ARMA 13-241

Effect of confining pressure on permeability during deformation andfailure of several rocks under compression
Alam A.K.M. B., Niioka M., Fujii Y.
Rock Mechanics Laboratory, Faculty of Engineering, Hokkaido University, Japan
Copyright 2013 ARMA, American Rock Mechanics Association
thThis paper was prepared for presentation at the 47 US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 23-26
June 2013.
This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review ofthe paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, ormembers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMAis prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. Theabstract must contain conspicuous acknowledgement of where and by whom the paper was presented.

ABSTRACT: Triaxial test was carried out under confining pressure of 1-15 MPa at 22°C by using an ultra compact triaxial cell on
Shikotsu welded tuff, Kimachi sandstone and Inada granite. The permeability was measured by using constant flow or transientpulse method. In case of tuff with increasing stress, permeability decreased to residual strength state. In case of sandstone
permeability began to decrease, reached minima before peak stress, began to increase again and reached almost stable value underresidual strength state; The permeability from consolidation to residual strength state increased under 1 MPa, was almost the samevalues under 5-10 MPa with some exceptions, it decreased under 11-15 MPa. For granite, the permeability and stress relationshipwas the same as sandstone; The permeability ratio of consolidation to residual strength decreased with increasing confiningpressure up to 9 MPa, afterward the permeability began to increase under 11-15 MPa. From the experimental results, sealability canbe improved in EdZ for all the rocks. For tuff improvement of sealability can also be expected in EDZ; for sandstone it isachievable if enough support pressure is applied but for granite it cannot be expected.

1. INTRODUCTION
Around underground excavations, there occur theExcavation disturbed Zone (EdZ) and Excavation
Damaged Zone (EDZ) due to excavation activities [1].
The porosity and permeability may increase or decreasei.e., sealabiliy may deteriorate or improve in these zones[1, 2]. Rock, before or around peak stress under triaxial
compression can be recognized as EdZ or EDZ as shownin Fig. 1. The objective of this study is to clarify theconfining pressure effect on permeability behavior
considering a wide range of rocks i.e., Shikotsu weldedtuff, Kimachi sandstone, and Inada granite as HMprocesses under compression.

2. EXPERIMENT
2.1 Specimen and Sample Preparation
Three types of rock block were considered and theproperties are in Table 1. Firstly, the P-wave velocity of
the rock blocks was measured with 140 kHz sensors toknow the anisotropy, and core boring was carried out inthe direction of the slowest P-wave velocity. Finally, thecore specimens having the diameter of 30 mm and 60mm in length with parallelism of 2/100 were preparedfrom these rock blocks.

(a)

(b)

Fig. 1. EdZ and EDZ around an excavation. (a) Initial state (b)Final (residual strength) state.

Table 1. Rock blocks and properties are shown as "value(number of specimen) ± standard deviation"
Name ofrock

Effective
porosity (%)

UCS (saturated)(MPa)

Shikotsu
weldedtuff

35.6 (2) ± 1.65

13.53 (2) ± 2.74

Kimachisandstone

18.35 (2) ± 1.98

20.53 (2) ± 2.35

Inadagranite

0.63 (2) ± 0.05

then the confining pressure was applied, up to the target
consolidation pressure (Figure 3(b)).

ΔPΔP

180.85 (2) ± 16.93
αln ΔP

Afterward, the specimens were made fully pure water
saturated. Two stainless steel end-pieces, having acentral hole to allow the flow of water through thespecimen were attached with each saturated specimen byvinyl tape, and two cross type strain gauges were glued
to the center of the opposite sides of the specimen tohave strain measurements. Silicone sealant was coatedon the specimen and the end-pieces attached specimen
was jacketed with heat shrinkable tube. Then the sample
was kept in water for 24 hours.

t(b)

(a)

2.2 Experimental Setup
For the triaxial experiment, an ultra compact triaxial celland two stainless steel attachments (upper and lower)
having the facilities of water flow as well as porepressure measurements were used for the experiment(Fig. 2 (a)). The constant flow of water and porepressure measurement within the end-pieces attachedsample were maintained by using a syringe pump which
was connected to the lower attachment. In constant flow
method, the upper attachment was open to theatmosphere for drainage of water (Fig. 2 (a)). For
transient pulse method, the upper attachment wasconnected to an accumulator (Fig. 2 (c)) and the syringe
pump acted as another accumulator for downstream.
Axial stress was applied by a loading frame whereas theconfining pressure was maintained by a double ballplunger pump with a relief valve.

