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Development of an Automated Large Scale Direct Shear Testing Machine for Rock

238

DEVELOPMENT OF AN AUTOMATED LARGE SCALE DIRECT SHEAR TESTING MACHINE FOR ROCK

K.S. Rao

Professor, Department of Civil Engineering, Indian Institute of Technology, New Delhi, India. E-mail: [email protected] A.K. Shrivastava Lecturer, Department of Civil Engineering, Delhi College of Engineering, Delhi, India. E-mail: [email protected] Jattinder Singh Managing Director, Hydraulic & Engineering Instruments, Delhi, India. E-mail: [email protected]

ABSTRACT: The shear behaviour of planar rock joints can be investigated in the laboratory by using a conventional direct shear apparatus where the normal load is kept constant (CNL) during the shearing process. However, shearing of non planar rock joint where dilation is restricted by the surrounding rock mass does not take place under Constant Normal Load (CNL) but rather under variable normal load where stiffness is constant called Constant Normal Stiffness (CNS) boundary condition. The shear strength of non planar discontinuities calculated under CNS boundary condition will be significantly higher than what is calculated under CNL boundary condition.

In the past most of the researchers have carried out the test under Constant Normal Load (CNL) boundary condition. The equipment developed in the past for CNS boundary condition was either having difficulty in changing the boundary conditions or it was not useful for wide variety of rock joints.

An automated servo controlled large scale direct shear testing machine on rock has been developed and fabricated to find the shear characteristics of rock under constant normal load and Constant normal stiffness boundary conditions at IIT Delhi. The Boundary condition can easily be changed by changing the input data in direct shear software. The Normal and shear load is applied through an electro hydraulic servo actuator unit which works on closed loop principle. The displacements are measured by LVDT’s mounted on the specimen. The data acquisition system has 16 channels, 2 channels for load cell, 6 channels for LVDTs and remaining 8 channels are free for additional input. The data acquisition system converts the mechanical and electrical signals in to the digital data. The output of signal is connected to CPU via cord. The load and deformation values are stored at desired intervals as note pad data. The direct shear software developed is having the facility to collect data and plot online graphs.

Using developed equipment shear tests have been conducted under CNL and CNS boundary conditions, on the modeled rock joint having asperity angle 0°, 15–15°, 15–30°, 30–15°, 30–30° and 30–60° at different normal stress. The test results of only modeled rock joint with asperity angle 15–30°, at different normal stress under both CNL and CNS boundary conditions are discussed in the study due to space constraint. 1. INTRODUCTION

The correct evaluation of shear strength of rock joints plays an important role in the design of excavations in rocks, stability analysis of rock slopes and design of rock socketed piles. The shear behaviour of planar rock joints can be investigated in the laboratory by using a conventional direct shear apparatus where the normal load is kept constant (CNL) during the shearing process. This particular mode of shearing is suitable for situations where the surrounding rock freely allows the joint to shear without restricting the dilation, thereby keeping normal stress constant during shearing process. Shear testing under a Constant Normal Load (CNL) boundary condition is only beneficial for cases such as non-reinforced rock slopes.

However, for non planar discontinuities, shearing results in dilation as one asperity overrides another, and if the surrounding rock mass is unable to deform sufficiently, then an inevitable increase in the normal stress occurs during shearing. Therefore, shearing of rough joints under such circumstances no longer takes place under Constant Normal Load (CNL), but rather under variable normal load where stiffness of the surrounding rock mass plays an important role in the shear behaviour. This particular mode of shearing is called as shearing under Constant Normal Stiffness (CNS) boundary conditions. For deep underground opening or rock anchor-reinforced slopes, shear tests under CNL condition are not appropriate. A more representative behaviour of joints would be achieved if the shear tests were carried out under boundary conditions of Constant Normal Stiffness (CNS).

