Study on Dynamic Properties of Rock Discontinuity using Dynamic Direct Shear Test Machine

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1 Study on Dynamic Properties of Rock Discontinuity using Dynamic Direct Shear Test Machine Jun Yoshida a*, Ryunoshin Yoshinaka b, Takeshi Sasaki a, Masahiko Osada c a SUNCOH Consultant Co. Ltd., Tokyo, Japan b Professor Emeritus of Saitama University, Saitama, Japan c Associate Professor of Saitama University,Saitama, Japan * jun@suncoh.co.jp (co rresponding author s ) Abstract In hard rock, many rock discontinuities such as bedding plane, joint, fissure et al. are distributed, and influence the strength and deformability of rock mass. In Japan, after the 2011 off the Pacific coast of Tohoku Earthquake, a new method of dynamic analysis for discontinuous rock mass has been proposed. The dynamic strength and deformability of rock discontinuity which used as the input parameters are very important in this analysis. The authors have developed a new dynamic load-testing machine, for the purpose of investigating the response to the earthquake motion of rock discontinuities. This test machine is a direct shear test machine of the shear-box type, and makes it possible to perform load-testing of cyclic shear stress ±50kN and positive normal stress +50kN. Both dynamic direct shear tests and the dynamic normal deformation tests are also possible. The control system makes it possible to change the parameters such as cyclic numbers of loading, stress amplitude, frequency of loading wave and type of loading wave. This paper reports on the dynamic shear deformability and strength of rock discontinuities under dynamic direct shear testing. We performed 2 types of dynamic direct shear tests on rock discontinuities. One is the Multi-stage amplitude dynamic direct shear test, as carried out usually on intact core specimens. The other was Increase amplitude dynamic direct shear test that we had previously proposed. The Increase amplitude dynamic direct shear test involves progressively increases loading of the cyclic shear stress amplitude. Test specimens were natural rock discontinuities sampled from bored-cores and artificial discontinuities made by mortar. From these tests results, the dynamic shear strength exceeds the static shear strength for rough natural rock discontinuity. However, the clear difference is not recognized among both for flat natural rock discontinuity. Keywords: Dynamic loading test machine, Rock discontinuity, Dynamic direct shear test, Dynamic shear strength, Dynamic diagonal shear stiffness 1. Introduction Many rock discontinuities are distributed in hard rock such as bedding planes or joint planes, and influence the strength and deformability of the rock mass (Müller,1963;Talobre,1967). After the 2011 Tohoku Earthquake off the Pacific coast, a dynamic analysis method for rock foundations such as for very important facilities and on large rock slopes was required. With regard to this dynamic analytical method for discontinuous rock mass, the problems due to the conventional elastic analysis methods being insufficient are pointed out, but a new analytical method for rock foundation and rock structure has been proposed in recent years (Iwata et al., 2012;Yoshinaka et al., 2012). However, in analysis and design using these analytical methods, dynamic strength and deformability of the rock discontinuity become the input parameters for analysis of the problem. Not much data has been accumulated on dynamic strength and deformability of rock discontinuities in the past, unlike that for the static parameters from laboratory core tests or in-situ tests. Furthermore, exclusive test machines are not generally available. Particularly, regarding dynamic cyclic load test machines for investigating the rock discontinuities that are assumed to occur in an earthquake, there are only a few research papers(celestino and Goodman,1979;Crawford and Curran,1981;Gillete et al.,1983;kana et al.,1991;lee et al.,2001;boulon et al., 2002). But, among these research papers, dynamic testing under the condition of assumed real seismic motions is almost non-existent.

2. Development of dynamic loading test machine The authors has been developed a dynamic loading test machine for the capable of dynamic direct shear and dynamic normal deformation tests on rock

The shear stress wave is a full-amplitude wave that has Fig.1 Type of dynamic loading tests for rock discontinuity positive and negative amplitude, and the sine wave with a predetermined frequency.

