lvdt cell stress measurements - in situ rock stress
TRANSCRIPT
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In situ rock stress measurements from existing tunnels with LVDT-cell
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LVDT-cell version II
Eight radialLVDT sensors
Rock and celltemperature sensors
Electronics
O-ring basedmounting system
Mountingtool
Batteries & USB-memory
Online cable
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Measurement location and hole layout
Mine niche -400 m
Raisebored shaft-310 m
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OC-12 cm - 63 cm
SC 80 cm- 10 cm- 50 cm
SC 90 cm- 66 cm - 10 cm
SC 75 cm- 46 cm -15 cm
OC- 50 cm - 10 cm
R1
R2R3
R4
R6
R5
S-tunnel
TBM
Measurement location and hole layout
TBM-tunnel-450
Drill and blastS-tunnel-450 m
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LVDT measurement
Measurement holes
Selecting measurement location- sparcely fractures- middle of blast round
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Overcoring to by pass EDZ- raise bore or TBM: 0 cm- drill and blast: 25-50cm
Pilot / installation hole- Ø 126 mm- min free length 35 cm
Overcoring- Ø ≥ 200 mm
min 5 cm
Min OC length 35 cm
EDZ
LVDT-probe
Hole dimensions
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Calibration of the cell
Measurement phases
Drilling the 126 mm pilot hole
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Overcoring
Cooling
Biaxial testing
Measurement phases
Biaxial testing
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3D photogrammetry
3D-photogrammetry
Profiles
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Defining the measurement holeLocations and orientations
Y=North
R1
R3R4
R5
Building the 3D-model
Building the 3D-model for inversion
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Calculation of in situ state of stress
- best fit inverse solution between measured and simulated convergences- requires 3D-numerical simulations of geometries ( BEM, FEM, DEM )- assumes linear elastic isotropy or known transverse isotropy- analytical solution for surface measurements on circular excavation
( considered as gigantic overcoring measurement )
For 3D-model- 3D-photogrammetric model- all holes can be in the same model if
far enough from each other
Interpretation
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For the inversion
To get orthogonal displacements components at each LVDT sensor head
- i.e., 6 × 3 displacements for each head location
Six runs:
1) sEE = 1MPa2) sNN = 1MPa3) sUU = 1MPa4) sEN = 1MPa5) sNU = 1MPa6) sUE = 1MPa
- other five components are set to zero- measured mean E and n
u1N,sij
u1E,sij
u1U,sij
Interpretation
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Inversion
In the case of linear elasticity the LVDT sensor head displacements caused by any in situ stress state, ie. ( k×sEE, l×sNN, m×sUU, n×sEN, o×sNU, p×sUE ),can be constructed by superimposing the multiplied displacement components caused by each unit stress tensor:
ui(ksEE, lsNN, msUU, nsEN, osNU, psUE ) = k×ui(sEE=1) + l×ui(sNN=1) + m×ui(sUU=1) +n×ui(sEN=1) + o×ui(sNU=1) + p×ui(sUE=1),i=E,N,U
-> Best fit between measured and calculated convergence can be foundusing a focused iterative search
Interpretation
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-> side coring can be used- to prevent ring disking
4.98e-002
9.10e-002
1.21e-001
9.10e-002
4.98e-002
2.82e-002
2.17e-002
2.82e-002
126.700
200.000
36.650
Total
Displacement
m
0.00e+000
8.67e-003
1.73e-002
2.60e-002
3.47e-002
4.33e-002
5.20e-002
6.07e-002
6.93e-002
7.80e-002
8.67e-002
9.53e-002
1.04e-001
1.13e-001
1.21e-001
1.30e-001
100
50
0-5
0-1
00
-350 -300 -250 -200 -150 -100 -50 0 50 100 150
deformed shapes
orignal shapes
Solution method does not require full stress release,because solution uses displacement differences between the phases before and after coring
Side coring
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Side coring
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CASE 1Äspö Hard rock laboratory
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Äspö Hard Rock Laboratorymeasurements in well known stress state (-450m level)
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
TBM - bearing 248°, plunge 8°
TASS - bearing 218°, plunge 0.6° (up)- drill and blast
TBM
TASS
TBM
TASS
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Sidecoring responce
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
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Stability of LVDT probe readings
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
LVDTs looking from tunnel to the measurement hole
Measurement location
OC_Start
OC_End, 35 cm
0
5
10
15
20
25
30
35
40
45
50
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
18:00 19:30 21:00 22:30 0:00 1:30 3:00 4:30 6:00 7:30 9:00
Te
mp
era
ture
(C
)
Dia
me
tric
de
form
ati
on
(m
m)
R1-50 - 90 (1+5)
R1-50 - 135 (2+6)
R1-50 - 00 (3+7)
R1-50 - 45 (4+8)
OC_Start
OC_End, 35 cm
Values for calc.
