Update on Geosynthesis Activities Phase II Geomechanics. December 8, 2009 Ottawa

Transcription

1 Update on Geosynthesis Activities Phase II Geomechanics December 8, 2009 Ottawa

2 Phase II Geomechanics Activities Update 1. Seismicity Bruce Network Monitoring (CHIS) Seismic Hazard Assessment (AMEC Geomatrix) 2. Geomechanical Characterisation Laboratory Testing (CANMET) Field Measurements (Intera) 3. Long-term Cavern Stability Analysis Model Approach - Geometry, Properties and BC s (Itasca/Queen s) Interim Results - UDEC and Phase 2 4. Long-term Shaft Seal Analysis Model Approach - Geometry, Properties and BC s (Itasca) Interim FLAC 3D Results HDZ/EDZ/EdZ - Formation 2

3 Bruce site - Mirco-Seismic Monitoring Installed 3 Borehole seismograph array in August 2007 No major activity (>2.5M) within 150 km radius since installation Maximum historical magnitude (4.2M 99 km) recorded 2005 Seismic Hazard Assessment by AMEC - Geomatrix in progress Low Seismic Hazard consistent with stable Canadian Shield 2008 Historical Records (GSC, 2009) 3 2/x

4 Geomechanical Characterization Laboratory Testing Program for DGR-1 to DGR Uniaxial Compression Tests on 22 Formations 11 Long-term Strength Degradation Tests on Cobourg Formation 18 Triaxial Compression Tests on 3 Formations 12 Cross-anisotropic Tests on 3 Formations 45 Brazilian Tests on 6 Formations 35 Direct Shear Tests on 6 Formations 57 Free Swelling Tests and 4 Semi-confined Swelling Tests on 6 Formations 30 Abrasiveness Tests on Cobourg Field Testing Program 52 Slake Durability Tests on shale formations in DGR-1 to DGR-4 Axial and diametric Point Load Index Testing at regular borehole intervals P and S wave measurement on rock cores at regular intervals Standard rock core/discontinuity inspection and RQD logging In-situ stress estimated by observed borehole breakout and stress induced fracturing Borehole geophysical logging 4

5 Testing of Ordovician Barrier Shales Depth from Top of Cabot Head (m) Depth from Ground Surface (m, w.r.t. DGR2) UCS measurements reveal competent shale cap rock Characterized by mean peak strengths between 20 and 45 MPa Crack Initiation (CI) stress ~40%UCS in all formations Strength is anisotropic 0 UCS (MPa) Queenston Georgian Bay Blue Mountain Collingwood Queenston Fm Georgian Bay Fm. Blue Mountain Fm

6 Depth from Top of Cobourg (m) Depth from Ground Surface (m, w.r.t. DGR2) Testing of Cobourg Host Rock 114 MPa (56 tests; s 25 MPa) UCS measurements reveal competent argillaceous limestone Characterized by mean peak strength of ~114 MPa Mean UCS substantially higher than regional average Mean CI stress of 45 MPa (~40%UCS) Isotropic Rock behaviour Cobourg Fm. UCS (MPa) DGR-2 15 DGR DGR DGR-2 LSD DGR-3 LSD DGR-4 LSD Proposed Repository Horizon

7 Depth from Top of Cobourg (m) Rock Strength Distribution in Cobourg Fm. Depth from Ground Surface (m, w.r.t. DGR2) Depth from Top of Cobourg (m) Depth from Ground Surface (m, w.r.t. DGR2) 100 day strength degradation testing revealed no change in CI Long-term rock strength approximately equal to mean CI 45 MPa 0 Crack Initiation Stress (MPa) Crack Initiation Stress (%UCS) 0% 20% 40% 60% DGR DGR-2 DGR-3 DGR-4 DGR-2 LSD DGR-3 LSD DGR-4 LSD Proposed Repository Horizon DGR-3 DGR-4 DGR-2 LSD DGR-3 LSD DGR-4 LSD Proposed Repository Horizon

8 Shear Stress (MPa) Shear Stress (MPa) Shear Strength of Intact Rock and Bedding Partings Peak and residual strength envelopes for Cobourg and Collingwood Fms. Cobourg Fm. Collingwood Fm Cobourg - intact sample Residual Strength Peak Strength C = 1.2MPa f = 75 o Collingwood - intact sample Peak Strength Residual Strength C = 0.8MPa f = 75 o Normal Stress (MPa) C = 0MPa f = 40 o Normal Stress (MPa) C = 0MPa f = 30 o 8

