地震作用下边坡岩体动力稳定性数值模拟

 以金沙江龙蟠边坡变形岩体为例,采用人工合成地震动为输入条件,并考虑超静孔隙水压力的作用,运用离散单元法进行边坡岩体的全时程动力分析,探讨地震作用下边坡岩体的动力响应规律及稳定性。数值模拟结果表明：地震惯性力的作用引起剪应力的积聚效应;岩体结构面对岩质边坡的地震动力响应起控制性作用;边坡最危险状态是地震刚刚开始时由静态到动态所发生的巨大系统改变之时;地震作用导致岩体变形破坏的主要机制是剪应力积聚和超静孔隙水压力累积效应的耦合作用。同时,计算结果还表明,龙蟠边坡变形岩体在遭受地震时不会发生整体性的远程滑动,这一结论对金沙江宽谷坝址的抉择具有重要的工程应用价值。     

Dynamic properties and liquefaction behaviour of cohesive soil in northeast India under staged cyclic loading

Estimation of strain-dependent dynamic soil properties, e.g. the shear modulus and damping ratio, along with the liquefaction potential parameters, is extremely important for the assessment and analysis of almost all geotechnical problems involving dynamic loading. This paper presents the dynamic properties and liquefaction behaviour of cohesive soil subjected to staged cyclic loading, which may be caused by main shocks of earthquakes preceded or followed by minor foreshocks or aftershocks, respectively. Cyclic triaxial tests were conducted on the specimens prepared at different dry densities (1.5 g/cm³ and 1.75 g/cm³) and different water contents ranging from 8% to 25%. The results indicated that the shear modulus reduction (G/G_max) and damping ratio of the specimen remain unaffected due to the changes in the initial dry density and water content. Damping ratio is significantly affected by confining pressure, whereas G/G_max is affected marginally. It was seen that the liquefaction criterion of cohesive soils based on single-amplitude shear strain (3.75% or the strain at which excess pore water pressure ratio becomes equal to 1, whichever is lower) depends on the initial state of soils and applied stresses. The dynamic model of the regional soil, obtained as an outcome of the cyclic triaxial tests, can be successfully used for ground response analysis of the region.     

动剪切模量比与剪切波速对地震动影响及等量关系研究

采用数值模拟方法,研究动剪切模量比和剪切波速对地震动的影响问题,并以对地表加速度反应谱影响为标准研究动剪切模量比变化与剪切波速变化间的对等关系,同时建立了等价关系式,分析中考虑了场地类别、场地遭遇地震强度和地震输入波型的影响。结果表明:①动剪切模量比误差和剪切波速误差对地震动都有着重要影响,在土体强非线性和软弱场地中影响更为强烈,实测的动剪切模量比和剪切波速比真实值低时较比真实值高时对地表反应谱影响更大,三类场地0.1g地震输入时动剪切模量比6%误差或剪切波速25%误差的影响已经不可忽视;②对不同的场地类别、场地地震烈度和地震输入波情况,动剪切模量比单独变化与剪切波速单独变化对反应谱的影响都具有对等关系;③动剪切模量比误差对地震动的影响程度要大于剪切波速误差,二者对地表反应谱的敏感性相差3～5倍,绝对误差相同时动剪切模量比降低情况下与剪切波速的敏感程度相差更大。     

浅层点振源作用下饱和砂层的超孔隙水压力

地震时,由于向上传播的地震波的作用,可能在饱和松砂中产生很高的超孔隙水压力,导致砂土发生液化。但至今还缺乏地震时土中超孔降水压力发生过程的实测资料。作者利用振动水冲器在施工过程所产生的振动,记录其周围饱和松砂中超孔隙水压力的产生过程,发现在这种振动条件--浅层点振源,形成的超孔隙水压力,基本上是由两部分组成的。即脉动孔隙水压力与应变孔隙水压力。脉动孔隙水压力是振动纵波在水中直接传播的结果。振动的纵、横波还通过土骨架向外传播,传播过程使土骨架产生应变势。应变超孔隙水压力是上耦合于骨架剩余应变势的反映。这两部分的比例与震中距及砂土密度有失。超孔隙水压力场与地面加速度场相似,基本上是震中距的幂函数.     

