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Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. P-369.)
SHEAR BEHAVIOUR OF INFILL JOINT UNDER CNS BOUNDARY CONDITION
A.K.Shrivastava, Assistant Professor, Department of Civil Engineering, DTU, Delhi, Email: [email protected]
K.S.Rao, Professor, Department of Civil Engineering, IIT, Delhi, Email: [email protected]
Ganesh. W. Rathod, Research Scholar, IIT, Delhi, Email: [email protected]
ABSTRACT: The presence of the joints in the rock, changes the behaviour of the rock as these joints are irregularly
distributed and it creates anisotropy in the strength of the rock mass. These joints in the rock make the rock mass weaker,
more deformable and permeable, hence reduces the shear strength. The presence of infill or gouge material in these joints
further reduces the shear strength. Therefore use of appropriate shear strength parameter of rock joints plays an important
role in the design of deep underground openings, stability analysis of rock slopes, socketed piles in rock and anchored rock
slopes. The evaluation of correct shear strength parameters becomes difficult if rock joints are filled with gouge material. In
this paper the influence of infill on the shear behaviour of non planar rock joint under constant normal stiffness (CNS)
boundary conditions is discussed. An automated servo controlled large scale direct shear testing machine on rock, which
has been designed and developed at IIT Delhi [1] is modified to study the affect of infill and their thickness on the shear
behaviour . The tests were performed on the modelled infilled rock joint of plaster of Paris having the asperity 150-150 of
5mm thickness under CNS boundary conditions. A detailed account of shear deformation behaviour of infilled non planar
rock joint under CNS boundary conditions is discussed in this paper.
INTRODUCTION
Rock is heterogeneous and quite often discontinuous, i.e.
rock masses are typically jointed. These rock joints are
mechanical discontinuities of geological origin. These
discontinuities may be in the form of joints, faults, bedding
planes or other recurrent planar fractures, which may be
filled with gouge material. The mechanical and engineering
properties of these joints depend upon whether these joints
are clean or filled, open or close. The presence of infill or
gouge material in these joints reduces the shear strength.
Sources of infill material include products of weathering or
overburden washed into open joint, water conducting in
discontinuities, precipitation of minerals from the ground
water, by-products of weathering adulterations along joint
walls, crushing of parent rock surfaces due to tectonic and
shears displacements, and thin seams deposited during
formation. In general, infill materials may consist of
partially loose to completely loose cohesionless soil or fine
grained clay. Normally fine-grained clays are more
frequently found as fillers and are more troublesome in
terms of structural stability. Thickness of the infill material
varies from micrometers to several meters and it plays an
important role in shear behaviour. In tectonically crushed
zones, the infill thickness may exceed several meters.
LITERATURE REVIEW
Despite their frequent natural occurrence, filled
discontinuities have been studied much less, perhaps
because of the difficulties arising from the preparation of
the sample and testing or due to increased number of
variable parameters. Due to limited research, it is a
common practice to assume the shear strength of an infilled
joint equal to the infill material alone, regardless of its
thickness. [2] reported that the shear strength of the infilled
joint is lower than that of the infill material. Hence this
assumption will lead to unsafe designs.
The shear behavior of infilled joint can be distinguished on
the basis of the interaction between the joint surfaces. The
most obvious effect of a filling material on the shear
behaviour is to separate the discontinuity walls and thereby
reduce rock to rock contact. The shear behaviour of infilled
joints depends on many factors and the following are
probably the most important [3]:
a) Mineralogy of the filling material,
b) Grading or particle size,
c) Over consolidation ratio (for clay filling only),
d) Water content and permeability,
e) Wall roughness,
f) Thickness of infill,
g) Fracturing or crushing of wall rock.
Besides the properties of the constituent materials,
thickness of the infill material is the most important
parameter controlling the shear strength of joint. Several
investigations have reported that thicker the infill, the lower
the joint strength. Filled joint have three thickness ranges,
which are, interlocking, interfering and non-interfering.
Interlocking when the rock surfaces come in contact,
interfering when there is no rock contact but the strength of
the joint is greater than that of filler alone and non-
interfering when joint behaves as the filler itself. The
thickness of infill material does not influence much on
shear behavior of planar joint but for rough joints it
influences appreciably.