IGC 2009, Guntur, INDIA


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2. LITERATURE REVIEW

The general criterion to determine the appropriate shear strength of rock joints was a great concerned for many years by researchers and field engineers. In the last decades numerous shear models have been proposed to find out the shear strength of rock joint. The researches in the last decades have improved the understanding of rock joint behavior. The limitations in the past research have given the sight for further improvements. These models available in the literature fail to appropriately determine shear behavior of rock either due to limitations of boundary condition or they are very simple like Patton (1966), or depend on empiricism like JRC models given by Barton (1973), or require estimation of complex input parameter like Ladanyi & Archambault (1970). In past several studies have been made to examine the shear behavior of rough rock joint under CNS conditions like Indrartna et al. (1998, 1999, 2003), Jiang et al. (2004) etc. But these equipments developed have some limitation either due to difficulty in putting the springs of required stiffness between the load cell and the sample or to change the set of the springs according to the normal stiffness of the surrounding rock like Indrartna et al. (1998, 1999, 2003), Johnston et al. (1987). The studies under CNS conditions are available mostly on small scale sample and asperities are triangular with equal angle for both the sides and roughness having some JRC value.

In the present study, a servo control large scale direct shear testing machine is designed and fabricated which is capable of conducting test on large prismatic specimens through friction free rigid platens under constant normal load and constant normal stiffness boundary conditions. The normal stiffness can be feed before the test of the specimen in the computer according to stiffness of the surrounding rock mass. Shear tests have been performed on modeled rock joint by newly developed equipment to study the influence of boundary conditions (CNL and CNS) on the shear behavior of the rock.

3. SERVO CONTROL LARGE SCALE DIRECT SHEAR APPARATUS

A servo control large scale direct shear apparatus is designed and fabricated for testing the rock joint under different boundary shown in Figure 1. The apparatus consists of three main units such as loading unit, hydraulic power pack with servo valve and data acquisition and controlling unit as shown in Figure 2. These units are discussed as below.

Fig. 1: Side View of the Direct Shear Testing Machine

Fig. 2: Servo Control Large Scale Direct Shear Apparatus

Note: All dimensions are in mm.

3.1 Loading Unit

Loading unit (Fig. 3) of apparatus consist of shear box, spherical seating platen, load cell for normal and shear load, displacement measurement, bottom plate and spacer plate, casting mould and asperity plate. The systematic description of each component of the loading unit is as below:

517

340

L.V.D.T.

Ø24

0

177

DISP. SENSOR ±20mm

SI-714 187.04

SYSCON

L.V.D.T.

85

98

45

L. 600xW. 710

485

S

S

SI-714 187.04

DISP. SENSOR ±20mm

SYSCON

LOAD CELL

L. 600xW. 710

5085

Ø85

165

Ø54

1066

1816

NOTE:- *L.=LENGTH *W.=WIDTH *H.=HEIGHT *Ø=DIA

L.V.D.T.

1000KN LOAD CELL

L. 1400

1847 CD

50

L. 2050

19H

. 585

xW. 7

00

Fig. 3: Line Sketch of the Loading Unit of the

Equipment

Loading unit

Power pack with servo valve

Dat

a ac

quis

ition

500 mm

3160

500m

m

500m

m

2450


240

3.1.1 Shear Box

The servo control large scale direct shear apparatus consist of two steel box i.e. upper shear box and lower shear box. The size of shear box is 300 mm × 300 mm × 448 mm, with a gap of 5 mm between the boxes. The needle linear bearing of 5 mm thickness is placed in between the shear boxes to maintain the 5 mm gap between the boxes and avoid any frictional forces during shearing to the sample.

At the centre of the lower shear box a movable circular head of 150 mm diameter is fitted in the piston of 100 mm diameter. An additional hydraulic ram is incorporated for lowering of the specimen inside the shearing box and also for extracting the sample after shearing. Figure 4 shows the sketch of upper and lower shear box.

NEEDLE LINEAR BEARING

385

300 5530

198

300 42.5

385

Ø150

Ø100

26

245 20

3

Fig. 4: Sketch of Upper and Lower Shear Boxes

The head of the piston is so placed that after the placement of the sample it will be flushed with the lower shear box. The shearing assembly of the equipment can be placed in the testing position or sample placement and removal position. The controlled shifting in the position of the shearing assembly can be achieved by pushing the equipment between the two guide bar placed at two ends of the equipment.

