Simulating damage evolution and fracture propagation in sandstone containing a preexisting 3-D surface flaw under uniaxial compression

Preexisting flaws and rock heterogeneity have important ramifications on the process of rock fracturing and on rock stability in many applications. Therefore, there is great interest in numerical modelling of rock fracture and the underlying mechanisms. We simulated damage evolution and fracture propagation in sandstone specimens containing a preexisting 3-D surface flaw under uniaxial compression. We applied the linear elastic damage model based on the unified strength theory following the rock failure process analysis code. However, in contrast to the rock failure process analysis code, we used the finite element method with tetrahedron elements on unstructured meshes. It provided higher geometrical flexibility and allowed for a more accurate representation of the disk-shaped flaw with various flaw depths, angles, and lengths through locally adapted meshes. The rock heterogeneity was modelled by sampling the initial local Young's modulus from a Weibull distribution over a cubic grid. The values were then interpolated to the computational finite element method mesh. This method introduced an additional length scale for the rock heterogeneity represented by the cell size in the sampling grid. The generation of three typical surface cracking patterns, called wing cracks, anti-wing cracks, and far-field cracks, were identified in the simulation results. These depend on the geometry of the preexisting surface flaw. The simulated fracture propagation, coalescence types, and failure modes for the specimens with preexisting surface flaw show good agreement with recent experimental studies.     

Mohr space and its application to the activation prediction of pre-existing weakness

The activation coefficient equations in the"activation criterion of pre-existing weakness"are relatively complex and not easy to apply to specific applications.The relative activity of pre-existing weaknesses is often critical in geological analysis.The Mohr circle can be used only in two-dimensional stress analysis.By applying the"activation criterion of pre-existing weakness"and combining it with numerical analysis,we establish the correspondence between the pole(n,n)of a pre-existing weakness plane and its orientation in"Mohr space".As a result,the normal stress(n)and shear stress(n)of a pre-existing weakness plane can be readily expressed in Mohr space.Furthermore,we introduce the method and procedures for predicting the activation and relative activation of pre-existing weaknesses in Mohr space.Finally,we apply the Mohr space method and compare the predictions to sandbox modeling results and 3D seismic data.The results show that Mohr space can be used in stress analysis to estimate the activation of a pre-existing weakness in any triaxial stress state.     

从华北克拉通中、西部结构的区域差异性探讨克拉通破坏

详细的深部结构信息是深入认识华北克拉通显生宙改造和破坏的重要依据。基于密集流动地震台阵和固定台网记录的远震P波和S波接收函数资料,获得了跨越华北克拉通东、中、西部的3条剖面的岩石圈和上地幔结构图像,揭示了克拉通不同区域深部结构特征的显著差异。与东部普遍减薄的岩石圈(60～100km)相比,中、西部表现出厚、薄岩石圈共存的强烈横向非均匀性,既在稳定的鄂尔多斯盆地之下保留着厚达200km的岩石圈,又在新生代银川—河套和陕西—山西裂陷区存在厚度<100km的薄岩石圈,差异最大的厚、薄岩石圈仅相距约200km。岩石圈厚度在东、中部边界附近的约100km横向范围内显示出20～40km的迅速增加。岩石圈厚度的快速变化与地表地形从东向西的突然改变以及南北重力梯度带的位置大致吻合,并对应于地壳结构、地幔转换带厚度和660km间断面结构的快速变化。这种从地表到上地幔底部深、浅结构的耦合变化特征表明,东西两侧区域在显生宙可能经历了不同的岩石圈构造演化和深部地幔动力学过程。克拉通东部薄的地壳、岩石圈和厚的地幔转换带以及复杂的660km间断面结构可能与中生代以来太平洋板块深俯冲及其相关过程对这一地区岩石圈的改造和破坏有关;而中、西部存在显著减薄的岩石圈这一观测结果,并结合岩石、地球化学资料表明,克拉通岩石圈改造和减薄不仅发生在东部,而且可能影响了包括中、西部在内的更广泛的区域。岩石圈薄于100km的中、西部裂陷区可能与先前存在于岩石圈中的局部构造薄弱带相联系。这些古老岩石圈薄弱带可能经历了后期构造事件的多次改造,并在新生代印度—欧亚陆陆碰撞过程中被进一步弱化、减薄,最终造成地表裂陷。另一方面,中、西部总体较厚的地壳、岩石圈以及正常偏薄的地幔转换带表明,同太平洋深俯冲对东部的作用相比,包括印度—欧亚大陆碰撞在内的多期热-构造事件对该地区的构造演化影响相对较弱,不足以大范围改造和破坏高强度的克拉通岩石圈地幔根,从而造成了该地区现今岩石圈结构的高度横向不均匀。     