Most research results show that when infill thickness is less
981
Shrivastava, Rao & Rathod
than the initial asperity height, infill governs the
development of shear plane until rock-to-rock contact
occurs. Once, the rock came into contact, shear strength is
governed by the asperity angle and the strength of the rock
surface. When the infill thickness is greater than the
asperity height there will be no interaction between the rock
to rock and shear behavior of the joint will be governed by
shear behavior of the infill alone. There is a critical ratio of
thickness to the asperity (t/a) after which the influence of
rock is diminished and shear behavior is controlled by infill
alone. The critical t/a ratio reported as 2 by [4], 1 by [5], 1.6
by [6]. [7] based on shear test on bentonite filled joint under
constant normal stiffness (CNS) and constant normal load
(CNL) condition concluded that critical t/a ration obtained
by CNS test is significantly smaller than that of CNL test.
[8] reported that the strength of clay infill joint increases
with the increase of surface roughness where as sand filled
joint is less affected by surface roughness. However the
overall shear resistance of the joint was reduced as a result
of increasing infill thickness. Various shear strength model
has been proposed to predict the shear behavior of infilled
joint under CNL and CNS boundary condition. [9]
developed a semi empirical method for predicting the shear
strength of infilled joints.
As Compared to research work conducted on infilled joints
under constant normal road (CNL) conditions only limited
research has been carried out under CNS condition in the
past. [5] based on the review of the past work on infilled
joints indicated the importance of CNS testing on the shear
behaviour of the infilled joints. Hence, an experimental
setup is developed to study the shear behavior of infill non
planar joint under CNS boundary conditions.
LABORATORY MODELLING OF ROCK JOINTS
Preparation of Rock Joint Sample
It is difficult to interpret the result of direct shear test on
natural rock because of difficulty in repeatability of the
sample. To overcome this difficulty laboratory samples are
made from plaster of Paris. Plaster of Paris is selected
because of its universal availability and its mouldability
into any shape when mixed with water to produce the
desired asperity. The basic properties of the model material
at 60% of the moisture by weight and 14 days air curing is
determined by performing the test on 38 mm diameter and
76 mm height sample. The average uniaxial compressive
strength ( c) is 11.75 MPa and tangent modulus, Et 50% is
2281 MPa for the cured sample and based on [10]
classification the rock model is classified as EL.
The asperity plates of different angles like 00-00,150-150 and
300-300, have been designed and fabricated to produce
desired asperity in the sample. The plaster of Paris with
60% of the moisture is mixed in the mixing tank for 2
minutes and then the material is poured in the casting
mould which is placed on the vibrating table. Vibrations are
given to the sample for a period of 1 minute and then the
sample is removed from the mould after 45 minutes and
kept for air curing for 14 days before testing.
Selection of Infill Material
The rock joints are seldom clean, it is filled with the
weathered material from the parent rocks or transported
material. The infill material is selected to simulate the field
conditions. In the present work combination of fine sand
and mica dust both passing through 425micron sieve and
plaster of Paris is selected. The selected composition is
plaster of Paris 40%, fine sand 50% and mica dust 10%
mixed together with water 45% by weight of total mass of
the material. The uniaxial compressive strength of the 7
days air cured infill material is 3.47 MPa and direct shear
tests carried on the infill material gave friction angle and
cohesion, 28.80 and 0 respectively.
Preparation of Infill joint
The infill joint with required thickness is created on the
sample with the help of infill mould. The samples are
placed on the mould and tighten at suitable point so that the
required thickness of the infill material is created (Fig. 1 a)
.The infill material is spread over the lower sample and the
asperity plate is put over the infill material and the asperity
plate is compressed from the top with the help of C- clamps
so that the uniform pressure is applied on the sample and
the same asperity is created on the infill material (Fig. 1 b
and c). The upper mould is now placed over the lower
mould with the help of the guide rod and movable screw the
correct placement and thickness of the infill material is
insured .The whole assembly is now compressed from the
top with the help of C- clamp, after 30 minutes the sample
is removed from the mould and kept for air curing for 7
days before testing (Fig. 1 d).
SHEAR BEHAVIOUR OF INFILL JOINTS
To study the shear behaviour, tests on infilled rock joints
were performed using the direct shear apparatus developed
by [1] as shown in Fig. 2 on 150-150 asperity joint having
asperity height 5mm under three initial normal stresses of
0.1, 1.02 and 2.04 MPa for infill thickness 0 and 5mm. The
normal stiffness (k) of the surrounding rock joints is set to
be 0 and 8 kN/mm for CNL and CNS boundary conditions
respectively.[11] conducted direct shear tests on samples
with different asperity under CNL and CNS boundary
conditions and concluded that at low shearing rate i.e. <
0.5mm/min, there is no effect of shearing rate on the peak
shear stress and at shearing rate > 0.5mm/ min, the effect is
to increase the peak shear stress with increasing shearing
rate for both the conditions. Hence, in the present study the
shearing rate is fixed at 0.5mm/min.
