土钉墙地基承载力问题探讨

土钉墙墙底地基土的承载力验算是土钉墙支护设计的一项重要内容。国内的工程实践中,通常将土钉墙地基承载力与坑底土抗隆起验算合并考虑。针对具体案例,通过Plaxis3D有限元数值模拟,分析研究了土钉墙底部土体发生地基承载力失稳的破坏模式、破坏荷载以及土钉墙墙底应力分布特点等,探讨了依据我国相关规程进行土钉墙坑底隆起或地基承载力计算可能存在的问题。借鉴国外加筋土挡墙地基承载力计算的一般方法,将土钉墙作为荷载倾斜、偏心的刚性基础对待,利用荷载倾斜、偏心条件下传统刚性浅基础的地基承载力的Meyerhof解和Vesic解,对土钉墙地基承载力进行了计算和对比,通过对比发现,Meyerhof解更接近实际,据此,提出了土钉墙地基承载力计算的合理模式。     

倾斜荷载作用下层状非均质地基的极限承载力

通过一定的室内模型试验对倾斜荷载作用下层状非均匀地基的极限承载力进行了试验研究.引入荷载倾斜影响系数,对于层状非均质地基的极限承载力计算公式进行了修正,给出了倾斜荷载作用下的地基极限承载力计算公式.根据模型试验结果,对所建立的计算公式进行验证.结果表明,对于倾斜荷载作用下的层状非均匀地基,根据修正后的Meyerhof承载力公式所得到的计算结果能够较好地与试验结果吻合.     

条形荷载下不排水土坡破坏模式判定及极限承载力估算

为了确定边坡放置地基的极限承载力,采用弹-理想塑性有限元,分析了不排水土坡(u=0)的坡肩上放置条形基础的地基极限承载力。结果表明,不排水土坡地基极限承载力仍可沿用Terzaghi水平地基极限承载力计算公式qu=cuNc。不考虑重度条件下,土坡地基的破坏模式类似于水平地基破坏模式,承载力系数Nc与极限分析法计算的结果一致;考虑重度条件下,可用土坡实际状态的Taylor数与其极限状态的Taylor数的差值N作为判别指标,当该值较小时将发生边坡破坏模式,较大时发生地基破坏模式,找出了两种破坏模式的N分界点,确定了Nc与N的关系,建立起坡肩作用条形荷载下不排水土坡极限承载力计算公式。     

深置矩形基础与地基共同作用解析分析

基于集中力作用下的Mindlin解位移公式,推导得到均布荷载作用下的地基柔度系数,对地基上的四周自由板与地基共同作用进行简化计算,得出了考虑埋深时地基基础共同作用的解析公式。在基础板为刚性和弹性、荷载为集中荷载及均布荷载的情况下,对考虑埋深时的Mindlin解与不考虑埋深时的Boussinesq解的计算结果进行比较,表明,埋深比对于基础沉降、地基反力的分布、基础板的弯矩等都有一定的影响。给出了考虑埋深时的沉降修正系数公式,以供工程应用参考。     

倾斜与偏心荷载作用下裙板式基础破坏包络面研究

联合采用Swipe加载模式与固定位移比加载模式,对裙板式浅基础在倾斜与偏心荷载两种加载条件下的承载性能进行了比较系统的平面应变有限元分析,主要探讨了裙板式基础埋深、地基土不排水强度的非均质性对基础在竖向荷载V与水平荷载H、竖向荷载V与弯矩M加载平面内的破坏包络面的影响,揭示了地基在不同荷载分量组合条件下的失稳破坏机制。结果表明,基础埋深与地基土强度的非均质性对于地基失稳模式有较大影响,但V-H和V-M平面内的破坏包络面形状具有较好的归一化特性,并分别建议了裙板式基础在倾斜和偏心荷载作用下的破坏包络面方程。     

偏心荷重下转动浅基承载力的严密解法

根据偏心荷载作用下地基失稳的特征,构建了偏心荷重下地基极限状态时土体中不连续应力场和速度场,分析了土体中不连续应力场和速度场的特点。基于上、下限原理的推论,建立计算模型,构建求解地基极限承载力的算法,通过解三类边值问题,求得偏心荷重下转动浅基极限承载力的严密解,该解满足刚性基础转动的速度边界和其他已知的边界条件。算例表明解垂直偏心荷载下地基极限平衡问题的解法是可行的,且结论可靠。     

基于上限法的地基极限承载力计算模式探讨

当前地基极限承载力理论研究及实践应用的基本框架,仍采用的是Terzaghi所建议的三项叠加法,即分别考虑土体粘聚力、超载和地基土重对承载力的贡献,然后将三项进行叠加,由于土体并非线弹性材料,应用叠加原理在理论上是欠妥的。从理论上讲,采用整体考虑影响承载力的各种因素直接进行极限承载力计算更为恰当。为探讨采用三项叠加法与整体考虑各种影响因素直接计算的差别,基于多块体上限方法,分别采用两种计算方法对中心荷载作用下粗糙及光滑基础地基极限承载力问题进行了计算,探讨两种计算方法的差异,并从地基破坏面所反映出的信息揭示了两种计算存在差异的内在原因。     

