Measurement of natural stresses in a Provence mine (Southern France)

ENGINEERING GEOLOGY ELSEVIER

EngineeringGeology44 (1996) 77-92

Measurements of natural stresses in a Provence mine (Southern France) Patrick Gaviglio a,,, Pascal Bigarre b, Hafid Baroudi b, Jack-Pierre Piguet b, Raymond Monteau c a Laboratoire de Gdosciences, Universit~ de Franche-Comt~, 16 route de Gray, 25030 Besan¢on cedex, France b Laboratoire de Mbcanique des Terrains, 1NERIS-Ecole des Mines, Parc de Saurupt, 54042 Nancy cedex, France H B C M Unit~ d'Exploitation de Provence, BP 1, 13590 Meyreuil, France

Received 21 June 1995; accepted 23 February 1996

Abstract

Stress measurements were carried out in the Arc syncline using drifts in a Ignite mine. Eleven sites were investigated using the fiat jack .and hydraulic fracturing (or stimulating) methods. Two stress states were found to coexist, one isotropic, the other highly anisotropic. The orientations of the principal stresses are not homogeneous and an orientation ranging from E-W to NE-SW predominates locally. This does not accord with the regional stress field. The vertical stresses are systematically underestimated.

1. Introduction

Measurements of natural stresses were made in galleries of the Gardanne lignite mine in Provence. This mine is located in the Arc syncline E - W trending (Fig. 1). The sites lie within a small area in the autochthonous part of the Arc syncline, just ahead of the thrust fault of the Etoile nappe (Fig. 2) and partly beneath the parautochthonous thrust wedge of Gardanne (Durand and Guieu, 1980). The depths range from 390 to 1260 m. Structurally speaking, two sites belong t o a zone (A) where the strata dip to the north; the other sites are in zone B where the predominant dip is to the west (Fig. 2). The measurement campaign was equivalent to estimating the existing stress pattern in a relatively * Correspondingauthor. 0013-7952/96/$15.00 © 1996ElsevierScienceB.V. All rightsreserved PH S0013-7952 (96)00041-5

well-known geological structure. Eleven sites were investigated between 1980 and 1993. The techniques applied were the fiat jack method (four sites) and hydraulic fracturing (seven sites). The results show complex conditions, but do provide some indication on the stress values and illustrate a few major trends. The relation of the observed stress state to the geological history and structures is not treated in this paper.

2. Measurement methods 2.1. The flat j a c k 2.1.1. Principle and application

The method involves artificially releasing the stresses parallel to the wall of a gallery, measuring

78

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

>- ARC

svN cLIN *

.4"

ETOILE L ¢ ,-l~-

110k m

•l

3

[~

2

['~

1

~....--41"-

~

-

+

+

+

+

+

+

"r

I

THRUST F A U L T STRIKE FAULT

-

SLIP

.,~.~"'ANTICLINE .JSYNCLIN E .I.. ~ *

SITE

LOCATION

Fig. 1. Structural sketch of Provence (after Tempier, 1987) and location of the sites where measurements were carried out. AP, Aixen-Provence; M, Marseille. 1, metamorphic basement and Permian sediments; 2, Triassic; 3, Mesozoic and Cenozoic sedimentary cover.

the deformation caused and cancelling this out by applying a controlled pressure (Rocha et al., 1966; Rochet, 1976; Paquin et al., 1978; Jaeger and Cook, 1979; Bertrand and Durand, 1983). The stresses are released by cutting a slot; the initial state is restored by pressurising a hydraulic jack (Fig. 3). The pressurisation value is considered as the value of stress normal to the plane of the slot. The displacement of the slot edges is monitored by means of an extensometer. The presence of any shear components can be demonstrated by examining the deformations parallel to the edges (Bonvallet, 1978). Stress determination at a particular site obviously requires a number of slots to be produced with different orientations (Figs. 4A and B).

2.1.2. Determining the tensor of natural stresses This is based upon the use of a finite element model for calculating the stress values at the points where they were actually measured around the gallery in question (Piguet et al., 1982; Piguet, 1983). The model is two-dimensional: it takes into account the geometry of the gallery and the mechanical properties of the rock. The geometry is very carefully recorded, sometimes by means of photographs. The method for estimating the natural initial tensor of the stresses involves finding the boundary conditions of the model which minimize the differences between calculated and measured stresses. The boundary conditions are expressed by the magnitudes of two principal stresses in the plane

79

P. Gaviglioet al./Engineering Geology 44 (1996) 77-92

.................. ,! ' ! ! i ,

F~_: ."/~.i i : :.'.. i :~!~ :

'IF,,,..,,,-"" ~o~,,o, ~_

, ~'

Fig. 2. Geologicalsetting of the area where the stress measurement sites are located. G, Gardanne; Etoile; Etoile thrust nappe; STF, Safre thrust fault; GTW, Gardanne thrust wedge; DTF, Diote thrust fault; MF, Meyreuilfault (strike slip fault). (A,B) Structural zones definedin the text. 1, Upper Jurassic; 2, Campanian; 3, Maastrichtian (3b, Begudian; 3r, Rognacian); 4, Eocene;5, Oligocene. All the sites are situated, at depth, in the Campanian limestones. of calculation, and by their orientations (Fig. 4C). Since the calculation is two-dimensional assuming plane strain conditions, the third principal direction is parallel to the axis of the gallery (i.e., perpendicular to the plane of the model) and its value is estimated in the same way by suitably adjusting the measured and calculated stresses in the wall (measurements, in this case, by vertical slots in the sidewalls). The influence of several factors on the estimation of the stress tensor through the proposed method has been tested (George and Piguet, 1981). Two main aspects were concerned: the accuracy of the calculation and the validity of the basic assumptions.

