Why measure in-situ stress?

Transcription

1 C. Derek Martin University of Alberta, Edmonton, Canada Why measure in-situ stress? Engineering analyses require boundary conditions One of the most important boundary conditions for the analysis of underground excavations is in-situ stress σ 1 Failure FOS= Strength Stress In Situ Stress State σ 3 1

2 What is in-situ stress? F 1 F 2 F 3 Boundary conditions Intact rock Excavation F n Water flow Discontinuities Total Stresses = In situ stress + Excavation-Induced stress Definitions Normal Stress (σ) Shear Stress (τ) σ xx Stress at a point τ yz σ zz τ zx τ zy τ xy τ xz σ yy σ xx σ yy τ yx The normal and shear forces acting at a point in the rock mass are represented by three normal and six shear stresses. σ zz 2

3 Definitions (cont d) Normal & Shear Stresses 3-dimensions σ τ τ τ σ τ τ τ σ xx xy xz yx yy yz zx zy zz σ xx τ σ xy yy Symmetrical The components in a row are the components acting on a plane; for row 1, the plane on which σ xx The components in a column are the components acting in one direction; for column 1, the x direction. acts. τ τ σ xz yz zz σ 1 Principal Stresses 0 σ σ 3 Planes of Principal stress have no shear stresses. Example: Excavation surface Co-ordinate ordinate systems Geotechnical Engineering -Right-hand systems -Compression positive -Tension negative 3

4 Data presentation Measurements Borehole E º σ1 ¾ 1 /T r/pl σ2 ¾ 2 /T r/pl σ3 ¾ 3 /T r/pl ν Depth (m) (MPa) MPa/ ± / ± MPa/ ± / ± MPa/ ± / ± /237/ /339/ /106/ /238/ /136/36 7.9/349/ /233/ /142/ /049/ /244/ /145/ /324/44 Magnitudes and Orientation Orientation Data Magnitude Data Stress Sigma 1 Sigma 2 (MPa) Plunge (º) σ Sigma σ 2 0 σ Trend (º) Need to now the in-situ stress in the plane of a tunnel for plane strain analysis 4

5 Useful stress equations Kirsch equations Max. Tangential Stress σ θmax =3σ 1 -σ 3 = 3(k-1)σ 3 Stress ratio k = σ 1 /σ 3 Min. Tangential Stress σ θmin =3σ 3 -σ 1 = 3(1-k)σ 3 Stress path Stress path 5

6 The World Stress Map Project Web Site: wsm.physik.uni-karlsruhe.de/ Types of Data: earthquake focal mechanisms (63%) well bore breakouts and drilling induced fractures (23%) in-situ stress measurements (overcoring( overcoring, hydraulic fracturing, borehole slotter (9%) young geologic data (from fault slip analysis and volcanic vent alignments (5%) Europe 6

7 The Alps Role of stress data bases for new projects Can we rely on these stress data bases Canadian Example Compiled by CANMET:Measurements to 2500 m 7

8 North American Stresses Site C URL Sudbury CANMET Stress database Sudbury Basin Measurements Onaping Depth 2500 m? 8

9 CANMET Database: Magnitudes Canadian Shield Sigma 1 Sigma 2 Possible stress ratios Linear Fit Nonlinear Fit σ v σ v σ H σ H Normal Faults σ v > σ H Thrust Faults σ H > σ v 9

10 Sweden Aspö HRL Sweden s stress database 0 Sigma 1 (MPa) Sigma 2 (MPa) Sigma 3 (MPa)

11 SKB s Äspö Hard Rock Laboratory Stress & water-bearing Fractures σ 1 11

12 Stress & Subhorizontal fractures Aspo HRL Forsmark URL (Canada) Stress magnitudes versus depth Summary from detailed measurements 12

13 AECL s URL Canada Stress orientations & Geological structure 13

14 What about σ 3 Predicting Stress Sigma 3 Horizontal Stress Displacement Boundary condition 14

15 Weak rocks Western Canada Cretaceous Clay Shales Marine origin Thrust Fault Site C Stresses (Canada) Plan Plan Section 15

16 Nagra s Mont Terri Lab Mont Terri Opalinus Clay Shale 16

17 In-Situ Stress Tensor Measurements Rosas1 Stress Sigma 1 Sigma 2 Sigma 3 (MPa) Trend (º) Gives σ n =4.6 MPa acting perpendicular to the bedding Plunge (º) Slotter Stress Sigma 1 Sigma 2 Sigma 3 (MPa) Trend (º) Gives σ n =0.4 MPa acting perpendicular to the bedding Plunge (º) Topography and σ 3 Valley above the Rock Lab cuts down to the Southwest to elevations 100m below the Rock Lab at St Ursanne. σ Distance along profile Negative values to South of Rock Lab Elevation (masl) Distance Along Profile (m) Easting Elevation Elevation Northing 17

18 Opalinus Clay (Geological Structures) In-situ Stress at Mont Terri Factors affecting in situ stress 1. Does geological structures affect in situ stresses? YES 2. Do different measurement methods used yield comparable in situ stress fields? Sometimes 3. What is the importance of residual stresses, i.e. stresses locked into the rock mass? For engineering not important A B C Fracture Rock mass 18

19 Stress modelling Stress modelling at Aspo HRL a) b) c) EW-1b d) EW-1a NE2 EW3 NE2 I EW3 L NE1 EW1a G EW1b H NE1 N Measurement methods Overcoring Triaxial Strain Cells Borehole Deformation gauge Slotter Fracturing Hydraulic Fracturing Sleeve Fracturing Undercoring Convergence 19

20 Undercoring Developed at AECL s URL when traditiional methods failed Originally used 8 CSIRO cells Involves a large rock-mass volume Used when traditional overcoring is affected by: Micro-cracking and nonlinear strains Core disking Expensive method requiring sufficient redundancy to constrain the solution Measure strain, such as CSIRO cells and Determine the stress tensor that produces the strains using a 3D Numerical Program, e.g., Flac3D Depends on the small strain region to determine the tensor hence redundancy is quite important. Undercore method at Mont Terri 20

21 Effect of Scale Residual strain AECL Residual Strain Experiments The residual strain measured was <5% of the stress magnitudes. Not of engineering significance at the URL 21

22 Tunnel convergence measurements Convergence and Stress Assumptions: -Elastic response -Estimate of response ahead of tunnel 22

23 Observations Evidence for stress orientations Borehole breakouts Core disking Stress fracturing around tunnels Magnitudes can be constrained but no absolute values can be given at this time! Borehole Breakouts 300-mm-diameter borehole 23

24 Breakouts: BDT4 Rosas1 Stress Tensor Note: Borehole influenced by tunnel Borehole Breakouts: 75-mm diameter 24

25 Breakout examples No Breakout Irregular Breakout Classic Breakout Core Disking Region of Stress concentrations σ min σ max Core disking 25

26 Core disking & Scale 1.2-m-diameter 300-mm-diameter Same Stress state Conclusion: Core disking Independent of scale 76-mm-diameter Lessons from stress measurement programs Stress databases only provide trends Stress must be measured if stress-related consequences important. Requires specialists who are knowledgable with various techniques. Large-scale measurements (Undercore) preferred over small-scale measurements such as overcoring When using Overcoring Glued triaxial strain cells for hard rocks Borehole deformation gauge for soft rocks When σ 1 >0.2σ c limit for elastic behaviour is close for crystalline rocks Hydraulic fracturing for deep boreholes (500 m overcore tests now possible) Do not discard results (without justification) they might be valid 26