In situ rock stress measurements from existing tunnels with LVDT-cell

LVDT-cell version II

Eight radialLVDT sensors

Rock and celltemperature sensors

Electronics

O-ring basedmounting system

Mountingtool

Batteries & USB-memory

Online cable

Measurement location and hole layout

Mine niche -400 m

Raisebored shaft-310 m

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

LVDT measurement

Measurement holes

Selecting measurement location- sparcely fractures- middle of blast round

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

Calibration of the cell

Measurement phases

Drilling the 126 mm pilot hole

Overcoring

Cooling

Biaxial testing

Measurement phases

Biaxial testing

3D photogrammetry

3D-photogrammetry

Profiles

Defining the measurement holeLocations and orientations

Y=North

R1

R3R4

R5

Building the 3D-model

Building the 3D-model for inversion

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

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

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

-> 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

Side coring

CASE 1Äspö Hard rock laboratory

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

Sidecoring responce

Experience with a new LVDT-Cell to measure in-situ stress from an existing tunnel

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

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

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

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

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

)

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

www.smcoy.fimatti.hakala@smcoy.fi