The upper shear box is fixed and the joint is sheared by moving the lower box. The upper box can only move in the vertical direction. The lower box is fixed on a rigid base through bearings, which can move only in horizontal direction.

3.1.2 Spherical Seating Platen

The spherical seating platen is provided to apply uniform normal load to the sample and four LVDTs are placed at the marked point on the platen.

3.1.3 Load Cell

The apparatus is fitted with two actuators for normal and shear load. Both the actuators can be loaded either on Strain basis or Stress basis. Holding facility is also provided in both the actuators for keeping the load constant irrespective of any deformation taking place in the sample. The whole assembly that is the shearing actuator and shearing box, slides on the guided rollers after fixing of the sample.

The normal load cell is having the loading capacity 500 kN and displacement capacity of 75 mm. The displacement and application of desired normal load can be done either by manual operation or through the direct shear software. The normal load cell applies almost uniform normal stress on the shear plane.

The shear load is applied through transverse load cell of loading capacity 1000 kN and displacement capacity of 100 mm.

The loading is static (ramp) type in both load and displacement control mode and the rate of static (ramp) is 1 kN/s to 10 kN/s in load control mode and 0.01 mm/min to 10 mm/min in displacement control mode. The load accuracy is ± 0.5% of full load capacity.

3.1.4 Displacement Measurement

Displacements are monitored and measured through LVDTs (linear variation displacement transducers). Six number of LVDTs are used for this purpose. Four LVDTs are used to measure the normal displacement and provide a check on specimen rotation about an axis parallel to the shear zone and perpendicular to the shearing direction. Degree of joint closure and dilation angle can also be obtained from these measurements. Two LVDTs are used to measure the shear displacement. These displacement devices have adequate ranges of travel to accommodate the displacements, ± 20 mm. Sensitivities of these devices are 0.001 mm for both normal displacement and shear displacement.

3.1.5 Bottom Plate and Spacer Plates

Bottom plate of thickness 15 mm is placed on the lower shear box and sample is placed above that plate. This plate can easily move up and down by the hydraulic ram provided and the sample can be removed from the shear box and placed inside the shear box.

The two spacer plate of thickness 143 mm and 108 mm is provided to fill the gap at bottom and top on the shear box respectively. Initially the thickness of the lower and upper sample used for testing was 150 mm but as this required huge mixing of the plaster of paris (model material used for

UPPER SHEAR BOX

LOWER SHEAR BOX


241

the rock joint), and also difficult to maintain the uniform quality throughout the volume of sample. Hence, the thickness of lower and upper sample is reduced to 60 mm to 65 mm and for deciding this thickness the guidelines of ASTM D 5607 (2006) is followed. These spacer plates also insure that asperity of the sample lies in the gap of 5 mm left in between the shear box.

3.1.6 Casting Mould and Asperity Plate

Casting mould of the size 299.5 mm × 299.5 mm × 85 mm is used for casting the sample. The casting mould is open from the top and bottom, the mould is placed on the casting platform.

The asperity plate of equal angle asperity and unequal angle asperity is designed. This is designed to create 5 mm thick equal angle and unequal angle asperity on the sample. Figure 5 shows the detail of one typical 15–30° asperity plate.

18.66

30°15°

?

8.66

5

27.32

299

299

25

20

Fig. 5: Details of 15–30° Asperity Plate

3.2 Hydraulic Power Pack with Servo Valve

Hydraulic power pack supplies required flow and pressure for the actuation of the actuator. It has an oil tank of adequate capacity, double vane type pump powered by a three phase motor. The electro hydraulic servo valve provides actual position control or load control selectable in the control electronics. The operation of the system is under control of a command source signal. This signal is generated in the function generator and is processed at a speed of 10 KHz in the PID controller. Figure 6 shows the sketch of hydraulic power pack with electro hydraulic servo valve.

3.3 Data Acquisition and Controlling Unit

Data is acquired in computer through built in 32 bit PCI bus advanced data acquisition card. It has eight channel system which convert the mechanical and electrical signal into the digital data. Out of sixteen channels, six channels are for LVDTs, two channels are for load cell and remaining channels are free for additional input. The output signal is connected to CPU via cord and the load and deformation

values are stored at desired intervals as note pad data. The software used is having the facility to collect data and plot online graph between load and displacement, and also online display of load and displacement readings.

