ADVANCED CYCLIC TRIAXIAL AND BENDER ELEMENT TEST EQUIPMENT

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1 KHBO Departement of Industrial Sciences Zeedijk Oostende Belgium INNOVATIE FORUM GEOTECHNIEK, 22 NOVEMBER 2005, ANTWERP ADVANCED CYCLIC TRIAXIAL AND BENDER ELEMENT TEST EQUIPMENT Christian Karg and Wim Haegeman Contact: Abstract This paper presents the setup of an advanced cyclic triaxial testing device, including bender element test equipment and the possibility of local strain measurements. The equipment is used to investigate cylindrical soil samples. The purpose of a cyclic triaxial test is to determine dynamic properties of soil calculated from measured values of the applied stresses and deformations of the sample. With the cyclic triaxial testing device it is possible to apply either a load or a strain controlled axial cyclic load. To improve the accuracy of measurements as well as minimize influences of clearance and deviations on the edge of the sample local strain sensors will be employed. Therefor on-specimen LVDT s will be used. In addition a bender element system is installed to determine dynamic properties of the soil. In a bender element test a sinusoidal shear wave is sent from the top of the sample to the bottom. The measurement of time delay between sent and arriving wave allows calculation of shear wave velocity and shear modulus of soil. The two different test procedures allow for comparison of the gathered data and confirmation of measurements. This paper describes the setup of the testing device, explains the general mode of operation for advanced cyclic triaxial testing, and shows some results on tested sand samples. In addition the application of remote control software to steer the testing process is outlined. The whole test procedure can be controlled by a common personal computer. Although the testing process might be time consuming it has to be monitored continuously. Thus it is very helpful to be able to observe the state of a running test via the World Wide Web. INTRODUCTION Vibrations in the built environment are a matter of growing environmental concern, especially in densely populated areas. Increasing dynamic impacts of different nature combined with a growing environmental concern and the enhancement of a sustainable development policy, have resulted in an increased interest and awareness for the problem of vibrations in the built environment among the population and public authorities. Besides traffic induced vibrations, vibrations due to construction and industrial activities are also a growing matter of concern. There is an increasing demand, for example, to use vibratory pile driving as an alternative to impact pile driving in densely populated areas. Vibrations caused by blast loading, for example, 1

2 due to the controlled explosion of mines, are a potential threat for nearby buildings. Other industrial activities as looms and printing presses are responsible for high vibration levels. Of major interest for the investigation of the effects of vibrations on the built environment is the observation of the cyclic behavior of the soil. Effects like permanent settlements, densifications and loss of strength under cyclic loading caused by the mentioned vibration sources have to be examined. Dynamic parameters of the soil are determined in order to predict the soil behavior under dynamic excitations (see scheme in Fig. 1). Vibration Sources Built Structure Vibrations Dynamic Excitations For this purpose a cyclic triaxial testing device is installed to investigate cylindrical soil samples on their dynamic properties. With the equipment it is possible to apply dynamic stress conditions to the sample through axial cyclic loading in vertical direction. In addition bender element tests are performed on the same sample allowing the comparison of results from two different tests under the same test conditions. Objective of this paper is to present the general setup of the testing device, to explain the test procedure as well as the data analysis, and to give a survey of the potentials of the test equipment. The dynamic loading options are outlined. The advantages of local strain Soil Sample Dynamic Properties? Laboratory Investigations Fig. 1: Overview measurements are described as well as the application of remote control software for monitoring the progress of the test procedure. Some test results on reconstituted sand samples are presented. TESTING DEVICE The test device includes the cyclic triaxial testing apparatus, the local strain transducer set, and bender element test equipment. While the local strain measurements are connected to the data acquisition system of the cyclic triaxial device, the bender element test is performed using a separate system. In this way the results of cyclic triaxial and bender element tests are completely independent of each other while they are performed on the same sample under identical test conditions. 2

