Radial deformation and failure of stabilised soft clay under uniaxial compression

This study examined the performances of four newly developed binders in stabilising soft Finnish clay by considering both the mechanical behaviour and environmental effect (i.e., carbon footprint). Thirty-six compacted clay-binder mixtures were investigated via uniaxial compression tests, during which photogrammetric scans were performed at axial strains of 0, 1, 3, and 7.5%. The testing protocol enabled the characterisation of full-field radial deformations and fracture developments. Experimental results demonstrated that varying the amounts and types of industrial by-products used in the production of binders has substantial potential to reduce carbon footprints. In terms of radial deformations, their distributions were highly non-uniform at different heights and loading stages. Radial displacements increased continuously at axial strains smaller than 3% (corresponding to peak strength point), whereas such a continuity disappeared afterwards, and global failures appeared. Based on three-dimensional (3D) reconstructions, the observed failures were categorised into three modes: inclined shearing (IS), axial splitting (AS) and hybrid shearing-splitting (HSS). For the studied specimens, shearing was the most common mechanism leading to failure.     

Dynamic failure analysis on granite under uniaxial impact compressive load

High strain-rate uniaxial compressive loading tests were produced in the modified split Hopkinson pressure bar (SHPB) with pulse shaper on granite samples. It was shown that the failure of the granite cylinder was typical tensile splitting failure mode by sudden splitting parallel to the direction of uniaxial compressive loading at different strain rates. Besides, it was concluded that not only the strength of granite increased, but also the fragment size decreased and the fragment numbers increased with the increasing strain rate. To quantitatively analyze the failure phenomena, the numerical calculation based on a dynamic interacting sliding microcrack model was adopted to investigate the influence of microcrack with the different initial crack length, crack angle, crack space and friction coefficient on the macro-mechanical properties of granite under different strain rates. Accordingly, the strain-dependency of the compression strength and the fragmentation degree of granite was explained reasonably.     

花岗岩在单轴冲击压缩荷载下的动态断裂分析

利用脉冲整形器改进后的分离式Hopkinson压杆(SHPB)系统,对新加坡Bukit Timah地区的花岗岩圆柱形试样进行了高应变率下的单轴压缩实验。实验结果发现:随着应变率的增加,不仅花岗岩材料的抗压强度增大,而且以轴向拉伸劈裂为主要破坏形式的破碎程度也有所提高,表现为碎块的尺寸减小和数量增加。针对上述花岗岩的动态特性,采用多裂纹相互作用的动态滑移型裂纹模型定量的分析了不同应变率下,材料的微裂纹的初始长度、角度、初始裂纹间距以及裂纹面的摩擦系数等微裂纹特征对材料动态强度及破碎的影响,将岩石类材料的宏观动力学特性与其细观微结构联系起来,合理地解释了花岗岩的动态强度及破碎程度的应变率相关性。     

Dynamic failure analysis on granite under uniaxial impact compressive load

High strain-rate uniaxial compressive loading tests were produced in the modified split Hopkinson pressure bar (SHPB) with pulse shaper on granite samples. It was shown that the failure of the granite cylinder was typical tensile splitting failuremode by sudden splitting parallel to the direction of uniaxial compressive loading at different strain rates. Besides, it was concluded that not only the strength of granite increased, but also the fragment size decreased and the fragment numbers increased with the increasing strain rate. To quantitatively analyze the failure phenomena, the numerical calculation based on a dynamic interacting sliding microcrack model was adopted to investigate the influence of microcrack with the different initial crack length, crack angle, crack space and friction coefficient on the macro-mechanical properties of granite under different strain rates. Accordingly, the strain-dependency of the compression strength and the fragmentation degree of granite was explained reasonably. __________ Translated from Chinese Journal of Geotechnical Engineering, 2007, 29(3): 385–390 [译自: 岩土工程学报]     

