逆断层地表变形及避让距离的离心模型试验研究

断层同震错动引起的地表变形破坏是工程场地评价中的难点之一,如何合理避开同震地表破裂带是工程选址必须解决的问题,关键在于能否准确预测发震断层错动过程中的地表变形演化特征.基于土工离心模型试验过程中获取的高精度地表监测数据,定量分析给出了断层面倾角为60°且厚度约为40 m的上覆砂土在不同基岩位错下的地表变形和坡度变化规律...     

正断层错动诱发单桩破坏及避让距离研究

历次地震实例表明基岩断层错动诱发建筑桩基破坏,针对桩基础的近断层破坏机理认识不足。采用土工离心机试验,研究正断层错动引起上覆砂土中单桩基础的破坏。试验详细量测单桩及土体变形,监测桩身轴力及弯矩随基岩错动量的变化规律。试验数据表明,当错动量为0.4 m时,单桩与桩周土体协同变形,桩顶出现显著位移。随着错动量增加,上覆土体变形集中于破裂带,使得桩顶位移显著减少。该破裂带在基岩和土体的交界面上偏离基岩错动方向,与水平面呈80°方向发展至地表,并在单桩靠近断层上盘一侧地表出露。上覆砂土变形可分为静止区、剪切区和刚体位移区。当单桩位于剪切区附近,桩身受弯变形使得桩顶向上盘一侧倾斜。针对正断层错动,单桩在下盘一侧和上盘一侧离开基岩断层线的安全避让距离分别为15 m和10 m。     

地下断裂对不同土质上覆土层的工程影响

采用土工离心模拟技术研究4种不同土质的上覆土层在地下断裂时的性状反应,分别模拟基岩垂直位错与水平位错两种基岩断裂形式,其最大模拟位错量分别达4.5,3.0m,由此获得以下试验结果:(1)当基岩断裂位错时,张性的正断错位比水平错位对上部土层的开裂影响要大;(2)上部覆盖为单一的软–中等强度的土层比粗细相间沉积的土层影响要大;(3)当断裂位错1～3m时,土层开始出现裂缝,其破裂高度均在20m以内;当断裂位错增大到4.5m时,土层破裂向上发展,但均在30m以内。本文的分析与结论对于场地断裂工程抗震评价及有关规范的修订具有参考价值和指导意义。     

土层条件对基岩反应谱放大效应的研究

选择了福州盆地二类场地和三类场地内不同土层厚度的10个钻孔,根据福州市地震危险性分析结果,用三角级数迭加法合成50年超越概率10%的基岩地震动3条,将其调幅后输入上述10个代表性钻孔中,得到不同峰值输入下、不同土层厚度土层的反应谱放大倍数(土层地表反应谱除以基岩地表反应谱)。     

地层错动引起的上覆砂层变形特性的离心试验研究

地层错动引起的基岩上覆土体变形对地表的建筑物、地下管线和隧道的安全带来潜在危险。为减小这种危险给人们的生命财产带来的损失,掌握地层错动引起的上覆土层的变形行为规律,通过土工离心试验,采用PIV图片测距技术,研究不同基岩错动量下竖向正断层错动造成的地表变形特点和断层裂缝在砂层中传播的规律。通过分析试验得到位移和应变场,得到以下几点认识:(1)不均匀沉降地表宽度(剪切区)并不随基岩错动位移的增大而线性增大,尤其在下盘,其宽度增大幅度很小;(2)基岩向下错动给上盘带来的干扰和破坏大于下盘;(3)裂缝传播整体传播方向偏于上盘。主裂缝的总体传播方向与断层平面的夹角约等于土体在土层平均围压下的剪胀角。当裂缝传至土体上部时,传播角会向上盘偏转而变大。     