3 Cyclic Triaxial Testing Apparatus A common triaxial test is performed for the determination of the shear characteristics of a soil sample. After saturation and consolidation to the required stress, in general the sample is sheared by increasing the vertical load until failure. In cyclic triaxial tests the option of cyclic axial loading is added to determine the dynamic characteristics of soil. The cyclic triaxial testing equipment (Fig. 2) consists of the loading frame with test cell, a control and data acquisition system (CDAS), and software to operate the test procedure via PC. Further on several hardware components are installed supplying and controlling the requested air pressure and providing deaired, demineralised water. 800 kpa is the maximum pressure to apply; at this time the frequency of axial cyclic loading has a range of 0-1 Hz. However, the loading piston is controlled pneumatically and Fig. 2: Cyclic triaxial testing apparatus works at a maximum loading frequency up to 70Hz and a maximum axial load of +/- 5 kn measured by a submersible load cell. Further development will therefore be to increase the loading frequency to about 20 to 30 Hz. The range of actuator deformation is 30 mm. The equipment allows saturation, isotropic and anisotropic consolidation, cyclic, and also conventional triaxial tests on samples with a diameter of 50 and 100 mm, respectively. The CDAS works with a digital resolution of 16bit for the controlling as well as for data acquisition and a sampling rate of 50 to 100 samples per cycle. With the current configuration of the equipment the minimum target deformation is about 10-2 % axial strain using 50 mm samples. According to the actuator range the maximum deformation to apply is about 10% axial strain. Combined with the bender element test equipment applying strains lower than 10-3 % a wide range of cyclic strain is covered with this testing device. Local Strain Measurements To measure local strains an on-specimen transducer set (LVDT s) shown in Fig. 3 is used. Using only a simple external deformation measurement the clearance of the loading device might influence the measurements. Also deviations at the edge of the soil sample are supposed to be minimized using these local strain measurements. One further advantage is the possibility of measuring radial deformations for determination of Poisson s ratio. Apart from these advantages it can be shown that the application of on-specimen transducers improves the accuracy of measurements in axial direction by factor 2.5 compared to the single application of external 3

4 lgauge Fig. 3: Local strain transducer Sample tube Axial LVDT #1 Radial belt Rubber ring Axial LVDT #2 deformation measurements. The on-specimen transducers are installed directly on the sample. The version used in the present testing device allows fixing the transducers without puncturing the surrounding rubber membrane. Different other methods require a penetration of the rubber membrane which might cause leakages and thus failure of a test. However, to the author s knowledge only few people measured local strains during cyclic loading, especially in axial direction. Further investigation will be performed to find optimal fixing methods, to proof correct local strain measurements and to relate external to local measurements correctly. Bender Element Testing The vertical arrival time bender element tests are performed with separate equipment. A single sinusoidal wave pulse is generated using the soundcard of the laboratory PC together with a MathLab routine. The generated signal is amplified to a peak-topeak amplitude of 20 V and sent to the top cap bender element (1), Fig. 4. Caused by the electrical signal the top cap element bends in horizontal direction. This bending generates a shear wave propagating towards the bottom of the soil sample where the arriving wave generates bending of the bottom bender element (2), also shown in Fig. 4. The resulting electrical signal is recorded with an PC oscilloscope. Both, sender and receiver signal are recorded for later analysis. The time delay between sender and receiver signal allows calculating the shear wave velocity and the shear modulus of the specimen. Remote Control Software Once the specimen is installed in the triaxial cell and the test is started, it may take some time until the final state of the respective test stage is reached. Especially for samples showing low permeability, saturation and consolidation may be time consuming. Thus it is very useful to be able to observe the progress of these test stages without being present in the laboratory. Since the test procedure is steered and monitored by the controlling software, installed on a common personal computer, the actual state can be checked simply by having a look to the monitor. Using any known 1 2 Receiver Sender Fig. 4: Bender element test Loading piston Top cap Porous stone Sample with membrane Bottom platen Triaxial cell 4