Measured convergences
LVDT pair dL at OC-stop (µm) dL/1 h (µm) dL/12 h (µm)
(1+5) 41 2 0
(2+6) 63 0 -2
(3+7) 19 2 2
(4+8) -2 3 3
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Results - TASS biaxial tests
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
0
20
40
60
80
100
R1 R2 R3 R4 R6
You
ng
's M
od
ulu
s (G
Pa
)
Sample
Surface A
Surface B
Deep A
Deep B
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
R1 R2 R3 R4 R6
Po
isso
n's
rat
ion
()
Sample
Surface A
Surface B
Deep A
Deep B R1
R2R3
R4
R6
R5
Elastic parameters from LVDT pilot cores
- no diffrence in mean values between EDZ and deep samples
- no difference related to location( stress state )
- EDZ samples have higher variation
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Results - in situ stress orientation
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
Sigma_1
Sigma_2
Sigma_3
North
East Trend 90
dip=60 dip=30 dip=0
Sigma_1
Sigma_2
Sigma_3
North
East Trend 90
dip=60 dip=30 dip=0
TASS Axis
TASS TBM
TBM Axis248°
Deepsolid signals
Surfaceopen signals
Christiansson &Jansson (2003)
Christiansson &Jansson (2003)
Constrainedto be H/V
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Results - in situ stress magnitudeNote, Vertical bars are for sH, sh and sV according to Christiansson & Jansson (2003)
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
24.6
18.9
13.6
13.4
9.2
9.8
0 10 20 30
Deep,Final values
Deep,OC Stop values
Surface,Final values
Surface,OC Stop values
Principal stress ( MPa )
Sigma 1 Sigma 2 Sigma 3
TASS, drill and blast
Deep,Final values
Surface,Final values
Principal stress (MPa)
TBM
25.6
22.0
14.1
0 10 20 30
Final, All
Final, All, H/V
All, OC-Stop
Principal stress ( MPa )
Sigma 1 Sigma 2 Sigma 3
Final values
OC stop values
Final values,constrainedto be H/V
Principal stress (MPa)
Deep,OC stop values
Surface,OC stop values
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Quality of the solution
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
TASS, drill and blast TBM
y = 1.01xR² = 0.84
y = 0.67xR² = 0.82
-60
-40
-20
0
20
40
60
80
100
-60 -40 -20 0 20 40 60 80 100
Cal
cula
ted
Co
nve
rgen
ce (
mic
rost
rain
)
Measred Convergence (microstrain)
Deep, final
Surface, final
y = 0.98x
R² = 0.97
-100
-50
0
50
100
150
200
-100 -50 0 50 100 150 200
Measured convergence (microstrain)Measured convergence (microstrain)
Cal
cula
ted
co
nve
rgen
ce (
mic
rost
rain
)
Cal
cula
ted
co
nve
rgen
ce (
mic
rost
rain
)
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Summary
Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel
- deep measurements have excellent agreement with traditional borehole techniques
- the higher internal error and distortion of surface measurement solution supports the existence of an excavation disturbed zone (EDZ)-> minimum measurement depth should be 50 cm
- clear advantages of the methodology are the capability to manage with short boreholes and a compact drill rig, and avoiding the issues associated with gluing and the time needed for curing
- method also involves large volume, avoids effect of small scale heterogeneity
σH
MPa
σH trend
(RT90)
σh
MPa
σv
MPa
Christiansson &
Jansson (2003)
24 ±5
136°
10 - 13
12
This study
Deep, > 0.5 m
23-24
136°-139°
12-13
10-11