Simulation of drilling‐induced compaction bands using discrete element method

Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory‐scale drilling tests in some high‐porosity quartz‐rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture‐like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production. This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture‐like breakouts and to identify the role of far‐field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre‐existing far‐field stresses. Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture‐like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture‐like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far‐field stresses. Copyright © 2013 John Wiley & Sons, Ltd.     

The application of satellite differential SAR interferometry-derived ground displacements in hydrogeology

The application of satellite differential synthetic aperture radar (SAR) interferometry, principally coherent (InSAR) and to a lesser extent, persistent-scatterer (PSI) techniques to hydrogeologic studies has improved capabilities to map, monitor, analyze, and simulate groundwater flow, aquifer-system compaction and land subsidence. A number of investigations over the previous decade show how the spatially detailed images of ground displacements measured with InSAR have advanced hydrogeologic understanding, especially when a time series of images is used in conjunction with histories of changes in water levels and management practices. Important advances include: (1) identifying structural or lithostratigraphic boundaries (e.g. faults or transitional facies) of groundwater flow and deformation; (2) defining the material and hydraulic heterogeneity of deforming aquifer-systems; (3) estimating system properties (e.g. storage coefficients and hydraulic conductivities); and (4) constraining numerical models of groundwater flow, aquifer-system compaction, and land subsidence. As a component of an integrated approach to hydrogeologic monitoring and characterization of unconsolidated alluvial groundwater basins differential SAR interferometry contributes unique information that can facilitate improved management of groundwater resources. Future satellite SAR missions specifically designed for differential interferometry will enhance these contributions.     

广州市轨道交通二十一号线工程路基抗震性测试研究

传统路基动模量法大都进行单路基抗震性能分析,缺乏复合路基抗震性能分析,存在较高的偏差。对广州市轨道交通二十一号线工程路基抗震性进行测试,分析该地地质条件后,通过标准贯入实验法判断原始路基土质液化情况,使用瑞典条分法测试原始路基的抗震性能,判别出该路基液化土层滑动安全系数低、抗震稳定性差,应对其进行复合路基加固。通过圆弧滑动法对加固后的复合路基中碎石桩桩网结构路基以及CFG桩桩网结构路基进行抗震性测试。实验结果表明,所提方法抗干扰性能强且测试结果精度高。     

强夯振动加速度的量测及现场试验研究

结合延安新区强夯加固地基工程,进行强夯振动加速度测试现场试验。选取填方区场地,结合地形设计监测方案,以测试强夯波沿水平方向和斜坡的传播规律。在选定试验段的某级填方场地上布置多组加速度传感器,运用多个8通道24位高速采集卡记录强夯振动加速度在平面和斜坡的传播和衰减过程,探讨强夯振动波沿水平面和斜坡的传播规律以及能级对振动加速度的影响。结果表明:径向和竖向加速度值均随与夯点距离的增大而减小;在与夯击点距离相同处,强夯振动波沿水平方向传播的径向和竖向加速度值要大于沿斜坡方向;随着能级的增大,强夯产生的夯击波增强,在与夯击点相同距离处产生的径向和竖向加速度均明显增大。     

凸轮型冲击旋压钻头成孔机理研究

挤密钻头是潜孔锤冲击挤密钻进技术的关键所在,其结构形式直接影响着钻进效率、成孔质量和应用效果。应用非线性有限元理论,对动载荷作用下土体的弹塑性和压缩性的变化规律进行了分析。研究发现,常规钻头在冲击挤密钻进过程中产生的桩效应是导致钻进效率降低的主要原因。为了克服桩效应的影响,利用ANSYS/LSDYNA软件对阶梯式钻头和凸轮旋压钻头的钻进过程进行数值模拟分析,并通过实钻试验进行验证。数值模拟分析和试验结果均表明：凸轮旋压钻头能够有效避免钻进过程中土体回弹问题,降低桩效应对钻进效率的影响。试验结果与模拟结果相吻合,表明数值模拟的结果相对可靠。     