循环振动对饱和粉土初始动剪模量的影响

初始动剪模量是土体多种结构性因素的综合反映,除固结压力和孔隙比(密度)这两个主要因素外,还受循环振动影响。以往对该因素的研究偏重于干砂,且预振时施加的剪应变幅多低于10-3;对于饱和土体,尤其是对经受过较大振幅振动后土体初始动剪模量的研究十分有限。本文利用安装弯曲元的动三轴仪,在等向固结条件下对不同相对密度的饱和粉土进行不排水循环三轴试验,同时监测试样的剪切波速,获得循环动力加载过程中土的初始动剪模量。为保证试验结果的可靠性,同时进行了共振柱试验,比较确定弯曲元接收信号的到达点;评估了不同孔压发展程度下试验暂停时间对测试结果的影响。结果表明:在一定次数大振幅循环荷载作用下观察到初始动剪模量有别于相同应力下固结后得到的模量值,出现附加衰减或增长,正是循环荷载作用下土微观结构变化的外部宏观表现。这种附加变化由土颗粒接触行为决定,受到试样初始条件和振动幅值等多种因素的影响,表现出不同的变化模式。     

Relationship between Shear Wave Velocity and Stresses at Failure for Weakly Cemented Sands During Drained Triaxial Compression

Small strain shear modulus (G_max) has been a parameter of choice used to assess the strength and deformation behavior of cemented and other sensitive soils. The influence of density, effective confining stress, stress anisotropy, and cement content on shear wave velocity (v_s)/shear modulus has been studied extensively and published. There are, however, very few studies on the effects of cement/strength degradation during shear on the shear wave velocity/shear modulus, which may be important for reliable and accurate prediction of mechanical behavior of cemented sands. The objective of this study is to evaluate the effect of cement degradation on shear wave velocity/shear modulus by measuring continuously the shear wave velocity during shear. A laboratory testing program was performed using samples of silty sand artificially cemented with Ordinary Portland Cement (OPC). Shear wave velocity was measured continuously within the triaxial cell during the shear phase using torsional ring transducers. G_max was calculated using the shear wave velocity and the corresponding density during shear. Results from this study suggest that G_max reaches a peak value before σ′₁ reaches a failure stress and this behavior is believed to be an indicator of bond breakage or destructuring. G_max calculated at various stages during shear showed that the cement and modulus degradation can be represented by a simple index using G_max. The results of this study suggest that there may be a unique relationship between small strain shear modulus and effective stresses at failure for dilative soils implying that in situ shear wave velocity measurements may be used to estimate effective stress strength parameters or as a precursor to failure in weakly cemented soils.     

基于流固耦合模型分析库水位对尾矿坝的影响

尾矿库水位是影响坝体安全的重要因素之一,针对库水位变化对尾矿坝稳定性的影响问题,建立考虑超孔隙水压力作用下的流固耦合数学模型,利用有限差分程序FLAC3D进行数值求解.模拟了某尾矿库在不同库水位时的稳定性,得出库水位的升高对尾矿坝影响规律.即随着库水位的升高,尾矿坝浸润线升高,安全系数降低,尾矿坝坝体中部的剪应力逐渐增...     

反压对土体强度特性的影响试验研究及其影响机理分析

为提高试样饱和度,室内试验普遍采用反压饱和法,但现行规范中对其反压大小未做具体规定。该文利用竖向-扭转双向耦合剪切仪及全自动真三轴仪,针对福建标准砂(相对密实度30%、58%、70%)及细砂,进行了不同反压下单调排水与不排水剪切试验,分析不同反压下砂土的应力-应变关系、超静孔隙水压力发展规律。试验结果表明：反压对剪胀性砂土的固结不排水剪切强度具有显著影响,且剪胀性越大,其影响更显著;反压对剪缩性砂土的不排水抗剪强度影响不显著;反压对砂土固结排水抗剪强度的影响相对较小;反压对砂土有效抗剪强度指标基本没有影响。本文认为反压对剪胀性砂土的固结不排水抗剪强度产生影响的原因是：反压不同时,土体中的气液混合体的体积模量并不相同,因此相同的体胀趋势或相同的体积回弹下所引起的超静孔隙水压力不同,进而直接影响有效围压及其砂土抗剪强度;而土体发生剪缩时,气液混合体越接近刚体,其体积模量对反压的依赖性越小,因此,相同的剪缩趋势或相同的体积压缩量下,所产生的超静孔隙水压力与反压的大小没有显著的依赖关系,因此在剪缩状态下,反压对砂土的应力-应变关系及抗剪强度没有显著的影响。在固结排水试验中无论反压设定多大,不存在超静孔隙水压力,有效围压与反压大小无关,因此反压对砂土排水抗剪强度没有显著影响。     

