Download Effect of acidic water on physico-mechanical behaviour of rock

Indian Journal of Engineering & Materials Sciences
Vol. 6, April 1999, pp. 66-72
Effect of acidic water on physico-mechanical behaviour of rock
TN Singh, S K Singh, A Mishra, P K Singh & V K Singh
Department of Mining Engineering, Institute of Technology, Banaras Hindu University,
Varanasi 221 005, India
Received 22 September 1998; accepted 28 Janllary 1999
The ph ys ico-mechanical properties of rocks are important input parameters for designing of mining and civil engineering
construction projects. The presence of acidic water in excavated areas ·pose serious problem and makes the excavation
process more difficult . In the present study, sandstone rock specimens were tested in the laboratory in dry, wet and acidic
environment. The uniaxial compressive, tensile, shear strength, cohesion and angle of internal fricti on were determined in
acidic water conditions. This study reveals that the acidic water changes the behaviour and strength of rock. The strength
decreases linearly from neutral pH to acidic ·condition. The angle of internal friction increases with decreasing pH values.
The physico-mechanical properties of rock plays
significant role in planning and designing of any
mining and civil engineering constructional works,
which are the basis of economic evaluation of
resource recovery and construction industries .
Rock properties are important to exploratory
drilling, drilling and blasting, cru shing, hauling,
production and communition for man y reasons. The
state of the art about the strength propert ies related to
acidic environment has not been reviewed properly in
recent years . The presence of acidi c water at mine
fac es in Indian geo-mining conditions very often pose
time consuming interruptions in the excavation
process. If the water get acidic, it affect s most of the
mining operations and the efficiency of operation gets
retarded in many folds . The drilling rate is also
affected. Many of the penetrated strata are chemically
sensitive leading to potential drill bit failures .
In Indian coal industry, the acid mine drainage
poses serious problem and adversely affects the
durability of machines and hamper the productivity.
But very little. work has been done particularly in
Ind ia to know the impact of acidic water on the
physico-mechanical behaviour of rock mass. This
work involves not only fluid flow network and
fracture diffusion but also the liquid rock interactions.
Rock can be generally identified as an aggregate
of minerals. These particles may be held together by
the binding material, i.e., matrix , at the grain
boundaries and by interlocking of grains. The grain
size affects the strength of rocks. It was found that as
the grain size .increases, specimen is expected to be
weaker in terms of load per unit area at breakage.
Geological discontinuities like fissures, joints and
fault zones are prone to intrusion by surface water
and/or hot acidic solution from springs. Hence,
chemical alteration in such zones is likely to take
place. The chemical action of acidic water whi ch
enters in the rock from the surface is referred to as
chemical weathering. Hot water intruding the voids
in rock, normally experienced at great depth, is said to
bring about hydrothermal alteration. The apparent
structure of rock depends on the combined 10caJ
strength of the matrix and the contact area among the
grams.
l
Boozer and Hiller reported that tbe deformational
behaviour of Indiana limestone and Navajo sandstone
are affected by fluids which are strongly absorbed on
the surface of the grains of these rocks. ll1e
predominant effect of these fluid s was observed, as
decrease in the ultimate strength of sandstone and a
decrease in the yield stress in the case of Indiana
limestone, which deformed in ducti le manner. In
certain cases, it was observed that the type of failure
of Indiana limestone was ductile when the specimen
was saturated with an inert liquid and brittle when the
limestone was saturated with Oleic acid.
The effect of moisture content on the uniaxial
compressive strength of quartzitic sandstone is shown
in Fig. 1. The uniaxial compressive strength is almost
inversely proportional to the surface tension of
different acidic liquids with which the specimen is
2
saturated . Fig. 2 shows the effect of different liquids
on the compressive strength of sandstone 3 . This figure
SINGH et al.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS
40
1Itt..... r PZOs
CI
CL
%
o
Oodtcy! a ... oni". thloridt
6
Sod h,. tarbonate
o
AI".iniu. chlorid.
67
35
~
g.