9 Swelling Potential (% strain per log cycle of time) Swelling Characteristic Swelling Potential (% strain per log cycle of time) Shale no swelling in formation water only in fresh water Swelling decreases as calcite content increases Swelling can be suppressed by confining pressure Tests on Cobourg and Sherman Fall samples show zero swelling potential Fresh Water Formation Water 1.6 Queenston - Vert. 1.4 Queenston - Vert. 1.4 Queenston - Horiz. Georgian Bay - Vert. 1.2 Georgian Bay - Vert. Georgian Bat - Horiz. Georgian Bat - Horiz. Blue Mt. - Horz. 1.2 Blue Mt. - Vert. 1 Cobourg - Vert. & Hoz. Blue Mt. - Horz. Sherman Fall - Vert. & Horz Sherman Fall - Vert. & Horz. Cobourg - Vert. & Horz.Sheet1!$N$106 Horizontal Swelling - S. Ont. data (Lo 1989) Vertical Swelling - S. Ont. data (Lo 1989) Queenston - Horiz. Horizontal Swelling - S. Ont. data (Lo 1989) Vertical Swelling - S. Ont. data (Lo 1989) Calcite Content (%) Calcite Content (%) 9

10 Long Term Cavern Stability Numerical analyses of cavern (DGR) stability, applying knowledge from Phase I analyses were conducted to explore four scenarios: time-dependent strength degradation only base case time-dependent strength plus additional effects of gas pressure build-up; additional effects of seismic ground shaking; additional effects of glacial loading; and combined effects. 10

11 Objectives of Long-Term Analysis Expected performance through 60 ka (long-term degradation, pore pressure, pre-glacial) Expected performance beyond 100 ka (1-2 glaciations + seismic shaking) Worst case impact on repository safety (Multiple glacial cycles + multiple seismics) Possible scenario of total collapse of cavern and pillar failure (1,000,000 yrs +) 11

12 Cavern Stability Models Room and Pillar Models Regional Stress Flow and Displacements Detailed Rock Behaviour Full Repository Models 12

13 Column Models vs Repository Model Need to reduce model coverage to increase resolution, accuracy and complexity of behavioural simulation within pillar Parametric study shows that displacements in column model are a minimum of x larger than displacements in repository model Column model is conservative due to lack of arching above repository 13

14 Shear Stress (MPa) Strength of Rock, Bedding and Latent Fractures Depth from Top of Cobourg (m) Depth from Ground Surface (m, w.r.t. DGR2) Strength of pre-existing bedding planes derived from lab tests 16 Cobourg - intact sample 14 Residual Strength Rock matrix strength Crack Initiation Stress (MPa) from lab testing 0 Bed Peak Strength DGR DGR-3 DGR-4 25 DGR-2 LSD DGR-3 LSD DGR-4 LSD Normal Stress (MPa) Pre-defined segments for induced fractures are modelled with strength properties of host rock 14

15 Depth from Top of Cobourg (m) Depth from Ground Surface (m, w.r.t. DGR2) Crack Initiation and Lower bound Long Term Strength in Cobourg Fm. Long-term rock strength of 45 MPa is assumed (= Mean CI) 0 5 Crack Initiation Stress (MPa) Rate of decay and long-term minimum are functions of confinement DGR-2 20 DGR DGR DGR-2 LSD DGR-3 LSD DGR-4 LSD CI/UCS s/ucs Long-term Lower Bound = CI 15

16 Damage Zone Definitions Subdivided Disturbed Rock into 3 zones Highly Damaged Zone (HDZ)- zone where macro-scale fracturing may occur. The effective HDZ permeability is dominated by the interconnected fracture system and may be significantly greater than the undisturbed rock mass (rockmass permeabilities increase by a factor of 100+) New fractures and bedding slip are continuous at the excavation scale. Identified by yield with significant plastic strain, fracture dilation, tensile failure, joint aperture increase and significant drop in internal stresses (Alternate Terminology - Excavation Fracture Zone (EFZ) Excavation Damaged Zone (EDZ)- zone with hydromechanical and geochemical modifications inducing changes in flow and transport properties (i.e., rock mass permeabilities increase by factor of Damage includes micro-fractures, minor slip on discontinuities - identified by yield (exceedance of damage criterion) with only limited plastic strain and/or limited internal stress reduction Excavation disturbed Zone (EdZ) - possible hydromechanical and geochemical modifications, without changes in flow/ transport properties Material is still elastic and no damage is induced (Alternate Terminology Excavation Influence Zone (EIZ) 16