2 2. Development of dynamic loading test machine The authors has been developed a dynamic loading test machine for the capable of dynamic direct shear and dynamic normal deformation tests on rock discontinuities in Fig.1 (Yoshida et al., 2013). Dynamic direct shear tests are performed by loading a predetermined shear stress wave pattern under constant normal stress. The shear stress wave is a full-amplitude wave that has Fig.1 Type of dynamic loading tests for rock discontinuity positive and negative amplitude, and the sine wave with a predetermined frequency. In this test, both a normal actuator and shear actuator are performed under stress control. This test machine is capable of Constant amplitude dynamic direct shear tests, Multi-stage amplitude dynamic direct shear tests and the earthquake wave direct shear tests that are generally conducted, and of the Increasing amplitude dynamic direct shear tests proposed by the authors. Dynamic normal deformation tests are performed using loading with normal stress of predetermined wave pattern. The normal stress wave is the semi-amplitude wave that has only positive amplitude, and a sine wave with a predetermined frequency. 2.1 Description of test machine The dynamic loading test machine developed by the authors is classified as a direct shear test machine of the shear box type. Both normal load actuator and shear load actuator are performed under stress control and displacement control. So this test machine makes possible both static loading tests and dynamic loading tests. Test specimens are made from bored-cores that have been obtained from geological surveys. Therefore, shear boxes have internal dimensions 150mm 150mm square, to house a core 100mm in diameter. Fig.2 shows the system of the dynamic loading test machine and Fig.3 shows the dynamic loading test machine. The dynamic loading test machine system consists of three units: 1) load actuator unit, 2) oil supply unit and 3) control-measurement unit. A high-performance Digital Servo Controller with CPU exhibits functions of a high-speed feedback system. 1 LOAD ACTUATOR UNIT 2 OIL SUPPLY UNIT 3 CONTROL-MEASUREMENT UNIT Fig.2 System of dynamic loading test machine Fig.3 Overview of test machine Fig.4 and Fig.5 show the load actuator unit. To prevent sympathetic vibrations from the test machine due to dynamic loading, the height of the pedestal base of the load actuator unit is reduced. Furthermore, to increase the rigidity, the base frame and top frame are made of cast iron. Specimens are mounted on the upper shear box and lower shear box in such a way that the joint becomes

3 horizontal. A normal load is applied downward perpendicular to the joint plane and a shear load is applied horizontally. The point of action of the normal load is attached to the center of the joint plane and the shear load is applied along the horizontal axis via the center of the joint plane. Fig.4 Drawing of load actuator unit Fig.5 Overview of load actuator unit The normal load actuator is a hydraulic cylinder of capacity 50kN. The hydraulic cylinders are equipped with a load cell for measuring the applied force and an internal LVDT for monitoring the stroke. Measured and monitored data are sent to the Digital Servo Controller with CPU, and used for controlling the hydraulic cylinder. The shear load actuator is a double-rod type hydraulic cylinder of capacity ±50kN. The hydraulic cylinders are equipped with load cells and internal LVDT, too. A double-rod type hydraulic cylinder is chosen to perform changes smoothly between positive and negative loading, and to equalize the operation of positive and negative loading. Table.1 shows the specifications of the load actuator unit. Table.1 Specification of load actuator unit Functions Normal Load Actuator Shear Load Actuator Specification Type : Hydraulic Cylinder Load : +50kN Frequency : static load~5.0hz Displacement rate : 10mm/sec Stroke : φ80mm (+100mm stroke) Control : Load control/displacement control Type : Hydraulic Cylinder (Double-rod type) Load : ±50kN Frequency : static load~5.0hz Displacement rate : 8mm/sec Stroke : φ80mm (±60mm stroke) Control : Load control/displacement control Fig.6 shows the simplified mechanism of the load actuator unit for shear boxes. The upper shear box is bolted to the box shaped press frame, and a normal load acts through the press frame by actuator. The press frame is controlled only for the movement in the vertical direction using two rails for a linear guide and roller bearings. The lower shear box is bolted to the base plate on the linear guide and roller bearing set on the pedestal base, and the shear load acts through the base plate by actuator. The base plate is controlled only for the movement in the shear direction using two rails for a linear guide and roller bearings. As a result, only two directions of displacement are relatively permitted to affect the discontinuity (2 degrees of freedom). A turnbuckle mechanism is installed on the reaction frame to restrain any wobble from occurring on the press flame for transmitting the normal load at the time of cyclic loading with the shear loading actuator. In this way, the clearance

