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A review of in situ stress measurement techniques with reference to coal measures rocks
Mining Science and Technology, 2 (1985) 191-206
191
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A REVIEW OF IN SITU STRESS MEASUREMENT TECHNIQUES WITH REFERENCE TO COAL MEASURES ROCKS K.D. Erer and A. Heidarieh-Zadeh Department of Mining Engineering, University of Nottingham, University Park, Nottingham NG 7 2RD (U. K.) (Accepted N o v e m b e r 1, 1984)
ABSTRACT The paper discusses rock stress measurement techniques with special reference to Coal Measures rock formations. A classification of various stress measurement methods is put forward and comments made on their applicability to
longwall mining stressfield situations. Test resuits using a borehole hydraulic cell are presented and discussed following a trial at a Midlands Colliery.
INTRODUCTION
upon the availability and suitability of a stress measuring technique. In fact, no single technique has, as yet, proved to be universally acceptable. It is difficult to apply one design of cell to a wide range of rock types. The stress relieving technique for absolute stress measurements is very common. When the nature of coal is taken into consideration the application of the stress measurement technique to coal mine situations should be carefully examined, especially regarding the obtaining of reliable strain or stress data from field instrumentation programmes.
Recent developments in coal mining have called for mine designs and excavation constructions to possess high degrees of reliability and this has prompted the need to know more about the nature of field measurements, especially regarding the prediction of the structural stability of any underground excavation. Studies aimed at improving the stability of the access roadways of a longwall panel will benefit substantially from a fairly accurate knowledge of the in situ state of stress. In situ stress measurements are carried out to determine the directions and magnitudes of either the 'absolute rock stress', which exists in the rock mass prior to mining, or the 'relative stress' (i.e. induced stress). Any study of in situ rock stress depends 0167-9031/85/$03.30
IN SITU STRESS M E A S U R E M E N T TECHNIQUES The basic principle of most rock stress measuring techniques has been dependent
© 1985 Elsevier Science Publishers B.V.
192 upon the measurement of deformation from which rock stresses are computed with the aid of the relevant formulae. Various stress measurement techniques for a wide range of rock types are given in Table 1. A large number of instruments have been developed, some of which have enjoyed considerable success and popularity in m a n y countries. Apart from the 'soft inclusions' of absolute stress measurement techniques, monitoring of stress changes in longwall mining has been carried out by means of 'hard inclusion' borehole stressmeters. The former has a rigidity less than that of the host rock, and therefore deforms under the field pressure, whereas the latter has an elasticity modulus greater than at least twice that of the host rock.
TECHNIQUES FOR MEASUREMENT OF ABSOLUTE STRESS OF IN SITU ROCK Stress relieving method In principle, a borehole under rock pressure is 'stress-relieved' by drilling a larger concentric hole (i.e. overcoring operation) [1]. The deformation of the inner hole, which is a function of the field stress, is measured to obtain the original state of stress within the rock. The borehole deformation gauges measure changes in length of one or more diameters of a borehole in which they are installed. They are of the soft inclusion type, and therefore, a change in deformation of the borehole will cause a change in the instrument itself. These gauges possess a piston mechanism which contacts with the wall of the borehole, and resultant deformation is sensed in terms of strain. In order to calculate absolute stress both the elastic modulus and Poisson's ratio of the host rock requires to be known. The borehole deformation gauges developed by the U.S. Bureau of Mines. are probably the most popular types [2]. The installation proce-
dure involves a 38 m m diameter borehole which will expand when it is overcored concentrically by a 150 m m diameter borehote. The strain measurements are taken regularly until the gauge is completely overcored. If the diametrical changes of the borehole in three directions are measured, the principal stresses in a plane perpendicular to the borehole axis can be determined. Application of this gauge to longwall mining requires careful consideration of the following points: (a) Drilling operations in Coal Measures rocks, particularly in coal, need special expertise. Overcoring operations in such rocks should be performed with great care, otherwise the entire work will have to be repeated perhaps a n u m b e r of times. (b) The overcored portion of the borehole should be removed from the test site in order to determine the elastic properties of the coal. (c) Unlike hard rock, the inner wall of a borehole drilled in coal has a rough surface and is easily disturbed during instrumentation, therefore an accurate measurement of the changes in the borehole diameters is difficult to obtain. (d) Intrinsically safe electrical equipment should be employed. (e) Existence of water in the borehole and its influence on strain readings should be taken into account. Another method involving overcoring is to make measurements of the changes in diametrical deformation on the bottom surface of a borehole. Instruments utilised in this method are known as "strain cells". Strain gauges are bonded to the flattened surface and overcoring is then carried out by continuing the borehole [3]. The "Doorstopper" has become the most popular gauge for this purpose. It has a rectangular strain rosette containing the three strain gauges, arranged at certain angles, bonded to the rock by araldite. Another cell known as the " C S I R Triaxial Cell" contains three strain gauge rosettes directly bonded
TABLE1 Insitustress measurementtechniques
Stress Measurement
Measuring Principle
Procedure
Stress relieving
Measurement of changes in the diameter(s) of a borehole (- Borehole Deformation gauges
Overcore the gauge w i t h i n a central borehole
U,S,B,M * Deformation
Overcore a cy]inder with
L.N.E.C. * gauge
Measu~abl~ Stress [omDoneo:~
Name of Instrument
Transducer
g
s t r a i n gauges
Bonded s t r a i n qauges
Bonded s t r a i n gauge rosettes
Maihak * v i b r a t i n g wire cel]
X
S t r a i n gauges attached to canti]evers or rings
CSI,R. Mark [ and II
I: e~ectrica~ cos strai~ ; I : L.V.D.T.
Sibek's c e l l
s t r a i n gauges
X
U n i v e r s i t y of Liege c e l l
L.V,D,T
X
Four-comp deformation g a u g e
s t r a i n gauges
Photo e l a s t i c soft i n c l u s i o n
s t r a i n gauges
cell
Measurement of s t r a i n s at the bottom face of a borehole (: borehoie strain cells)
Ye~~gS?
! V i b r a t i n q ~ire transducer
Gris~old's c e ! l
Dvercoring operation is carried out
X fYear 1961)
Overcore a rosette gauge bonded on the bottom of a hole
"Ooorstopper"
Overcore a borehole w i t h s t r a i n gauges on i t s w a l l s
C.S.I.R, * Triaxial cell
*
bonded s t r a i n gauge rosette
bonded s t r a l n s t r a i n gauges
',ature of 3tress ~ea~Jemert
~bso!ute
Author: "anufacturer
I Country: Year
Application to cnal m i n i n q
Remd~kS
Coal p i l l a r s
currently available
relative
Nerrii, Ceter~on
U.S.A, (Ig61]
gocha and $ i l v e r ~ o (L,NE,C.)
