侧移土体被动桩成拱效应及主动侧土压力计算

采用三维数值模型模拟土体水平位移条件下被动桩的成拱效应。分析结果表明,被动桩的成拱效应和桩间距关系最为密切,桩间距较小时,侧移土体在桩周附近呈现出明显的土拱效应,应力等值线密集,相互连通成一整体,并向两端突出,桩间距增大时土拱效应减弱;随着作用于土体侧向荷载的增大,土拱效应增强,桩周应力区增大。在数值分析的基础上,以T.Itos土压力为依据,考虑土体的实际分层,探讨了考虑土拱效应的土压力计算方法,给出了被动桩计算模式,最后将简化方法与数值分析的桩侧土压力进行了对比。     

端承粘弹性桩纵向振动的轴对称解析解

将桩和土体视为粘弹性介质,在频率域求得了桩轴对称纵向振动时动力响应的解析解,进而得到桩头复刚度的解析表达式.通过数值计算,给出了桩头动刚度因子和等效阻尼随激励频率的响应,考察了土层粘性、桩长径比、桩土模量比、桩泊松比和粘性等参数对桩头刚度因子和等效阻尼的影响.研究表明:由于考虑了桩的径向变形效应以及土层对桩的径向力作用...     

考虑渗流影响的球孔扩张理论解答

为研究沉桩过程中渗流作用下球孔扩张引起的土体力学响应,基于桩端应力为三轴压缩状态下的空间滑动面(SMP)准则和非相关联流动法则,考虑渗流和剪胀特性等因素的综合影响,结合弹塑性边界条件推导了饱和无黏性土球孔扩张问题的弹塑性解答。利用体积守恒并考虑塑性区的弹性变形条件,得出了极限扩孔压力的理论解答。在此基础上,通过与不考虑渗流影响的球孔扩张解答对比,研究了渗流体积力对球孔扩张中应力和塑性区半径的影响。研究结果表明:随着渗流体积力的增大,扩孔压力和塑性区半径均减小;剪胀特性对土体的位移和变形影响显著,随着剪胀角的增加,扩孔周围土体的径向位移减小,并沿径向逐渐趋于零;随着土体强度的增大,扩孔压力明显增大。该解答可为富水工程中进行扩底沉桩等岩土渗流问题提供必要的理论依据。     

基于微结构指标主成分分析的沉桩挤土效应研究

前期研究已认识到,土的宏观力学性质及其表现从本质上应取决于土的微观结构。在结构性较强的软土中沉桩,桩周土体内部结构会发生显著变化,土体的强度与变形性质是这种内在变化的宏观表现,研究土体微观结构与宏观力学行为变化之间的关系,对认知土体的力学性质,从微观出发去认识沉桩挤土效应的机理,指导工程实践具有重要地理论和现实意义。本文基于天津滨海地基土实际静压桩工程,在沉桩的不同时刻、沿桩身的不同位置取桩周土体原状土样进行室内三轴固结不排水剪切试验,得到土体强度指标参数,同时进行对应的微观结构试验,得到垂直与水平方向的10个微结构指标。采用主成分分析方法,在微结构指标中提取3个主成分,较好地分析了土体微结构特征。研究表明: 3个主成分与黏聚力之间存在较好的相关关系,而与内摩擦角之间的相关性相对较弱; 第一主成分对各微结构信息的提取比较充分,第二、第三主成分是对第一主成分未反映信息的进一步补充。同时主成分分析表明,土体微结构性质对强度性质起控制作用,在沉桩过程中,近地表和下部土层宏观力学指标表现出了相反的变化规律。主成分分析方法较好地表述了土体的微结构性质,为进一步从微观入手解释沉桩挤土效应机理提供了有力依据。     