4 between the roller bearings of press frame can be minimized on both sides. The shear box is comprised of an upper box and lower box. Therefore, the shear boxes have the internal dimensions of 150mm 150mm square, for housing a core with dimensions of 100mm diameter. To control deformation as much as possible the shear box is produced by gouging. A wedge system is set up in the box to fix the specimen. Fig.7 and Fig.8 show a drawing of the shear box and specimens set in boxes. The test specimen is produced by filling a molding box with mortar, to produce a core sample of size of 50mm~100mm in diameter. Fig.6 Mechanism of load actuator unit Fig.7 Drawing of shear box Fig.8 Test specimens set in boxes (Sandstone joint) Fig.9 Natural Discontinuities (Mudstone bedding plane) Fig.10 Artificial discontinuities (Tension crack of mortar)

5 2.2 Test specimens In this study, test specimens were used Limestone joint, Mudstone bedding plane and Sandstone joint from natural rock discontinuity. These discontinuities were sampled from bored-core with a digging aperture diameter of 116mm and 86mm, and the outer diameters of the core-samples were φ90mm and φ60mm. Fig.9 shows test specimens of natural rock discontinuity. In addition, because it is expensive and time consuming to sample the natural rock discontinuities and it is difficult to secure an adequate amount of samples with uniform property; artificial discontinuities made from mortar were used for comparative tests of the strength and deformability of discontinuities. Artificial discontinuity is a tension crack produced by tensile breaking of mortar core for outer diameter of φ100mm. Fig.10 shows test specimens of artificial discontinuities The properties of the discontinuities of the test specimens are summarized in Table.2. Table.2 Properties of rock and discontinuities for test specimens Rock Type Limestone Mudstone Sandstone Mortar Discontinuity Type Joint Bedding Joint Joint Tension crack Direction Low-angle Low-angle Low-angle High-angle Roughness (JRC for observation) 10~16 4~8 6~10 14~18 8~12 Interlocking B C C A A Compressive strength σc (MPa) (JRC : Barton and Chouvey,1977, Interlocking : defined authors) 2.3 Dynamic shear tests Fig.11(a) shows the result of the Constant amplitude dynamic direct shear test, and Fig.11(b) shows the result of an Earthquake wave dynamic direct shear test. The upper figures show the time history waveform of the shear stress applied by actuator, and the lower figures show the shear stress shear displacement curve (shear hysteresis). From these results, it can be said the objectives that the development of a test machine are all accomplished: a shear stress wave with both positive and negative amplitude, a stable stress wave at high frequency, reproduction of deformation in the micro deformation region, dynamic load testing using earthquake vibrations. (a) Constant amplitude (b) Earthquake wave dynamic direct shear test dynamic direct shear test Fig.11 Results of dynamic direct shear tests