L

Fig. 3. Flat jack devicefor measurement of natural stresses.

2.1.2.1. Accuracy o f the calculation. This depends mainly on the geometry of the drift and on the mechanical characteristics of the rocks. The real precise geometry of the drift, determined by measurements of sections, is taken into account, including irregularities due to damaged zones. Making the calculations for an homogeneous material, if the real material displays heterogeneities in mechanical characteristics (beds with very contrasted elastic moduli and Poisson's ratios), produces major errors which may amount, in

80

P. Gaviglio et aL /Engineering Geology 44 (1996) 77-92

A - Lips of the slot horizontal Crown angle> 0

.

B

-

.

.

2.1.2.2. Validity of the basic assumptions.

.

Lips of the slot are dipping

Crown lips of slot

. . . .

extreme situations, to 50% of the calculated values. In the present case, serious attention was paid to possible heterogeneities as faults or more rigid strata in the studied sites: none was observed. The only bed which could have an influence is the lignite seam. Site no. 3 was the closest (10 m below the seam) but the tests indicated no significant influence.

.~ longitudinal axis

C-

The three assumptions are: the homogeneity of the initial stress field; the orientation of one principal stress parallel to the axis of the gallery; and the elastic behaviour of the material. The homogeneity of the initial stress field in a cubic volume with a 20 m long edge is reasonable provided mechanical heterogeneities are avoided as indicated above. The assumption of one principal stress parallel to the axis of the gallery is necessary because of the two-dimensional calculation method. The tests carried out show that the maximum error concerning the stress values does not exceed 20% if the angle between the principal stress and the axis is smaller than 20 ° . This error is in the range of the admitted indetermination of the method and has shown to be in general good agreement with results of other methods. The linear elastic behaviour is an absolutely necessary condition. It was considered when sites were selected: they were located in recently excavated galleries and the behaviour was tested when the measurements were carried out. It was a criterium for selecting data.

2.2. Hydraulic fracturing or stimulation 2.2.1. Principle and application

Fig. 4. (A,B) Geometrical description of the planes of slots used for flat jack tests. (C) Sketch of the model used for determining the original stresses in the plane perpendicular to the axis of the drift. The purpose is to find the boundary conditions (crl and a3) producing in the model the state of stress fitting the best the pressures recorded on the slots.

This method is based upon the creation or opening of fractures by water pressure in boreholes. In the case of hydraulic stimulation, the closure and re-opening pressures are used to determine the stresses normal to the fracture planes (Haimson, 1978; Bertrand and Durand, 1983; Cornet and Valette, 1984). Water is injected into a section of the borehole isolated with packers. From the continuous recording of pressures and flow rates it is possible to analyse the events which affect the stressed section of ground (Fig. 5). The significant pressures are those which are recorded

81

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

~_HighPressureroom pipe [=.=~ HighPreuure packers pipe ~ Pressurized air

1~~(~

__J_ filter

~

n

- "11" I

manometers

accumulator

LP~HP

II

Pressurerecorder "

I

Water Fig. 5. Hydraulic fracturation devicefor measurement of natural stresses.

at low flow rate; re-opening is obtained with a very slow increase in pressure, under nearly static conditions. The orientation of pre-existing fractures is determined from cores taken during drilling, and that of the newly created fractures from an imprint of the borehole wall obtained with a packer. 2.2.2. Determining the tensor of natural stresses Two methods were used: the instantaneous shut-in pressure (ISIP) method and the hydraulic test on pre-existing fractures (HTPF) method. In practice the second was used for dealing with all the data. The former is used for a borehole parallel to one of the principal stresses (usually vertical) and, in the case of a fracture, parallel to the borehole axis. The minor principal horizontal stress (ah) is then considered to be perpendicular to the fracture and equivalent in value to the closure pressure (ere):

ah =Pro

(1)

The major principal horizontal stress at! is given

by: a H = 3ah -- PreQ

(2)

P,=Q is the re-opening pressure at high flow rate and these equations are valid on the assumption that the rock behaves elastically.N o fluidpressure exists in the investigatedpart of the field. In the case of the H T P F method the stress normal to the fracture plane (an) is assumed to be equal to the closure pressure or to the re-opening pressure at low flow rate (Pr~q) an =(n)t [a] (n)

(3)