级配碎石填料大三轴试验及累积塑性应变预测模型

级配碎石作为重载铁路基床表层的主要填料,其受列车荷载的影响最大。因此,研究级配碎石在循环荷载作用下的动力行为及累积塑性应变演化特征变得尤为重要。首先,通过制备不同细粒含量的级配碎石填料,开展一系列大型动三轴试验,探究细粒含量、围压及动应力幅值对循环荷载作用下试样累积塑性应变的耦合影响机制。其次,基于塑性安定理论,确定不同应力水平下试样的动力行为,得到考虑围压及细粒含量参数的塑性蠕变状态临界动应力计算模型。最后,结合试验数据,建立考虑应力水平及细粒含量参数的塑性蠕变动力行为累积塑性应变预测模型,并明确各参数的物理意义。其研究成果可为既有重载铁路路基健康状态评估及考虑强度、变形综合控制的路基结构设计提供参考。     

小型土石坝加密抗液化离心机振动台试验研究

地震作用下土石坝液化易导致坝坡失稳滑移等严重后果,加密法是常用的抗液化手段之一。针对坝趾压重与坝壳翻压两种坝身加密加固方法,开展了离心机振动台试验,分析了不同加密型抗液化处理的小型土石坝坝坡地震响应规律。试验结果表明,由于高水头作用下坝坡底部土体软化,未处理坝坡加速度放大系数沿高程先减小后增大,而加密坝坡加速度放大系数沿高程逐渐增大,且坝坡表面处加速度存在表面放大现象。坝趾压重和坝壳翻压提高了坝身有效应力,降低地震产生的超静孔压比,有效防止土体液化。未处理坝坡在峰值加速度为0.24g地震作用下即发生坝趾液化现象,而加密坝坡在峰值加速度为0.24g和0.45g下均未发生液化。未处理坝坡整体侧向位移大,加密处理后,在峰值加速度为0.24g下坝坡整体表现为竖向位移。坝趾压重区坝趾水平位移明显减小,坝壳翻压区坡顶沉降减小了50%。试验结果验证了坝趾压重和坝壳翻压的抗液化效果,为小型土石坝抗震加固设计提供了参考。     

TWO-PHASE OIL MIGRATION IN COMPACTING SEDIMENTARY BASINS MODELLED BY THE FINITE ELEMENT METHOD

The upstream-weighted finite element method with lumped mass matrix is applied to the modelling of oil migration in compacting sedimentary basins. An implicit formulation is made in Lagrangian co-ordinates of a pressure, saturation and a temperature equation, which is based on immiscible two-phase flow of oil and water. The formulation accounts for the compaction of the sediments, the generation of oil from solid organic material (kerogen), the eventual pore space generated by kerogen breakdown, and the density variations of the fluids which may set up thermal convection. The model is validated by comparison with results from a one-dimensional (1D) fractional flow-based migration model. A 2D case example showing oil expulsion from source rocks, and the filling of a trap is presented. The mass balance of the model is easily checked because all oil in the basin originates from breakdown of kerogen. Compared with other alternatives, the simple upstream-weighted finite element method is suggested as a possible first choice for a numerical method for the modelling of oil migration in compacting sedimentary basins. It easily deals with the complex geometry of a basin, it yields reasonably good results, is simple to implement, and the same implementation applies to all spatial dimensions. © 1997 by John Wiley & Sons, Ltd.     

多边形断层系及其研究现状

多边形断层为一类非构造断层,主要分布于深水环境下的细粒沉积物中。它的形成与沉积物的早期压实作用和流体排出作用有关。多边形断层系的识别主要通过时间切片分析和地震剖面解释等手段进行。研究表明,多边形断层系发育一般要经历3个阶段,即超压封存箱的形成、密度反转和多期水力压裂作用。对多边形断层系的研究有助于泥岩压实、油气运移、泥岩中流体活动状态以及对深水砂体分布和几何形态特征的分析等。由于我国还未发现有多边形断层系的存在,希望本文的介绍能为我国开展此类断层研究时有所帮助。     

基于遗传优化神经网络的高速公路路基沉降量预测

控制路基沉降是公路工程中的一个关键技术问题,而路基沉降与其影响因素之间存在着线性、非线性关系。当输入自变量较多时,用传统神经网络建模容易出现过拟合现象,导致网络模型预测精度较低。针对此问题,本文用遗传算法对神经网络模型的权值和阈值进行优化,同时讨论遗传参数的设定对输出结果的影响。通过对成南高速的实测数据进行仿真,试验结果表明:优化后的BP神经网络具有较高的预测精度,预测效果明显优于传统神经网络模型的输出结果,该预测方法可作为高速公路路基长期沉降预测的一种有效辅助手段。