Crack initiation of granite under uniaxial compression tests: A comparison study

In this study, a combination of acoustic emission (AE) method (AEM) and wave transmission method (WTM) is used to investigate the behaviors of AE and ultrasonic properties corresponding to initial fracturing in granitic rocks. The relationships of AE characteristics, frequency spectra, and spatial locations with crack initiation (CI) are studied. The anisotropic ultrasonic characteristics, velocity distributions in different ray paths, wave amplitudes, and spectral characters of transmitted waves are investigated. To identify CI stress, damage initiations characterized by strain-based method (SBM), AEM and WTM are compared. For granite samples, it shows that the ratio of CI stress to peak strength estimated by SBM ranges from 0.4 to 0.55, and 0.49–0.6 by WTM, which are higher than that of AEM (0.38–0.46). The CI stress identified by AEM indicates the onset of microcracking, and the combination of AEM and WTM provides an insight into the detection of rock damage initiation and anisotropy.     

单轴压缩条件下锚杆影响脆性岩体破裂的细观机制

锚杆支护系统是控制深部脆性围岩动力灾害的重要措施,但锚固理论研究仍滞后,锚杆支护下的脆性岩体破坏问题困扰着深部岩体工程实践。根据实际工程中锚杆支护下脆性围岩的浅表局部破坏特点,通过室内相似模型试验研究单轴压缩条件下锚杆杆径影响完整脆性岩体的破坏特性,试验表明,锚杆杆径对脆性岩体弹性模量和强度的提升存在最优匹配的特点,一味强调增大锚杆直径并不能达到理想的围岩控制效果;锚杆改变了脆性岩体单轴压缩破坏模式,宏观上由劈裂破坏转为剪切破坏,杆径对试样剪切破坏的程度有所影响。从细观角度,建立了含两条固有主裂纹的裂纹扩展分析模型,加锚试样单轴压缩破裂模式的改变,可以归结为锚杆锚固止裂效应对试样内部裂纹扩展的抑制作用,使翼裂纹与主裂纹长度比η变小。根据最易开裂角度ζ的计算结果,翼裂纹较长时,翼裂纹朝外载作用方向扩展,产生劈裂破坏,翼裂纹较短时,翼裂纹偏离外载作用方向扩展,产生剪切破坏。从细观上很好地解释了锚杆改变脆性岩体破裂模式的作用机制。     

Time-dependent tests on intact rocks in uniaxial compression

The influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine. An experimental data processing method for such creep tests is suggested. The correction formulas to determine the rheological model parameters are derived for the case when load relaxation is considered. Creep and relaxation tests under uniaxial compression on four types of rocks are conducted using an electronic hydraulic servo-controlled stiff testing machine. The creep and relaxation laws of the different rocks are compared. The complete stress–strain curves for red sandstone specimens are obtained at nine strain rates from 2.43×10⁻⁶ to 4.38×10⁻³/s. The effects of strain rates on rock strength and limit strain are discussed. Empirical equations to evaluate the strain rate dependence of rock mechanical properties are presented.     

Stiffened steel plates under uniaxial compression

The stability of steel plates stiffened with tee-shape sections under uniaxial compression and combined uniaxial compression and bending was investigated using a finite element model. The emphasis of the work presented in this paper was to find the parameters that uniquely describe the strength and behaviour of stiffened steel plates. A finite element model, validated using the results of tests on full-size stiffened plate panels, was used to investigate the scale effect for five dimensionless parameters. The parameters investigated were: the transverse slenderness of the plate, the slenderness of the web and flange of the stiffener, the ratio of torsional slenderness of the stiffener to the transverse slenderness of the plate, and the stiffener-to-plate area ratio. Average magnitude residual stresses and initial imperfections were assumed for this study.A parametric study covering a wide range of dimensionless parameters indicated that stiffened steel plates do not fail by stiffener tripping unless a bending moment is applied to create flexural compressive stresses in the stiffener. Although plate buckling and overall buckling were found to lead to a very stable post-buckling behaviour, the interaction between these two buckling modes was found to give rise to a sudden loss of capacity following initial plate buckling. The plate transverse slenderness, the stiffener slenderness-to-plate slenderness ratio, and the stiffener-to-plate area ratio were found to have a significant effect on this behaviour.A comparison of the numerical analysis results with API and DnV design guidelines indicates that the guidelines predict stiffened steel plate capacity with various degrees of success, depending on the governing mode of failure. Neither guidelines address the potential interaction-buckling phenomenon.     