Pf= = a . or eroq = a . where n is the vector normal to the fracture, (n)t its transform and [a] the tensor of natural stresses to be determined, expressed in a fixed frame of reference (Comet and Valette, 1984). The SURFRACT program (Lizeur et al., 1993) was used to calculate the tensor of stresses at each site by solving the system of equations Y = A . a , where Y is the vector constituted with the n measured data (here pressures) and a is the vector

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

82

constituted by the six unknown values of the stress tensor. Six measurements at least are necessary ( n > 6 ) to find a solution of the system leading to six equations. Assuming that one principal stress is vertical, a calculation can be run with only four measurements. The S U R F R A C T program provides two numerical solutions. The first one is based on the least squares optimization (the best solution minimizing the difference between observed and calculated data, i.e., pressures). The second method is based on the analysis of sets of solutions obtained by grouping together data six by six (statistical analysis of combinations of results) In the present case SURFRACT was used with the least squares option.

for hydraulic fracturing were made from the roadways mined in the lignite seam; the tests were realized at a depth ranging between 10 and 35 m below the wall of the virgin seam (Fig. 6). In every case the measurements took place before mining reached the investigated site and changed the stress conditions (except for sites 1 and 2): at a minimum distance of 600 m from the mine-workings (except for site 6:200 m). The dip of the strata does not exceed 10°, except at sites 9 and 10 where it reaches 16 and 17° . All sites lie in limestones of the upper Cretaceous (Durand, 1980). Zone A (sites 1 and 2) has been less strained than the rest of the lignite field (zone B) during its geological history (Gaviglio, 1985, 1987). The lithology, in each site, is homogeneous.

3. Site conditions and characteristics

The overall characteristics of the sites are shown in Table 1. The fiat jack method was used in drifts (sites 1 and 2: direction N135; sites 3 and 4: direction Nl15 and N60). The vertical boreholes

4. Results of the measurement campaign

The detailed results are given in Tables2-12: for each site these give the geometrical characteris-

Table 1 General characteristics of the sites Site no.

Method a

Gallery

Depth (m)

Elevation/ seam (m)

Bedding (line of m a x i m u m slope)

No. of useful tests

Date

N305 N000 N270 N250 N245

8 5 6 10 0

nov. 1980 dec. 1980 nov. 1979 rnay1980 may 1986

10°

N300

6

june 1986

10

N 280

5

apr. 1987

9 '~

N280

4

nov. 1990

17°

N 292

6

june 1992

16~

N283

6

oct. 1992

6~

N280

4

nov. 1993

Dip 1 2 3 4 5

F.J. F.J. F.J. F.J. H.F.

6

H.F.

6

H.F.

8

H.F.

9

H.F.

10

H.F.

11

H.F.

T.B. 90 T.B. 90 T.B. 33 T.B. 235 voie 25 Etoile voie 75 b Etoile Sud voie 83 Estaque Nord voie 19 Eguilles voie 59 Estaque Sud recoupe60-59 Estaque Sud Arbois

390 390 600 550 600 900 1030 1140 1200 1130 1260

~F.J., flat jack; H.F., hydraulic fracturation. Sites 1 and 2, zone A; sites 3-11: zone B.

60 150 - 10 50 - 10 -25 - 10 - 25 - 15 - 30 -13 - 25 - 10 - 28 - 12 -34 - 10 -28

6~ 6 9 :~ 9" 6~

Azimuth

P. Gaviglio et aL/Engineering Geology 44 (1996) 77-92

83

tics, the measured values and the observations for each slot or each hydraulic fracturing test. SEAM

5m

4.1. Flat jack measurements

10 m

Hydraulic fracturing Location of the test zone in a bore-hole

N

35 m

Fig. 6. Location of the hydraulic fracturation tests below the lignite seam.

The data obtained from a certain number of slots were discarded owing to surface scaling, cracking phenomena or irreversible behaviour probably due to undetected rock failure (George and Piguet, 1981). This is the case, for example, of slots 1 and 7 at site 3, and 4 and 11 at site 4 (Tables 4 and 5). The crown slots made at T.B. 33 (site 3) gave zero pressure values owing to very high separation of the shelly limestone along the bedding planes; however these values had to be retained to make the calculation possible, since these slots were the only ones orientated along the axis of the gallery, and provide information on the transverse horizontal stresses.

Table 2 Flat jack, T.B. 90 site 1 Slot no.

Orientation of the slot H/V

1 2 3 4 5

H H V H H

6 6

V H

8 9

V V

Angle (°)/axis (transversal)

Height/ ground (m)

Measured pressure (MPa)

Elastic modulus (MPa)

Number of cycles

0.70 0.50 0.65 0.50

13.1 6.2 5.1 13.5 22.2

5 300 10 370 11 140 36 540 20 500

2 2 2 2 1

0.60 0.85

11.6 18.2

35 000 28 800

2 2

90 90

0.65 0.40

6.6

14700

2

88 25

0.65 0.70 2.90

9.3 8.2

0

2.90

7.6

Angle (°)/axis (longitudinal)

1 -2 90 -4 4

90 -3

10 11 12

H V CV

0

13

CV

90

H, horizontal; V, vertical; C, crown.

1

?