5 remote control software this supervision can be carried out from any computer connected to the internet. Even the steps to the next test phase can be performed via the remote control as long as no manual changes on the test equipment itself are necessary. TEST PROCEDURE The current research investigates the dynamic behavior of reconstituted samples of granular soils. Cyclic triaxial and bender element test equipment is used to determine the dynamic properties of the soil as damping ratio and shear modulus under different cyclic loading conditions. Further on the long term behavior of the soil is a matter of interest. Therefore the accumulated strain caused by very high numbers of low strain cycles will be investigated. In close cooperation with the manufacturer further development of the test equipment, especially to increase the loading frequency and to improve the local strain measurements during cyclic loading, will be an important issue as well. Reconstitution of the Sample, Saturation and Consolidation, Bender Element Testing Several methods of reconstituting soil samples of granular material are known. For tests presented in this paper the undercompaction method after Ladd (1978) is used. Sand samples with a height of mm and a diameter of 49.1 mm are reconstituted in eight layers of constant thickness using an internal three-part split mold. The objective of the saturation phase of the test is to fill all voids in the specimen with water without undesirable prestressing of the specimen or allowing the specimen to swell. Therefor the specimen is flushed with deaired water from bottom to top to replace the air in the pores by water. Saturation is accomplished by applying a back pressure to the specimen pore water to drive remaining air into solution. During consolidation the specimen is maintained in an isotropic or other known state of stress according to the operators specification. The static load on the piston to maintain isotropic consolidation is applied automatically by controlling software of the system. With the specimen drainage valves closed, the maximum back pressure is held constant and the chamber pressure is increased until the difference between the chamber pressure and the back pressure equals the desired effective consolidation stress. For anisotropic consolidation an additional axial stress is applied to the specimen. Once the specimen is under the requested stress conditions the drainage valve is opened. The consolidation is assumed to be finished when the volume change equals zero and the pore water pressure stays constant at the back pressure level. When consolidation is accomplished the bender element test can be performed. If the sample is not destroyed during cyclic loading it is appropriate to redo the bender element test after every cyclic loading. In this way structural changes or a loss of strength can be detected. 5

6 Cyclic Triaxial Testing After consolidation of the soil sample and the optional bender element test, cyclic loading is started. The parameters for cyclic loading are chosen by the operator. The controlling and data acquisition software provides certain predefined testing options according to respectively standard ASTM D3999 (method A or B) and standard ASTM D5311. Following the standards the cyclic loading is to be performed under undrained conditions. Thus only consolidation stress conditions, loading frequency and amplitude are to be defined by the operator. During cyclic loading the cell pressure is kept constant. The cyclic load is to be applied with a sinusoidal waveshape following several recommendations according to the mentioned standards. Standard ASTM D3999 describes methods for the determination of the modulus and damping properties of soils using the cyclic triaxial apparatus. This standard provides two test methods. Method (A) permits the determination of the shear modulus and the damping ratio using a constant cyclic load. Method (B) permits the determination of the shear modulus and the damping ratio using a constant cyclic stroke. The test methods fit for undisturbed as well as for reconstituted soil samples of both fine-grained and coarse-grained material. The amplitude of the cyclic load can be given either by the stress or the strain level to be applied to the specimen. After standard ASTM D3999 the specimen is loaded cyclically with 40 cycles and the first half cycle in compression. Standard ASTM D5311 describes a test method for the investigation of the load controlled cyclic triaxial strength (also called the liquefaction potential) of soil using the cyclic triaxial testing apparatus. This test method fits for undisturbed as well as for reconstituted soil samples and is generally applicable for testing cohesionless free draining soils of relatively high permeability. After standard ASTM D5311 the specimen is loaded cyclically with the first cycle applied in compression. The load is to be cycled until either cyclic double amplitude vertical strain exceeds 20%, the single amplitude strain in either extension or compression exceeds 20%, 500 load cycles are exceeded, or the load wave form deteriorates beyond acceptable values. Beside these predefined loading options, user defined settings can be made using the options of the control software of the testing equipment. In this case it is possible to use several waveshapes for the cyclic loading. Triangular waveshapes as well as pulses can be applied. Even self-programmed waveshapes are applicable. The operator can choose between drained or undrained loading, number of cycles, and termination stress or strain. The load can be applied alternatively in compression and extension, compression only, or extension only. DATA ANALYSIS AND SOME TEST RESULTS The analysis of test results of a test on a reconstituted sand sample under isotropic stress conditions is outlined here. The material investigated is coarse sand. The effective stress applied is σ = 300kPa. Test conditions according to standard ASTM eff D3999 (method A) are used. The cyclic stress has a peak-to-peak amplitude of 0.5σ eff 6

