Borehole breakouts and compaction bands in two high-porosity sandstones

We compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field principal stresses. Tablerock sandstone has a porosity of 28%, and is composed of 55% quartz and 37% weaker feldspar grains. Grain cementation is substantial through microcrystalline quartz. Critical far-field stresses induce failure around boreholes in the form of V-shaped (dog-eared) breakouts, the result of dilatant intra and trans-granular microcracking subparallel to both the maximum horizontal far-field stress and to the borehole wall. On the other hand, boreholes in Mansfield sandstone, which has similar porosity (26%), but contains mainly quartz grains (90%) held together primarily by spot-sutured contacts, fail by developing fracture-like breakouts. We show evidence suggesting that Mansfield sandstone breakouts are preceded by the development of a narrow (several grain diameters) localized compaction zone along the direction of the least horizontal stress, which is where the maximum compressive tangential stress concentration occurs. Failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band. The type of cementation, mineral homogeneity, grain strength, and sphericity appear to be major factors in the formation of compaction bands. Some breakout dimensions in both rocks are correlatable to the far-field principal stresses, and can be potentially used (in conjunction with other information) as indicators of their magnitudes.     

单向碳纤维增强环氧基复合物中纤维破坏的荷载传递微观机制:第三部分复合性能的多尺度再现

第三部分介绍了多尺度再现方法。该方法考虑了与复合物退化相关的非常重要的微观现象,包括纤维的断裂、界面剥离、纤维之间的超负荷以及模具的黏弹性性能。分析结果用以精确预测单向碳纤维增强环氧基复合物中纤维的宏观破坏以及量化相同材料压力容器中的损伤累积。该方法还介绍了由于纤维的破坏形成的声发散行为,以及如何预测压力容器的残余寿命。     

岩石劈裂与岩石破坏性质的不稳定性

 岩石劈裂是裂纹受轴向主压缩荷载引起的裂纹尖端开裂所致,这种开裂与Griffith理论的分析结果不同,它是一种与岩石压缩剪切破坏不同的压缩张拉破坏。基于Griffith裂纹分析方法,证明了裂纹尖端存在小拉应力极值,结合岩石破坏的渐进性特点,说明岩石在压缩过程中可以形成张拉破坏(劈裂破坏),这一破坏形式是修正的Griffith强度理论以外的另一种破坏形式。给出了岩石劈裂破坏的条件,即裂纹纵向荷载高且侧压力很小或为0,且没有后续荷载的进一步作用或荷载时间短,并通过岩石厚壁圆筒卸荷试验验证了该岩石劈裂破坏条件的合理性。基于能量原理,分析岩石卸荷在无外功条件下张拉破坏与压缩破坏的能量耗散特征,给出岩石破坏时的能量关系式;从能量的积聚与耗散角度,揭示了煤岩体动力冲击、巷道围岩突然破坏等动力破坏现象产生的机制。     

Effects of borehole stress concentration on elastic wave velocities in sandstones

The stress redistribution that occurs in the vicinity of a borehole may lead to damage or failure of the rock. If the vertical stress is a principal stress, the principal stresses at the wall of a vertical well are the hoop, radial, and vertical stresses. The hoop stress varies with position around the wellbore and takes its maximum compressive value at the azimuth of the far-field minimum horizontal stress. This may lead to compressive failure at this azimuth if the compressive strength of the rock is exceeded. Recent developments in sonic logging allow the variation in elastic wave velocities with radius and azimuth around a borehole to be characterized. Since elastic wave velocities in sandstones are sensitive to changes in stress, owing to the presence of stress-sensitive grain boundaries within the rock, this allows the changes in stress to be monitored. An expansion of the elastic compliance of the grain boundaries to first order in stress shows that a radially polarized, vertically propagating shear wave is more sensitive to radial stress than to hoop stress. Close to the wellbore, however, large changes in stress occur, and any nonlinearity in the variation in compliance of the grain boundaries as a function of stress becomes important. This nonlinearity increases the sensitivity to the wellbore pressure, and can cause significant reductions in the velocity of compressional and shear waves in the vicinity of the wellbore.     

The local and global instability and vibration of systems subjected to non-conservative loading

The paper presents the results of theoretical and numerical studies on the slender, geometrically nonlinear system supported at the loaded end by a spring of a linear characteristic and subjected to non-conservative (generalized Beck's) loading. The boundary problem was formulated based on the Hamilton principle and then solved using, due to the nonlinearity, the small parameter method. The bifurcation force depending on system parameters and local and global instability regions has been determined in the study, and the characteristic curves have been plotted in the load-free vibration plane.     