断层错动数值模拟中的几个关键影响因素

到目前为止,人们将研究重点更多地放在了地震引发的地面震动对结构的影响上,而对下覆断层错动导致上覆土层破裂的机理方面研究甚少。文章运用有限元软件ABAQUS,针对倾滑断层,对自由场-断层体系进行了数值模拟,总结了断层错动数值模拟问题应当注意的关键影响因素,包含以下四点:准静态法、网格尺寸、应变软化和材料阻尼。通过计算对比分析,发现未全面考虑以上四个影响因素时,计算结果对断层错动时上覆土的响应估计不足。     

土层条件对基岩峰值放大效应的研究

在福州市的Ⅱ类场地和Ⅲ类场地中选出10个不同土层厚度的代表性钻孔,进行了一维土层地震响应分析,得到不同峰值基岩地震波输入下,不同场地类别、不同土层厚度的峰值放大倍数,为福州市地震评估提供指导。     

断层错动对上覆土体及隧道影响的模型试验

通过砂箱模型试验,采用FBG传感器,研究隧道距基岩面不同距离时隧道顶部和底部纵向应变变化规律,试验结果表明:随着隧道距基岩面距离的增加,隧道变形极值点的位置向错动迹线左侧区域移动,出现在错动迹线左侧0～15 cm的范围内;在上覆土体厚度一定的情况下,隧道距基岩面距离存在着一个最不利值,该位置的隧道衬砌在断层错动时,被破坏的可能性最大;随着隧道距基岩面距离的增加,断层传播到地表的水平传播距离减小。     

地震动时-频特性对土层场地地震反应的影响

地震动非平稳特性对土层反应有重要影响。为了能够近似定量控制基岩地震动输入特性,文章首先基于时-频包线函数建立能够表征地震动频率非平稳特性的地震动拟合方法;然后选择3条主频率变化不同的加速度记录,引入两种不同的包线函数分别模拟地震动强度或频率非平稳特性,设计了用于土层地震反应分析的三种输入方案,拟合完成42条加速度时程;最后,采用非线性场地反应计算方法对三类土体模型进行计算,分析基岩地震动频率非平稳性对地表PGA和加速度反应谱的影响。结果表明：①文章基于时-频包线函数建立的地震动拟合方法能够近似定量控制目标地震动强度与频率非平稳特性,可用于基岩地震动输入的拟合;②当基岩地震动主频率变化显著时,不考虑频率非平稳则存在低估地表地震反应的风险,且场地越硬、基岩峰值越大,低估程度越显著;③地震动频率时变特征对土层地表反应谱高频部分(2～5Hz)的影响与地震动高频成分集中的时间和土层性质相关：当基岩地震动高频成分集中在强震段时,采用仅考虑强度非平稳的拟合方法可在较硬场地得到相对保守的结果;④地震动频率随时间的变化对反应谱放大系数的影响与场地类型和反应谱周期有关,当基岩峰值较大时,对较硬场地5Hz左右的频率成分影响更大,但对三、四类场地条件,则对1Hz左右的土层反应影响更大。因此在拟合基岩地震动时,特别是输入地震动的峰值较大时,应结合结构自振周期、场地条件、工程需要等因素,考虑基岩地震动输入的频率随时间的变化特征,采用全非平稳方法进行人工地震动模拟。文章的研究成果对工程实践中合理设置基岩地震动模拟参数具有重要的指导意义。     

地震场地的综合评定方法

本文给出了一个考虑场地土层的刚度、覆盖土层厚度及软弱夹层、液化土层等因素对场地地震动影响的综合评定方法。该方法根据场地土层的刚度和覆盖土层的厚度，按模糊评判方法确定场地指数μ值，并建立μ值与设计地震反应谱的关系。当场地指数确定时，就可确定用于抗震设计的反应谱。对于场地土层中含有软弱夹层或液化土层的情况，本文还给出了考虑这些影响因素，对场地指数或设计地震动参数进行修正的方法。     