2.3 Experimental Procedure
The saturated sample was inserted into the triaxial cell.
Then the upper and lower attachments were attachedwith it. Afterward the axial stress and confining pressure
were introduced.
In the triaxial test, confining pressure (Pc) was used
to make σ2 = σ3 and the axial stress (σ A) acted as σ1(Figure 3(a)). In consolidation state, the axial stress
was equal to the confining pressure, by making the stresssituation as σ1 = σ2 = σ3. To reach the desiredconsolidation pressure the axial stress was increased first,tΔPαln ΔPt(c)t(d)

Fig. 2. Experimental setup. (a) Ultra compact triaxial cell andaccessories for constant flow method (b) Basis forpermeability measurement in constant flow method. (c)Schematic diagram for transient pulse method (d) α- gradientof time-ln ΔP in transient pulse method.

After reaching the target consolidation pressure, it waskept in this condition for 24 hours at 22°C temperatureby using air conditioner. On the consolidated samples,
constant strain rate controlled compression wasintroduced at a rate of 10-5 s-1 (0.036 mm/min) until theaxial strain reached to 10% for tuff and 7% for sandstone
and granite.

hydraulic system of upstream and downstream (pa/m3),
A: cross-sectional area (m2).
σA=P c
σA=PC

3. RESULTS
3.1 Shikotsu welded tuff
In case of tuff, with increasing stress permeability
decreased having slight disturbance around the peak load
due to rapid volumetric change (Fig. 4).
Before compression (at the end of 24 hrs. consolidation),
permeability under 15 MPa was the lowest (Fig. 5(a)).
In residual strength state, permeability under 1 MPa was1

where q: flow rate (m3/s), µ: fluid viscosity (pa∙s), A:
cross sectional area (m2) and dP/dL: pressure gradient(Pa/m).
The permeability of Kimachi sandstone and Inada
granite was measured by the transient pulse method. The
measurement was conducted at a stroke interval of 0.2
mm up to the peak load and 0.6 mm after peak load. In
this case, pore pressure of 1 MPa was maintained in thewhole system during consolidation and compression. At
the point of permeability measurements, the valve which
was connected to the accumulator was closed (Fig. (c))
and 0.5MPa pore pressure was applied in thedownstream and waited for 30 min. From the gradient oftime-ln ΔP (Fig. 2(d), permeability was calculated byusing the following equation considering theapproximate solution of Brace [3].

KL(G1 G2 ) A
2Where K: permeability (m ), µ: viscosity (pa∙s), L:
length of the specimen (0.06 m), : gradient of thetime-ln P curve (s-1), G1 and G2: stiffness of the30

10-14

2010-1510-161000123456s-78910o (%)

Fig. 4. Stress and permeability relation of Shikotsu welded tuff.

larger. No apparent confining pressure dependency wasobserved for other conditions (Fig. 5(b)). The
permeability ratio of consolidation to residual strengthstate, showed a smaller decrease for 1 MPa and for otherconfining pressures the decrease magnitude was almostsame (Fig. 5(c)).
After compression
10-13Consolidation
10-13
10-14

10-15

10-162AdPdL

5 MPa
1 MPa

10-13

Permeability ( m )KqPermeability (m 2)

Constant flow of pure water at a rate of 0.3 ml/min wasmaintained in the axial direction by using the syringe
pump during the period of the consolidation andcompression in constant flow method for Shikotsu
welded tuff. The permeability K (m2) was calculated bythe following equation.

