岩石在循环荷载作用下的强度及变形特征

本文着重研究了岩石在循环荷载作用下的强度及变形特征。实验结果表明,岩石在循环荷载作用下的强度低于其静力强度,而这个循环疲劳强度极限与振幅、频率有关;岩石在循环荷载作用下的变形特征类似于静力蠕变特征。     

Mixed-Mode Fracturing of Rocks Under Static and Cyclic Loading

Static diametrical compression tests conducted on inclined cracked chevron notched Brazilian disc (CCNBD) Brisbane tuff specimens showed that the notched cracks at the centre of the specimens opened (Mode I) up to 30° crack inclination angle (β), whereas crack closure (Mode II) started for β > 33°, and closure became more pronounced at even higher β of 45° and 70°. Both the experimental and numerical results showed that the crack initiation angle (θ) was a function of the β. Scanning electron microscope (SEM) images showed that fatigue damage on cyclic loading of Brisbane tuff is strongly influenced by the failure of the matrix due to both intergranular and transgranular fracturing.     

Fatigue Behavior of Granite Subjected to Cyclic Loading Under Triaxial Compression Condition

A series of laboratory tests were performed to examine the fatigue behavior of granite subjected to cyclic loading under triaxial compression condition. In these tests, the influences of volumetric change and residual strain on the deformation modulus of granite under triaxial cyclic compression were investigated. It is shown that the fatigue behavior of granite varies with the tendency for volumetric change in triaxial cyclic compression tests. In the stress–strain space, there are three domains for fatigue behavior of rock subjected to cyclic loading, namely the volumetric compaction, volumetric dilation with strain-hardening behavior, and volumetric dilation with strain-softening behavior domains. In the different domains, the microscopic mechanisms for rock deformation are different. It was also found that the stress level corresponding to the transition from volumetric compaction to volumetric dilation could be considered as the threshold for fatigue failure. The potential of fatigue deformation was compared with that of plastic deformation. The comparison shows that rocks exhibit higher resistances to volumetric deformation under cyclic loading than under plastic loading. The influence of residual strain on the fatigue behavior of rock was also investigated. It was found that the axial residual strain could be a better option to describe the fatigue behavior of rock than the loading cycle number. A constitutive model for the fatigue behavior of rock subjected to cyclic loading is proposed according to the test results and discussion. In the model, the axial residual strain is considered as an internal state variable. The influences of confining pressure and peak deviatoric stress on the deformation modulus are considered in a term named the equivalent stress. Comparison of test results with model predictions shows that the proposed model is capable of describing the prepeak fatigue behavior of rock subjected to cyclic loading.     

Behavior of Undisturbed Residual Soil under Cyclic Loading

In order to determine behavior of Xiamen residual soil under cyclic loading, triaxial test with undrained conditions has been conducted in this paper. First, some important dynamic parameters such as strength, elastic modulus and damping ratio have been determined. Secondly, behavior of stress-strain, stress path and excess pore water pressure under cyclic loading has been analyzed. Finally, both the normalization and the determination of dynamic strength index ∅d and ?d for residual soil have been discussed.     

Pile Head Cyclic Lateral Loading of Single Pile

This paper presents an elastic continuum model using an extended nonlinear Davies and Budhu equations, which enables the nonlinear behavior of the soil around the long elastic pile to be modeled using a simple expression of pile-head stiffness method. The calculated results were validated with the measured full-scale dynamic field tests data conducted in Auckland residual clay. An idealized soil profile and soil stiffness under small strain (i.e. shear modulus, G _s and shear wave velocity, V _s of the soil) determined from in situ testing was used to model the single pile tests results. The predictions of these extended equations are also confirmed by using the three-dimensional finite-element OpenSeesPL (Lu et al. in OpenSeesPL 3D lateral pile-ground interaction: user manual, University of California, San Diego, 2010). A soil stiffness reduction factor, G _s /G _s,max of 0.36 was introduced to the proposed method and model. It was found to give a reasonable prediction for a single pile subjected to dynamic lateral loading. The reduction in soil stiffness found from the experiment arises from the cumulative effects of pile–soil separation as well as a change in the soil properties subjected to cyclic load. In summary, if the proposed method and model are accurately verified and properly used, then they are capable of producing realistic predictions. Both models provide good modelling tools to replicate the full-scale dynamic test results.     