Portugal, (1974)
Maihak Company
Griswold
LeemaP CCSI~,R.)
Sibek
(!960)
U.S,A
(1962-63)
Not aDDlica~le
Out of date
S. Africa (1960)
Coal pillars, not reliable
Out of date
Czechoslovakia
rib pillars
~1960)
France (1975)
Crouch Fairhurst
I2.S.A (1967~
Riley Goodman Nolting
C u r r e n t l y used. A r a l d i t e is used in the borehole
W', Germany
Bonnechere
('J.S B.!I)
Soft inclusion
and face
Out of date
Variation of U S.B,M. tria~ial
'
cell
U.S,A (1976-7)
Leeman
S. A f r i c a
app]icable to
very Dopu:ar
(C.S.[R,)
(197])
Coal Measures rock
in manycountries
Leeman (C.S,[.R,)
S. A f r i c a (1971)
Overcoring is difficult in coal
Determination of complete stress tensor i n a s i n g l e boreho]e
S t r a i n s at the bottom of a
t borehole
Photoelastic biaxial cell
W,N.I/~,I.
strain cell
Change in magnetic I permeability with stress
Overcore a
rectangular. solid "
gbservation
Overcore a
of photoelastic stress fringes
hard inclusion with down hole polariscope
Strain gauges encapsulated i n an epoxy resin cylinder
P~rtial
relieving of stress
Photoelasticity
Overcoring operation
Host's
cell
Bonded strain gauges
Magnetost,'iction
C.S,I.R.O. * hollow inclusion ceil
gauge body
Strain gauges bonded to the
Photoelastic glass stressmeter
Photoelastic s t r e s sfringes
Soft i n c l u s i o n cell
Electrical resistance s t r a i n gauges
X
L.N.E.C, Flat-jack
Measurement of displacement c~used by a s l o t cut wall
U.S.B,M. flat-jack
Electrical resistance strain gauges
Cylindrical Jack
Hydraulic oil within a core
CStress compensation)
Measurement of displacement of s t r a i n gauges due to u n d e r c o r i n g
Undercore the strain gauge rosette on the surface of a rock face
East inclusion method
Overcore a hard i n c l u s i o n to freeze stresses i n t o it
Borehole deepening method
Pressure-bag method
Three p a i r s of s t r a i n gauges
Soft inclusion gauge
Detection of bending
Monitor the radial deformation of deepening boreheles
Locating t h e rubber pressure bag i n the bottom end of a boreho]e d r i l l e d in a pillar
X
cantilver beams with bonded I
X
Displacement of a h y d r a u l i c fluid
x
[
s t r a i n gauges
The rubber pressure bag
I
l
Hawkes and
Sh)Dody
U.SI,q (1964)
Intrinsically safe for use in coal minin~
Boreho]e should not exceed 5 m
U.S.S.R. (1965)
I t was used in lengwall Mining
Modified versions are bein 9 used
The f i r s t r e l i a b l e gauge employed
Sweden (]958)
Wet 3ndOt n! c Ki '^alton
Roberts and
Australia (1976)
U.K. (196d-5)
Hawke~
Applicable to longwall
mining
Observation at fringes are d i f f i c u l t in deep boreholes
Blackwood
Australia (]976)
Rib p i l l a r s of 3ongwall ~ane]s
Similar to LNEC gauge developed by Rocha
Rocha, Lopes and Silva (L.~.E C,)
Portugal {1965)
Simply and
No g r o u t i n g Semi-circle slots. Reusable at ~ncreasinq depths of same slot
Panek (U.S,B.~.)
U.S.A (1961)
Jaeger and Cook
U,S.A (1966-9)
DuvalI and Nooker
U.S.A {1974)
Variant of the "Ooorstopper" I n s t a l l a t i o n is d i f f i c u l t supplementary method
~iley Goed~an
U.S.A (~gz7)
Stresses ere measured in the laboratory
Ge La Cruz and Goodman
U.S,A (]970)
Measurements should be performed as close as possible to the bottom
Baar
W. Germany (1960)
and e f f e c t i v e l y used App)icabie to Coal Measures rock
in coal p i l l a r s up to cm a depth 14-15
easily i n s t a l l e d in coal
Supplementary method
~ieJsuremetlt of stress changes by hard inclusions
Borehole inclusion stressmeter
Measuren~nt of changes in t r a n s d u c i n g element due to rock stress variations
Ports' stressmeter
Electrical resistance s t r a i n gauges
Grid stressmeter
S t r a i n gauges in the form of a wheatstone bridge
M,R,D.E. borehole plug
Foil resistance strain gauges
Hast's r i g i d inclusion
Electrical res strain gauges
Glass Stressmeter
Phetoelastic fringes
Spherical rigid inclusion
Three strain gauge rosettes
Vibrating * wire stressmeter
Vibrating wire
U S.B,M. * Encapsulated i borehole hydraulic stressmeter
Hydraulic fracturing i n a bQrehole
Rock fracture
Rock fracture
Core diseing
Measure strain to fracture a borehole with a borehole jack
Measure the velocity of sound in rock
Other stress ~easurement technicques
Correlation between r o c k oroperties and stress
Resistivity, rock noise
Detection of yield pGint Isotope method I
Close hydraulic circuit
X
Pott~
LI.K (1960)
Salamon
U.K (1960)
4ilson
U.K
widely used in coal mlnes
NOt c u r r e n t l y available
(1960) Sweden (1958)
"Poberts and :awkes
U.K. (1964}
The ~vl,S. Geological survey
U,S.A (1968)
Hawkes
U,S.A [1973)
suitable for coal mines
Illumination difficu]ities erise
may
CeTI is qrouted in the borehole
Applicable to coal mines Favourable gauges at present
U.S. Bureau of Mines
U.S.A
F~irnurst Haimson
(U.S.A,) (1965) and (1978)
3bert and Stephenson
U.S.A (1955)
De La Cruz
U,S.A (~978)
Obert and buvall
U.S.A (1957)
~anagawa Hayashi Nakasha
Japan (1976)
Oreyer Borchart
W, Germany (1957)
Borecki Kidyibinski
Poland (1966)
Favourab]e for coat mines
Current stress measuring method
Supplementary methods of stress measurement In rock
t~ I kCD OO
199 along the circumference of a chosen diameter of a borehole. To utilise these two strain cells in longwall mining operation the following problems must be effectively resolved: (1) the difficulty in bonding the gauges to Coal Measures rocks; (2) the difficulty in obtaining an intact core with a diameter up to 40 mm and with a length of 25-50 mm; (3) the unknown behaviour of the field stress at the bottom region of the borehole [4]. The "Doorstopper" requires a shorter length of core than its triaxial counterpart and therefore it is more adequate for use in coal mines.