考虑土体三维强度特性的静压桩周超孔压解析及演变

 将饱和黏性土中静压沉桩过程近视看作柱孔不排水扩张问题,在充分考虑土体三维强度特性的条件下采用SMP准则改进的修正剑桥模型,推导得出柱孔扩张引起超孔压的基本解答。在此基础上,考虑桩周土竖向和径向固结,建立空间轴对称固结方程的定解条件,采用分离变量法求得桩周超静孔隙水压力消散的级数解答。分析桩周土体超静孔隙水压力随时间和空间的演变规律,揭示应力历史、径向和竖向固结系数以及剪切模量等因素对初始超孔压的产生和随后的固结速率的影响规律,并通过实例验证本文解答的合理性和适用性。通过与现场实测对比,本文解答较好地反映了静压桩周土体超静孔隙水压力的演变规律。此外,桩周土体的超静孔隙水压力随距桩侧径向距离增大呈对数衰减。剪切模量和竖向固结系数对桩周土体固结速率影响较小,而土体超固结比和径向固结系数对固结速率影响较为显著,表明超孔压消散主要发生在径向。研究成果对静压桩承载力的确定具有一定的指导意义。     

初始静孔隙水压力对砂土静动力剪切特性影响的试验研究

选取福建标准砂和滹沱河细砂,利用空心圆柱扭剪仪开展了一系列不同初始静孔隙水压力条件下的不排水循环扭剪试验和单调扭剪试验,着重探讨初始静孔隙水压力对超静孔隙水压力发展及其不排水抗剪强度的影响。试验结果表明:初始静孔隙水压力对超静孔隙水压力的发展产生显著的影响,从而影响砂土的静动力剪切特性。具体地,在不排水循环剪切过程中,初始静孔隙水压力越大,其超静孔隙水压力发展和变形发展越快;在不排水单调剪切过程中,初始静孔隙水压力越大,在砂土剪胀阶段产生负超静孔隙水压力越大,从而使砂土的强度显著提高。基于试验结果,初步探讨了初始静孔隙水压力对超静孔隙水压力及静动力剪切特性的影响机理。研究表明,研究地下水位以下土体(准饱和土)静动力剪切特性尤其是研究液化问题时,应充分考虑初始静孔隙水压力对砂土抗液化强度的影响,室内试验应根据砂土所处的地下水位深度来决定初始静孔隙水压力(反压)的大小。     

交壅沟蓄水坝基础开挖控制爆破设计

本文介绍了柳东新区交壅沟蓄水坝基础及边坡石方开挖的爆破施工中,采用的松动爆破技术及施工安全措施。防止了因爆破震动和冲击波、飞石对临近桥梁造成危害,保护了主干道的交通安全,取得了预期的效果。     

Ko固结饱和土沉桩引起的超静孔隙水压力

根据饱和粘土中沉桩特点,用柱孔扩张理论模拟其贯入过程,将扩张后周围土体分为塑性区和弹性区,由柱孔扩张基本平衡方程和边界条件,采用能够考虑天然状态土体实际固结性状的K_o固结土体本构模型的屈服面方程为屈服准则,以对数应变考虑桩周土体发生的大变形,结合Henkel孔压公式,推导出沉桩后桩周超静孔隙水压力的理论表达式。通过算例分析了土体剪切模量、临界状态应力比、超固结比和静止侧压力系数对超静孔隙水压力的影响。同时,与基于修正剑桥模型解答和工程实例进行对比分析,理论计算值和现场实测值吻合较好,表明理论结果的合理性与实用性。     

软土固结中波速与固结度的关系及其工程应用

本文首先应用室内共振柱研究了浙江沿海软粘土在固结过程中剪切波速与固结度的关系,由试验结果发现波速增值△υ_s＇与△p＇/p＇（△p＇为围压增量,p＇为初始有效固结压力）之比与固结度有一线性关系,这为应用现场波速检测软土地基加固效果提供了实验依据。本文然后通过镇海电厂堆煤场砂井预压工程,将实测孔隙水压力消散得到的固结度与现场跨孔试验测得的波速计算的固结度进行比较,显示了该方法有良好的工程应用前景。     

土层非线性非平稳有效应力动力响应可靠度分析

在非平稳一致地震输入下对非线性土层进行了有效应力动力响应可靠度分析。通过虚拟激励法,将非平稳地震加速度功率谱密度作为基岩上的虚拟输入,并利用Hardin-Drnevich双曲线模型考虑土层的非线性特性,迭代计算得到土单元的剪切模量与阻尼比。在此基础上,运用有限元方法对土层进行了地震响应分析,通过比较4种方法获得等效峰值剪应力和剪应变来计算等效动力剪切模量和振动孔压。结果表明,采用基于概率平均法的插值方法与确定性分析所得的平均值较为接近,说明由于孔压的不断上升与土层有效应力的降低,可液化地基的地震响应变为非平稳随机过程。     