Micromechanics of rock damage: Advances in the quasi-brittle field

Constitutive models play an essential role in numerical modeling and simulation of nonlinear deformation, progressive damage and failure of rock-like materials and structures. Recent advances in the quasi-brittle field show that upscaling methods by homogenization have provided a new efficient way to derive macroscopic formulations of rocks from their microstructure information and local properties and then to model nonlinear mechanical behaviors identified at laboratory. This paper aims first at relating the mechanical phenomena on sample scale to their respective mechanisms on microscale. Main focus is put on unilateral effects due to crack's opening/closure transition, material anisotropy induced by crack growth in some preferred directions and multiphysical coupling at microcracks. After a brief introduction to the linear homogenization method and its application to crack problems, we present some recent advances achieved in the combined homogenization/thermodynamics framework, including anisotropic unilateral damage-friction coupling, theoretical failure prediction in conjunction with deformation analyses, poromechanical coupling, analytical solutions and numerical implementation with application to typical brittle rocks.     

Counter-intuitive collapse of single-layer reticulated domes subject to interior blast loading

A commercial finite element package, LS-DYNA, was employed to simulate the response of a single-layer reticulated dome to an interior blast. The dome, which was initially stressed by static preloading, encountered an interesting phenomenon namely counter-intuitive collapse, which was found during the blast analysis. The Johnson–Cook constitutive model for mild steel was used to identify different failure modes of the dome for more than 2430 samples. A seemingly counter-intuitive collapse was identified due to dynamic instability. This unusual collapse was explained using total potential curves, and the critical blast load was defined. The effect of different static preloads, which is the other determination factor of counter-intuitive collapse, was investigated. A relationship between collapse and static/dynamic loads was also obtained. The results indicate that single-layer reticulated domes, with large initial stresses, may collapse at lower dynamic loads than expected.     

基于围岩本体Mogi-Coulomb强度准则的层理性岩层斜井井壁稳定模型

 层理性地层中进行大斜度井施工的井壁失稳问题较为突出,在传统井壁稳定模型基础上,以弱面对岩石强度的弱化作用实验为依据,引入欧拉变换充分考虑三维地应力方向的任意性,利用Mogi-Coulomb准则强化中主应力对围岩本体强度的影响,结合单弱面强度准则建立分析层理性地层斜井井壁稳定问题的模型,并进一步给出计算斜井坍塌压力与破裂压力的方法与公式。实验与计算结果表明：当加载方向与弱面夹角为30°时岩石强度最低;Mogi-Coulomb准则因考虑了中主应力的影响而对围岩本体强度的估计更为有效;考虑层理弱面影响的井壁坍塌破坏区域明显增大,破坏位置也发生改变;取得最小坍塌压力值的钻井方向在空间中与层理面并非简单的垂直关系;空间中关于地应力主平面对称的井孔破裂压力相同。利用所建模型进行安全泥浆压力窗口的计算可为安全钻井以及斜井轨迹设计提供理论依据。     

Volumetric analysis of rock mass instability around haulage drifts in underground mines

Haulage networks are vital to underground mining operations as they constitute the arteries through which blasted ore is transported to surface. In the sublevel stoping method and its variations, haulage drifts are excavated in advance near the ore block that will be mined out. Numerical modeling is a technique that is frequently employed to assess the redistribution of mining-induced stresses, and to compare the impact of different stope sequence scenarios on haulage network stability. In this study, typical geological settings in the Canadian Shield were replicated in a numerical model with a steeply-dipping tabular orebody striking EW. All other formations trended in the same direction except for two dykes on either side of the orebody with a WNW–ESE strike. Rock mass properties and in situ stress measurements from a case study mine were used to calibrate the model. Drifts and crosscuts were excavated in the footwall and two stope sequence scenarios – a diminishing pillar and a center-out one – were implemented in 24 mining stages. A combined volumetric-numerical analysis was conducted for two active levels by comparing the extent of unstable rock mass at each stage using shear, compressive, and tensile instability criteria. Comparisons were made between the orebody and the host rock, between the footwall and hanging wall, and between the two stope sequence scenarios. It was determined that in general, the center-out option provided a larger volume of instability with the shear criterion when compared to the diminishing pillar one (625,477 m³ compared to 586,774 m³ in the orebody; 588 m³ compared to 403 m³ in the host rock). However, the reverse was true for tensile (134,298 m³ compared to 128,834 m³ in the orebody; 91,347 m³ compared to 67,655 m³ in the host rock) instability where the diminishing pillar option had the more voluminous share.     

波散射问题求解中消除多次透射公式漂移失稳的一种措施

在采用多次透射公式求解结构-地基体系地震反应等波散射问题时,其稳定性是人们所关注和研究的重要方面。针对其漂移失稳机制的研究表明,在将总波场分离为入射波场和散射波场从而实现波动输入和多次透射边界条件施加的过程中,采用连续介质解析解给出的入射波场与计算中的实际入射波场存在较大差别,会导致漂移失稳现象的出现。基于此,提出了一种直观的改进措施,即利用多次透射边界建立边界计算区,从而获得入射波离散数值解,并应用于波动的输入过程。数值实验验证了该措施可以有效地消除计算中的漂移失稳现象,给出更为合理的计算结果。     