RETURN LINEFILTER

PRESSURELINE

RELIEF VALVE

SERVO VALVE

PRESSURESWITCH

PRESSURE LINE

PRESSURE LINEFILTER

RELIEF VALVE

TO NORMAL LOAD ACTUATOR TO SHEARING

ACTUATOR

RETURN PIPE

Fig. 6: Hydraulic Power Pack with Electro Hydraulic

Servo Valve

Controlling unit consists of signal conditioning unit and controlling unit. Signal conditioning unit receives the output signal from the various transducers (Load Cells and LVDT’s) and amplifies and process that signal as per the requirement and transfer it to computer through connecting cables where it is accepted by the data acquisition system.

Control is on Stress or Strain basis. It consists of two dedicated servo-controller card that gives the desired processed signal through the two different P.I.D controllers to the servo valve to operate in stress or strain mode. It also sends the signal to computer and accepts the command from the software to operate in desired manner. The parameters like rate of loading, safety limits for load, stiffness of the surrounding rock mass are initially programmed through the software.

In this apparatus constant normal load and constant normal stiffness control conditions are reproduced by an electro hydraulic servo-valve which under the control of an electronic controller controls the application of hydraulic power to a linear actuator to provide the programmed force to the test specimen (Fig. 7). The programmed force keeps changing depending upon the stiffness of the surrounding rock joint entered as a input data in the direct shear software and the dilation and horizontal displacement data collected through sixteen channel data acquisition system. The increased load during the progress of testing is calculated by the following equation:

Pn (t + ? t) = Pn(t) + Kn (Y–Y') (1) Where, Pn (t + ? t) = Normal stress at any time interval t + ? t Pn(t) = Normal stress at any time interval t Kn = Stiffness of the surrounding rock mass Y–Y' = Dilation resisted by the surrounding rock mass.


242

Fig. 7: Working of Electro Hydraulic Servo Actuator

At any time t + ? t this increased load is updated in the software and the increased load is applied to the sample. The CNL boundary condition is obtained by making stiffness of the surrounding rock as zero in the input data of direct shear software.

4. LABORATORY MODELLING OF ROCK JOINTS

4.1 Selection of Model Material for Rock Joints

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, mouldebility into any shape when mix with water and easy to produce the desired asperity. The basic properties of the model material is as shown in Table 1.

4.2 Preparation of Asperities on the Sample

The asperity plates of different angles like 0°, 15–15°, 15–30°, 30–15°, 30–30° and 30–60°, 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 sample is demoulded from the mould after 45 minutes and kept for air curing for 14 days before testing. Figure 8 shows air curing of the sample.

Table 1: Physical and Engineering Properties of the Model Material

S. No. Properties Representative

value

1. Dry density in kN/cu.m 12.34

2. Initial setting time in min 28

3. Final setting time in min 56

4. Unconfined compressive strength of intact cylinder 38 mm dia. and 76 mm height (s ci) after 14 days air curing in MPa

11.75

5. Poisson’s ratio, at 50% of ultimate stress

0.22

6. Tangent modulus, Et50 in MPa 2281

7. Modulus ratio (Et50/s ci) 194

8. Deere-Miller (1966) classification EL


243

Fig. 8: Air Curing of the Samples

5. LABORATORY TESTING OF ROCK JOINTS

To study the shear behavior of the rock joints the tests were conducted under CNL and CNS boundary conditions on the developed and fabricated large scale direct shear testing apparatus on 15–30° asperity joint under three initial normal stress of 0.1 MPa, 0.3 MPa and 1.1 MPa. The normal stiffness of the surrounding rock joints is set to be 8 kN/mm and rate of shearing is maintained as 0.5 mm per min during the test.

5.1 Effect of Boundary Conditions on Strength Parameter

The effect of CNL and CNS boundary conditions on Mohr—Coulomb strength parameter is assessed by plotting the peak shear strength against the corresponding initial normal stress as shown in Figure 9. The test results reflect that peak cohesion under CNS condition is always more than that under CNL condition and there is no appreciable effect of boundary condition on the peak friction angle.

