POTENTIAL STABILITY ISSUES THAT COULD AFFECT PERFORMANCE OF BRIDGE FOUNDATION UNITS Short Term: � Bearing Failure due to Imposed Loading Long Term: � Ground Loss into Shafts or Mine Voids � erosion of shaft plugs � dissolution of limestone � roof cave � Increased Loading Related to Regional Groundwater Lowering 1
Potential Mine Shaft Locations 2
LONG TERM STABILITY ISSUE RELATED TO SHAFT CLOSURES Q R e Weak, non-bearing H Strong, competent h 3
SHORT TERM STABILITY – BEARING FAILURE MECHANISMS PUNCHING FAILURE COMPOSITE BEAM BENDING 4
PUNCHING FAILURE ANALYSIS Q B e Punching Failure: Weak, non-bearing FS = Resisting Force / Driving Force = π B e H (c + σ N (tan Φ )) Q + W o +W r H Where: Strong, competent c = cohesion h σ N = normal stress Φ = friction angle W o = weight overburden W r = weight rock Weak, compressible 5
FINITE ELEMENT ANALYSES Performed by Wyllie & Norrish Rock Engineers 1. Used Phase 2 Ver 5.048 software from Rocscience Inc. to perform sensitivity analysis. 2. For the model replaced twin bearing pads with a single circular footing with radius of 4.8m and with same total load and bearing pressure. 3. Footing simplification to enable axisymmetric model – 3D model that is rotationally symmetric about the line of loading. 4. Groundwater assumed to be at top-of-rock. Incorporated by using buoyant unit weight for the rock mass. 6
Finite Element Analyses – Roof Stability over Void FAIR QUALITY 10 m beam depth 20 m void width Self weight “Stable” FAIR QUALITY 10 m beam depth 40 m void width Self weight “Marginal” 7
Finite Element Analyses – Roof Stability over FAIR QUALITY Void with Foundation 10 m beam depth Load 20 m void width Foundation Load “Stable” FAIR QUALITY 10 m beam depth 40 m void width Foundation Load “Marginal” 8
GOOD QUALITY 5 m beam depth 60 m void width Foundation & “Unstable” Embankment Load GOOD QUALITY 10 m beam depth 60 m void width Foundation & Embankment Load “Marginal” Finite Element Analyses – Roof Stability over Void with Foundation and Embankment Load 9
ROCK BEAM – VOID DIMENSION SENSITIVITY BASED ON FINITE ELEMENT ANALYSES Key: Unstable Marginal Stable Void Width (20m) Void Width (40m) Void Width (60m) Beam Rock Foundation Embankment Foundation Embankment Foundation Embankment Thickness Quality Load & Foundation Load & Foundation Load & Foundation Self Weight Self Weight Self Weight Only Load @ 10m Load @ 10m Load @ 10m (m) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) Poor 5 Fair Good Poor 10 Fair Good Poor 20 Fair Good Poor 40 Fair Good 10
Q B e CLOSED FORM PLATE Overburden / Embankment BENDING ANALYSIS (after Wyllie, 1999) H Strong, competent VOID r v Bending Failure: FS = Resisting Rockmass Tensile Strength / Induced Tensile Stress ( σ t ) Where: σ t = 6M/H 2 M = (Q + W o + W r /2) [(1+ ν )log e (r/r 0 )+1], B>H, then r o = B/2 4 π B<H, then r o = [1.6(B/2) 2 +H 2 ] 0.5 – 0.67H ν = Poisson’s ratio, r v = radius of void, W o = weight of overburden, W r = weight rock 11
Unstable Marginal Stable Void Width (20m) Void Width (40m) Void Width (60m) Beam Rock Foundation Embankment Foundation Embankment Foundation Embankment Thickness Quality Load & Foundation Load & Foundation Load & Foundation Self Weight Self Weight Self Weight Only Load @ 10m Load @ 10m Load @ 10m (m) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) Poor 5 Fair Good Poor 10 Fair Good Phase2 Finite Element Poor 20 Fair Analysis Good Poor 40 Fair Good Unstable FS<1.0 Marginal FS<2.5 Stable FS>2.5 Void Width (20m) Void Width (40m) Void Width (60m) Beam Rock Foundation Embankment Foundation Embankment Foundation Embankment Thickness Quality Load & Foundation Load & Foundation Load & Foundation Self Weight Self Weight Self Weight Only Load @ 10m Load @ 10m Load @ 10m (m) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) (0.5 MPa) (0.696 MPa) Poor 0.03 0.02 0.02 0.01 0.02 0.01 5 Fair 0.28 0.21 0.22 0.16 0.19 0.14 Good 1.53 1.12 1.16 0.85 1.02 0.74 Poor 0.12 0.09 0.08 0.06 0.07 0.05 Closed Form Plate Bending 10 Fair 1.29 0.96 0.93 0.69 0.8 0.6 Good >5 >5 5 3.73 4.3 3.21 Analysis Poor 0.61 0.47 0.39 0.3 0.32 0.24 20 Fair >5 >5 4.27 3.29 3.52 2.71 Good >5 >5 >5 >5 >5 >5 Poor 4.39 3.55 1.9 0.77 1.43 1.15 40 Fair >5 >5 >5 >5 >5 >5 Good >5 >5 >5 >5 >5 >5 Comparison of Analytical Approaches 12