23 070 14 700

2 2

9 880

2

18 000

2

Observations

Influence of shear stresses; rock broken during the 1st cycle Influence of shear stresses; rock broken during the 1st cycle; Open fractures Not exploitable results

Rock broken during the 1st cycle

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

84

Table 3 Flat jack, T.B. 90, site 2 Slot no.

Orientation of the slot H/V

14 15 16 17 18 19 20 21

H H V CV V CV H H

Angle (°)/axis (transversal)

Angle (°)/axis (longitudinal)

Height/' ground (m)

0 0

1.30

90 80 65 84 0 15

Measured pressure (MPa)

Elastic modulus (MPa)

Number of cycles

10.7 8.4 6.6 6.1 6 6 11

11 260 13 710 15 420 4 650 10 600 9 800 12000

2 3 2 2 2 2 2 0

Observations

No reversibility

Bad reversibility Broken jack

H, horizontal; V, vertical; C, crown. Table 4 Flat jack, T.B. 33, site 3 Slot no.

Orientation of the slot H/V

1 2

H H

3

V

4 5 6 6 8 9

V H CV V H CV

Angle (°)/axis (transversal)

Height/ ground (m)

Measured pressure (MPa)

Elastic modulus (MPa)

0.00 0.00

0.0 0.0

90

0.00

0.0

85

0.00 0.00 0.00 0.00 0.00 0.00

0.0 0.0 0.0 0.0 0.0 0.0

970 17 850 45 600 53 060 80 000 46000 30000

Angle (°)/axis (longitudinal)

0 -2

3 90 -20 90

0 84 0

26 000 37 000

No. of cycles

Observations

Measurement close to a shaft - discarded

Decohesion of the shelly limestone Spalling - discarded Decohesion of the shelly limestone

H, horizontal; V, vertical; C, crown.

The results of the calculations are shown in Table 1313. In every case one can see, according to the optimization method described above, that one of the principal directions is subvertical. For the northern section, the results obtained from site 1 are the most representative (George and Piguet, 1981) owing to the larger number of slots and better rock behaviour (Tables 2 and 3). The stress state at sites 1 and 2 can be regarded as nearly isotropic. With regard to the sites in the south (3 and 4) the order of magnitude of the calculated stresses is the same in both cases, ~2 is vertical and the principal stress direction assumed to be parallel to

the axis of the drifts is al (Piguet, 1983). However the data from site 4 are more reliable, since no major degradation of the ground was observed. Since no significant shear component was detected during measurements it is reasonable to consider the directions of the principal stresses nearly parallel and perpendicular to the axis of the drift. Of course the true directions cannot be found out exactly, because of the method of interpretation, but it is possible to consider that in this part of the field al is close to the direction of the drift N60 (no more than 20 ° from this direction). This is in agreement with the results obtained by hydraulic fracturing further west.

P. Gaviglioet al./Engineering Geology44 (1996) 77-92

85

Table 5 Flat jack, T.B. 235, site 4 Slot

Orientation of the slot

no.

H/V

1

H

2

V

3 4 5 6

H V H O

7 8 9 10 11 12 13

H V V H V CV CO

Angle (°)/axis (transversal)

Angle (°)/axis (longitudinal)

0

90 0 51 0 59 0 90 90 0 32 81 65

25

Height/ ground (m)

Measured pressure (MPa)

Elastic modulus (MPa)

No. of cycles

0.00 0,00

0.0 0.0

43 500 84 000

2 2

0.00 0.00 0.00 0.00

0.0 0.0 0.0 0.0

29 450 34 500 20 500 64 350

2 2 2 2

0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.0 0.0 0.0 0.0 18? 0.0 0.0

58 170 42 900 42 000 17 300

2 3 2 2 2 2 2

12 500 9 825

Observations

Fracturing of the rock at 20 MPa Estimated pressure Open fissures - discarded Fracturing of the rock at 28 MPa discarded

Fracturing of the rock at 21 MPa Fissuration, bad reversibility - discarded

H, horizontal; V, vertical; O, oblique; C, crown.

Table 6 Hydraulic fracturing: Etoile Roadway 25, site 5 Test Depth Fracture line of maximum slope Fracturing Pressure of Reopening pressure (MPa) pressure instantaneous no. (m) Azimuth (°) Dip (o) (MPa) shut in (MPa) High flow rate Low flow rate 1

0.00

-

2

0.00

2 fractures

-

0.0

10.0

0.0

0.0

Extension pressure with sawtooth peaks at 16 MPa. (Partial opening of the oblique fracture) 12-13

0.0

0.0

8.5-10

0

0.0

-

0

0

3

0.00

N 316 No print

4

0.00

-

5

0.00

N 313

76

0.0

15-16

0.0

13-14

6

0.00

N 313

76

-

0.0

-

14.0

Observations

Fracture along the bedding (shelly limestone)

51

The azimuths are counted from 0 to 360 ° clockwise.

Probably fracture along the bedding 6 successive fractures, no drop in pressure when shutting 12 MPa drop after fracturing Influence of test no. 5

P. Gaviglio et at. ~Engineering Geology 44 (1996) 77-92

86

Table 7 Hydraulic fracturing: Etoile Roadway 75 B, site 6 Test no.