Partial relieving of stress The stress-relieving method for absolute rock stress measurement is apparently superior to other means of measuring in situ rock stresses. However, partial stress-relief techniques such as flatjacks [5,6] and "undercoring" [7,8] are alternatively used for in situ rock stress measurements (Table 1). The latter is a new technique and is gaining popularity in hard rock conditions.
Rock fracturing method The most popular rock fracturing method is that of "hydraulic fracturing". Its installation procedure is to seal off a section of a vertical borehole and to apply a fluid pressure to the interior of the borehole section until the inner wall fractures [9]. Stresses along a plane normal to the axis of the borehole are thus determined. This method is applicable to hard rocks such as granite and soft rocks such as clay. However, hydrofracturing of coal cannot be regarded as a suitable technique of determining stresses in every circumstance.
Complementary techniques Correlations between the rock properties and the applied stress can be utilised as c o r n -
plementary techniques (Table 1). Among these, seismic methods have been widely employed in order to determine stress distributions within coal pillars.
BOREHOLE I N C L U S I O N STRESSMETERS FOR M O N I T O R I N G STRESS CHANGES IN ROCK Three main features of the borehole stressmeters are: (1) high rigidity; (2) longterm stability; and (3) contact with the wall of the borehole through the entire body of the instrument. Two currently available instruments for measuring stress changes in rock are the IRAD vibrating-wire stressmeter and the U.S. Bureau of Mines hydraulic pressure cell (Table 1). The former is a very sophisticated and expensive device. However, it gives a good picture of stress changes when a longwall advances [10]. It has a hollow cylinder containing a coil, magnet and a yoke used to vibrate a wire stretched diametrically. The stressmeter is preloaded by a plate and wedge mechanism. The wire has a natural frequency. In order to measure changes in the frequency of the wire the coil plucks the wire. A change in the length of the wire due to diametrical change of the borehole is related to a change in rock stress in the direction of the wire. The U.S.B.M. borehole pressure cell is also very popular to monitor stress changes in longwall mining.
FIELD INSTRUMENTATION Field site Encapsulated hydraulic borehole stressmeters were employed in order to monitor stress changes on the H11's longwall advancing face at Cotgrave Colliery, in South Nottinghamshire, U.K. The face was in the Deep Hard Seam at a depth of about 530 m below the
200
surface (Fig. 1.). The coal in the adjacent faces had already been taken. The immediate roof was a thick layered mudstone overlain by a siltstone (Fig. 2). The floor was also of mudstone underlain by thick layers of siltstone and sandstone. Test locations and positions of the stressmeters are shown in Fig. 3. In the test location (A) four cells, namely 2,5,6 and 8 were installed in the rib pillar of the tailgate. In the location (B), 60 m behind the face line, a single stressmeter was installed in the solid coal.
hydraulic circuit (Fig. 4). The cell is inserted into the borehole and orientated in order to measure either vertical or horizontal stress within coal. Following the grouting process the cell is prestressed in order to make an intimate contact between the wall of the borehole and the entire outer surface of the cell body. A n y change in the diameter of the borehole due to a change in rock stress will cause a change in volume of the hydraulic circuit. Consequently, the internal pressure in hydraulic systems will change, thus the displacement of the fluid will indicate a pressure change in the pressure gauge.
Encapsulated hydraulic pressure cell Construction of cell The encapsulated hydraulic pressure cell comprises a copper bladder and a pressure gauge which are connected to form a closed
For construction of the cells a standard procedure was utilised [11]. Materials em-
S3's
S3's
N
Hollygate Lane
"'-:-.....
HII's
"~'~..~..~ --....~
//
"~..
a
d Fig. 1. The location plan of H l l ' s district, Cotgrave Colliery.
2bo 300
201 Mudstone
~ ......- [ , , . . . ' '
..
. . . . . .
,',
•
Mudstone
, , . . . ' . , . .
•
•
Sandstone
; , ' . . ~ •
,
•
,'
. •
• .
Siltstone
.. . . .
., .
. ..
, ,
Grey Mudstone, S i l t s t o n e
-52-Roadway
Deep Hard Coal
Location
Cannel Canneloid Mudstone
ployed for the cells are given below: 1. bladder: soft copper tube, 2.54 cm (i. dia)*; 2. feed pipe: soft copper tube, 7 mm (i. dia) and 6 mm (i. dia); 3. tubing: (a) plastic pipe: 40 m m (i. dia) and 19 mm (i. dia); (b) steel pipe: 4 mm (o. dia)*; 4. oil for the hydraulic circuit; 5. encapsulation material: 'Celfix' 100 (epoxy resin); 6. orientation rod: steel; 7. installation rods; 8 bleed valve; mild steel, 4 mm (i. dia); 9. screwed joints and taps; 10. pressure gauges: ENERPAC: uppei limit 70 MPa.
Siltstone
0
1
2m
* i.dia = inside d i a m e t e r , o.dia = o u t s i d e diameter.
Fig. 2. Section of s t r a t a at H 1 1 ' s district. Location
Location
(A)
(B)
I 1 !
o
x~
o
o
o
o
-1 _
3.60 m
4.26 m
i$
--2J
u6___~
4.87
J
16.5 m , i,
5m l m
"-~6
45 m
1F F
Location
(B)
I
I
E
=I 2 Fig. 3. L a y o u t of b o r e h o l e s at H 1 1 ' s m a i n g a t e a n d tailgate at C o t g r a v e Colliery.
lk
202
~gmm L19mE[~ j
19ram Elbow
,
..~_
' Io\ ......... 'I
Fig. 4. Details of hydraulic pressure cell.
60
I
pI/
3 []
Z L, .,i.o
50
z~
'7 zx
~'--I°
£
/
~I -- ~oc
E]"
0
/
//,
/
&
~~.
o
X A
~/
#
/
/ 0 ,
J
0
, / pj / o
.,o
-"
o
[]
/o
f I lO
20
30 Bladder
40 pressure
50
60
(MPa)
Fig. 5. Laboratory calibration of encapsulated hydraulic bladder using different angles, c~, between the applied stress and the bladder surface.