CONFIRMED VOID ANALYSIS Foundation Beam Rock Mine Bridge Bent Type (prelim) Thick (m) Quality Horizon 6140 1 B2 0.9 Fair Chaotic 6150 3 B2 1.5 Fair Chaotic 6149 8 B2 5.1 Good Chaotic 6140 2 B2 20.0 Poor Chaotic 6149 7 B2 22.6 Good Chaotic 6149 1 B2 42.5 Fair Deep 6140 3 B2 44.5 Fair/Good Deep 6148 1 A 45.5 Good Deep 6149 6 B1 47.0 Good Deep 13
CONCLUSIONS – APPLICATION TO FOUNDATION DESIGN 1. Deep mine workings with “fair” or “good” rock quality should not represent a stability issue. 2. Deep mine workings with extreme lateral continuity and “poor” quality rock cover will require grout improvement (e.g. bridge A6149 – A6140 confluence). 3. Upper chaotic mine workings will require site specific probe and treatment approach during construction. Targets elevations for treatment can be provided in advance. 4. A number of locations will be suitable for spread footings subject to ground verification during construction. 14
Spread Footing Micropile Footing Presentation of Ground Conditions for Foundation Design 15
Foundations for Bents – Exploration and Pretreatment Anticipated Grout Type Use Versus Ground Classification and Intensity of Treatment. HMG HMG HMG 30% Relati ve 100% 40% Consumption Anticipated of ea ch Grout LMG Type LMG 70% 60% High Medium Treatment Low Intensity Intensity Inte nsity Intensity Ground Type 1 Type 2 Type 2 Classification 16
Anticipated Footing Types and Pretreatment Method Low Intensity (7): A6140 - EB3 Treatment – Spread Ftgs A6148 – EB1, B2, B3, EB4 A6149 – EB9 A6150 – EB1 Medium Intensity (6): A6149 – B3, B6 Treatment – Micropile Ftgs A6150 – B2 A6165 – EB1, B2, EB3 High Intensity (10): A6140 – EB1, B2 Treatment – Micropile Ftgs A6149 – EB1, B2, B4, B5, B7, B8 A6250 – B3, EB4 17
Structure Site Plan – 3 MSE Walls, 3 Box Culverts, and 5 Bridges MSE Wall MSE Wall A7264 A7263 MSE Wall A7265 Br. A6165 Br. A6149 Br. A6150 Br. A6140 Br. A6148 Br. A7262 Br. A7261 Br. A7260 18
MSE Wall A7263 MSE Wall Plan Layout 19
MSE Wall Typical Section at Bridge End Bent 20
Ground Pretreatment at MSE Walls • Philosophy – Locate and treat unforeseen mine features and/or voids prior to wall construction • Construction Procedure – Drill primary holes (30-meters ea.) under the front face of wall – Drill primary holes (20-meters ea.) in predetermined pattern under the footprint of the wall reinforced mass – Grout primary holes utilizing Low Mobility Grout (LMG) • Small fissure grouting is not necessary 21
Typical Pretreatment Pattern at MSE Wall 22
Ground Pretreatment at Bridge Embankments • Philosophy – Locate and treat unforeseen mine features and/or voids below embankments around bridge foundations • Construction Procedure – Drill primary holes (30-meters ea.) in predetermined pattern shown on drawings – Grout primary holes utilizing Low Mobility Grout (LMG) • Small fissure grouting is not necessary 23
Typical Pretreatment Pattern at Bridge Embankments 24
Ground Pretreatment at Box Culverts • Philosophy: – Locate and treat unforeseen mine features and/or voids below box culvert bottom slab • Construction Procedure – Drill primary holes (20-meters ea.) in predetermined pattern within the footprint of each box culvert – Grout primary holes utilizing Low Mobility Grout (LMG) • Small fissure grouting is not necessary 25
Typical Pretreatment Pattern at Box Culverts 26
Bridge A6140 Bridge Layout Showing Foundations 27
Bridge A6148 Bridge Layout Showing Foundations 28
Bridge A6149 Bridge Layout Showing Foundations 29
Bridge A6149 Bridge Layout Showing Foundations 30
Bridge A6150 Bridge Layout Showing Foundations 31
Bridge A6165 Bridge Layout Showing Foundations 32
Ground Pretreatment at Bridge Foundations • Philosophy: – Locate and treat unforeseen mine features and/or voids at bridge footings – Verify the nature of the rock mass under each footing – Treat the ground: • To improve mechanical properties of the rock mass • Limit subsequent micropile grout takes • “Intensity” of Ground Pretreatment – Low Intensity – “Type 1” Ground Conditions – Medium Intensity – “Type 2” Ground Conditions – High Intensity – “Type 2” Ground Conditions 33
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