.•
C
OJ
•
a.•
.~
II
II
30
e0
c;"
pH vclue
Fig. 2-Uniaxial compressive strength versus pH value of
immersion fluids forsandstone 3
)(
,1
.2
c
25
~
20L-__J -__ __
- 0.4 -0.2
.0.2
0.0
~
~~
__~__- L__~
.0.4
.0.6
Moisture content. % by wei ght
Fi g. I -Uniaxia~ compressive strength versus moisture content for
Quartzitic sandstone 2
also shows the importance of pH on the compressi'l(e
strength of the silica rock.
Two principal theories have been proposed to
explairl the mechanism by which the liquids affect
the compressive strength of various rocks. Rehbinder
4
et al. proposed that the strength is altered by a
change in the surface-free energy of the developing
crack or fracture due to absorption of the
5
Iiquid.Westwood proposed that the liquid alters
dislocation density and mobility around the cracktip
as a function of the zeta potential of the liquid-rock
system and thus inhibits or enhances crack
propagation. There is still a controversy about the
merics of the two mechanisms.
The most important external factor affecting shear
strength is the magnitude of the effective normal
stress (G,J acting across the weak plane filled with
water having different pH value. This value of Gil may
lie in the range of 0 .1 - 2.0 MN/m2 (1 - 20 kg/cm 2) for
most of the rocks encountered in solving engineering
problems. But for tectonophysics studies involving
shear strength of laboratory induced faults, stress
levels may range even up to 100 - 2000 MN/m 2
(1 - 20 kbt The higher value of normal stress means
a higher shear strength.
Fluid pressure influences rock mass stability by
slaking and softening the clays along with the
discontinuity of surfaces, thereby causing a significant
reduction in the shear strength of the joint and by
infilling tension cracks, exerting hydrostatic forces,
along the crack surfaces. Swelling of rock mass,
excavated undrained may produce strain and so
7
causes failure .
Influence of fluid pressure on cohesive and
frictional properties of the discontinuity depends upon
the nature of the filling or cementing materials. In
many hard rocks and sandy materials, these properties
are not significantly altered by water but clays, shales,
mudstones and si milar materials exhibit significant
chan ges with changes in moisture content6,8 .
Q
According to Sharp and Maini , it was found that
seepage of water caused by drainage into an
engi neering excavation had far reaching consequences
in cases where sinking ground water level caused
settlement of stmctures founded on overlying clay
deposits.
The physico-mechanical properties of rock play an
important role il1 any long or short term stability.
Several researchers have done extensive work on the
different aspects of physico-mechanical properties
and
have
discussed
about
the
fracture
mechanism 10.1 I. The effect of acidic water on phys ico
mechanical properties has
not attracted much
attention earlier. Some of the Indian coal mines
where acidic mine water is encountered, the working
is difficult: Keeping in view of these important
aspects, it was decided to study the effect of different
pH values on the physico-mechanical properties of
Chunar sandstone in different geo-environmental
conditions .
The objective of the present work is to establish a
correlation between acidic water and physico
mechanical
properties of rocks and analyse the
fracture mechanism and modes of failure.
68
[NDIAN J. ENG. MATER. SCI., APRIL 1999
Material and Method
Rock core-specimens of Chunar sandstone were
prepared from a selected rock blocks. The NX size
cylindrical rock cores were prepared with the help of
diamond core drilling machine. These core specimens
were smoothened on specially modified lathe Il"!achine
to avoid parallelism defects. Then, these specimens
were dried, weighed and later submerged into acidic
water having different pH values from 2 to 7 till full
saturation. After ten days it was found that the weight
of the saturated specimen became constant, thereby
indicating full saturation . The moisture content of the
specimens were determined with the help of the
following formula :
Moisture content(%)
~-~I
~I
specimens were tested to determine the uniaxial
compressive strength on universal testing machine.
The uniaxial compressive strength of dry specimen
was calculated as 79.83 MPa. The mode of failure
was conical wedged shaped which was due to the end
constraint by the loading platens(Fig 3) .
The other specimens were also tested at pH values
2, 4, 6, and 7 and a graph was plotted between pH
values and the corresponding uniaxial compressive
strength obtained (Fig. 4). The highest va lue was
obtained at pH 7 whereas lowest at pH 2. Fi g. 4
indicates that the strength increases with pH. It was
probably due to increase in open spaces among the
intragranular bindings. The corrosive nature o f the
x 100
where, W, is the weight of the specImen after
saturation, and Wd is the weight of the specimen just
after oven drying at 105 DC for 24 h.