StressPermeability (m2 )2.4 Permeability Measurement

15 MPa10 MPa● ● Permeability

Differential stress (MPa)Fig. 3. The triaxial test and the experimental procedure. (a)Thetriaxial text (b) Procedure to reach the target consolidationpressure and compression.

10-1410-15
10-160051015
Effective confining pressue (MPa)(a)

Permeability incresed or decreased (%)

(b)
(a)

51015

Effective confining pressure (MPa)Consolidation to After compression
300(b)

200
100
0-100

0

51015

Effective confining pressure (MPa)(c)
Fig. 5. Permeability after consolidation (a), after compression(b) and it’s changes (c) of Shikotsu welded tuff.

6010-1740-181020-191024680Stresss - o (%)

Minimum
10-16

10-17

10-17

10-20

10-21051015

After compression

2Permeability ( m )10-16-1710-18

1010-1910-20051015

Effective confining pressure (MPa)

(c)

2-20

10-21051015

Effective confining pressure (MPa )Permeability incresed or decreased (%)

(a)

10100

10-210246

Minimum

10-18
10-19
10-20
510

After compression
10-16200051015

Effective confining pressure (MPa)

(d)

10-19
10-20
10-21051015

Effective confining pressure (MPa)(b)Consolidation to Afer compression

6000

10-17

4000

10-18

2000

10-19
10-20
051015

Effective confining pressure (MPa)Fig. 7. Permeability after consolidation (a), at minimum (b),after compression (c) and it’s changes (d) of Kimachi
sandstone.

10-1815(a)300

010-17

Effective confining pressure (MPa)Consolidation to Afer compression

100

0

10-16

10-17

10-210(b)-100200-20Consolidation

10-19

Efective confining pressure (MPa)

10-19

10-16

10-18

10300

Before compression, the permeability decreased
with increasing confining pressure (Fig. 9(a)). The
minima value of permeability decreased withincreasing confining pressure (Fig. 9(b)). In residualPermeability (m )

10-19

10-18

Fig. 8. Stress and permeability relation of Inada granite

210-18400s - o (%)

Permeability ( m )2Permeability (m )Permeability (m2)Consolidation
10-16

101,3 MPa

-17

10-22

Fig. 6. Stress and permeability relation of Kimachi sandstone.

Before compression (at the end of 24 hrs. consolidation),the permeability under confining pressure of ≥ 7 MPa
was slightly smaller than under confining pressure of ≤ 5MPa (Fig. 7(a)). After 7 MPa the minima was smaller
than under confining pressure of ≤ 5 MPa (Fig. 7(b)). In
residual strength state, with increasing confiningpressure permeability decreased (Fig. 7(c)).

9,11 MPa

10-1620● ● Permeability 13,15 MPa 5,7 MPa

Permeability (m )Permeability (m2)

10-16

In case of granite, with increasing stress, permeability
began to decrease and reached to minima before peakstress, afterward it began to increase as sandstone. Under
residual strength state, it was almost at a stable value(Fig. 8).

Permeability incresed or decreased (%)

StressDifferential stress (MPa)13,15 MPa 5,7 MPa
9,11 MPa 1,3 MPa● ● Permeability

3.3 Inada Granite

Differential stress (MPa)In case of sandstone, with increasing stress, permeability
began to decrease and reached to minima before peakstress. Afterward it began to increase. Under residual
strength state, the permeability reached almost at a stablevalue (Fig. 6).

The permeability ratio of consolidation to residual
strength state suggests that permeability increased under
1 MPa, showed almost the same value under 5-10 MPa
with some exceptions, under 11-15 MPa it decreased(Fig. 7(d)).

