The cerchar method of pressiometric testing

Mining Science and Technology, 2 (1985) 229-233

229

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE CERCHAR METHOD OF PRESSIOMETRIC TESTING J. Arcamone and J.J. Tritsch Mining Techniques Division, CERCHAR, B.P. No 2, 60550 Verneuil-en-Halatte, Oise (France) (Received November 1, 1984; accepted November 13, 1984)

ABSTRACT

The paper describes the pressiometer and its application for investigating rock deformation modulus as related to in situ elasticity, and the

general condition of rocks in relation to their state of fracture in Coal Measures strata affected by mining operations.

1. I N T R O D U C T I O N

2. ME TH OD

Ever since the 1960s, pressiometric measurements have been regularly applied in civil engineering, particularly in soils, where they are used for assessing the m a x i m u m pressure threshold and deformation modulus of the ground in situ. The technique was pioneered by L. Menard, who designed a variety of appliances which were originally adapted for measurements in soils and subsequently for rock materials. The instrumentation was, however, unsuitable for measuring the high pressures found in fractured strata. Cerchar's Strata Control Department has designed and developed a much less complicated system for use in the Coal Measures. This paper describes the measuring technique and examines the results obtained in the light of two examples where the method has been applied.

The instrument (see Fig. 1) is made up [1] of: - - a Petrometallic rubber sleeve, which acts as the pressiometer; - - a n Enerpac pump for increasing pressure in the sleeve. The pump is connected to a pressure gauge for measuring the pressure applied; - - a volumeter attached to the pump for measuring the volume of oil injected. The sleeve is inserted into a hole drilled in the rock. The device is very easy to handle underground, but is not able to provide a highly accurate measurement of the rock's deformation characteristics because: - - t h e proximity of the bottom of the borehole and micro-asperities on the coal cause secondary effects, - - t h e pressiometer is formed by a single cell and there is no way of eliminating pressure variations at each end of the sleeve. The deformation modulus of the rock

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230

Fig. 1. Pressiometricequipment.

material in situ is obtained by measuring the difference in volume 2XV in relation to the difference in pressure Ap during the pseudoelastic plateau (Fig. 2). Certain precautions must be taken because above a given pressure the rubber sleeve reaches a maximum diameter and errors are apt to occur in the test results. Furthermore, this pressure limitation helps to avoid any modification of the rock material after several loading-unloading cycles have been applied. The following moduli are defined for interpreting the results

ep-

2xP

~x~ v

P+ ~P Vo

. . . . . . . . . . . .

(1) o

and

E=

where I, is the estimated Poisson ratio for the material and V is the volume of the pressiometric cell at the half-way stage during the

2(1 + ,,)Eo

(2)

Fig, 2. P r e s s i o m e t r i c curve.

Pr

P/

231

pseudo-elastic plateau. The Ep modulus is known as the pressiometric modulus and the E modulus is the deformation modulus. Interpretation is made using Ep (pressiometric modulus when pressure is applied) and E v (pressiometric modulus when pressure is decreased). The characteristic reversibility index of the rock material in its elasto-plastic state is considered as a function of the above moduli

(3)

E;/( E; - E; )

The higher the index, the less fractured (in a plastic state) is the rock material.

3. TYPICAL RESULTS

~JS ODC Hole 4 sound pt/laF

'~ 4 000

Hole 3

2 ooo I 5

J 10

Ep ÷

Ep--Fp*

"-,5

H o le

4 :

sound p i l l a r

~3

Hole 3 Hole 2 H o le I : Fractured pillar i J --

5

10

15

Depth Era)

Fig. 4. Change in Reversibility Index as a function of depth.

On the basis of the results obtained on the sound pillar, the coal's deformation modulus is calculated as follows: E = 2(1 + v)Ep = 2(1 + 0.35)700

In order to test the method, calibration tests were carried out at the A u m a n c e colliery [2]. Three pillars with known characteristics were chosen: one pillar was perfectly sound and therefore considered almost totally elastic; another, situated in the vicinity of caving, was visibly fractured and the third was between the two extremes. Figures 3 and 4 show the pressiometric moduli and the reversibility indices obtained on the three pillars. They show that the parameters have very different values according to the condition of the pillars. The results confirm the value of this test as a tool for characterizing the in situ fracture condition of the strata.

~ 6 0DO

J

~6

Hole 2 Hole I : £rsctured pillar I I lm 15 Depth ( m )

Fig. 3. Change in modulus as a function of depth.

= 1,890 MPa. Similar values were found during laboratory trials for calculating the coal's elastic moduli (approx. 2000 MPa). Although the test is a simplified procedure, it does provide a meaningful assessment of the material's in situ elastic modulus. The test was also used at the Aumance colliery for describing the condition of pillars in the mine working as caving advanced, with a view to anticipating pillar design as mining deepens. The investigation was carried out using several drill holes and the results are summarized in Figs. 5 and 6 on the basis of the iso-modulus and iso-reversibility curves. The results reveal that the fractured, plastified zone around the edge of the pillar extends about 5-10 m. The order of magnitude of this value is very similar to that predicted by calculations. This result shows that the calculation model can be used for predictions. A similar investigation was carried out at the coal face of vertical seams in the Lorraine Coal Basin, in order to assess the deconsolidated area extending above the support. In Anna I seam, for example, the investigation

232

~ > ~ ' ~ J ~i / / × / / / / / / / / ,

9 ; ,;~;fl

Scale ~ 10 m

Scale ~ 10 m

Fig. 5. Iso-modulus curves.

Fig. 6. Iso-reversibility curves.

was undertaken on four holes drilled immediately behind the coal face to depths of between 0 and 9 m [1]. According to the

results summarized in Fig. 7, which show the change in the moduli depending on the depth of the drill hole, the coal is deconsolidated up

SO0

1~,--,

~ i

-~

I I

I

I

I

lap= is00

Test No. 3

j

- - ' ~ .._- ,7oo

500

~=

1680

Coo/ face Ep in MPa v =

4400 300

0.3

Ep = 1 8 6 0

I

2OO lee

]

0

,

/f

]f'q

.

P/astif[ed

.

.

.

.

.

,

,

,

height at the coel face ; the day aftec mimng , accocding to expansion measucements

l 600 -~ 500

~-" " u,

|

[

Ep =

~

, I

~

E

14400 ~ p

Test N o . , ;

Ep = ' 11090 890

!

cp

=} 14430

Coal face

4400 300 2O0 100

,

,

Oist;oce bet, weeo the, mo~th of th,e hole ~.d fhe (owef eod of, the,pre;sio..,~teZ 2

3

4,

5

6

7

8

Fig. 7. Moduli obtained during pressiometric tests (Anna 1 seam--Vouters Colliery).

,9

10

11

233

to a height of approximately 1.5 m. This is approximately the value that was estimated on the basis of expansion measurements.

solidated or fractured or plastified zones and elastic zones. The instrument has the advantage of being simple to use and easy to handle in the underground environment.

4. C O N C L U S I O N REFERENCES The pressiometer used for the tests has two practical applications: --assessing the deformation modulus of the material, which, provided certain precautions are taken, is assimilated to the in situ elasticity modulus, --determining the condition of the rock material by distinguishing between decon-

1 J. Arcamone, R. Poirot and R. Schwartzmann. Pressiometry used in mining investigations (in French). Paper presented at the International Symposium on In Situ Testing (Essais en Place), Paris, 1983. 2 J.J. Tritsch. Results of the pressiometric measurements undertaken in Panel IV at the Aumance colliery (in French). Cerchar Working Report to the Aumance Colliery, February 1984.