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تعداد صفحات این فایل: ۴۱ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Effects of loading rate and initial stress state on stress–strain behavior of rock fill materials under monotonic and cyclic loading conditions~~en~~

Abstract

This paper studies loading rate effect on stress–strain behaviors of high compacted rockfill materials. The large scale triaxial-dynamic equipment was used for this propose. To check the equipment capabilities, tests have been carried out on elastic spring and damping rubber specimens. The laboratory tests results indicate that the loading rate effect in elastic metal spring is negligible, while it has considerable effects on stress–strain behaviors in damping rubber, and especially in rockfill materials. The laboratory test results on rockfill materials have shown that dry specimens have higher E and D values compared to the saturated samples. Anisotropic specimens have higher Young’s modulus and less damping ratio in comparison to isotropic samples. Generally, the influence of the main parameters on E and D in rockfill specimens is initial vertical effective stress and loading frequency, respectively. The high compacted rockfill specimen exhibits a non-destructive response to the application of cyclic loading below a threshold axial strain level of <0.005%. Since the frequency effect was observed in the strain levels lower than 0.005%, and there is not meaningful build up of pore pressure in this strain level, therefore any effect of excess pore water pressure on the frequency effect is negligible

۱ Introduction

Damping rubber and rockfill materials have many useful applications in civil engineering structures. For example, the damping rubber used as elastomeric bearing for seismic isolation of structures and bridges. Generally, damping rubber exhibits both nonlinear and viscous properties. Effects of temperature [1], strain amplitude [2] and strain rate [3] on modulus and damping ratio of damping rubber at strain higher than 5% were investigated previously. Moreover, modulus and damping ratio decrease as temperature and strain increase, whilst damping ratio and especially modulus increase as strain rate increases.

Furthermore, gravelly materials that have higher damping ratio compared to clay and sand materials are frequently used in the earth fill structures and rockfill dams. The seismic design of these structures at earthquake-prone regions requires determination of the stress–strain properties under monotonic and cyclic loading conditions of the used materials. The strength and deformation behavior of coarse granular media under monotonic condition were studied previously [4–۷].

Over the past two decades, many investigators published cyclic triaxial shear test results for gravels [8–۲۰]. In general, in cyclic loading using triaxial equipment, each stress–strain curve, which is named hysteresis loop, can be described by Young modulus (E) and damping ratio (D) curves. The word hysteresis comes from ancient Greek and means ‘‘lag’’ or ‘‘delay’’; the time lag between a driven cyclic stress and the driving cyclic strain.

Many researchers [21,22] have shown that the dynamic properties of soils cannot be considered frequency independent in the earthquake frequency bandwidth, 0.01–۳۰ Hz [23], even for low strain level excitations [24] as it is common practice in geotechnical engineering [25]. Generally, the higher modulus is obtained from Resonant Column and Bender Element tests compared to triaxial and torsional shear tests. This may be due to higher loading frequency effect. Frequency effect on damping is not observed in triaxial and torsional shear element tests in previous research and can be explained as follows: most element tests are performed at very low frequencies of less than 1 Hz. In this range, perhaps the effect of frequency is negligible except for a creep effect which may be observed at a very low frequency. Generally, in Resonant Column test, damping is measured under free vibrations. It not necessary that the behavior of the material should be the same under free and forced vibrations and the material in forced vibration might not dissipate energy as expected according to the minimum principle. Recent test results show high damping ratio even at low strain levels (<0.001%) in non Resonant Column tests [21,24]. Besides, higher damping ratio was observed at the resonant frequency [21]. Loading frequency and waveform effects on modulus and damping ratio of the three high compacted modeled rockfill materials were studied by means of a large scale triaxial testing [19]. The laboratory test results have shown that the modulus, and especially the damping behavior, is influenced by the frequency of loading. The D value for sinusoidal waveform is slightly higher than those of the triangle waveform. Furthermore, the D value of rectangle waveform is considerably higher than sinusoidal and triangle waveforms [19].

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