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: aksrivastava@dce.ac.in

K.S.Rao, Professor, Department of Civil Engineering, IIT, Delhi, Email: raoks@civil.iitd.ernet.in

Ganesh. W. Rathod, Research Scholar, IIT, Delhi, Email: ganesh.rathod@yahoo.co.in

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

983

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

REFERENCES

1. Rao, K.S., Shrivastava, A.K. and Singh, Jattinder,

(2009a), Development of an automated large scale

direct shear testing machine for rock, IGC, 238-

244. 2. Kanji, M.A. (1974), Unconventional laboratory

tests for the determination of the shear strength of soil-rock contacts, Proc. 3rd Congr. Int. Soc. Rock Mech., Denver, 2, 241-247.

3. Shrivastava, A.K. and Rao, K.S., (2009). Shear

behaviour of jointed rock: A state of Art, IGC, 245-

249.

4. Phien-wej, N., Shrestha, U.B. and Rantucci, G.

(1990), Effect of infill thickness on shear behaviour

of rock joints, in Proceedings of the International

Symposium on Rock Joints, Loen, Norway, Barton, N. and Stephansson, O. (eds), Balkema, A.A.,

Rotterdam, 289–294.

5. De Toledo, P.E.C. and De Freitas, M.H. (1993),

Laboratory testing and parameters controlling the

shear strength of filled rock joints, Geotechnique,

43, 1–19.

6. Indraratna, B. and Haque, A. (1997), Experimental

study of shear behaviour of rock joints under

constant normal stiffness conditions, Int. J. Rock

Mech. Min. Sci., 34, Paper No. 141.

7. Indraratna, B., Haque, A. and Aziz, N. (1999),

Shear behaviour of idealized joints under constant

normal stiffness, Geotechnique, 40, 2, 189-200.

8. Kutter, H.K. and Rautenberg, A. (1979), The

residual shear strength of filled joints in rock, Proc.

4th Int. Congr. Rock Mech., Montreux, 1, 221-227.

9. Welideniya, H. S. (2005), Laboratory evaluation

and modeling of shear strength of infilled joints

under constant normal stiffness (CNS) conditions,

PhD Thesis, University of Wollongong, Australia.

10. Deere, D.U. and Miller, R.P. (1966), Engineering

classification and index properties of rock,

Technical Report No.AFNL-TR-65-116, Air Force

Weapons Laboratory, New Mexico.

11. Rao, K.S., Shrivastava, A.K. and Singh Jattinder,

(2009b), Universal large scale direct shear testing

machine for rock, INDOROCK, 157-168.

12. Shrivastava, A.K. and Rao, K.S. (2011). Shear

behaviour of non planar rock joints, 14th ARC on

Soil Mechanics and Geotechnical Engineering,

Hong kong, China.

984