The variations in shear stress were continuously recorded
with the shear displacements for infill joint. Test results
(Fig. 3) indicates that at low initial normal stress (Pi) CNL
conditions under predict the peak shear stress but with
increase in the Pi i.e. (Pi) > 0.1 uniaxial compressive
strength
982
Shear behavior of infill joint under CNS boundary conditions
(a)
(b)
(c)
(d)
Fig. 1 Preparation of infill surface: (a) fixing the
adujustable mould to the sample. (b) compressing the infill
material. (c) Lower sample with infill material (d) Air cured
infilled sample with 150-150 asperity.
( c) there is no effect of boundary conditions on the shear
behaviour of infill joints. Peak shear strength of infill joints
are compared with experimental results of [12] on
clean/unfilled joint as shown in Fig. 4.
1
2
3
1. Loading unit 2. Hydraulic power pack with servo valve
3. Data acquisition and controlling unit
Fig. 2 Photograph of Large scale direct shear machine
0
0.5
1
1.5
2
0 5 10 15
Sh
ea
r st
ress
(M
Pa
)
Shear displacement (mm)
CNL Pi=0.10 CNL Pi=1.02 CNL Pi=2.04
CNS Pi=0.10 CNS Pi=1.02 CNS Pi=2.04
Fig. 3 Shear behaviour of 5mm thick infill joint.
0
0.5
1
1.5
2
2.5
3
0 0.2 0.4 0.6 0.8 1
Peak shear stress (MPa)
Thickness/ asperity
CNL 0.10 CNL 1.02
CNL 2.04 CNS 0.10
CNS 1.02 CNS 2.04
Fig. 4 Variation of peak shear stress for clean and infill
joint.
All
Pi in
MPa
Shear behavior of infill joint under CNS boundary conditions
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Shrivastava, Rao & Rathod
It is observed that under both CNL and CNS conditions the
peak shear stress of the infill joint is less than that of the
clean joint. This is because of reduction in the contact
between the joints due to presence of the infill material. The
increase in thickness of the infill material reduces the
influence of joints and increases the influence of infill
material on the shear behaviour. The shear strength
envelope of the infilled joint is as shown in Fig. 5. The
shear strength envelope for infilled joint is curvilinear for
both CNL and CNS condition and there is no effect of
boundary conditions at Pi > 0.1 c. It is due to complete
crushing of asperity and infill materials at Pi > 0.1 c and
joint start behaving like a planar joint, which does not have
effect of boundary conditions on peak shear strength.
Strength envelope for the unfilled/clean joint is also
curvilinear as shown in Fig.6 for both CNL and CNS
conditions and the curvature of the strength envelope
changes with increase in Pi. The curvature of the strength
envelope is same upto low normal stress i.e Pi < 0.1 c and
after that the curvature of the strength envelope is decreased
and approaches more towards the linearity for both CNL
and CNS boundary conditions. But the effect is more
visible in the CNS condition as increase in normal stress on
shearing plane caused complete breaking of the asperity
and the joint starts behaving like a planar joint.
0.0
0.5
1.0
1.5
2.0
0.0 0.5 1.0 1.5 2.0 2.5
Pea
k s
hea
r s
tress
(M
Pa
)
Initial normal stress (MPa)
CNL CNS
Fig.5. Strength envelope for infill joint.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.5 1.0 1.5 2.0 2.5
Peak shear stress (MPa)
Initial normal stress (MPa)
CNS CNL
Fig.6. Strength envelope for unfilled/clean joint.
CONCLUSIONS
The infilled mould is designed and fabricated to create the
infill of required thickness between the samples to study the
shear behaviour of infilled joints. The experimental results
on these samples indicate that the presence of the infill
material significantly reduces the shear strength of the
joints for both CNL and CNS conditions as compared to the
clean/unfilled joints. The strength envelopes for infilled
joints are curvilinear for both CNL and CNS conditions.
There is no effect of boundary conditions on peak shear
strength for Pi > 0.1 c
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