203 gradually decreased due to malfunctioning and finally dropped to zero. When the face was 110 m ahead of the test site a fall in stresses was observed and the decreases in cell pressure continued for one month. In the test location (B) the cell showed a similar performance to those cells in the location (A). A peak pressure value of 48 M P a was obtained from the gauge. The following conclusions can be drawn from this field study: (1) The peak strata pressure could achieve four times the overburden pressure. (2) Some difficulties were encountered during the installations of the cells. Firstly, great care was necessary during instrumentation so that the cell b o d y and tubing were not damaged during the insertion of the cell assembly into the boreholes; there is a considerable risk of leakages occurring inside the borehole. Secondly, drilling deep straight boreholes is a rather difficult operation in weak Coal Measures strata, especially when the test site is in a fault zone. Thirdly, a
Calibration The cell response to an applied load was calibrated in the laboratory in changing the angle between the direction of the load and the surface of the bladder. A steel block containing an encapsulated bladder was tested between the plates of a compression testing machine. Three separate tests were carried out by altering the angle to 90 ° , 60 ° and 30 ° (Fig. 5). Linearity between the vertical load and the cell response was observed.
RESULTS AND CONCLUSIONS The results from both test sites are shown in Figs. 6 and 7. In the location (A) setting pressures of between 5.5 and 7 M P a were used. The peak pressure values were obtained when the face was about 90 m away from the test location. These maximum pressure readings were 2 to 3.5 times greater than the overburden pressure. The pressure in cell 5
/
N "E
%"
/ /
(D
g-
B No. 6
/1! /
/
c~
No. 2
@
-
=,.m/" ....:
.~r'"
~ "X~. ",.. • XNo. 8
j] 65
Borehole numbers
1O ~
20
30
40
50 60 70 Distance along the gate (m)
80
go
lO0
II0
~zi~IHIzz~zI1ii~iI~1~~r11~zz1II~I~I1H~H~~I1I~1HII/II~II1~1~H~zH~inbye ,I,I,,,111~,,,,,,,,,,'',,H,HIH',,,,,IHH,,HFace outbye
Fig. 6. Stress changes during mining at Hll's tailgate.
advance
~
]6.5m deep
(no. 2)
5m deep
(no. 5)
lm deep
(no, 8)
5mdeep
(no. 5)
1
204
@
1
(D
@
/ o
0
J \ o
o
%
®
8~
~0
=.
\ @
\
~.
o
_o
/
_o
/ @ I
i
I
!
|
o
(ecU4)
ssa~]S
=o o ,4D
r~
205
virtually perfect contact is required between the cell and the wall of the borehole. (3) There was a delay in registering stress in the hydraulic ceils. This is ascribed to the following factors: (a) ineffective grouting in the b0rehole; (b) frequent interruption of mining due to faults in the first 100 m of excavation, which slowed the rate of the face advance. (4) Although there was a delay, after a second attempt of repressurisation of the cells, three of the cells provided satisfactory results when the face was 50 m away from the test location (A). The cell in location (B) also responded to delayed strata stress changes. However, it is always risky to install a cell in such a long borehole in very weak rock conditions. (5) Apart from the difficulties in installing these cells this technique is suitable for monitoring stress changes in longwall mining. Its form is simple and comparatively cheap to manufacture. It is safe for use in underground coal mines. However, it is unsuitable for deep boreholes (greater than 15 m) owing to the difficulty of aligning the cell with respect to the direction of the stress, based on the experience gained at Cotgrave Colliery in weak and faulted ground conditions.
GENERAL CONCLUSIONS The utilisation of various types of stress measuring methods in underground coal mines is possible provided certain restrictions which relate to the different nature of coal with respect to other Coal Measures rocks are borne in mind. In order to use such an instrument in coal mines a number of factors should be taken into account including the following; the assumption dealing with the elastic properties of coal; the calculation of stresses; the orientation of the instrument; influence of the overcoring operation on the stress measurements; and general physical problems of in-
stallation and effective use of the techniques in providing reliable results. It is concluded that most underground coal mine sites permit a reasonable degree of success in performing in situ stress measurements, although careful site planning and choice of stress measurement technique need due consideration.
ACKNOWLEDGEMENT The authors wish to thank Professor Atkinson, Dr. B.N. Whittaker and Dr. R.N. Singh of the Department of Mining Engineering, University of Nottingham, for their help given during the project and also to the management staff and personnel at the N.C.B. Cotgrave Colliery, for the excellent co-operation and assistance with the underground tests.
REFERENCES 1 N. Hast, The Measurement of Rock Pressure in Mines. Sveriges Geologiska Undersokning, Ser C, 560 (1958), 183 pp. 2 L. Obert, R.H. Merrill and T.A. Morgan, Borehole deformation gauge for determining the stress mine rock. U.S. Bur. Mines Rep. Invest., 5978 (1962) 90-95. 3 E.R. Leeman, Borehole rock stress measuring techniques. J. S. African Min. and Metall., 65 (Sept. 1964) 45-81. 4 E.R. Hoskins, An investigation of strain rosette relief methods of measuring rock stress. Int. J. Rock Mech. Min. Sci. 4 (1967) 155-164. 5 L.A. Panek, Measurement of rock pressure with a hydraulic cell. Min. Eng., 13 (1961) 282-285, 6 M. Rocha, J.J.B. Lopes and J.N. Da Silva, A new technique for applying the method of the flatjack in t h e determination of stresses inside rock masses. Proc. First Congr. of Int. Society for Rock Mech., Lisbon, Vol. 2, 1966, pp. 57-65. Publ. L.N.E.C., Lisbon. 7 E.N. Lindner and J.A. Halpern, In situ stresses in North America. Int. J. Rock Mech. Min. Sci., 15 (1978) 183-203. 8 Y.T. Lavie and F. Van Ham, Accuracy of strain measurements by the undercoring method. 3rd Conf. Int. Society for Rock Mech., Denver, Colorado, U.S.A., 1974, pp. 474-480.
206 9 B.C. Haimson, The hydrofracturing stress measuring technique-method and recent field results in U.S.A. Int. J. Rock Mech. Min. Sci., 17 (1980) 81-88. 10 I. Hawkes and W.V. Bailey, Design, develop, fabricate, test and demonstrate permissible low cost cylindrical stress gauges and associated components capable of measuring change of stress as a function
of time in underground coal mines. U.S. Bur. Mines Rep. Invest., (Nov 1973), 105 pp. 11 T.C. Miller and R. Sporcic, Development of hydraulic device for measuring relative pressure changes in coal during mining. U.S. Bur. Mines. Rep. Invest., 6571 (1964), 33 pp.