Depth (m)

Fracture line of maximum slope Azimuth (°)

Fracturing pressure (MPa)

Pressure of instantaneous shut in (MPa)

0

14

10-12

Dip (")

Reopening pressure (MPa) High flow rate

Low flow rate

14-12

11 12

1

0.00

-

2 3

0.00 0.00

N 220 N 261

43 90

23 23

2O

4

0.00

N 220

90

23

19-20

23

5

0.00

N 155

41

28.5

21-22

24

6

0.00

N 228

68

21.5

Oscillating extension pressure Opening of 2 bedding planes

18 19

-

One vertical fracture but no drop of pressure No drop of pressure after fracturing Oscillating evolution of the pressure One oblique splitting fracture

21 22.5

-~

Observations

20

The azimuths are counted from 0 to 360° clockwise. Table 8 Hydraulic fracturing: Estaque Nord Roadway 83, site 7 Test

Depth

no.

(m)

Fracture line of maximum slope Azimuth (°)

1

0.00

N

2 3 4 5 6 7

0.00 0.00 0.00 0.00 0.00 0.00

N N N N N N

Dip (°)

16 334 77 34 69 337 349

Fracturing pressure (MPa)

35 8 27 20 31 90 90

Pressure of instantaneous shut in (MPa) 19.5 19,5 21 20.5 21 17 17

Reopening pressure (MPa) High flow rate

Observations

Low flow rate 19 20 21 20.5

-

16.5 18

The azimuths are counted from 0 to 360 ° clockwise. Table 9 Hydraulic fracturing: Eguilles Roadway 19, site 8 Test no.

Depth (m)

Fracture line of maximum slope Fracturing pressure Azimuth (o) Dip (°) (MPa)

Pressure of instantaneous shut in (MPa)

1 2 3 4 5

13.05 13.05 15.05 16.20 17.50

N N N N

6 7 8

18.80 21.30 24.60

N 96 N 7 -

10 102 335 235

57 55 57 69 and 60

22.5 30 40.8 23.5 36

19.5 22.5 30 22.2 28.6

56 54 -

24.5 24 27.5

23.5 22.8 24.5

The azimuths are counted from 0 to 360 ° clockwise.

Reopening pressure (MPa)

Observations

High flow rate Low flow rate 22.2 27.5

21.5

23.5

23

No fracture print Test using bentonite Test using bentonite Test using bentonite, 2 fractures

24.5 24 27.5

27.2

No fracture print

P. Gaviglio et aL/Engineering Geology44 (1996) 77-92

87

Table 10 Hydraulic fracturing: Estaque Sud Roadway 59, site 9 Test Depth no. (m)

Fracture line of maximum slope Fracturing Pressure of Reopening pressure (MPa) pressure instantaneous Azimuth (o) Dip (°) (MPa) shut in (MPa) High flow rate Low flow rate

Observations

1

0.00

N291

59

21

22.5

25

Test using water, other tests carried out with bentonite

2

0.00

N287

48

20

21 17

24 18

3

0.00

N304

31

23

4

0.00

N271

40

24

5

0.00

N305

74

23

15 19.5 19.5 19.5 19.5 19

17 22 21 21 21 21

6

0.00

N339

43

25

7

0.00

N73

69

20

21.5 23 18

23 23 19

Several parallel fractures No full fracturing One fracture Several fractures One fracture Fracturing pressure not accurate

The azimuths are counted from 0 to 360° clockwise.

Table 11 Hydraulic fracturing: Estaque Sud 60-59, site 10 Test no.

Depth (m)

1 2 3 4 5 6 7

12.45 17.85 19.45 0.00 0.00 0.00 0.00

Fracture line of maximum slope Azimuth (°)

Dip (°)

N N N N N N N

30 18 36 53 7.5 7.5 36

133 354 340 341 312 238 103

The azimuths are counted from 0 to

360 °

Fracturing pressure (MPa)

Pressure of instantaneous shut in (MPa)

Reopening pressure High flow rate

(MPa)

Observations

Low flow rate

18 18.8 19.5 20 21 23 23.5 clockwise.

4.2. Hydraulic fracturing measurements I n m o s t cases the tests consisted in s t i m u l a t i n g pre-existing fractures. Several d a t a c o u l d n o t be considered: the n u m b e r o f tests a c t u a l l y u s e d for the d e t e r m i n a t i o n is given in T a b l e 1. T h e d a t a f r o m site 5 were d i s c a r d e d o w i n g to the existence o f a single f r a c t u r e c r e a t e d o u t s i d e a few o p e n stratification p l a n e s ( R e v a l o r , 1986). Because o f the i n a d e q u a t e n u m b e r o f fractures w i t h a p p r e c i a -

b l y different o r i e n t a t i o n s a t sites 7, 8 a n d 11, it was necessary to c o n s i d e r t h a t one p r i n c i p a l direction was vertical in o r d e r to e s t i m a t e the stress state. T h e extension o f this a s s u m p t i o n to all the sites p r o d u c e d the results in Table 14. T h e c a l c u l a t i o n s d o n e for sites 6, 9 a n d 10 s h o w e d t h a t the o r i e n t a t i o n s a n d the values o f the p r i n c i p a l stresses d o n o t differ v e r y m u c h f r o m those o b t a i n e d w h e n the vertical is a s s u m e d to be a p r i n c i p a l d i r e c t i o n ( T a b l e 15).