胶结黏土中正断层扩展的变形和孔压变化研究

 基岩断层错动而引起的上覆土体变形会对地表以及地下结构物造成破坏。上覆土体的胶结特性和隐伏断层的存在,使得断层错动的变形机制更为复杂。采用滤纸技术模拟隐伏断层,通过2组离心机试验研究胶结土体中正断层裂缝的扩展机制及伴随的孔压变化规律。对土体的变形分析和孔隙水压力的监测加深了对地裂缝的认识。研究发现,正断层错动引起胶结黏土的变形机制为受弯变形。隐伏断层使得土体受弯区变小、土体破坏程度加剧。胶结黏土受弯形成的地裂缝分为张拉裂缝和剪切裂缝,这些地裂缝的发展为超孔压的消散提供了优势路径。     

四川汶川8.0级地震地表破裂与震害特点

 2008年5月12日四川汶川8.0级地震发生在青藏高原东缘的龙门山构造带上。通过野外实地调查与测量发现,汶川8.0级地震的地表破裂发生在龙门山构造带中的北川—映秀断裂之映秀南西—平武南坝石坎子段,彭县—灌县断裂的都江堰向峨—安县桑枣段也同时发生了同震地表破裂。北川—映秀断裂上的地表破裂长约220 km,表现为逆冲–右旋走滑运动特征,最大同震位错在6 m左右,水平与垂直位错量大致相当,与先期的研究成果一致;彭县—灌县断裂上的同震地表破裂长约100 km,表现为右旋–逆冲运动特征,最大垂直位错在2 m左右,水平与垂直位错量之比为1∶1～1∶3。地震地表破裂的空间分布及同震位错特征表明,汶川8.0级地震系北川—映秀断裂的逆冲–右旋运动所导致,并同时牵动了彭县—灌县断裂发生同震地表破裂。地震灾害的特点主要表现为地震波导致的强地面运动破坏、地震地表破裂带直接撕裂、地震导致的次生地质灾害(如崩塌和滑坡等)摧毁或掩埋建(构)筑物和堰塞湖等。根据地震考察资料并参考InSAR和强震仪记录,勾绘的该次地震等震线长轴方向沿龙门山构造带呈N40°～50°E方向延伸,出现了3个XI度的破坏区,具单侧多点瞬间破裂的典型特征,导致了四川北部、甘肃和陕西南部地区灾害较正常地震衰减破坏的显著加重。     

Distinct element simulations of earthquake fault rupture through materials of varying density

Surface manifestations of earthquake fault rupture are strongly affected by the dilatant response of the soil deposit overlying the bedrock fault displacement. The granular material’s in-situ void ratio and effective confining stress affect its dilatancy, and hence, its stress-strain response and ductility. Distinct element method (DEM) assemblages of 3D, non-spherical particles are prepared with different void ratio distributions, and their dilatancy is characterized using direct shear test simulations. DEM simulations capture the response of sand in centrifuge experiments of earthquake fault rupture propagation. Macro-scale mechanisms of ground deformation and micro-scale mechanisms of shear band formation during dip-slip fault rupture propagation are analyzed through particle rotations, homogenized strains, frictional dissipation, and particle displacements. The brittle and ductile responses of granular media undergoing fault rupture are related to changes in the coordination numbers in each particle assemblage. The deformational characteristics of a metastable fabric in the loosest particle assemblages and a stable fabric in the densest particle assemblages are revealed through the accumulation of energy dissipated through friction. The normalized strong contact forces are also greater in magnitude in the loosest particle assemblages and greater in number in the densest assemblages.     

Performance of geosynthetic-reinforced soil foundations across a normal fault

This paper presents a series of model tests on geosynthetic-reinforced soil (GRS) foundations across a normal fault. The aim was to evaluate the performance of reinforced foundations as a mitigation measure for surface faulting hazards. Experimental tests modeled a 3-m thick foundation in prototype subjected to a fault displacement up to 90 cm. Test variables included the number of reinforcement layers, reinforcement stiffness and location, and foundation height. Digital image analysis techniques were applied to determine the ground settlement profile, angular distortion, shear rupture propagation, and mobilized reinforcement tensile strain at various magnitudes of fault offset. Test results revealed that compared with the unreinforced foundation, reinforcement inclusion could effectively prevent the shear rupture propagating from the bedrock fault to the ground surface. It also spread the differential settlement to a wider influential zone, resulting in an average reduction of 60% in the fault-induced angular distortion at the ground surface. The maximum angular distortion decreased as the foundation height, number of reinforcement layers, and reinforcement stiffness increased. Relationships between the maximum angular distortion and maximum mobilized reinforcement tensile strain with fault displacement were therefore established. Based on the findings from this study, design suggestions and implications are discussed.     