5.2 Effect of Boundary Conditions on Shear Behavior

The shear stress and shear displacement relation for three initial normal stresses under different boundary conditions are shown in Figure 10. It is observed that CNL boundary condition always under estimate the peak shear stress of the joint relative to CNS boundary condition, because normal stress acting on the specimen is increased during shearing as shown in Figure 11.

6. CONCLUSIONS

An automated large scale direct shear testing machine for rock has been developed and fabricated to study the shear behavior of the rock under CNL and CNS boundary conditions.

Fig. 9: Strength Envelop Under Different Boundary

Conditions

Fig. 10: Shear Behavior of the Joint under CNL and CNS

Boundary Conditions


244

Fig. 11: Variation of Normal Stress of the Joint under CNL

and CNS Conditions

The developed machine is used to study the shear behavior of large scale modeled rock joints with asperity of 15°–30°

under both CNL and CNS boundary conditions. The advantages of this apparatus and some of the test results obtained from this apparatus are summarized below: (a) The developed apparatus can easily reproduce the CNL

and CNS boundary conditions, by just changing the input parameter normal stiffness in the direct shear software.

(b) In the present apparatus the stiffness of the surrounding rock mass to simulate different types of rock joint can easily be changed in the direct shear software, in some of the earlier developed equipment for different stiffness of the surrounding rock mass, different stiffness spring has to be inserted. Hence for different rock joints different spring set has to be prepared, but this is not the case with the developed apparatus.

(c) This apparatus can work under stress or strain controlled mode. Load and displacement graphs and readings are online displayed.

(d) A series of tests conducted on the jointed model rock with 15–30° asperity indicate that CNS conditions yield a higher peak shear stress than that under CNL condition for the same initial normal stress.

(e) There is no effect of boundary condition on the peak friction angle but the peak cohesion is more for CNL boundary conditions than CNS.

REFERENCES

ASTM (2006). “Standard Test Method for Performing Laboratory Direct Shear Strength Tests of Rock Specimen under Constant Normal Force”, ASTM Intrnational, D 5607–02, 1–12.

Barton, N. (1973). “Review of a New Shear Strength Criterion for Rock Joints”, Engineering Geology, 7, 287–332.

Indraratna, B., Haque, A. and Aziz, N. (1998). “Laboratory Modelling of Shear Behaviour of Soft Joints Under Constant Normal Stiffness Condition”, Geotechnical and Geological Engineering, 16, 17–44.

Indraratna, B., Haque, A. and Aziz, N. (1999). “Shear Behaviour of Idealized Joints under Constant Normal Stiffness”, Geotechnique, 40, 2, 189–200.

Indraratna, B. and Haque, A. (2000). Shear Behaviour of Rock Joints, Rotterdam: Balkema.

Indraratna, B. and Welideniya, H.S. (2003). “Shear Behaviour of Graphite Infilled Joints Based on Constant Normal Stiffness (CNS) Test Conditions”, Proc. 10 th Congr., Int. Soc. Rock Mech.—Technology Roadmap for Rock Mechanics, Johannesburg, 1, 569–574.

Jiang, Y., Xiao, J., Tanabashi, Y. and Mizokami, T. (2004). “Development of an Automated Servo-Controlled Direct Shear Apparatus Applying a Constant Normal Stiffness Condition”, Int. J. Rock Mech. Min. Sci.; 41, 275–286.

Johnston, I.W., Lam, T.S.K. and Williams, A.F. (1987). “Constant Normal Stiffness Direct Shear Testing for Socketed Pile Design in Week Rock”, Geotechnique, 37 (B1) 83–87.

Ladanyi, B. and Archambault, G. (1970). “Simulation of Shear Behaviour of a Jointed Rock Mass”, In: Rock Mechanics; Theory and Practice (Somerton, W.H., ed.), 105–125.

Patton, F.D. (1966). “Multiple Modes of Shear Failure in Rock and Related Materials”, Ph.D. Thesis, University of Illinois, Urbana.

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