88

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

Table 12 Hydraulic fracturing: Arbois Sud Roadway 83, site 11 Test no.

Depth (m)

Fracture line of maximum slope Azimuth (°)

Dip (°)

Reopening pressure (MPa)

Fracturing pressure (MPa)

Pressure of instantaneous shut in (MPa)

High flow rate

1

0.0

N 179

8

18

17

17

2 3 4

0.0 0.0 0.0

N39 N6 N294

7 7.5 3.5

24 29 29

22 28 27.5

22 27 28

5 6 7 8

0.0 0.0 0.0 0.0

N 171 N155 N88 N 128

19 90 90 90

31 30 30 33

29.5 29 29 31.5

? 28 29 32

Observations

Low flow rate Influence of the gallery 2 parallel fractures 1 fracture with a low dip Several fractures

Several fractures

The azimuths are counted from 0 to 360 ° clockwise. Table 13 Natural stresses obtained by the flat jack method (sites 1, 2, 3, 4) Site no.

1 2 3 4

Module (MPa)"

crl - a 3 (MPa)

al

~2

a3

8 6.5 16 19

8 6.5 7 10

7 3 2.5 5

1 3.5 13.5 14

Direction

trV/p.g.h

al

a2

a3

N 135 N 135 N lI5 N60

V V V V

N45 N45 N25 N 150

0.82 0.64 0.39 0.71

V is vertical direction; p. g. h is the theoretical stress calculated after the depth, using a specific gravity of 2.5. avertical stress: bold. Table 14 Natural stresses obtained by hydraulic fracturing Site no.

6 7 8 9 10 11

Module (MPa) a

t r l - a3 (MPa)

al

a2

a3

40 32 31 22 45 34

17 19 23 20 20 27.5

12 16 20 18 15 20

28 16 11 4 30 14

direction

~V/p'g.h

al

a2

~3

N150 N70 N104 N 170 N40 N 100

N60 V N14 V V V

V N160 V N80 N130 N10

0.53 0.73 0.00 0.00 0.00 0.00

V: vertical direction, p . g . h is the theoretical stress calculated after the depth using a specific gravity of 2.5.aVertical stress: bold.

4.3. General remarks In zone B (sites 3-11 ), the stress states obtained are highly anisotropic and the major principal stress a a is always horizontal. In most cases the vertical stress corresponds to a 2. Only the stress

state determined at site 9 approaches isotropy, the values obtained reaching 18, 20 and 22 MPa. With this exception, the values of al (from 22 to 45 MPa) are appreciably higher than the values of tr3 (from 12 to 20 MPa): the ratio al/a3 lies between 1.6 and 3.3.

P. Gaviglio et al./Engineering Geology44 (1996) 77-92

89

Table 15 Determination of the natural stresses (verticaldirection not chosen as a principal direction) Site no.

Module (MPa) #1

6 9 10

44 23 50

#2

18 20 20

Orientation a3

9 10 17

#1

#2

Azimuth

Dip

Azimuth

Dip

Azimuth

Dip

N330 N12 N235

6 23 16

N237 N270 N331

25 26 20

N72 N137 N108

64 54 64

Table 16 Natural stresses: two groups of sites Stress

Module ( M P a )

Sites 7, 8, 10, 11 al 43 a2 22 a3 19 Sites 6 and 9 #1 25 #2 19 #3 15

a3

Orientation Azimuth

Dip

N265 N141 N03

18 60 24

N341 N251 N71

0 12 78

Fig. 7 shows the relation between stress and depth. The vertical stress is systematically smaller than the theoretical stress calculated as p . g . h . This phenomenon shows up clearly independantly of the method used. The ratio grip" g" h, calculated on the assumption that the average specific gravity of the ground is 2.5, lies between 0.4 and 0.9 (Tables 13 and 14). This phenomenon has already been reported in other geological environments (Josien et al., 1987). Inspection of the directions of the principal horizontal stresses shows a high degree of heterogeneity (Fig. 8). A simulation using the S U R F R A C T computer program was applied to all the data taken together, i.e., assuming that the stress state is homogeneous throughout the zone investigated: it clearly indicated that this is not the case. More simulations showed that a distinction can be drawn between two groups of sites: a homogeneous group consisting of sites 7, 8, 10 and 11 (districts of Estaque