中粒辉长岩单轴三轴压缩试验研究

研究了中粒辉长岩在饱和、风干两种状态下的单轴和三轴压缩强度变形特性。在单轴和三轴压缩试验中得到了应力与各种应变之间的关系曲线,采用回归分析得到了强度准则和强度参数c,φ值,分析了围压和含水量对中粒辉长岩的影响。     

Acoustic emission characteristics and energy mechanism in karst limestone failure under uniaxial and triaxial compression

Bulletin of Engineering Geology and the Environment - Acoustic emission (AE) tests were performed on saturated karst limestone under uniaxial and triaxial compression. Mechanical properties, AE...     

Deformation and failure characteristics of sandstone under uniaxial compression using distributed fiber optic strain sensing

This paper investigates the deformation and fracture propagation of sandstone specimen under uniaxial compression using the distributed fiber optic strain sensing (DFOSS) technology. It shows that the DFOSS-based circumferential strains are in agreement with the data monitored with the traditional strain gage. The DFOSS successfully scans the full-field view of axial and circumferential strains on the specimen surface. The spatiotemporal strain measurement based on DFOSS manifests crack closure and elastoplastic deformation, detects initialization of microcrack nucleation, and identifies strain localization within the specimen. The DFOSS well observes the effects of rock heterogeneity on rock deformation. The advantage of DFOSS-based strain acquisition includes the high spatiotemporal resolution of signals and the ability of full-surface strain scanning. The introduction to the DFOSS technology yields a better understanding of the rock damage process under uniaxial compression.     

Study of failure and scale effects in rocks under uniaxial compression using 3D cellular automata

A numerical model for simulating the rock fracturing process has been developed and implemented in the three-dimensional code EPCA^3D, which is based on the earlier EPCA^2D (elasto-plastic cellular automaton). EPCA^3D has the ability to simulate the initiation, propagation and coalescence of cracks in the failure processes of rocks. Using this code, we studied the failure processes of simulated rock specimens with different sizes and shapes in three-dimensional space. It is concluded that the ‘scale effect’ is not directly related to the heterogeneity, if the rock specimens have the same homogeneity. If the platen-rock elastic mismatch effect in physical testing is not considered in the numerical simulations, there is almost no scale effect for rock specimens with different sizes and shapes. However, by considering the platen-rock interaction in the simulation of the rock failure process with the EPCA^3D code, the modeling results reproduced the well-known phenomenon of scale effects found in physical experiments.     

Strength of composite flysch samples under uniaxial compression

Bulletin of Engineering Geology and the Environment - This paper presents the results of uniaxial compression strength tests performed on samples composed of sandstone and siltstone discs. Samples...     

聚丙烯纤维混凝土单轴压缩性能研究

通过低速加载得到了不同条件下聚丙烯纤维混凝土的应力与应变关系曲线,通过对聚丙烯纤维与基体混凝土的界面力学分析,得到了聚丙烯纤维在混凝土中的适用范围,对单轴压缩下的聚丙烯纤维混凝土进行了细观机理研究,得到了单轴压缩混凝土的本构方程。     

Estimation of uniaxial compressive strength from point load strength,Schmidt hardness and P-wave velocity

Uniaxial compressive strength is considered one of the most important parameters in the characterization of rock material in rock engineering practice. The study investigated correlations between uniaxial compressive strength and point load index, P-wave velocity and Schmidt hardness rebound number together with the effects of core diameter size. A total of 150 core samples at five different diameters (54, 48, 42, 30 and 21 mm) were obtained from sandstone, limestone and cement mortar. Ten saturated samples at each diameter (length:diameter ratio 2:1) were prepared from each of the three materials. The best correlations were found between uniaxial compressive strength and point load or Schmidt hammer values. The closest relationship was observed for the 48 mm diameter cores.      