17 Modelling Strategies Two Models Finite Element Analysis Material is a non-linear continuum Bedding is modelled as discontinuity Discrete fractures: Not modelled, or Modelled as discontinuities to allow discrete separation Materials are modelled as plastic rockmasses (upper shales) or as brittle materials (limestones) Discrete Element Analysis Material is a non-linear discontinuum Bedding is modelled as discontinuity Discrete fractures: Modelled as discontinuities capable of full separation 17

18 Finite Element Results Long Term Response EdZ (circa 40 ka) EDZ HDZ After 1 Glacial Period (circa 70 ka) 18

19 Finite Element Results Post Glacial EDZ HDZ Peak of 2nd Glacial Period (circa 120 ka) EDZ HDZ After 7 Glacial Period (ca.100 s of ka) 19

20 Sigma One [MPa] Pillar Support Capacity (Vertical Stress) Pillar Wall has failed In Situ Maximum (vert) Stress in Pillar Pillar Core acting as full support GLACIAL PEAKS Peak 7 Peak 6 Peak 5 Peak 4 Peak 3 Peak 2 Peak 1 END OF OPERATION (Long Term) Post Closure Pre-Glacial 0 Wall Distance [m] Pillar Centre Model shows that Pillar Core will remain functionally intact for at least 7 Glaciations 20

21 Sensitivity to Bedding Plane Model Shear Stress (MPa) 2 nd Interglacial CI = 40MPa K=1.5 Phase I (Itasca, 2008) << Cohesional Bedding C=1MPa Phi=30deg RED = EDZ WHITE = HDZ (shear strain) Cobourg - intact sample Residual Strength Peak Strength Normal Stress (MPa) Red bedding=slip Phase I/IIa Data (Revised) << Frictional Bedding C=0.1MPa Phi=60deg 21

22 Sensitivity to In-plane Stress Ratio Pre-Glacial State (Black = EDZ, Red Lines= Slip) Stress Ratio State after 1 Glaciation 22

23 Sensitivity to Fracture Style Random Fracturing Vertical Fracturing Test of Sensitivity to Fracture Generation Style. Random Fracturing is Conservative in terms of HDZ generation 7th Glacial Peak 23

24 Observed Nature of Pillar Damage - Limestone Actual Fractures in Limestone Pillars Vertical with concave curvature 24

25 Long -term damage predictions Long-term Strength + 7 Glaciations (Pillars still functional) Damage Zone + Factor of Safety for Intact Strata 25

26 Discrete Element Model (1) Rock allowed to fracture into blocks EDZ 1.6m Degraded Waste 6m Long-term Strength = Mean CI = 45 Mpa (no rock support) 26

27 Discrete Element Model (2) EDZ HDZ EDZ HDZ Degraded Waste 6m Long-term Strength = Mean CI = 45 MPa, No PP/Gas, 2 Full Glacial Periods 27

28 Pore Pressure and Gas Pressure Assumptions: Very low porosity rock Pore pressure drops when crack opens Fractures equilibrate with cavern Gas pressure builds inside cavern Formation pore pressure calculated from hydrogeological model +Time dependant strength loss +Glaciation +Pore Pressure +Gas Pressure 28

29 Effect of Seismic Shaking (Phase I) 100,000 year state (no glaciation) Effect of Seismic Shaking (Cave of yield zone) *Note Bulking Response 29

30 Worst Case Long-term State Need to investigate long-term ultimate state many glacial periods and seismic events. Assumption that eventually (>>100,000 yrs), Cavern roof will undergo substantial collapse and pillars will lose load carrying capacity. Key to overall stability over indefinite time frame (1,000,000 yrs +): Ability of broken limestone beds and blocks to bulk and choke off further collapse. Resultant displacements will not cause any rupture of overlying/underlying shales. 30

31 Natural Analogues Roof collapses in limestone caverns with minimal drainage and water flow Note degree of volume expansion and bulking The collapse on the left has evolved over 100,000 years. 31

32 Extreme Low Strength Scenario FORCED Collapse with extreme parameters LSD + Glaciation VERY Weak rock (CI= 32MPa), 2 m floor clearance Complete Collapse and Choking due to Bulking 32

33 Extreme Worst Case (Displacements) Average 40cm displacement of upper Cobourg boundary Column Model Vertical Displacements shale Cavern Pillar Cavern Full collapse and bulking of blocky ground (15 to 20% bulking factor) Bulked limestone chokes cavern and prevents further failure Vertical Closure = 40cm in column model (note % overprediction) EDZ/HDZ does not propagate into overlying shales shale 33