6 3. Results of dynamic direct shear tests 3.1 Test methods In this study, Multi-stage amplitude dynamic direct shear tests and Increase amplitude dynamic direct shear tests were carried out. Increasing amplitude dynamic direct shear tests were proposed by the authors. (1) Multi-stage amplitude dynamic direct shear test Multi-stage amplitude dynamic direct shear testing is performed by loading with M stages of N cycles of sine waves in succession, under a constant normal stress. After loading 10 stages with 5 cycles of sine waves, we planned so that shear stress amplitude would reach the targeted strength. We set the target strength as the static shear strength,τs of the same discontinuity. So, set the shear stress amplitude of the first stage to S= 0.1τs. Thereafter, the shear stress amplitude was increased stepwise: 0.2τs,0.3τs,0.4τs, 0.5τs. In reality, the number of loading stages may exceed 10, because shear stress loading is continued until dynamic shear failure. Fig.12 shows the loading wave of the shear stress amplitude using Multi-stage amplitude dynamic direct shear tests. (2) Increase amplitude dynamic direct shear test Increase amplitude dynamic direct shear test was performed by loading with sine waves that gradually increased the shear stress amplitude, under a constant normal stress. It was planned so that the shear stress amplitude would reach the targeted strength (static shear strength,τs of same discontinuity), after N times cycles of loading waves. We assumed the number of loading cycles N=50 to match the Multi-stage amplitude dynamic direct shear test. In reality, loading cycles may exceed 50, because shear stress loading is continued until dynamic shear failure. Fig.13 shows the loading wave for the shear stress amplitude using Increase amplitude dynamic direct shear tests. This test method was able to perform a stable deformation, Fig.12 Fig.13 Loading wave of shear stress amplitude used in Multi-stage amplitude dynamic direct shear tests (5-waves/10-stages) Loading wave of shear stress amplitude used in Increase amplitude dynamic direct shear tests (50-waves) because the shear stress amplitude was gradually increased. This is the reason why the authors proposed this test method. 3.2 Test results Fig.14 shows the results of Multi-stage amplitude dynamic direct shear tests used Limestone joints. The limestone joints in the tests were rough plane (JRC=10~16 for observation) and were interlocking well (rank B). The frequency of the shear stress wave was 1.0 Hz, the shear stress amplitude arrived at the targeted strength (static shear strength, τs) after loading with 10 stages and 5 cycles of waves. According to Fig.14(a), the shear stress was not reached to the predetermined stress amplitude for both positive and negative sides at 12th-stage (1.2τs) exceeding targeted strength. Simultaneously, shear displacement increased rapidly at the same time (see Fig.14(c)). According to Fig.14(b), most of the variation was not recognized as normal stress initially. However, the dispersion increases rapidly exceeding 11th-stage or more. This tendency was seen frequently with the rough and interlocked discontinuities. According to Fig.14(e), shear hysteresis is

7 approximately a linear spindle shape from the first stage to the 10th-stage, then the hysteresis becomes a reverse s-shape from the 11th-stage. Finally, hysteresis becomes a parallelogram with a reverse S-shape at the 12th-stage where shear stress was not reached to the predetermined stress amplitude. According to Fig.14(g), dilation curve presents an approximately symmetric concave shape. Fig.14(f) shows stress-path. Though normal stress has a little variability, it almost shows a constant value until it reach to the dynamic peak shear strength. The dynamic peak shear strength was defined by Yoshida et al.,2014. Fig.14 Example of Multi-stage amplitude dynamic direct shear test results (Limestone joint, JRC=10~16, Frequency=1.0Hz, Normal stress,σn=3.0mpa)

8 Fig.15 shows the results of Increase amplitude dynamic direct shear tests for a mortar tension crack. The tension crack was a rough plane (JRC=8~12 for observation) with very well interlocking (rank A). It had a general tendency to follow the same pattern as multi-stage amplitude dynamic direct shear tests result, but the shear hysteresis of Fig.15(e) changes continually. Fig.15 Example of Increase amplitude dynamic direct shear test results (Mortar tension crack, JRC=8~12, Frequency=1.0Hz, Normal stress,σn=1.0mpa)

9 3.3 Dynamic shear strength Fig.16 shows results of Multi-stage amplitude dynamic direct shear test and Increase amplitude dynamic direct shear test for a Limestone joint of rough discontinuity (φ60mm core). Fig.16(a) shows the failure criterion for the Multi-stage amplitude dynamic direct shear test results, and is represented positive dynamic peak shear strength,τp+(d) (symbol ) and negative dynamic peak shear strength,τp-(d) (symbol ). In Fig.16(a), solid lines represent the approximate lines of the peak strength, Mohr-Coulomb s failure criterion. The dashed line represents the approximate lines of static peak strength obtained from static shear tests. As a result, for the dynamic strength of an uneven joint, a value exceeding the static strength is obtained. Fig.16(b) shows the results of the increasing amplitude dynamic direct shear test, and these results are similar to a former result. Fig.16(c) shows both test results together; no remarkable difference is obtained between both test results. In this study, we express test results by linear model, for the purpose of using it for a real design. Fig.17 shows the results of the tests for a Mudstone joint of flat discontinuity (φ60mm core). It is clear there is little difference between both test results for the Mudstone joint. Furthermore, the dynamic strength slightly exceeds the static strength. Fig.16 Failure criterion for rough discontinuity (Limestone joint, JRC=10~16, Frequency=1.0Hz) Fig.17 Failure criterion for flat discontinuity (Mudstone bedding plane, JRC=4~8, Frequency=1.0Hz)