The physico-mechanical properties of Chunar
sand stone spec Imens like uni ax ial compress Ive
strength , Brazilian test for tensile strength , shear
stren gth , triaxi al strength etc were determined at
different pH values in the laboratory to understand the
behaviour of rock and their fracture mechani sm as per
ISRM code.
Results and Discussion
Effect of pH
The
011
unia xia l compressive strength
cylindrical
NX
sI ze
Chunar
sand stone
Fig. 3-Mode of failure of specimen under compre3sion test
70
..
60
c.
~ 50
L::
c;,
c:
~
.
cii 40 .
>
'iii
..a.
on
E
0
'-l
..
30
.~
><
'c
20
:J
y = -0. 1306x3 + 1.4582; + 0.8168x
R2 =t
10
-+-
33.3481
O+---------~--------_+--------~----------r_--------~--------~---------
o
2
3
4
5
pH of the acidic water
Fig. 4--Variation of uni axial compressive strength with pH for
acidic water (Saturated Chunar Sandstone)
6
7
SINGH et al.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS
acidic water with sandstone may also be responsible
for decrease in the uniaxial compressive strength.
The fracture pattern was not changed much due to
change in the pH values. In all the cases, initial
fracture extension started near the center of the
specimens, where the axial stress is greatest and the
lateral constraint is least.
Effect of pH on tensile strength
Brazilian test was conducted to determine the
tensile strength of Chunar sandstone. In this test, a
circular disc is compressed to failure across a
diameter (Fig 5). It was found that at pH 2 the tensile
69
strength was 3'.23- MPa which was lowest while that
of dry specimen it was 5.06 MPa which was highest.
Fig. 6 shows that the tensile strength increases as the
pH value increases though the best fit do not follow
the linear path exactly. The fracture initiated from the
central part of the disc and propagated radially. Two
types of cracks were observed in the failed specimen.
First diametrical crack which was present in each case
but in addition, there were secondary cracks, which
are less symmetrical and complete. It was because of
the bounce in the platen caused by sudden reduction
in resistance to the applied load at failure. It was also
observed that the contact area also influence the
origin of the cracks. The mineral (Si0 2) in this case
reacted with H2 S04 thus decomposing and creating
more intergranular spaces filled with liquid, which
contributes fOT the decrease in strength of the
specimen . It is also possible that liquid influence the
surface energy of the specimens and as new sur face
originates during loading of the specimen, the liquid
which wet the surface of the rock invariably decreases
the surface energy of the rock specimen and hence the
tensiIe strength of the rock.
Eff("ct of pH on shear strength of the rock
Fig. 5-Tensile failure of specimen under diametrical mode of
fracture
The cylindrical Chunar Sandstone specImen was
tested on uni versal testing machine with double shear
4.5,---------------------------------------------------___________________ ~
4 ·
3.5
'"
3 ·
a.
::!:
.r:.
c;,
2.5
c:
~
Ui
.!!
c:
2
·iii
.,
I-
1.5 .
y = -O.009X3 + O.143x2 - 0.436x +" 3.602
R2 = 1
0.5
O~------_+--------~------~--------~--------~------~------~
o
2
3
4
5
6
7
pH of acidic water
Fig. 6--Variation of tensile strength with pH for acidic water
(Saturated Ch unar Sandstone)
70
INDIAN J. ENG . MATER. SCI. , APRIL 1999
case(Fig. 7). The shear strength of the Chunar
sandstone specimen were determined in dry condition
and after submerging in solutions of different pH
values as in the case of compressive and tensile
strength. The shear strength of dry specimen was
calculated and fou nd to be 12.71 MPa, whereas
lowest value of shear strength was fou nd to be 4.6 1
MPa at pH 2. Fig. 8 shows that with increase in the
pH value the shear strength decreases, i.e., as the
acidity increases the shear strength decreases . T.e
mode of fractl!-re was compression-cum-shear mode.