7 and a loading frequency of 0.5 Hz. The recorded time history of stresses and deformations is translated to effective stresses and strains for further analysis (Fig. 6). From these axial values shear stress Stress [kpa] Effective stresses Cycle number Time [sec] Axial effective stress Radial effective stress Deviator stress Axial strain [%] Axial strain Cycle number ,04 2,02 2,00 1,98 1,96 1,94 1, Time [sec] Fig. 6: Graph of effective stresses and axial strain and shear strain can be calculated. For undrained test conditions it can be shown that the shear stress is half of the deviatoric stress, that is τ = 0.5 q, and the shear strain is 1.5 times the axial strain, that is γ = 1.5 ε ax. This relation is connected to Poisson s ratio υ = 0.5 in undrained test conditions. Shear stress [kpa] ,90 2,92 2,94 2,96 2,98 3,00 3,02 3,04 3, Shear strain [%] Cycle [-] G [MPa] D [%] 1 76,49 10, ,06 6, ,85 5, ,93 5, ,87 4, ,69 4, ,01 4, ,26 3, ,74 4,12 Fig. 5: Graph of hysteresis loops 7

8 The hysteresis loop is achieved by plotting shear stress against shear strain. It is used for the calculation of shear modulus and damping ratio for each cycle. The visual interpretation of Fig. 6 and Fig. 5 allows conclusions about the behaviour of the tested material as well. Nonlinear behavior and plastic deformations can be observed. Shear modulus and damping ratio are calculated using the following equations: DA G = τ and γ DA A loop D = (1) 4π A with variables as described in Fig. 7. To achieve these two values from the hysteresis loop the recommendations of standard ASTM D3999 are followed. A 1 n loop = i i+ 1 i + i+ 1 2 i= 1 ( τ τ ) ( γ γ ) τ γ centre, τ centre L = 1 A S L 2 Loop area S S = γ γ max L = τ τ max centre centre γ τ DA γ DA Fig. 7: Hysteresis loop with triangle area In the analysis of the bender element tests the time delay between sender and receiver signal t [msec] allows calculating the shear wave velocity v s [m/s] and the shear modulus G [MPa] of the specimen from the following equations: v h t t t s = and G = v ρ (2) 2 s c where: ht t tip-to-tip distance between the top and bottom bender element [mm] and ρ density of the consolidated specimen [kg/cm³]. c The signal is sent at four different frequencies, on the one hand to confirm the measurements, on the other hand to reduce the influence of a capacitive coupling 8

9 effect which can be observed in the receiver signal. In order to define the arrival time of the shear wave a certain point is selected. That point assigns the first deflection of the arriving wave and should be detected in all measurements. Fig. 8 shows the graph of the bender element test results with the time delay t and sender and receiver signal. Sender [V] 15,0 10,0 5,0-10,0-15,0 BE-test after consolidation, f = 15 khz 0,0 0,0-5,0-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0-1,0 v s = 270,6 m/s G 0 = 159,6 MPa t = 0,388msec Sender signal Time [msec] Receiver signal Fig. 8: Graph of bender element test Plotting the calculated hysteretic shear modulus of different tests against the corresponding shear strain illustrates the high capability of the test equipment. The well known degradation curve,, can be achieved performing cyclic triaxial tests at several strain levels. Applying the bender element test additionally completes the degradation curve from the linear elastic range at very low strain levels to the strongly nonlinear range at larger strains. 3,0 2,0 1,0-2,0-3,0 Receiver [mv] Fig. 9: Graph of degradation curve 9

10 SUMMARY The setup of advanced cyclic triaxial and bender element test equipment for the investigation of dynamic behavior of soil is described. Combining cyclic triaxial and bender element tests, local strain measurements and remote control software gives a good complement and a wider range of applications compared to conventional laboratory soil investigations. Test procedure as well as test options are introduced to give an overview about the potentials of the equipment. Some results illustrate the data analysis. ACKNOWLEDGEMENTS The work presented in this paper is obtained within the frame of SBO project IWT Structural damage due to dynamic excitations: a multi-disciplinary approach. The financial support of the Flemish Community is gratefully acknowledged. REFERENCES ASTM D : Standard test methods for the determination of the modulus and damping properties of soils using the cyclic triaxial apparatus ASTM D : Standard test method for load controlled cyclic triaxial strength of soil R.S. Ladd (1978): Preparing Test Specimen Using Undercompaction. ASTM, Geotechnical Testing Journal, Vol.1, No.1 10