191
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A REVIEW OF IN SITU STRESS MEASUREMENT TECHNIQUES WITH REFERENCE TO COAL MEASURES ROCKS K.D. Erer and A. Heidarieh-Zadeh Department of Mining Engineering, University of Nottingham, University Park, Nottingham NG 7 2RD (U. K.) (Accepted N o v e m b e r 1, 1984)
ABSTRACT The paper discusses rock stress measurement techniques with special reference to Coal Measures rock formations. A classification of various stress measurement methods is put forward and comments made on their applicability to
longwall mining stressfield situations. Test resuits using a borehole hydraulic cell are presented and discussed following a trial at a Midlands Colliery.
INTRODUCTION
upon the availability and suitability of a stress measuring technique. In fact, no single technique has, as yet, proved to be universally acceptable. It is difficult to apply one design of cell to a wide range of rock types. The stress relieving technique for absolute stress measurements is very common. When the nature of coal is taken into consideration the application of the stress measurement technique to coal mine situations should be carefully examined, especially regarding the obtaining of reliable strain or stress data from field instrumentation programmes.
Recent developments in coal mining have called for mine designs and excavation constructions to possess high degrees of reliability and this has prompted the need to know more about the nature of field measurements, especially regarding the prediction of the structural stability of any underground excavation. Studies aimed at improving the stability of the access roadways of a longwall panel will benefit substantially from a fairly accurate knowledge of the in situ state of stress. In situ stress measurements are carried out to determine the directions and magnitudes of either the 'absolute rock stress', which exists in the rock mass prior to mining, or the 'relative stress' (i.e. induced stress). Any study of in situ rock stress depends 0167-9031/85/$03.30
IN SITU STRESS M E A S U R E M E N T TECHNIQUES The basic principle of most rock stress measuring techniques has been dependent
© 1985 Elsevier Science Publishers B.V.
192 upon the measurement of deformation from which rock stresses are computed with the aid of the relevant formulae. Various stress measurement techniques for a wide range of rock types are given in Table 1. A large number of instruments have been developed, some of which have enjoyed considerable success and popularity in m a n y countries. Apart from the 'soft inclusions' of absolute stress measurement techniques, monitoring of stress changes in longwall mining has been carried out by means of 'hard inclusion' borehole stressmeters. The former has a rigidity less than that of the host rock, and therefore deforms under the field pressure, whereas the latter has an elasticity modulus greater than at least twice that of the host rock.
TECHNIQUES FOR MEASUREMENT OF ABSOLUTE STRESS OF IN SITU ROCK Stress relieving method In principle, a borehole under rock pressure is 'stress-relieved' by drilling a larger concentric hole (i.e. overcoring operation) [1]. The deformation of the inner hole, which is a function of the field stress, is measured to obtain the original state of stress within the rock. The borehole deformation gauges measure changes in length of one or more diameters of a borehole in which they are installed. They are of the soft inclusion type, and therefore, a change in deformation of the borehole will cause a change in the instrument itself. These gauges possess a piston mechanism which contacts with the wall of the borehole, and resultant deformation is sensed in terms of strain. In order to calculate absolute stress both the elastic modulus and Poisson's ratio of the host rock requires to be known. The borehole deformation gauges developed by the U.S. Bureau of Mines. are probably the most popular types [2]. The installation proce-
dure involves a 38 m m diameter borehole which will expand when it is overcored concentrically by a 150 m m diameter borehote. The strain measurements are taken regularly until the gauge is completely overcored. If the diametrical changes of the borehole in three directions are measured, the principal stresses in a plane perpendicular to the borehole axis can be determined. Application of this gauge to longwall mining requires careful consideration of the following points: (a) Drilling operations in Coal Measures rocks, particularly in coal, need special expertise. Overcoring operations in such rocks should be performed with great care, otherwise the entire work will have to be repeated perhaps a n u m b e r of times. (b) The overcored portion of the borehole should be removed from the test site in order to determine the elastic properties of the coal. (c) Unlike hard rock, the inner wall of a borehole drilled in coal has a rough surface and is easily disturbed during instrumentation, therefore an accurate measurement of the changes in the borehole diameters is difficult to obtain. (d) Intrinsically safe electrical equipment should be employed. (e) Existence of water in the borehole and its influence on strain readings should be taken into account. Another method involving overcoring is to make measurements of the changes in diametrical deformation on the bottom surface of a borehole. Instruments utilised in this method are known as "strain cells". Strain gauges are bonded to the flattened surface and overcoring is then carried out by continuing the borehole [3]. The "Doorstopper" has become the most popular gauge for this purpose. It has a rectangular strain rosette containing the three strain gauges, arranged at certain angles, bonded to the rock by araldite. Another cell known as the " C S I R Triaxial Cell" contains three strain gauge rosettes directly bonded
TABLE1 Insitustress measurementtechniques
Stress Measurement
Measuring Principle
Procedure
Stress relieving
Measurement of changes in the diameter(s) of a borehole (- Borehole Deformation gauges
Overcore the gauge w i t h i n a central borehole
U,S,B,M * Deformation
Overcore a cy]inder with
L.N.E.C. * gauge
Measu~abl~ Stress [omDoneo:~
Name of Instrument
Transducer
g
s t r a i n gauges
Bonded s t r a i n qauges
Bonded s t r a i n gauge rosettes
Maihak * v i b r a t i n g wire cel]
X
S t r a i n gauges attached to canti]evers or rings
CSI,R. Mark [ and II
I: e~ectrica~ cos strai~ ; I : L.V.D.T.
Sibek's c e l l
s t r a i n gauges
X
U n i v e r s i t y of Liege c e l l
L.V,D,T
X
Four-comp deformation g a u g e
s t r a i n gauges
Photo e l a s t i c soft i n c l u s i o n
s t r a i n gauges
cell
Measurement of s t r a i n s at the bottom face of a borehole (: borehoie strain cells)
Ye~~gS?
! V i b r a t i n q ~ire transducer
Gris~old's c e ! l
Dvercoring operation is carried out
X fYear 1961)
Overcore a rosette gauge bonded on the bottom of a hole
"Ooorstopper"
Overcore a borehole w i t h s t r a i n gauges on i t s w a l l s
C.S.I.R, * Triaxial cell
*
bonded s t r a i n gauge rosette
bonded s t r a l n s t r a i n gauges
',ature of 3tress ~ea~Jemert
~bso!ute
Author: "anufacturer
I Country: Year
Application to cnal m i n i n q
Remd~kS
Coal p i l l a r s
currently available
relative
Nerrii, Ceter~on
U.S.A, (Ig61]
gocha and $ i l v e r ~ o (L,NE,C.)