10 3.4 Dynamic shear deformability Fig.18 shows the definition of the Dynamic Diagonal Shear Stiffness,Ksd(d) and Attenuation,h in a hysteresis for one cycle. Similar to the hysteresis of Fig.14 and Fig.15, the Dynamic diagonal shear stiffness,ksd(d) decreases with increase in shear displacement full-amplitude,δdh (i.e. shear stress full-amplitude, Δτ). For details refer to Yoshida et al.,2014. Fig.18 Definition of Dynamic shear deformability 4. Conclusions The authors have developed a dynamic load-testing machine, for the purpose of investigating the response to earthquake motions of rock discontinuities. It achieves the objectives of: producing both amplitude positive and negative waves, stable stress waves at high frequency, reproduction of deformation in the micro deformation region, dynamic load testing using earthquake motions. In this study, Multi-stage amplitude dynamic direct shear tests and Increase amplitude dynamic direct shear tests that we had proposed were conducted using natural discontinuity specimens made from bored-core and artificial discontinuity specimens made by mortar. From these results, the dynamic strength exceeds the static strength for rough natural rock discontinuity. However, the clear difference is not recognized among both for flat natural rock discontinuity. The number of experiments is so small in this paper, further studies are needed. References Barton, N. and Choubey, V., 1977, The Shear Strength of Rock Joints in Theory and Practice, Rock Mechanics, Vol.10, pp Boulon, M., Armand, G., Hoteit, N. and Divoux, P., 2002, Experimental investigations and modelling of shearing of calcite healed discontinuities of granodiorite under typical stresses, Engineering Geology, Vol.64, Celestino,T. B. and Goodman,R.E., 1979, Path Dependency of Rough Joints in Bi-directional Shearing, Proc. 4th. ISRM Congress, Vol.1, Crawford, A.M. and Curran,J.H., 1981, The Influence of Shear Velocity on Frictional Resistance of Rock Discontinuities, Int. J. of Rock Mech. And Min. Sci. & Geomech. Abst., Vol.18, Gillete, D.R., Sture, S., Ko, H.Y., Gould, M.C., Scott, G.A., 1983, Dynamic Behavior of Rock Joints, 24th U.S. Symposium on Rock Mechanics, Iwata, N., Sasaki,T., Sasaki, K., Yoshinaka, R., 2012, Static and dynamic response analysis of rock mass considering joint distribution and its applicability, Proc. 12th ISRM Congress, Kana, D.D., Chowdhury, A.H., Hsiung, S.M., Ahola, P.A., Brady, B.H.G. and Philip, J., 1991, Experimental techniques for dynamic shear testing of natural rock joints, Proc. 7th. ISRM Congress, Lee, H.S., Park, Y.J., Cho, T.F. and You, K.H., 2001, Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading, Int. J. of Rock Mech. and Min. Sci., Vol.38, Müller, L., 1963, Der Felsbau, Ferdinand Enke Verlag, Stuttgart Talobre, J.A., 1967, La mécanique des roches, UNOD Yoshida, J., Yoshinaka, R., Tsubota, Y., Iwakoke, Y. and Nakashima,M., 2013, Development of dynamic test machine for rock discontinuity, 13th Japan Symposium on Rock Mechanics, Yoshida, J., Yoshinaka, R. and Sasaki, T., 2014, Study on dynamic shear strength and deformability of rock discontinuity using dynamic loading test machine, Proc. 42th Symp., Rock Mech., JSCE, Yoshinaka,R., Iwata,N. and Sasaki,T., 2012, Applicability of Multiple Yield Model to earthquake response analysis for foundation rock of large-scale structure, Jour. of JSCE,

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