花岗岩残积土在周期荷载下力学特性的实验研究

本文首先介绍厦门某地两种具有代表性的原状残积土(花岗岩残积土)室内三轴不排水动力剪切实验所得到的残积土在循环荷载作用下表现出来的诸如周期抗剪强度,动弹性模量以及等效阻尼比等动力学性质及其有关的重要参数;继而分析了花岗岩残积土在周期剪切过程中强度、变形、应力路径及孔隙压力等方面所具有的特点,以图揭示它们的内在规律;最后讨论了残积土周期荷载作用下归一化性状及动强度的确定方法。     

Behaviour of Backfill Undergoing Cementation Under Cyclic Loading

Geotechnical and Geological Engineering - Cemented paste backfill (CPB), a man-made soil undergoing cementation, is extensively applied to support underground mine openings or spaces and provide...     

Post-Cyclic Undrained Behavior of Compacted Composite Clay Subjected to Various Cyclic Loading Paths

Using a sequential procedure of undrained cyclic and post cyclic tests, the strength and stiffness degradation characteristics of compacted composite clays are studied immediately after various cyclic loading paths by triaxial and hollow cylinder tests. The effects of cyclic loading paths, sand contents, cyclic loading amplitude and confining pressure on the post-cyclic mechanical behavior of the composite clays are evaluated. The results point out different peculiarities which can be of interest in assessing the mechanical behavior of the composite clays under post seismic shaking. The results show that effect of cyclic loading on post cyclic pore water pressure build-up is significant when pore water pressure build-up is considerably lower than the associated value in monotonic loading. The effect of sand content and cyclic loading path on degradation of stiffness is more remarkable than shear strength. Test results also reveal that the effect of sand content on the post cyclic pore water pressure build-up is minor. However, when the aggregate content increases the shear strength increases.     

大理岩静态和特环荷载试件的电镜试验分析

In this paper, crack propagation laws were studied by means of observing marble specimens, which were conducted uniaxial static and cyclic loading tests, with electron-microscope. The studying results showed: (1) Different parts of rock specimen suffer differential degree damage; (2) Failure models and characteristics of rock specimens change with loading paths; (3) Rock cracks are to be closed, when amplitude of cyclic stress is smaller than “fatigue threshold stress” of rock. This is mainly testimony of “rock fatigue threshold stress” existence.     

大理岩静态和循环荷载试件的电镜试验分析

本文对单轴静态和循环荷载试验前后的大理岩试件进行了电镜试验分析,从中得出三个结论:(1) 试件中不同的部位,其破坏程度不同;(2) 对试件用不同的加载方式,其破坏方式和特征不同;(3) 在疲劳应力门槛值(疲劳应力极限值。以下,称疲劳应力门槛值)以下的循环幅值荷载作用下,岩石内部的裂隙等发生闭合,这是证明岩石疲劳应力门槛值存在的一个重要试验证据。     

Investigating the Effect of Cyclic Loading on the Indirect Tensile Strength of Rocks

This paper presents the results of laboratory experiments during the investigation of the stress–strain characteristics of Brisbane tuff disc specimens under diametral compressive cyclic loading. Two different cyclic loading methods were used: namely, sinusoidal cyclic loading and cyclic loading with increasing mean level. The first method applied the S–N curve approach to the indirect tensile strength (ITS) of rock specimens for the first time in the literature, and the second method investigated the effect of increasing cyclic loading on the ITS of rock specimens. The ITS of Brisbane tuff disc specimens was measured using the Brazilian tensile strength test. The reduction in ITS was found to be 33% with sinusoidal loading tests, whereas increasing cyclic loading caused a maximum reduction of 37%. It is believed that the fracturing under cyclic loading starts at contact points between strong grains and weak matrices, and that contact points at grain boundaries are the regions of stress concentration (i.e., indenters). Transgranular cracks emanate from these regions and intergranular cracks sometimes pass through the contact points. Once cracking begins, there is a steady progression of damage and a general ‘loosening’ of the rock, which is a precursor to the formation of intergranular cracks.     