Permeability (m 2)

3.2 Kimachi Sandstone

005

1015

Effective confining pressure (MPa)(d)(c)
Fig. 9. Permeability after consolidation (a), at minimum (b),after compression (c) and it’s changes (d) of Inada granite.

strength state, permeability decreased till confiningpressure of ≥ 9 MPa, and afterward it increased
again (Fig. 9(c)).
The permeability of consolidation to residual strengthstate decreased with increasing confining pressure up to9 MPa. Afterward it began to increase under confiningpressure of 11-15 MPa (Fig. 9(d)).

4. DISCUSSION
In case of of Shikotsu welded tuff, continuous decreaseof permeability was observed with increasing stress. The
reasons behind it would be the pore collapse andcrushing of grains by compression as the rock is very
porous and weak.
The decrease of permeability before peak stress wasobserved for Kimachi sandstone and Inada granite might
be because of microcrack closure, those were not parallelto the loading axis (Fig. 10). The increase ofpermeability after the minima was obviously due toinitiation and growth of microcracks in the loading(water flow) direction.
Plastic deformation of clay mineral could be one of themechanisms of the confining pressure dependant
permeability for Kimachi sandstone. On the other hand,elastic deformation would be one of the mechanisms for
Inada granite. Sub rupture planes which might appear
due to the rough and hard main rupture plane would be
the reason of increased permeability in high confiningpressures for Inada granite.

in the residual strength state, permeability underconfining pressure of 1 MPa was larger and no apparentconfining pressure dependency was observed for otherconditions but decreased for all cases.
In the case of sandstone, with increasing stress,
permeability began to decrease and reached to minima
before peak stress, afterward permeability began toincrease and under residual strength state, permeability
reached almost stable value; in residual strength state,permeability decreased with increasing confiningpressure. The permeability from consolidation toresidual strength state increased, under confiningpressure of 1 MPa, showed almost the same under 5-10
MPa with some exceptions, and it decreased under 11-15
MPa.
For granite, the permeability and stress relationship was
the same as sandstone but in residual strength state,permeability decreased till confining pressure of ≥ 9MPa, afterward it increased. The permeability ratio ofconsolidation to residual strength decreased withincreasing confining pressure up to 9 MPa, afterward it
began to increase under confining pressure of 11-15
MPa.
From the experimental results, considering stressredistribution, it can be stated that sealability can beimproved in EdZ for all the rocks. For tuff, improvementof the sealability can be expected in EDZ; for sandstone,
sealability improvement is also expected in EDZ if
enough support pressure is applied, but for granite,sealability improvement in EDZ cannot be expected.
These findings could contribute in rational design for
construction and maintenance of tunnels and waste
disposal repositories, although the authors recognize that
rupture planes of a rock specimen are different fromnatural fractures and/or joints in rock mass and further
considerations are required.
The CT imaging and microstructural analysis of thespecimens after axial compression, already have beencarried out. The results of the analyses as well as thoseof experiments under 80°C will be published in nearfuture.

Fig. 10. Permeability decreased before peak stress due toclosure of microcracks which are not parallel to the loading
axis.

ACKNOWLEDGEMENT
A part of this work was supported by KAKENHI(22560804).

5. CONCLUSION
The experimental results under 22°C can be summarized
as follows.
In the case of tuff, with increasing stress, permeability
decreased with slight disturbance around the peak load;

REFERENCES
1.

Tsang, C.-F., F. Bernier, C. Davies. 2005.
Geohydromechanical processes in the excavation
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waste disposal. International Journal of RockMechanics & Mining Sciences 42, 109-125.
2.

Fujii Y., Y. Ishijima, Y. Ichihara, T. Kiyama. S.
Kumakura, M. Takada, T. Sugawara, T. Narita, J.
Kodama, M. Sawada, E. Nakata. 2011. Mechanicalproperties of abandoned and closed roadways in theKushiro Coal Mine, Japan. International Journal ofRock Mechanics & Mining Sciences 48, 585-596.

3.

Brace W. F., J. B. Walsh, W. T. Frangos. 1968.
Permeability of granite under high pressure. Journal ofGeophysical Research 73, 2225- 2236.