s Ridge – Compared to Downie
Slide Base
Drainage Adit
Smaller slide mass - easier to effectively drain Single sliding plane (DMR) vs two + planes (Downie) Slightly better rock quality – less previous slide displacement Adit below slide in competent rock Nearly “fully” drained condition – effectively halted movements 32 Revelstoke Dam – Marble Shear Block
• 175 m high concrete gravity; with earthfill wing dam
• 1985 dam completion
• Slope instability on right bank (Marble Shear Block) 200,000 m3
• Potential to undermine spillway; block tailrace
33 Marble Shear Block - Geology
1. Quartzite / Mica Gneiss
2. Persistent planes of Pre-drainage Post-drainage weakness (foliation / shears) dipping down slope 13 mm/yr 3. Considerable groundwater
Risk Management 1. Risk to spillway / discharge facilities considered unacceptable
2. Drainage selected as remedial option
34 Marble Shear Block – Drainage
1. Spillway Structure
2. Sliding Plane
3. Adit (40 m long)
4. With ~1000 m of drainholes
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79AT24 79AT24A 79AT25A REVELSTOKE ROCKSLOPES 79AT25D 79AT25 Marble Shear Block - Piezometric Pressures Piezometers Near Section C (6+35) Threshold Levels TWSPZ56 August 19, 2015 1. Significant pressure reductions (up to 15 m) from drainage 2. Remedial effects are persisting over 15 years – lasting results 36 Marble Shear Block - Displacement
1. Minor Inclinometer – slide plane displacement prior to 3 drainage 2 installation (2mm/yr) 1 2. Drainage 1998-99
3. Slope displacement effectively stopped following drainage
37 LANDSLIDE REMEDIATION THROUGH DRAINAGE Lessons Learned – Summary: 1. Drainage has been effective – objectives achieved
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8 8 8 8 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
9 9 9 9 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
8633MP37 8633MP35 8633MP33 MICA RESERVOIR SLOPES 8629MP31 8633MP29 Dutchman's Ridge 8633MP27 Multi-Point Piezometers 86-33A Ports 27-37 (2 of3) PBS DMRPZ27 January 18, 2017
Tom Stewart, January 2017 LANDSLIDE REMEDIATION THROUGH DRAINAGE Lessons Learned – Summary: 2. Well positioned monitoring system required (plan/evaluate) Need to understand where water is located Detailed site investigation and monitoring required Needs to be installed before drainage
Tom Stewart, August 2015 LANDSLIDE REMEDIATION THROUGH DRAINAGE Lessons Learned – Summary: 3. Well positioned monitoring system required (plan/evaluate) 1. Need to conduct in situ testing during drilling to understand where water is located – 2. Identifies perched zones and downhole gradients that are otherwise missed 3. Allows planning for permanent piezometric monitoring. 4. Use of multi-port type piezometers highly effective (i.e. “Westbay” type). LANDSLIDE REMEDIATION THROUGH DRAINAGE Lessons Learned – Summary: 4. Drainage response typically confined to treated area only overage installation network spatially
Tom Stewart, August 2015 LANDSLIDE REMEDIATION THROUGH DRAINAGE Lessons Learned – Summary: 5. Plan for maintenance – successful results come from long- term commitment
• Lighting, ventilation, power supply
• Drainhole replacement (cleaning ineffective)
• Requirement to install additional monitoring
• Rock support
Tom Stewart, January 2017 LANDSLIDE REMEDIATION THROUGH DRAINAGE
Questions / Discussion ?
Tom Stewart, January 2017 BRITISH COLUMBIA – NORWEGIAN CONTEXT
British Columbia Norway 944,735 sq. km 304,282 sq. km 1900 km long 1347 km long 505 km wide 304 km wide
23. Mica Dam Mountainous 28. Revelstoke Dam Mountainous Lots of snow Lots of snow Lots of fjords Lots of fjords
3% of population of Norwegian descent
44