浅埋盾构隧道开挖面失稳大比尺模型试验研究

盾构隧道施工过程中支护压力不足会引起开挖面失稳。针对浅埋盾构隧道,采用大比尺物理模型试验对盾构开挖面稳定进行研究。试验通过控制支护板移动速度来模拟开挖面失稳变形过程,并对支护压力和地表变形进行监测,发现了支护力和地表变形发展的3个阶段。与此同时,采用数码摄像技术对失稳土体实时观测,并利用颗粒图像技术对图像进行处理得到失稳土体的位移增量场。对比发现,位移增量场的变化规律与支护力和地表变形的3个阶段相对应。     

中深部含软弱夹层的深基坑土钉支护失稳破坏数值模拟分析

以中核北京科技园综合楼深基坑项目在分层开挖与分层土钉支护中出现的边坡顶端土体开裂、坡底土体坍塌形式的失稳破坏现象为研究背景,运用FLAC3D有限差分软件进行模拟分析。数值模拟分析中考虑土层分布、基坑边坡坡度、土钉的排列方式、钢筋网喷射混凝土面层、含水层的孔隙水压力、分层开挖支护等因素,使模拟分析与实际相接近。根据基坑大小建立相应的基坑边坡三维模型,采用莫尔-库仑弹塑性模型计算基坑边坡在不同开挖支护阶段的位移变形,并通过后处理程序显示基坑整体、最大位移截面的位移云图,记录所设置的关键点处水平位移与沉降量。在工程实际分布开挖过程中,测量每次开挖支护所引起的边坡水平位移与沉降量。将数值模拟中所得节点水平位移、沉降量与两者的实测数值对比分析,由此判断基坑潜在滑裂面与可能出现坍塌的大致位置,为工程设计方案中关于变形量较大区域支护方式的整改提供重要参考。     

Statistics of damages to timber structures in Germany

The paper reports results of a comprehensive failure analysis with the focus on timber structures located in Germany. Data of 550 damages and corresponding circumstances were recorded, classified and evaluated. Statistical distributions of the structure characteristics and the kind of damages are described. The majority of the damages are distributed on cracks along the grain, by far, decay, tension and shear failure follow. In about 90% of the damages glulam is affected. Nearly one thousand damage-cause-relations show that constructions resulting in stresses perpendicular to the grain, alternating climates and overloading rank as one of the main reasons for damages. Spending more time on proper design and on careful details is seen as the key consequence of this analysis.     

带裙房的大底盘高层建筑桩筏基础沉降后浇带位置确定的浅析

在大底盘结构的桩筏基础中设置沉降后浇带既可以释放相当部分沉降差产生的不利影响,又可以满足建筑功能的要求,目前已被越来越多的工程应用。后浇带设置位置的确定受到诸多方面的限制,同时,位置的选择也影响其作用效果。文章对沉降后浇带的设置位置变化对筏板内力的影响作了一定程度上量化的计算,对两者之间相关性进行了研究。归纳了随位置变化,沉降后浇带作用的变化规律,提出了确定后浇带位置的建议,对今后工程应用提供了理论依据。     

Bearing capacity evaluation of small-diameter spiral piles in soft ground subjected to combined loads

In the current practices, the evaluation method for the bearing capacity of small-diameter spiral piles is a conservative method performed on the safe side, since it does not consider the integration effect with the surrounding ground due to rotational press-fitting. In addition, the evaluation method for the bearing capacity of spiral piles subjected to combined loads has not been established. In this study, a series of push-in, pull-out, and horizontal loading tests on spiral piles constructed on a soft viscous ground is conducted, and a method to easily consider the integration effect of the surrounding ground by rotational press-fitting is proposed. Subsequently, based on the proposed method, small-diameter spiral piles are modelled with a FEM analysis, various test results are reproduced, and coefficients for considering the integration effect of the surrounding ground in a numerical analysis are established. With the same FEM analysis method, using the established coefficients, the bearing capacity characteristics of vertical and batter spiral piles for combined loads are obtained. Finally, based on the results, formulas for the bearing capacity envelopes in the H-V-M space are proposed and the accuracy of the formulas are verified.     

Change in the hydromechanical characteristics of embankment material due to compaction state conditions

In recent years, earthquakes and heavy rains have frequently caused soil embankments to collapse. In order to prevent the collapse of embankments, it is well known that sufficient compaction and drainage control are necessary. Although numerous research findings have described compacted soils, compaction management has been essentially based on simple parameters such as dry density, degree of saturation or air content. It is important for the construction of a stiff embankment that the effect of compaction condition on the mechanical properties and inherent anisotropy of compacted soil should be recognized in detail. In the present study, the relationships between the compaction condition and the mechanical properties obtained from laboratory tests using saturated specimens are presented. Specifically, undrained monotonic and cyclic shear strength, shear modulus, and permeability are reported. The arrangements of soil particles were also observed with a microscope. From the observation of fabric characteristics of soil particles, an inherent anisotropy of compacted soil is discussed. Furthermore, a conceptualization of the relationship between the fabric of soil particles and the mechanical characteristics for each compaction condition is suggested.