Fig . 7-Modes of fai lu re und er shearing of th e specimen
Tensile stresses were also present for some moment
during the bending of the specimen under the loading.
High shear stress concentration is at the loading edge
and hence stress is not uniform or the .fracture plane.
This also follows the same trend as the case of the
compressive strength and tensile strength.
E ffect of pH on Triax ial Strengt h of the Rock
The Chunar sandstone specimens were tested in
triaxial stress condition to know the shear behaviour
of the rock at different pH va lue. It was observed that
as the pH va lue decreases, the angle of friction
increases . The lowest angl e of friction was 44° at pH
7, whereas 47° at pH value 2. It was probably due to
corrosion and di ssolving of certain grains and
interlocking binding materials because of submersion
of the rock specimen in acidic water. Hence, the
surface of the speci mens pose undulation which
increases the peak and residual shear stresses and
ulti mately increase the resista nce and fr iction ang le. A
graph between pH va lues and angle of internal
friction is given in Fig. 9.
Cohesive strength decreased as the medium gets
aci di c (F ig. 10) . All the specimen tested in triaxial
cell fractured alon g an oblique shear plane (Fig 11)
and thi s shear plane (was oriented at 30°+2° ) with the
12r--------------------------------------------------------------------------,
10 ·
-;;
·8
0.
~
r=.
en
c:
~
6
Vi
.,'"
~
J:
VI
4
2
y = 0.0584,' ·0.5673,' + 2.4103, +
R2 = 1
,.5j
o+---·------~--------~--------~--------~--------~~--------+_------~
o
2
3
4
5
pH of acidic water
Fig. 8- Va ri ation of shea r strength \vith pH for acid ic wa ter
(Saturated Chunar Sandstone)
6
7
71
SINGH el at.: PHYSICO-MECHANICAL BEHAVIOUR OF ROCKS
47.5
y = -O.0333x3 + OAx2 - 1.9667x + 49.6
R2::: 1
47
46.5
'0
c:
0
~
46
·c
....
....0
Q)
c;,
45.5
c:
<t
45
44.5
44
2
0
3
4
5
6
7
pH of acidic water
Fig. 9--Variation of angle of friction with pH of acidic water
(Saturated Chunar Sandstone)
250.00
Y = 9.81x + 137.34
R2 = 1
200.00
E
u
;;:
en
~
150.00
~
0,
c:
~
iii
Q)
>
100.00
VI
Q)
~
0
u
50.00
0.00
+-------t------+-----+------+-----+------If-------l
o
2
3
4
5
pH of acidic water
Fig . I C)- Va riation of cohesive strength wi th pH for acid ic water
(Saturated Chulla r S~lI1d s t o lle)
6
7
72
INDIAN 1. ENG. MATER. SCL, APRIL 1999
highest at pH 7 and lowest at pH value 2.
In all cases, pH values bears a linear refationship
with the physico-mechanical properties bf Chunar
sandstone. This study will be helpful in designing of
mine structure on surface as well as underground
areas in the region where. water gets acidic due to
pyritic contaminations.
Acknowledgement
One of the authors (VKS) acknowledges SIncere
thanks to CSIR for financial assistance.
References
Fig. II-Modes of fracture after triaxial loading
axial load direction. It was also observed that
compressive strength increased as the confining
pressure was increased. The rate of increase in the
strength of Chunar sandstone is higher at initial
confming pressure. As the confining pressure
increases further, the corresponding rate of increase in
strength continuously drops down.
Conclusions
The following important conclusions are drawn
from this study:
I The effect of pH value has significant role in the
physico-mechanical behaviour of the rock.
2 The uniaxial compressive strength and tensile
strength decreases as the medium becomes more
and more acidic.
3 The angle of friction determined by triaxial tests
increases with decreasing pH values.
4 The cohesive strength of Chunar sandstone is
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on the deformation behaviour of rocks subjected to triaxial
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4 Rehbinder P, Schreiner L & Zhijach K, Hardness reducer in
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on Percolation Through Fissured Rock, Stuttgart, IT-F,
1972,15.
10 Frankin J .A, Classification of rock according 10 its mechanical
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Note INIl41/65,1965