and Arbois); another group being formed by sites 6 and 9 (Table 16). The stress state at the group of sites 7, 8, 10 and 11 therefore corresponds to a clearly anisotropic horizontal compression with a direction east-west to northeast-southwest. This agrees with the data provided by the flat jack at sites 3 and 4 located slightly to the east. Otherwise it shows a marked difference from that prevailing near the Diote fault ( N N W - S S E for sites 6 and 9). This direction was determined only at two sites: it cannot be attributed the same degree of certainty. The co-existence of these two orientations, of which only the second complies with what is known of the regional stress state (Philip, 1980; Bethoux et al., 1988; Ritz et al., 1990; C o m e t and Burlet, 1992) cannot be explained at present. Formation of anisotropic states of stress and variations on short distances are common in sedimentary series because o f differences in mechanical characteristics from one bed to another (George and Piguet, 1981; Burlet and Ouvry, 1989). Interbedding of rocks with contrasted properties like coal or anhydrite amidst stiffer materials (Comet and Burlet, 1992) produces significant variations in magnitudes o f horizontal stresses but does not seem to change the directions much. Besides variations in elastic properties o f rocks, the fault plane properties have to be considered too (Enever et al., 1995): they control, to some extent, the orientations of the principal stresses. In such conditions the proposed determinations can be considered only as local states of stress representative of limited volumes of rock. Local heterogeneities, especially structural, should be

P. Gaviglio et aL/Engineering Geology 44 (1996) 77-92

90

measured

o l a3

-200 -400 A v

E

-600 m

@ -800

Q

m

Q

- 1000

• •

-1200

(a)

m []

• •

m •

B

-1400

o

,

i'o

20 '

40 '

30

50

o l o3 (MPa]

MEASURED VERTICAL STRESSES -200 Bm

-400

•

R^2 = 0,851

B

-600 D

y = - 221,89 - 43,140x

-800

-1000 -1200 -1400

(b)

10

't 3O

20

ov

(MPa)

MEASURED AND THEORETICAL VERTICAL STRESSES N%~--theoretical

G v

8-006"004-00 m~m ~ ,,,,," 0

-1000

-I 200

......... ?

-1400

(c)

0

10

.ov,., 20

30

40

Fig. 7. Variations of al, tr3 (a) and crv (b and c) versus depth,

P. Gaviglio et aL/Engineering Geology 44 (1996) 77-92

91

PRINCIPAL STRESSES

/

/

j ; ,/~

j

/,!; L~

®

/ *,~

:.'X ,.'~:-':'J

/

~,"..j L ~r~ ~J I ~ :

/

~

L

@

I

10~G

I

D i r e c t i u s and uuallultmles (MPa) of the principal Itrelles

sit, n . . w r

Fig. 8. Magnitudes and orientations of the principal stresses determined for each site. The shaded area is the mined out area today. Each site was far away from the coal-winning zones and the already exploited parts of the mine at the time of the measurements. The thick dotted line is the limit between zones A and B.

taken into account for explaining the observed deviations and differences in stress magnitudes. An interpretation based upon the large-scale mechanical behaviour of a rock mass compartmented by major geological discontinuities will have to involve modelling. Numerical modelling using modern methods of the distinct element type is being used for this purpose (Mamane, 1992). Further stress measurement campaigns are being prepared. The investigated sites, except 1 and 2, seem to be free from influence of mining operations since a minimum distance of 600 m was kept from the coal-winning zones or the already exploited parts of the mine. Only one site (no. 6) was located at a distance of 200 m; in that case it was estimated that the influence on the magnitude of the vertical

stress is less than 10%. All the sites were located west of the mine-workings, in drifts prepared for future exploitation. The tests on sites 1 and 2 were realized after mining took place in the area but they are located above the seam (60 and 150 m). It is reasonable to consider that they are presently undergoing only the load of the overburden.

5. Conclusion The data obtained from the 11 measurement sites provide a coherent view of the stress state. While in the northern part of the coal-field (zone A) the state can be regarded as isotropic owing to the overburden loading, the pattern in the southern sector is dearly anisotropic. The major principal

92

P. Gaviglio et al./Engineering Geology 44 (1996) 77-92

stress is practically horizontal in every case but two different orientations can be recognized. The predominant direction ranges from east-west to northeast-southwest while near the Diote fault it is closer to what is known about the regional compression state, i.e., north-south. As regards the vertical stress, the values obtained are systematically below the theoretical value.