大巴山前陆弧形构造的成因：来自砂箱实验的认识

基于砂箱实验方法,本文对大巴山前陆弧形构造的成因进行了探讨.实验结果表明,多种要素的共同作用造就了大巴山弧形构造的形成,这些要素包括:早期伸展背景下形成的弧形边界、两侧基底地块的砥柱作用、底部滑脱层等.此外,北大巴山早期的伸展构造对随后挤压背景下的构造组合可能具有重要的制约作用.结合前人研究,大巴山弧形构造的演化可分为...     

预制节理岩体试件强度及破坏模式的试验研究

采用相似材料模型试验对不同节理倾角、节理贯通度、节理组数、载荷应变率、试件长径比、节理充填物厚度及类型等7种工况下的预制节理岩体在单轴压缩下的峰值强度及破坏模式进行了研究。结果表明：节理岩体的破坏模式及峰值强度与节理构造形态密切相关。贯通节理岩体将产生沿节理面的剪切破坏或穿切节理面破坏,且与第1种破坏模式对应的岩体峰值强度更低。非贯通节理岩体的强度介于完整岩体和贯通节理岩体之间。随着平行节理组数的增加,岩体峰值强度逐渐下降。随着载荷应变率的增加,岩体峰值强度逐渐增大,相应地试件的破坏模式也变得更加复杂。试件长径比基本没有改变其破坏模式,完整试件仍主要是以张拉破坏为主,而节理试件仍以剪切破坏为主。随着长径比增加,试件峰值强度逐渐增大。随着节理充填物厚度增加,试件峰值强度降低。不同节理填充物对试件峰值强度也有一定影响。     

裂陷盆地断层的形成和演化——目标砂箱模拟实验与认识

摘要 断层的分布、活动和演化是裂陷盆地构造研究的核心,也是其油气勘探的关键。大量高精度的三维地震资料表明裂陷盆地内断层分布组合十分复杂,无法用经典构造地质学理论——Anderson模式作出合理的解释, 现有的模式（叠加模式和斜向伸展模式等）也还没有很好地揭示裂陷盆地断层复杂性的实质和断层形成和演化的内在规律。本文以渤海湾盆地张巨河构造带和北部湾盆地涠西南凹陷两个有高精度三维地震资料覆盖、断层分布与组合十分复杂并富含油气的典型裂陷盆地区为原形,进行了目标砂箱模拟实验,再现了目标区构造的基本特征。砂箱实验所揭示裂陷盆地断裂系统的基本特征是：1）基底先存构造（主要所控制的断层构成了盆地断裂系统的基本格架;2）断层的形成与演化决定于基底先存断裂的取向、区域伸展方向及伸展量。首先形成的是有刚性边界基底先存断裂控制的断层。其次,形成的是薄弱带基底先存断裂控制的断层。继而,形成的是小型基底先存断裂控制的断层。最后,在远离基底先存构造的区域,形成与伸展方向垂直的新生断层。3）伸展作用早期,断层的方位主要决定于基底先存断裂的方位。随着伸展量的增大,受伸展方向的影响越来越大,晚期形成的断层常反映区域伸展作用方向。4）不协调递进伸展变形过程是导致裂陷盆地复杂断层体系的根本原因。上述砂箱模拟实验揭示的裂陷盆地在不协调性伸展作用下断层的形成与演化规律可以利用“不协调性准则”从理论上给予解释,表明在裂陷盆地中具有普遍意义。     