桩-土接触效应及对桥梁结构地震反应的影响

目前有关涉及桥梁桩基础地震反应的研究大多是基于桩与桩侧土体之间无相对滑动、位移保持协调的假定。本文针对地震作用下桥梁桩基础的接触面效应及其对结构地震反应的影响问题，以某桥梁工程为背景，通过在桩-土交界面处设置接触单元来模拟桩-土间的接触非线性，建立了土-桩-桥梁结构相互作用体系的三维分析模型。利用这一模型，分析了地震作用下桩-土交界面处的动力反应形态，探讨了桩-土间的接触非线性及其对桥梁结构地震反应的影响。初步分析结果表明：在强震作用下，桩-土间会产生较强的接触非线性，在本文模型中，这种非线性主要表现为桩-土交界面处的滑移；考虑桩-土间的接触面效应将使结构的位移反应结果较基于桩-土间位移协调的情形有所增大。     

基于土拱效应的抗滑桩与护壁桩的桩间距分析

抗滑桩与护壁桩分别在滑坡治理和基坑支护中发挥着巨大的作用 ,其实施正是利用了土中的成拱效应 ,然而传统桩的设计虽考虑土拱效应的存在 ,但桩间距确定理论与方法并未建立起来。本文首先基于土体的极限平衡条件对滑坡推力作用下的土体中的成拱作用进行研究 ,得出了抗滑桩的最大桩间距公式 ,并以某一具体工程为例 ,对该最大桩间距的物理意义和可用性进行了讨论。在此基础上 ,给出了在考虑土拱效应的情况下合理桩间距的确定方法 ,并以此为据 ,对边坡加固设计提出一些建议.     

任意圈层径向非均质土中桩的纵向振动特性

研究三维轴对称条件下径向非均质土中桩的纵向振动. 首先将桩周土体沿径向分为任意圈层来考虑土体的径向非均质性,每个圈层土体均质; 然后结合边界条件和相邻圈层土体之间接触面上位移和应力连续条件,对任意圈层土体动力平衡方程由外而内逐层求解,进而利用桩-土完全耦合条件求解桩动力平衡方程,得到桩顶的频域响应解析解和时域响应半解析解; 最后通过对土体主要控制参数的研究,得出了土体径向非均匀性对桩-土动力响应的影响.      

饱和黏土中考虑土塞效应的柱孔扩张分析

为研究静压开口管桩的挤土效应,在考虑土塞效应的前提下,采用柱孔扩张理论对管桩压入饱和软黏土时的扩孔过程进行了分析。基于可以合理描述黏性土强度特性的拓展Lade-Duncan屈服准则,对柱孔扩张过程中的桩周土体进行了弹塑性理论推导,得到了桩周土体应力场、位移场、塑性区半径、极限扩孔压力和塑性区外侧边界径向位移的解析解。在此基础上,通过室内模型实验获得了软土中开口管桩的土塞高度,且运用等效替代法验证了理论解答的合理性;并通过引入土塞长度比对参数进行了分析。结果表明:在沉桩过程中随着土塞长度比的增大,塑性区半径、极限扩孔压力和塑性区外侧边界径向位移均逐渐减小;土体的剪切模量和黏聚力对沉桩挤土效应有显著的影响,挤土效应始终随着土塞效应的增强而减弱。该研究成果对开口管桩工程的实际应用具有一定理论指导意义。     

考虑横向惯性效应的非饱和土中单桩的竖向动力响应

基于非饱和土的动力控制方程,考虑横向惯性效应,建立了三相非饱和介质中嵌岩桩的竖向动力响应连续介质模型,对桩侧非饱和土的动力控制方程进行Laplace变换,在频域内,通过引入势函数、算子分解等手段对控制方程进行解析,得到了桩侧土体剪应力及竖向振动位移的表达式.结合桩基的竖向振动方程及桩–土接触面的连续性条件,使桩土耦合振动系统得以解答,最终在频域内得到了桩顶复刚度、导纳、桩–土系统振动位移及应力的解析解,借助Laplace逆变换得到了半正弦激励载荷下桩顶的速度时程曲线.最后,通过算例分析验证了计算结果的准确性,分析了横向惯性、泊松比、饱和度、长径比、桩土模量比等因素对桩基动力响应的影响.结果表明:(1)单桩动刚度、阻尼、导纳等变量随频率变化发生周期性振荡,在桩基各阶固有频率处发生共振;(2)泊松比、饱和度、长径比、桩土模量比等因素对桩基的动力响应有较大影响,且频率越大,影响越明显;(3)泊松比越大,单桩动刚度、阻尼、导纳的波动幅值及对应的频率越小,桩顶时程曲线中的桩底反射信号越弱;(4)饱和度越大,对应各动力响应的波动幅值越大,且桩底反射信号的波峰越大.     