References Bertrand, L. and Durand, E., 1983. Mesures de contraintes in situ: comparaison de diffSrentes mSthodes. C.R. Symp. Essais en place, Paris, vol. 2: 449-470. Bethoux, N., Cattaneo, M., Delpech, P.Y., Eva, C. and Rehault, J.P., 1988. M6canismes an foyer de sSismes en mer ligure et dans le sud des Alpes occidentales: r6sultats et interpr6tations. C.R. Acad. Sci. Paris, t. 307, II: 71-77. Bonvallet, J., 1978. CritSres de stabilitS des exploitations souterraines /~ faible profondeur. Application au cas des carriSres souterraines du Nord. ThSse Dr. IngSnieur INPL Nancy, 159 p. Burlet, D. and Ouvry, J.F., 1989. Discontinuit6s des contraintes en profondeur dams une s6rie s6dimentaire associ6e/t l'hStSrog6nSit6 des matSriaux. Rock at Great Depth, Maury and Fourmaintraux Ed., Balkema. 1065-1071. Cornet, F.H. and Burlet, D., 1992. Stress Field Determinations in France by Hydraulic Tests in Boreholes. J. Geophys. Res., 97 (B8): 11829-11849. Cornet, F.H. and Valette, B., 1984. In situ stress determination from hydraulic injection test data. J. Geophys. Res., 89 (B13): 11527-11537. Durand, J.P., 1980. Les s6diments fuv61iens du synclinal de l'Arc (Provence). Rev. Ind. Min6r., Suppl. no. de juin: 13-25. Durand, J.P. and Guieu, G., 1980. Cadre structural du bassin de l'Arc. Rev. Ind. MinSr., Suppl. June: 3-12. Enever, J.R., Yassir, N., Addis, M.A., Willoughby, D.R., Tan, C,P., and Schmidt, P., 1995. A note on the status of deep hole stress measurement/estimation research in CSlRO. In: K. Matsuki and K. Sugawara (coordinators), Int. Workshop on Rock Stress Measurement at Great Depth. Tokyo, September 30, 1995: 40-45. Gaviglio, P., 1985. A fault and stress field analysis in a coal mine (Gardanne, Bouches du Rh6ne, France). Tectonophysics, 113: 349-366. Gaviglio, P., 1987. Cons6quences de l'evolution structurale sur les caract6ristiques physiques des calcaires fuv61iens du bassin de l'Arc (Bouches du Rh6ne, France): fracturation, propri6t6s m6eaniques, Stats de contrainte. Rev. G-6ol. MSd., XIV, 3: 221-232. George, L. and Piguet, J.P., 1981. Influence de la profondeur et des facteurs naturels sur le comportment des ouvrages

miniers. Convention CECA no. 7220 AC/304, rapport final. Rapport CERCHAR 81-76, 1580, no. 11, 38 pp. Haimson, B.C., 1978. The hydrofracturing stress measuring method and recent field results. Int. J. Rock Mech. Min. Sci., 15: 167-178. Jaeger, J.C. and Cook, N.G.W., 1979. Fundamentals of Rock Mechanics, 3rd edition. Chapman and Hall, London, 593 pp. Josien, J.P., Piguet, J.P. and Revalor, R., 1987. Apport de la M6canique des Roches ~tla maitrise des ph6nomSnes dynamiques dans les mines. Proc. 6th Congr. Int. Soc. Rock Mech. MontrSal, Balkema 2, pp. 999-1004. Lizeur, A., Baroudi, H. and Baucheron, V., 1993. Data analysis for stress measurements by overcoring: new optimization techniques. Proc. International Symposium on Application of Computer Methods in Rock Mechanics and Engineering, 24-28 Maym 1993, Xian (China). Mamane, I., 1992. Influence de la fracturation sur la rSpartition des contraintes: modSlisatiou par la m6thode des 616ments distincts (UDEC). MSmoire de DEA GSnie Civil et Minier, INPL, Nancy, 93 pp. Paquin, C., Froidevaux, C. and Souriau, M., 1978. Mesures directes des contraintes tectoniques en France septentrionale. Bull. Soc. G6ol. France, (7) XX, 5: 727-731. Philip, H., 1980. Tectonique r6cente et sismicitS de la France: caract6ristiques g6odynamiques. In Evolutions g6ologiques de la France, 6dit6 par Autran et Dercourt. Coll. 7 du 26 Sme CongrSs gSologique international, M6m. B.R.G.M., 107: 42-46. Piguet, J.P., 1983. La mod61isation en m6canique des terrains et son application ~ l'exploitation miniSre. Doctorat Etat, 163 pp. Piguet, J.P. et al., 1982. Evolution des contraintes en fonction de la profondeur et des facteurs naturels. 7~ne Conf. Int. sur les Pressions de Terrains, INIEX 1983, pp. 587-611. Revalor, R., 1986. Units d'exploitation de Provence. Mesures de contraintes par fracturation hydraulique. Etoile-Etoile sud (May-June 1986). Rapport CERCHAR 86(1) 22.71.1132. no. 6 NM, 19pp. Ritz, J.F, Hoang-Trong, P., Rebai, S., Philip, H. and Herquel, G., 1990. Le s6isme du 26 d6cembre 1989 en M6diterran6e, au large de la C6te d'Azur: tectonique active en compression, perturbation de contraintes et inversion tectonique au niveau d'une marge continentale. C.R. Acad. Sci. Paris, t. 310, II: 1505-1512. Rocha, M., Lopes, J.B. and Da Silva, J.N., 1966. A new technique for applying the method of the fiat jack in the determination of stresses inside rock masses. Proc. 1st Congr. int. Soc. Rock Mech. Lisbon. t. 2: 57-65. Rochet, L., 1976. Auscultation des ouvrages et des massifs rocheux encaissants. In La mScanique des roches appliquSe aux ouvrages du g6nie civil. Document de formation continue de rEcole Nationale des Ponts et Chauss6es, Chapter 11: 183-214. Tempier, C., 1987. Mod61e de mise en place des structures provengales. Bull. Soc. (36ol. France, (8) III, 3: 533-540.