高压喷射灌浆技术在橡胶坝防渗工程中的应用

通过高喷灌浆修补垂直防渗土工膜缺陷的工程实例，介绍了高压喷射灌浆防渗墙的设计方法、施工工艺及优良的防渗效果。     

工程锚杆注浆质量无损检测技术研究与应用

采用应力波一维波动理论,对清江水布垭工地21根模型锚杆和74根工程锚杆进行了测试与研究,初步形成了工程锚杆无损检测方法,建立了波形特征与锚杆注浆密实度之间的关系,并从锚杆的有效锚固长度、注浆密实度及缺陷大小和位置三个方面评价锚杆的施工质量,确定了该工程锚杆无损检测验收标准。     

FOCAL MECHANISM SOLUTIONS AND STRESS FIELD OF THE 2019 CHANGNING,SICHUAN MAINSHOCK AND ITS MODERATE-STRONG AFTERSHOCKS(MS≥4.0)

A M_S6.0 earthquake with shallow focal depth of 16km struck Changning County, Yibin City, Sichuan Province at 22:55: 43(Beijing Time)on 17 June 2019. Although the magnitude of the earthquake is moderate, it caused heavy casualties and property losses to Changning County and its surrounding areas. In the following week, a series of aftershocks with M_S≥4.0 occurred in the epicentral area successively. In order to better understand and analyze the seismotectonic structure and generation mechanism of these earthquakes, in this paper, absolute earthquake location by HYPOINVERSE 2000 method is conducted to relocate the main shock of M_S6.0 in Changning using the seismic phase observation data provided by Sichuan Earthquake Administration, and focal mechanism solutions for Changning M_S6.0 main shock and M_S≥4.0 aftershocks are inferred using the gCAP method with the local and regional broadband station waveforms recorded by the regional seismic networks of Sichuan Province, Yunnan Province, Chongqing Municipality, and Guizhou Province. The absolute relocation results show that the epicenter of the main shock is located at 28.35°N, 104.88°E, and it occurred at an unusual shallow depth about only 6.98km, which could be one of the most significant reasons for the heavier damage in the Changning and adjoining areas. The focal plane solution of the Changning M_S6.0 earthquake indicates that the main shock occurred at a thrust fault with a left-lateral strike-slip component. The full moment tensor solution provided by gCAP shows that it contains a certain percentage of non-double couple components. After the occurrence of the main shock, a series of medium and strong aftershocks with M_S≥4.0 occurred continuously along the northwestern direction, the fault plane solutions for those aftershocks show mostly strike-slip and thrust fault-type. It is found that the mode of focal mechanism has an obvious characteristic of segmentation in space, which reflects the complexity of the dislocation process of the seismogenic fault. It also shows that the Changning earthquake sequences occurred in the shallow part of the upper crust. Combining with the results from the seismic sounding profile in Changning anticline, which is the main structure in the focal area, this study finds that the existence of several steep secondary faults in the core of Changning anticline is an important reason for the diversity of focal mechanism of aftershock sequences. The characteristics of regional stress field is estimated using the STRESSINVERSE method by the information of focal mechanism solutions from our study, and the results show that the Changning area is subject to a NEE oriented maximum principal stress field with a very shallow dipping and near-vertical minimum principal stress, which is not associated with the results derived from other stress indicators. Compared with the direction of the maximum principal compressive stress axis in the whole region, the direction of the stress field in the focal area rotates from the NWW direction to the NEE direction. The Changning M_S6.0 earthquake locates in the area with complex geological structure, where there are a large number of small staggered fault zones with unstable geological structure. Combining with the direction of aftershocks distribution in Changning area, we infer that the Changning M_S6.0 earthquake is generated by rupturing of the pre-existing fault in the Changning anticline under the action of the overall large stress field, and the seismogenic fault is a high dip-angle thrust fault with left-lateral strike-slip component, trending NW.