饱和粉土地基低强度混凝土桩振动沉管及静载试验

为研究饱和粉土地基低强度混凝土桩振动沉管施工产生的超孔隙水压力分布、消散规律及单桩和复合地基的承载特性,对滨州市饱和粉土地基进行了低强度混凝土桩的振动沉管和静载试验。结果表明:沉管振动下沉时,最大超孔隙水压力一般出现在桩端以上2~3 m;沉管振动下沉对桩周土体的扰动较小,最大超孔隙水压力与上覆土有效应力的比值仅为0.327;单桩振动拔管后15 min,临近超孔隙水压力的消散率可达到65%~75%;与设计规范的估算值相比,振动沉管成桩后单桩竖向极限承载力偏小,仅为估算值的55%~60%;低强度混凝土桩的加固作用明显,复合地基承载力与天然地基相比提高了约1倍。     

考虑流固耦合作用的高土石坝动力分析

在土石坝施工、蓄水和遭遇地震时,流固耦合作用对土石坝的静动力响应有重要影响,应在计算分析中有所考虑。以糯扎渡高心墙堆石坝为例,选用莫尔–库仑弹塑性模型来描述坝料的力学性质,并采用流固耦合的方法对该坝进行了静动力分析。静力分析中模拟了大坝施工和蓄水过程,然后基于静力分析得到的初始应力场,采用完全耦合的非线性方法研究了大坝的地震动力响应。该分析方法能够更为合理准确地描述土石坝在地震动作用下残余变形的发展及超静孔压的累积和消散过程。计算结果表明:超静孔隙水压力随地震过程逐渐累积,最大值出现在心墙的底部;由于鞭梢效应,加速度放大系数在坝顶处达到最大;水平和竖直方向的永久变形同样都是在坝顶处达到最大值。     

Investigation of mechanical properties of soft rock due to laboratory reproduction of physical weathering process

The focus of the present study is on soft rocks (moderately weathered granite and artificial rock) that have suffered physical weathering due to changes in temperature and confining pressure. Unconfined compression tests were conducted on moderately weathered granite, and triaxial compression tests were conducted on artificial rocks. Two test plans were conducted to study the effect of weathering. In the first plan, the specimens suffered weathering process cycles under unconfined conditions, followed by triaxial tests with different confining pressures (0 kPa, 30 kPa, 60 kPa, and 90 kPa). In the second plan, the specimens suffered weathering process cycles under a certain confining pressure (0 kPa, 30 kPa, 60 kPa, and 90 kPa), and the shear strength and initial Young's modulus in each weathering cycle was then studied. Finally, based on the formula of the shear wave velocity and initial Young's modulus, the relationships between normalized shear strength and normalized shear wave velocity were found. These relationships can be used in a further study to understand rock strength on site by detecting the shear wave velocity.The results of this study show that artificial rocks (cement treated sand, CTS) can be used as a homogeneous material to simulate soft rock. In the stress-strain curves, the initial Young's modulus showed no significant change when increasing the confining pressure. The initial Young's modulus showed a nonlinear decrease when the weathering process cycle increased. When soft rocks suffer the weathering process at a certain confining pressure, the relationship between normalized shear strength and normalized shear wave velocity was linear. When soft rocks suffer the weathering process at different confining pressures, the normalized shear strength under a lower confining pressure dropped faster than when the confining pressure was higher.     

饱和散体振动液化的波动机理研究

在有关假设条件下，给出了几个基本控制方程式，并得出了饱和散体中的波动传播方程。分析推导了波在无耗散情况下的势矢量方程和3种体波的速度表达式以及具有耗散情形的势矢量的普遍方式和P波与S波的衰减系数。通过砂和铁粉的振动液化试验观察并记录振动液化时，饱和散体中孔隙水压力的变化情况，结果表明：随着振动的施加，孔隙水压力迅速升高，并表现出一定的波动性。利用波动理论和试验结果，详细分析了振动过程中饱和散体介质的3种体波的传播对孔隙水和颗粒的作用情况，揭示了孔隙水压力迅速高和饱和散体波化的波动机理。     

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.     

随机地震荷载作用下黄土的液化特性

 以场地未来50 a超越概率10%的地震波作为输入激振波,对取自宁夏南部西吉县夏家大路喜家湾滑坡后壁的黄土开展动三轴试验,研究随机地震荷载作用下黄土中孔隙水压力的增长基本特性、影响效应及孔隙水压力增长规律。由试验结果可知,在随机地震荷载作用下,孔隙水压力对输入动荷载的响应具有明显的滞后性,其增长主要集中在地震荷载的有效持时范围内。输入地震荷载的幅值、有效持时以及固结压力对黄土的液化特性有较大的影响,幅值较大、持时较长、固结压力较小时,更有利于孔隙水压力的发展。由于孔隙水压力的增长是源于孔隙的压缩引起的,因此,可以将孔隙水压力比表示为残余应变的函数,据此,可得地震荷载作用下,黄土不同变形情况时孔隙水压力的发展水平及液化程度。