周期荷载作用下粘性土变形及强度特性述评

周期荷载作用下粘性土变形及强度特性的研究所涉及的内容非常广泛,而我国在该领域的研究成果还较少。文中对其国内外研究现状作了较为详尽的评述,以期引起岩土工作者足够的重视。     

Experiment and Study on Mechanical Property of Sandstone Post-peak Under the Cyclic Loading and Unloading

Engineering research has shown that the surrounding rock of deep roadways experienced many times of post-peak cyclic loading and unloading. So studying on rock mechanical characteristics of post-peak loading and unloading is helpful to control the deep surrounding rocks. The test using RMT-150B rock mechanics testing system, to experiment on the mechanical properties of sandstone of post-peak cyclic loading and unloading. The results show that: the stress–strain curves of post-peak cyclic loading and unloading have significant plastic hysteretic loops. The area of plastic hysteretic loops gloss back. The enveloping outer enclosure of cycle loading curve is the similar as the stress–strain curves of strain softening stage when the samples failure, which shows that post-peak failure of rocks have significant memory. With the increase of cycles, the cumulative deterioration parameters are gradually increased, and the ultimate bearing capacity, elastic of loading section of samples and cumulative deterioration parameters conform with the exponential attenuation function with constant term. With the increase of surrounding pressure, the total energy, dissipated energy and elastic energy of samples are gradually increased. With the increase of cycles, the total energy, dissipated energy and elastic energy of samples all are gradually decreasing, the rate of reduction decreases gradually. The samples exist in vertical splitting and transverse shear combination failure under the post-peak uniaxial cycles, or exist in shear failure under the post-peak triaxial cycles. The shear plane exists slip traces.     

Tensile Fracture Strength of Brisbane Tuff by Static and Cyclic Loading Tests

This research presents the results of laboratory experiments during the investigation of tensile strength–strain characteristics of Brisbane tuff disc specimens under static and diametral cyclic loading. Three different cyclic loading methods were used; namely, sinusoidal cyclic loading, type I and II increasing cyclic loading with various amplitude values. The first method applied the stress amplitude?cycle number (s–n) curve approach to the measurement of the indirect tensile strength (ITS) and fracture toughness (K _IC) values of rocks for the first time in the literature. The type I and II methods investigated the effect of increasing cyclic loading on the ITS and K _IC of rocks. For Brisbane tuff, the reduction in ITS was found to be 30 % under sinusoidal loading, whereas type I and II increasing cyclic loading caused a maximum reduction in ITS of 36 %. The maximum reduction of the static K _IC of 46 % was obtained for the highest amplitude type I cyclic loading tested. For sinusoidal cyclic loading, a maximum reduction of the static K _IC of 30 % was obtained. A continuous irreversible accumulation of damage was observed in dynamic cyclic tests conducted at different amplitudes and mean stress levels. Scanning electron microscope images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix because of both inter-granular fracturing and trans-granular fracturing. The main characteristic was grain breakage under cyclic loading, which probably starts at points of contact between grains and is accompanied by the production of very small fragments, probably due to frictional sliding within the weak matrix.     

Remote Sensing of Biologically Reworked Sediments: A Laboratory Experiment

The present study aims to test the application of remote sensing to address the impact of bioturbation on physical sediment properties. Therefore, a laboratory experiment was developed, using microcosms mimicking a marine intertidal water–sediment interface to test the influence of Corophium volutator densities on sediment properties. Three main variables (water content, clay content, and mean grain size) were measured in three treatments (no Corophium, 5,000 Corophium per square meter, and 20,000 Corophium per square meter) after 16 days of bioturbation. Results obtained with conventional—destructive—techniques showed a significant increase of water content and a significant, but small decrease of clay content in the presence of Corophium. The remote sensing technique detected the impact of Corophium on water content as an increase in absorption at 1,450 nm, but was not able to detect the animal impact on clay content. This study demonstrates that remote sensing data could be significantly modified by bioturbation activities and that remote sensing can be applied in the laboratory to address the impact of bioturbation on sediment properties. This possibly opens new perspectives for long-term experiments concerning the role of bioturbation on sedimentary processes.     

Experimental Study of Mechanical Behaviour of Rock Joints Under Cyclic Loading

Summary Evaluation of the effects of small repetitive earthquakes on the strength parameters of rock joints in active seismic zones is of interest of the designers of underground constructions. In order to evaluate these effects, it is necessary to study the behaviour of rock joints under dynamic and cyclic loadings. This paper presents the results of a systematic study on the behaviour of artificial rock joints subjected to cyclic shearing. More than 30 identical replicas have been tested using triaxial compression devices under different conditions of monotonic and cyclic loading. At the first stage a few samples have been tested in monotonic loading modes under various confining pressures and rate of displacement. In the second series of tests, small cyclic loads were applied on the samples for increasing number of cycles, frequency levels and stress amplitudes. These were then followed by monotonic loading again. The variations of maximum and residual shear strengths for each test have been studied. The results show increase of shear strength as a result of the increase in confining pressure and they display decrease of shear strength due to the increase of rate of loading, number of cycles, frequency levels and stress amplitudes.