基于广义SMP准则的球孔扩张理论解及其应用

通过广义空间滑动面(广义SMP)准则,结合非相关联流动法则对球孔扩张问题进行了理论分析。利用应力不变量推导了符合黏性土中球孔扩张的屈服准则;通过弹塑性区边界条件,推导了孔周土体应力场和考虑剪胀角影响的应变场、位移场。利用体积守恒和塑性区平均体应变表达式,推导出扩孔压力p与初始孔半径a0、扩孔后半径a三者之间的理论关系式,通过该关系式给出了求解球孔扩张问题的具体方法和极限扩孔压力pu的计算式。此外,运用镜像法推导了考虑半无限空间下的压密注浆挤土竖向位移计算公式。最后通过算例分析了球孔扩张问题中应力场分布、不同剪胀角下位移场分布,以及扩孔后半径a对塑性区半径rp和扩孔压力p的影响规律。结果表明,剪胀角是球孔扩张问题中的一个重要参数,随着剪胀角增大,径向位移减小,塑性区半径和扩孔压力均增大。     

从深基坑护壁桩桩身弯矩分析桩侧土压力

根据某高层建筑深基坑护壁桩从实测钢筋应力求得的桩身变矩,用数值分析的方法,对桩侧土压力的分布和大小进行了反分析,所得结果与经典土压力理论有显差别,中用土拱的概念对桩侧土压力的分布进行了解释,并给出了桩侧土压力建议图式。     

基于水平土拱效应的桩间挡土板土压力计算研究

基于``平板开缝-装配-焊接'工艺制备了以高聚物为基体的Kagome等蜂窝结构,并开展 了Kagome, 正三角形和菱形蜂窝结构的面内准静态压缩力学行为实验研究,实验过程中应用 CCD图像采集系统和图像相关法对试件进行了全场位移监测. 另外对比传统正六边形蜂窝, 采用数值分析技术,模拟了低速冲击下不同蜂窝结构坍塌行为. 实验结果和数值模拟均 揭示了在材料用量和结构尺寸完全相同的情况下,Kagome蜂窝结构的面内能量吸收性能优于 其它3种蜂窝结构,并发现了Kagome蜂窝压缩变形时所特有的局部蜂窝旋转变形. 研究结果 表明改变蜂窝形状和周期性排布会对蜂窝结构整体的变形模式以及能量吸收性能产生较大的 影响.     

Displacement of surrounding rock in a deep circular hole considering double moduli and strength-stiffness degradation

The problem of cavity stability widely exists in deep underground engineering and energy exploitation. First, the stress field of the surrounding rock under the uniform stress field is deduced based on a post-peak strength drop model considering the rock's characteristics of constant modulus and double moduli. Then, the orthogonal non-associative flow rule is used to establish the displacement of the surrounding rock under constant modulus and double moduli, respectively, considering the stiffness degradation and dilatancy effects in the plastic region and assuming that the elastic strain in the plastic region satisfies the elastic constitutive relationship. Finally, the evolution of the displacement in the surrounding rock is analyzed under the effects of the double modulus characteristics, the strength drop, the stiffness degradation, and the dilatancy. The results show that the displacement solutions of the surrounding rock under constant modulus and double moduli have a unified expression. The coefficients of the expression are related to the stress field of the original rock, the elastic constant of the surrounding rock, the strength parameters, and the dilatancy angle. The strength drop, the stiffness degradation, and the dilatancy effects all have effects on the displacement. The effects can be characterized by quantitative relationships.     

Constitutive modeling of soil-structure interface through the concept of critical state soil mechanics

The behavior of soil-structure interface can be crucial to the overall response of a soil-structure system. The numerical simulation of soil-structure interaction problem requires proper modeling of the interface. The similarity between the behavior of soil and interface is first analyzed in the present paper. With this similarity, the concept of critical state soil mechanics (CSSM), which has been successfully used in the modeling of soil behavior, is used to develop a constitutive model for soil-structure interface in the framework of generalized plasticity. The model is capable of modeling strain hardening, softening, normal dilatancy and stress-path dependency of interface between sandy soil and structures during shearing. The effects of normal pressure as well as density of sand are captured in the model. The performance of the model is verified with various experimental results. The unified modeling of the behavior of interfaces with different roughness, different density of soil and different normal pressures using the concept of CSSM is also successfully attempted.     