Portugal, (1974)
Maihak Company
Griswold
LeemaP CCSI~,R.)
Sibek
(!960)
U.S,A
(1962-63)
Not aDDlica~le
Out of date
S. Africa (1960)
Coal pillars, not reliable
Out of date
Czechoslovakia
rib pillars
~1960)
France (1975)
Crouch Fairhurst
I2.S.A (1967~
Riley Goodman Nolting
C u r r e n t l y used. A r a l d i t e is used in the borehole
W', Germany
Bonnechere
('J.S B.!I)
Soft inclusion
and face
Out of date
Variation of U S.B,M. tria~ial
'
cell
U.S,A (1976-7)
Leeman
S. A f r i c a
app]icable to
very Dopu:ar
(C.S.[R,)
(197])
Coal Measures rock
in manycountries
Leeman (C.S,[.R,)
S. A f r i c a (1971)
Overcoring is difficult in coal
Determination of complete stress tensor i n a s i n g l e boreho]e
S t r a i n s at the bottom of a
t borehole
Photoelastic biaxial cell
W,N.I/~,I.
strain cell
Change in magnetic I permeability with stress
Overcore a
rectangular. solid "
gbservation
Overcore a
of photoelastic stress fringes
hard inclusion with down hole polariscope
Strain gauges encapsulated i n an epoxy resin cylinder
P~rtial
relieving of stress
Photoelasticity
Overcoring operation
Host's
cell
Bonded strain gauges
Magnetost,'iction
C.S,I.R.O. * hollow inclusion ceil
gauge body
Strain gauges bonded to the
Photoelastic glass stressmeter
Photoelastic s t r e s sfringes
Soft i n c l u s i o n cell
Electrical resistance s t r a i n gauges
X
L.N.E.C, Flat-jack
Measurement of displacement c~used by a s l o t cut wall
U.S.B,M. flat-jack
Electrical resistance strain gauges
Cylindrical Jack
Hydraulic oil within a core
CStress compensation)
Measurement of displacement of s t r a i n gauges due to u n d e r c o r i n g
Undercore the strain gauge rosette on the surface of a rock face
East inclusion method
Overcore a hard i n c l u s i o n to freeze stresses i n t o it
Borehole deepening method
Pressure-bag method
Three p a i r s of s t r a i n gauges
Soft inclusion gauge
Detection of bending
Monitor the radial deformation of deepening boreheles
Locating t h e rubber pressure bag i n the bottom end of a boreho]e d r i l l e d in a pillar
X
cantilver beams with bonded I
X
Displacement of a h y d r a u l i c fluid
x
[
s t r a i n gauges
The rubber pressure bag
I
l
Hawkes and
Sh)Dody
U.SI,q (1964)
Intrinsically safe for use in coal minin~
Boreho]e should not exceed 5 m
U.S.S.R. (1965)
I t was used in lengwall Mining
Modified versions are bein 9 used
The f i r s t r e l i a b l e gauge employed
Sweden (]958)
Wet 3ndOt n! c Ki '^alton
Roberts and
Australia (1976)
U.K. (196d-5)
Hawke~
Applicable to longwall
mining
Observation at fringes are d i f f i c u l t in deep boreholes
Blackwood
Australia (]976)
Rib p i l l a r s of 3ongwall ~ane]s
Similar to LNEC gauge developed by Rocha
Rocha, Lopes and Silva (L.~.E C,)
Portugal {1965)
Simply and
No g r o u t i n g Semi-circle slots. Reusable at ~ncreasinq depths of same slot
Panek (U.S,B.~.)
U.S.A (1961)
Jaeger and Cook
U,S.A (1966-9)
DuvalI and Nooker
U.S.A {1974)
Variant of the "Ooorstopper" I n s t a l l a t i o n is d i f f i c u l t supplementary method
~iley Goed~an
U.S.A (~gz7)
Stresses ere measured in the laboratory
Ge La Cruz and Goodman
U.S,A (]970)
Measurements should be performed as close as possible to the bottom
Baar
W. Germany (1960)
and e f f e c t i v e l y used App)icabie to Coal Measures rock
in coal p i l l a r s up to cm a depth 14-15
easily i n s t a l l e d in coal
Supplementary method
~ieJsuremetlt of stress changes by hard inclusions
Borehole inclusion stressmeter
Measuren~nt of changes in t r a n s d u c i n g element due to rock stress variations
Ports' stressmeter
Electrical resistance s t r a i n gauges
Grid stressmeter
S t r a i n gauges in the form of a wheatstone bridge
M,R,D.E. borehole plug
Foil resistance strain gauges
Hast's r i g i d inclusion
Electrical res strain gauges
Glass Stressmeter
Phetoelastic fringes
Spherical rigid inclusion
Three strain gauge rosettes
Vibrating * wire stressmeter
Vibrating wire
U S.B,M. * Encapsulated i borehole hydraulic stressmeter
Hydraulic fracturing i n a bQrehole
Rock fracture
Rock fracture
Core diseing
Measure strain to fracture a borehole with a borehole jack
Measure the velocity of sound in rock
Other stress ~easurement technicques
Correlation between r o c k oroperties and stress
Resistivity, rock noise
Detection of yield pGint Isotope method I
Close hydraulic circuit
X
Pott~
LI.K (1960)
Salamon
U.K (1960)
4ilson
U.K
widely used in coal mlnes
NOt c u r r e n t l y available
(1960) Sweden (1958)
"Poberts and :awkes
U.K. (1964}
The ~vl,S. Geological survey
U,S.A (1968)
Hawkes
U,S.A [1973)
suitable for coal mines
Illumination difficu]ities erise
may
CeTI is qrouted in the borehole
Applicable to coal mines Favourable gauges at present
U.S. Bureau of Mines
U.S.A
F~irnurst Haimson
(U.S.A,) (1965) and (1978)
3bert and Stephenson
U.S.A (1955)
De La Cruz
U,S.A (~978)
Obert and buvall
U.S.A (1957)
~anagawa Hayashi Nakasha
Japan (1976)
Oreyer Borchart
W, Germany (1957)
Borecki Kidyibinski
Poland (1966)
Favourab]e for coat mines
Current stress measuring method
Supplementary methods of stress measurement In rock
t~ I kCD OO
199 along the circumference of a chosen diameter of a borehole. To utilise these two strain cells in longwall mining operation the following problems must be effectively resolved: (1) the difficulty in bonding the gauges to Coal Measures rocks; (2) the difficulty in obtaining an intact core with a diameter up to 40 mm and with a length of 25-50 mm; (3) the unknown behaviour of the field stress at the bottom region of the borehole [4]. The "Doorstopper" requires a shorter length of core than its triaxial counterpart and therefore it is more adequate for use in coal mines.