Investigation of mechanical properties of fractured marbles by uniaxial compression tests

Uniaxial compression tests(UCTs)on 34 naturally fractured marble samples taken from the transportation tunnels of Jinping II hydropower station were carried out using the MTS815 Flex test GT rock testing system.Rockburst proneness index WET is determined for the marble samples with the UCTs.According to the number,size and spatial structure characteristics of the internal natural fractures of the marble samples,fractures are basically divided into 4 types,namely,single fracture,parallel fracture,intersectant fracture and mixed fracture.The mechanical properties of naturally fractured rocks(4 types)are analyzed and compared with those of intact rock samples(without natural fractures).Experimental results indicate that failure characteristics of fractured rocks are appreciably controlled by fracture distribution or fracture patterns.In comparison with intact rocks,the failure of fractured marbles is a locally progressive failure process and finally rocks fail abruptly.Statistically,the uniaxial compressive strengths(UCSs)of rocks with single,parallel,intersectant and mixed fractures are 0.72,0.69,0.59 and 0.46 times those of the intact rocks,respectively.However,the elastic modulus of the fractured Yantang marbles is generally not different from that of intact rocks.But the elastic moduli of Baishan marble with single,intersectant and mixed fractures are 0.61,0.62 and 0.45 times those of intact rocks,respectively.Experimental results also indicate that WET of fractured marbles is generally smaller than that of intact marbles,which implies that rockburst intensity of fractured marble in field may be controlled to some extent.In addition,the bearing capacity of surrounding rocks is also reduced,thus the surrounding rocks should be supported or reinforced timely according to practical conditions.     

单轴压缩下混合片麻岩脆性破坏特征分析

岩石单轴压缩变形试验是地下工程勘察中的基本力学试验。本文根据混合片麻岩单轴压缩变形试验的峰前应力—应变曲线,把其破坏过程分为裂纹闭合阶段、线弹性变形阶段、裂纹稳定扩展阶段和裂纹非稳定扩展阶段,并采用滑点回归分析技术得到了混合片麻岩各阶段裂纹临界应力值,即:裂纹闭合应力为23MPa,裂纹起裂应力为45MPa,裂纹损伤应力为86MPa,最后进行了单轴压缩下混合片麻岩脆性破坏过程细观模拟分析,为工程支护设计和围岩破坏机理分析提供参考。     

黑色页岩巴西劈裂破坏的层理效应研究及能量分析

以重庆龙马溪组黑色页岩为试样,对黑色页岩进行了不同层理角度的巴西劈裂试验,研究了其抗拉强度和破坏形态的各向异性特征,分析了其变形破坏过程中的吸收能变化规律,揭示了层理角度、抗拉强度、AE能量与最终吸收能之间的关系。结果表明:1黑色页岩的抗拉强度受层理影响明显,90°时,抗拉强度最大,最小值则出现在30°附近。在0°≤θ≤30°和60°≤θ≤90°,抗拉强度值变化较小,而30°≤θ≤60°,其值随着层理角度增加而迅速增大;2试样破坏形态可以分为三类典型的裂纹,且低层理角度的试样仅出现单一种类的裂纹,而高层理角度的试样存在多种裂纹形态;3吸收能随着载荷增加呈非线性增长,根据吸收能增长速率的大小,发现黑色页岩发生拉伸滑移、滑移拉伸、纯拉伸、滑移和压张型拉伸破坏的剧烈程度依次减弱;4低角度试样的最终吸收能小,高角度试样的最终吸收能大,在30°≤θ≤60°,最终吸收能变化跨度最大,其与抗拉强度和AE能量均存在相应的二次非线性关系,为页岩气勘探开发提供了参考。