考虑土体损伤的挤土效应计算探讨

摘 要：圆孔扩张理论应用于静压桩沉桩、搅拌桩成桩过程周围土体的应力和变形分析,取得了一定的成果,但其理论分析结果和实验结果仍存在一定的偏差,主要原因在于理论推导时未考虑桩周一定范围内的土体受施工因素影响而产生的损伤。为更有效地发挥圆孔扩张理论在指导桩基施工中的作用,本文通过构造桩周土体粘聚力变化的表达式来考虑施工造成的土体损伤,基于连续介质力学的原理,依据边界条件确定了表达式中的土体损伤因子,得到了能够反映造成土体损伤主要因素的土体粘聚力变化表达式。基于圆孔扩张理论,并引入土体损伤因子,通过平衡微分方程的迭代计算,分析了成桩过程中的土体塑性区半径及应力分布,发现考虑土体损伤时,塑性区范围相对较大且应力的变化会相对较缓。通过不考虑损伤和考虑损伤的分析结果与既有理论的计算结果对比分析,表明本文提出的考虑土体损伤的分析方法简单合理,为精细化分析桩基施工对地基的影响提供了新方法。     

Critical state shear behavior of the soil-structure interface determined by discrete element modeling

The interface between soil and structure can be referred to as a soil-structure system, and its behavior plays an important role in many geotechnical engineering practices. In this study, results are presented from a series of monotonic direct shear tests performed on a sand-structure interface under constant normal stiffness using the discrete element method (DEM). Strain localization and dilatancy behavior of the interface is carefully examined at both macroscopic and microscopic scales. The effects of soil initial relative density and normal stress on the interface shear behavior are also investigated. The results show that a shear band progressively develops along the structural surface as shear displacement increases. At large shear displacement a unique relationship between stress ratio and void ratio is reached in the shear band for a certain normal stress, indicating that a critical state exists in the shear band. It is also found that the thickness and void ratio of the shear band at the critical state decreases with increasing normal stress. Comparison of the DEM simulation results with experimental results provides insight into the shear behavior of a sand-structure interface and offers a means for quantitative modeling of such interfaces based on the critical state soil mechanics.     

Effects of a roller and a tracked vehicle on the compaction of a high lifted decomposed granite sandy soil

The aim of this research was to innovate a new compaction machinery by comparing experimentally the effects of a two-axle, two wheel road roller and a tracked vehicle on the compaction of a decomposed granite sandy soil with a high spreading lift. By measuring the amount of sinkage of the terrain surface, the dry density distribution versus depth using a cone penetrometer, the normal earth pressure distribution versus depth using a stress state transducer (SST), the effects of the road roller and the tracked vehicle on the increment of the soil dry density were considered theoretically. It was observed that the tracked vehicle showed a larger amount of sinkage and a larger dry density distribution versus depth than the roller. The ratio of shear stress to normal stress was still large enough at the deep stratum, so that an optimal shear strain was developed on the whole range of the high lifted stratum and it increased the soil compaction density due to the dilatancy effect.     

粗粒土与结构接触面受载过程中的损伤

进行了粗粒上与结构接触面单调和循环加载试验,基于宏细观测量结果,扩展了损伤概念以描述该类接触面在受载过程中的物态演化,及由于物态演化导致的力学特性从初始状态到最终稳定状态的连续变化过程.揭示了接触面损伤的细观物理基础主要是接触面内土的颗牲破碎和剪切压密这两种物态演化;指出接触面的剪胀体应变可以划分为可逆性和不可逆性剪胀体应变两部分,其中不可逆性剪胀体应变可作为接触面损伤发展的宏观量度,因此其归一化形式可作为一种损伤因子的定义;提出了建立粗粒土与结构接触面一种损伤本构关系的基本思路.