Partial relieving of stress The stress-relieving method for absolute rock stress measurement is apparently superior to other means of measuring in situ rock stresses. However, partial stress-relief techniques such as flatjacks [5,6] and "undercoring" [7,8] are alternatively used for in situ rock stress measurements (Table 1). The latter is a new technique and is gaining popularity in hard rock conditions.
Rock fracturing method The most popular rock fracturing method is that of "hydraulic fracturing". Its installation procedure is to seal off a section of a vertical borehole and to apply a fluid pressure to the interior of the borehole section until the inner wall fractures [9]. Stresses along a plane normal to the axis of the borehole are thus determined. This method is applicable to hard rocks such as granite and soft rocks such as clay. However, hydrofracturing of coal cannot be regarded as a suitable technique of determining stresses in every circumstance.
Complementary techniques Correlations between the rock properties and the applied stress can be utilised as c o r n -
plementary techniques (Table 1). Among these, seismic methods have been widely employed in order to determine stress distributions within coal pillars.
BOREHOLE I N C L U S I O N STRESSMETERS FOR M O N I T O R I N G STRESS CHANGES IN ROCK Three main features of the borehole stressmeters are: (1) high rigidity; (2) longterm stability; and (3) contact with the wall of the borehole through the entire body of the instrument. Two currently available instruments for measuring stress changes in rock are the IRAD vibrating-wire stressmeter and the U.S. Bureau of Mines hydraulic pressure cell (Table 1). The former is a very sophisticated and expensive device. However, it gives a good picture of stress changes when a longwall advances [10]. It has a hollow cylinder containing a coil, magnet and a yoke used to vibrate a wire stretched diametrically. The stressmeter is preloaded by a plate and wedge mechanism. The wire has a natural frequency. In order to measure changes in the frequency of the wire the coil plucks the wire. A change in the length of the wire due to diametrical change of the borehole is related to a change in rock stress in the direction of the wire. The U.S.B.M. borehole pressure cell is also very popular to monitor stress changes in longwall mining.
FIELD INSTRUMENTATION Field site Encapsulated hydraulic borehole stressmeters were employed in order to monitor stress changes on the H11's longwall advancing face at Cotgrave Colliery, in South Nottinghamshire, U.K. The face was in the Deep Hard Seam at a depth of about 530 m below the
200
surface (Fig. 1.). The coal in the adjacent faces had already been taken. The immediate roof was a thick layered mudstone overlain by a siltstone (Fig. 2). The floor was also of mudstone underlain by thick layers of siltstone and sandstone. Test locations and positions of the stressmeters are shown in Fig. 3. In the test location (A) four cells, namely 2,5,6 and 8 were installed in the rib pillar of the tailgate. In the location (B), 60 m behind the face line, a single stressmeter was installed in the solid coal.
hydraulic circuit (Fig. 4). The cell is inserted into the borehole and orientated in order to measure either vertical or horizontal stress within coal. Following the grouting process the cell is prestressed in order to make an intimate contact between the wall of the borehole and the entire outer surface of the cell body. A n y change in the diameter of the borehole due to a change in rock stress will cause a change in volume of the hydraulic circuit. Consequently, the internal pressure in hydraulic systems will change, thus the displacement of the fluid will indicate a pressure change in the pressure gauge.
Encapsulated hydraulic pressure cell Construction of cell The encapsulated hydraulic pressure cell comprises a copper bladder and a pressure gauge which are connected to form a closed
For construction of the cells a standard procedure was utilised [11]. Materials em-
S3's
S3's
N
Hollygate Lane
"'-:-.....
HII's
"~'~..~..~ --....~
//
"~..
a
d Fig. 1. The location plan of H l l ' s district, Cotgrave Colliery.
2bo 300
201 Mudstone
~ ......- [ , , . . . ' '
..
. . . . . .
,',
•
Mudstone
, , . . . ' . , . .
•
•
Sandstone
; , ' . . ~ •
,
•
,'
. •
• .
Siltstone
.. . . .
., .
. ..
, ,
Grey Mudstone, S i l t s t o n e
-52-Roadway
Deep Hard Coal
Location
Cannel Canneloid Mudstone
ployed for the cells are given below: 1. bladder: soft copper tube, 2.54 cm (i. dia)*; 2. feed pipe: soft copper tube, 7 mm (i. dia) and 6 mm (i. dia); 3. tubing: (a) plastic pipe: 40 m m (i. dia) and 19 mm (i. dia); (b) steel pipe: 4 mm (o. dia)*; 4. oil for the hydraulic circuit; 5. encapsulation material: 'Celfix' 100 (epoxy resin); 6. orientation rod: steel; 7. installation rods; 8 bleed valve; mild steel, 4 mm (i. dia); 9. screwed joints and taps; 10. pressure gauges: ENERPAC: uppei limit 70 MPa.
Siltstone
0
1
2m
* i.dia = inside d i a m e t e r , o.dia = o u t s i d e diameter.
Fig. 2. Section of s t r a t a at H 1 1 ' s district. Location
Location
(A)
(B)
I 1 !
o
x~
o
o
o
o
-1 _
3.60 m
4.26 m
i$
--2J
u6___~
4.87
J
16.5 m , i,
5m l m
"-~6
45 m
1F F
Location
(B)
I
I
E
=I 2 Fig. 3. L a y o u t of b o r e h o l e s at H 1 1 ' s m a i n g a t e a n d tailgate at C o t g r a v e Colliery.
lk
202
~gmm L19mE[~ j
19ram Elbow
,
..~_
' Io\ ......... 'I
Fig. 4. Details of hydraulic pressure cell.
60
I
pI/
3 []
Z L, .,i.o
50
z~
'7 zx
~'--I°
£
/
~I -- ~oc
E]"
0
/
//,
/
&
~~.
o
X A
~/
#
/
/ 0 ,
J
0
, / pj / o
.,o
-"
o
[]
/o
f I lO
20
30 Bladder
40 pressure
50
60
(MPa)
Fig. 5. Laboratory calibration of encapsulated hydraulic bladder using different angles, c~, between the applied stress and the bladder surface.