厚表土薄基岩煤层开采覆岩运动规律

 厚表土薄基岩煤层采用综放开采后基本顶将难于形成稳定结构,发生滑落失稳导致工作面“压架”,在对薄基岩定义的基础上,综合采用实验室试验、理论分析、数值模拟和现场实测的方法对薄基岩煤层开采上覆岩层运动规律进行了研究。研究结果表明,若其他条件不变,“砌体梁”结构的稳定主要取决于基岩厚度和上覆表土层的力学性质及厚度,具有较大承载力的厚黏土层能与薄基岩组合形成稳定的结构,降低稳定结构所需最小基岩厚度。由此建立薄基岩工作面结构力学模型,并针对司马矿具体条件分析认为当表土为松散砂土,最小基岩厚度40 m;当黏土厚度40 m,最小基岩厚度20 m;黏土厚度30 m,最小基岩厚度30 m。据此提出首采面可安全开采,在现场工业性试验中得到成功验证。     

Centrifuge experiments for shallow tunnels at active reverse fault intersection

Tunnels extend in large stretches with continuous lengths of up to hundreds of kilometers which are vulnerable to faulting in earthquake-prone areas. Assessing the interaction of soil and tunnel at an intersection with an active fault during an earthquake can be a beneficial guideline for tunnel design engineers. Here, a series of 4 centrifuge tests are planned and tested on continuous tunnels. Dip-slip surface faulting in reverse mechanism of 60-degree is modeled by a fault simulator box in a quasi-static manner. Failure mechanism, progression and locations of damages to the tunnels are assessed through a gradual increase in Permanent Ground Displacement (PGD). The ground surface deformations and strains, fault surface trace, fault scarp and the sinkhole caused by fault movement are observed here. These ground surface deformations are major threats to stability, safety and serviceability of the structures. According to the observations, the modeled tunnels are vulnerable to reverse fault rupture and but the functionality loss is not abrupt, and the tunnel will be able to tolerate some fault displacements. By monitoring the progress of damage states by increasing PGD, the fragility curves corresponding to each damage state were plotted and interpreted in related figures.     

Intersonic shear rupture mechanism

According to the theory the rupture speed in solids for mode-I cracks is limited by the Rayleigh speed c_R, while mode-II cracks can propagate intersonically. These theoretical predictions and sustaining experiments were made for the idealized condition of a crack propagating along a predetermined weak and straight-line path in a homogeneous linear elastic material. In real materials, however, the mode-I crack speed has never been observed to exceed 0.65c_R. The reason for this is the natural tendency for physical cracks to follow a wavy path and for microbranching, which results in a significant increase in microcrack population and, consequently, in the fracture energy. At the same time, intersonic shear ruptures (mode-II cracks) have been reported for crustal earthquakes. It seems paradoxical because earthquake ruptures are normally associated with high complexity and extreme damage in the rupture zone.

The present paper shows that nature has provided special shear rupture mechanisms acting in hard rocks at high confining pressure that minimize the rupture energy, causing the increase in rupture speed. These mechanisms are different for primary ruptures (continuous thin ruptures with uniform structure) and general faults (complex discontinuous systems). The general faults propagate in a jump-like manner, forming a cascade of segments due to an advanced triggering mechanism. The advanced triggering mechanism triples the propagation speed of a general fault compared with primary fractures involved in the fault. The propagation of primary ruptures is facilitated by another mechanism, which is a fan-shaped self-equilibrated mechanism created on the basis of an echelon of rotating blocks representing the intrinsic nature of the shear crack structure. These two mechanisms acting in combination can provide intersonic propagation of general faults.