34 Effect of Collapse on Overlying Strata Repository model with collapsed pillars and bulking Average vertical closure across repository = 30cm Contours of Safety Factor for yield in intact surrounding strata (1.8 in Georgian Bay Shale, 1.4 in Blue Mountain) Queenston Shale Georgian Bay Shale Blue Mt. Shale Collapsed Zone Hinge >5.0 Cobourg Sherman Falls 34

35 Conclusions on Cavern Stability Cavern construction and operations in Cobourg will experience minor spalling in roof & flaking in walls. Through 100,000 yrs + : Pillar Core remains load bearing Vertical EDZ extent limited <8m, HDZ expands after each glaciation Worst Case (Ultra long-term) : Pillar disintegration Cavern roof collapse limited to 10m Bulking of broken limestone fills and chokes cavern Displacements within tolerance of overlying strata Repository zone remains contained 35

36 DGR Shaft Seal Analyses 1. Investigate long-term shaft seal stability and geomechanical evolution using FLAC 3D by Itasca 2. Determine the extent of EDZ of selected shaft seal sections under the following loading Strength degradation with time Seismic loading Glacial loading Gas pressure (effective stress analysis) Selected combinations of above 3. Five shaft seals simulated to explore: Rock mass response (i.e., varied formations) Specific seal behaviour (i.e. waterstop; asphalt; concrete bulkhead) In-situ stress environment Pore pressure response 36

37 Shaft seal analysis (FLAC3D) B9 WS1 B4 S3 B1 Shaft seal design FLAC3D model of WS1 37

38 Shaft Seal Analyses - Scenarios Shaft Seal Timedependent Time-dependent, Concrete Degradation & Glaciation Time-dependent, Concrete Degradation, Glaciation & Seismic Time-dependent, Concrete Degradation, Glaciation & Pore Pressure B9 X X TBC WS1 X X TBC B4 X X TBC S3 X X TBC X B1 X X Preliminary X 38

39 EDZ Distribution - B1 in Blue Mountain Fm. Time dependent strength degradation, glacial loading in 1M years. EDZ expressed as loss in cohesion No increase in EDZ under glacial and seismic loading Estimate EDZ of ~4m thick time-dependent degradation (1M years) time-dependent degradation (1M years) plus glacial loading 39

40 Evolution of EDZ Blue Mountain Fm. EDZ at vary pre- and post-closure stages EDZ at Shaft Seal B1 - Cutoff Bulkhead a = shaft radius 0.5a a 0.7a a 0.6a 1.7a 0.8a 0.8a 0.8a 1.7a 1.7a 1.7a 40

41 Effective Stress Analysis of B1 Pore Pressure Distribution at various distance from shaft After backfill/sealing After 100,000 years After 10,000 years After 1,000,000 years Time dependent strength degradation, glacial loading and gas/water pressure evolution in 1M years. Gas Generation Base Case with max. pressure of 6.8MPa Extent of EDZ could increase by 40% in effective stress analysis Estimate EDZ of ~5.5 m thick 41

42 EDZ Distribution S3 Queenston/Georgian Bay Fm. Time dependent strength degradation, glacial loading and gas/water pressure evolution in 1M years. No significant increase in EDZ extent after backfilling Estimated EDZ of ~2.5m thick After backfill/sealing After 10,000 years After 100,000 years After 1,000,000 years 42

43 Summary of Interim Shaft Seal Analysis A total of five seals were simulated over a period of 1M years. Time-dependent strength degradation (LSD) resulted in 25-50% increase in the extent of EDZ. Additional glacial loading coupled with LSD had minimal effect on the extent of EDZ. Effective stress analyses indicated that long-term gas/water pore pressure evolution combined with LSD and glacial loading could result in increase in EDZ by 40% for B1 Seal and a minor amount for S3 Seal. Seismic effect on the extent of EDZ is negligible. EDZ unlikely to extend beyond one radius behind shaft wall. 43

44 Proposed EDZ Dimensions & Permeabilities 8m 600mm Concrete Liner ~500mm HDZ ~2m~2m EDZ-i -12 K v =10 m/s EDZ-o -13 K v =10 m/s HDZ Highly Damaged Zone EDZ-i Inner Excavation Damaged Zone EDZ-o Outer Excavation Damaged Zone HDZ EDZ EDZ Compacted Bentonite/Sand Backfill -11 K =10 m/s EDZ Undisturbed Rock Mass -14 K v =10 m/s EdZ ~4m ~9m ~4m (Dossier Argille 2005) 44