203 gradually decreased due to malfunctioning and finally dropped to zero. When the face was 110 m ahead of the test site a fall in stresses was observed and the decreases in cell pressure continued for one month. In the test location (B) the cell showed a similar performance to those cells in the location (A). A peak pressure value of 48 M P a was obtained from the gauge. The following conclusions can be drawn from this field study: (1) The peak strata pressure could achieve four times the overburden pressure. (2) Some difficulties were encountered during the installations of the cells. Firstly, great care was necessary during instrumentation so that the cell b o d y and tubing were not damaged during the insertion of the cell assembly into the boreholes; there is a considerable risk of leakages occurring inside the borehole. Secondly, drilling deep straight boreholes is a rather difficult operation in weak Coal Measures strata, especially when the test site is in a fault zone. Thirdly, a
Calibration The cell response to an applied load was calibrated in the laboratory in changing the angle between the direction of the load and the surface of the bladder. A steel block containing an encapsulated bladder was tested between the plates of a compression testing machine. Three separate tests were carried out by altering the angle to 90 ° , 60 ° and 30 ° (Fig. 5). Linearity between the vertical load and the cell response was observed.
RESULTS AND CONCLUSIONS The results from both test sites are shown in Figs. 6 and 7. In the location (A) setting pressures of between 5.5 and 7 M P a were used. The peak pressure values were obtained when the face was about 90 m away from the test location. These maximum pressure readings were 2 to 3.5 times greater than the overburden pressure. The pressure in cell 5
/
N "E
%"
/ /
(D
g-
B No. 6
/1! /
/
c~
No. 2
@
-
=,.m/" ....:
.~r'"
~ "X~. ",.. • XNo. 8
j] 65
Borehole numbers
1O ~
20
30
40
50 60 70 Distance along the gate (m)
80
go
lO0
II0
~zi~IHIzz~zI1ii~iI~1~~r11~zz1II~I~I1H~H~~I1I~1HII/II~II1~1~H~zH~inbye ,I,I,,,111~,,,,,,,,,,'',,H,HIH',,,,,IHH,,HFace outbye
Fig. 6. Stress changes during mining at Hll's tailgate.
advance
~
]6.5m deep
(no. 2)
5m deep
(no. 5)
lm deep
(no, 8)
5mdeep
(no. 5)
1
204
@
1
(D
@
/ o
0
J \ o
o
%
®
8~
~0
=.
\ @
\
~.
o
_o
/
_o
/ @ I
i
I
!
|
o
(ecU4)
ssa~]S
=o o ,4D
r~
205
virtually perfect contact is required between the cell and the wall of the borehole. (3) There was a delay in registering stress in the hydraulic ceils. This is ascribed to the following factors: (a) ineffective grouting in the b0rehole; (b) frequent interruption of mining due to faults in the first 100 m of excavation, which slowed the rate of the face advance. (4) Although there was a delay, after a second attempt of repressurisation of the cells, three of the cells provided satisfactory results when the face was 50 m away from the test location (A). The cell in location (B) also responded to delayed strata stress changes. However, it is always risky to install a cell in such a long borehole in very weak rock conditions. (5) Apart from the difficulties in installing these cells this technique is suitable for monitoring stress changes in longwall mining. Its form is simple and comparatively cheap to manufacture. It is safe for use in underground coal mines. However, it is unsuitable for deep boreholes (greater than 15 m) owing to the difficulty of aligning the cell with respect to the direction of the stress, based on the experience gained at Cotgrave Colliery in weak and faulted ground conditions.
GENERAL CONCLUSIONS The utilisation of various types of stress measuring methods in underground coal mines is possible provided certain restrictions which relate to the different nature of coal with respect to other Coal Measures rocks are borne in mind. In order to use such an instrument in coal mines a number of factors should be taken into account including the following; the assumption dealing with the elastic properties of coal; the calculation of stresses; the orientation of the instrument; influence of the overcoring operation on the stress measurements; and general physical problems of in-
stallation and effective use of the techniques in providing reliable results. It is concluded that most underground coal mine sites permit a reasonable degree of success in performing in situ stress measurements, although careful site planning and choice of stress measurement technique need due consideration.
ACKNOWLEDGEMENT The authors wish to thank Professor Atkinson, Dr. B.N. Whittaker and Dr. R.N. Singh of the Department of Mining Engineering, University of Nottingham, for their help given during the project and also to the management staff and personnel at the N.C.B. Cotgrave Colliery, for the excellent co-operation and assistance with the underground tests.
REFERENCES 1 N. Hast, The Measurement of Rock Pressure in Mines. Sveriges Geologiska Undersokning, Ser C, 560 (1958), 183 pp. 2 L. Obert, R.H. Merrill and T.A. Morgan, Borehole deformation gauge for determining the stress mine rock. U.S. Bur. Mines Rep. Invest., 5978 (1962) 90-95. 3 E.R. Leeman, Borehole rock stress measuring techniques. J. S. African Min. and Metall., 65 (Sept. 1964) 45-81. 4 E.R. Hoskins, An investigation of strain rosette relief methods of measuring rock stress. Int. J. Rock Mech. Min. Sci. 4 (1967) 155-164. 5 L.A. Panek, Measurement of rock pressure with a hydraulic cell. Min. Eng., 13 (1961) 282-285, 6 M. Rocha, J.J.B. Lopes and J.N. Da Silva, A new technique for applying the method of the flatjack in t h e determination of stresses inside rock masses. Proc. First Congr. of Int. Society for Rock Mech., Lisbon, Vol. 2, 1966, pp. 57-65. Publ. L.N.E.C., Lisbon. 7 E.N. Lindner and J.A. Halpern, In situ stresses in North America. Int. J. Rock Mech. Min. Sci., 15 (1978) 183-203. 8 Y.T. Lavie and F. Van Ham, Accuracy of strain measurements by the undercoring method. 3rd Conf. Int. Society for Rock Mech., Denver, Colorado, U.S.A., 1974, pp. 474-480.
206 9 B.C. Haimson, The hydrofracturing stress measuring technique-method and recent field results in U.S.A. Int. J. Rock Mech. Min. Sci., 17 (1980) 81-88. 10 I. Hawkes and W.V. Bailey, Design, develop, fabricate, test and demonstrate permissible low cost cylindrical stress gauges and associated components capable of measuring change of stress as a function
of time in underground coal mines. U.S. Bur. Mines Rep. Invest., (Nov 1973), 105 pp. 11 T.C. Miller and R. Sporcic, Development of hydraulic device for measuring relative pressure changes in coal during mining. U.S. Bur. Mines. Rep. Invest., 6571 (1964), 33 pp.