复合加载条件下沉箱基础稳定性的三维效应

联合采用swipe加载模式与固定位移比加载模式,对于吸力式沉箱基础在水平荷载H与力矩M的复合加载条件下的稳定性进行比较系统的三维有限元分析,主要探讨了基础埋深与直径之比、地基土不排水强度的非均质性对于基础在H-M荷载空间内的破坏包络轨迹的影响,揭示了地基在不同荷载分量组合条件下的失稳破坏机制,并与平面应变假定下得到的结果进行了比较。计算结果表明：平面应变与三维情况下基础的破坏包络面形状有较大差异,分析基础稳定性时,必须考虑其三维效应。在三维情况下,非均质土中埋深与直径之比较小的基础的破坏包络面仍然会向负方向倾斜,已有的包络面方程明显高估这种情况下沉箱基础在正向水平荷载与力矩联合作用下的承载力,从而导致基础设计偏于不安全。     

条形基础极限承载力数值分析

根据塑性力学滑移线理论,推导了条形地基在极限平衡状态下的平衡微分方程,然后利用有限差分法推出地基土在极限状态时的有限差分公式,结合边界条件编制出地基承载力计算程序,该程序求解条形地基极限状态下的滑移线区域及相应的地基极限承载力值时可以考虑基础埋深、地基土重度、土的内摩擦角、基础与地基摩擦等参数。利用程序,得到了土的内摩擦角与地基承载力系数N? 的对应表,并全面讨论了基础埋深、地基土重度、内摩擦角及基础与地基摩擦角等参数对地基承载力和滑移线形状的影响,得到了一些有意义的结论。     

倾斜荷载作用下桶形基础承载力特性研究

针对吸力式桶形基础结构,借助大型通用有限元软件ABAQUS平台,建立了均质软黏土地基上倾斜荷载作用的三维弹塑性有限元计算模型。通过系统地计算分析,得到了单个倾斜荷载分量作用下桶形基础的承载力特性及其破坏机制。进一步研究了水平倾斜荷载与竖向倾斜荷载共同作用下的桶形基础承载性能,得到了该倾斜荷载加载方式作用下桶形基础在V-H平面内的破坏包络面,依此建立了倾斜荷载与偏心距e之间的三维破坏包络曲面。计算结果表明,偏心距e对竖向倾斜承载力的影响相对水平倾斜承载力较大;桶形基础承载性能随着偏心距e的增大而减小。     

输电线路掏挖基础极限上拔承载力变分解法

针对输电线路掏挖基础极限上拔承载力问题,根据塑性极限平衡原理,假设基础周围土体发生整体剪切破坏并服从Mohr-Coulomb破坏准则,建立静力学平衡方程式,并结合变分法原理求解不同加载条件下基础的极限抗拔力。通过变参数计算,分析土体强度及基础埋深比对中心上拔、偏心上拔及倾斜上拔时基础极限上拔力的影响。经归一化处理,分别给出水平力和弯矩存在对基础抗拔力折减的影响系数。当归一化弯矩与上拔力比值Nm 0.4时,? m减小速度放缓。相对于内摩擦角,黏聚力变化对? m的影响更明显。水平力影响系数? h同时受到土体内摩擦角、黏聚力以及基础埋深比的共同影响,规范建议值较变分解法的计算结果偏大。通过与规范公式法计算结果及试验结果进行对比,验证了所提方法的合理性和准确性。研究结果为偏心上拔及高露头基础顶部作用水平荷载时基础抗拔承载力的计算提供参考。     

Bearing Capacity of Embedded Strip Footings on Cohesionless Soil Under Vertical and Horizontal Loads

In the last decades a few attention was given to the evaluation of the bearing capacity of embedded footing under inclined loads on a frictional soil. This paper focuses on a numerical study using the finite-difference code Fast Lagrangian Analysis of Continua (FLAC), to evaluate the bearing capacity of embedded strip footing on a frictional soil. The soil is modeled by an elasto-plastic model with a Mohr–Coulomb yield criterion and associative flow rule; the effect of non-associativity of the soil on the bearing capacity is also investigated. The effect of the embedment is estimated though a depth factor, defined as a ratio of the bearing capacity of a strip footing at a depth D to that of a strip footing at the ground surface. The inclination effect is estimated by inclination factors, defined as the ratio of the limit vertical load for a footing under inclined loading to that of the vertically loaded footing. Both swipe and probe analyses were carried out to identify the vertical force–horizontal force (V–H) failure envelope. The results have been compared with those available in the literature.     

Bearing capacity of rectangular footings in uniform clay with deep embedment

This letter is concerned with the undrained bearing capacity of rectangular footings with various aspect ratios and embedment ratios in uniform clay. It covers thin plate foundations with low aspect ratios and high embedment depth with embedment ratio up to 150. The work is based on small strain finite element analysis (FEA). After verification of the FEA model against existing solutions of the bearing capacity factors of rectangular footings, a series of FEA results are obtained. Based on the FEA results, a simple formulation is proposed to calculate the bearing capacity factor for rectangle footing with different aspect ratio in any embedment depth, extending the existing solutions to cover a wider ranges of footing aspect ratios and embedment ratios.     

基于迈耶霍夫和汉纳冲剪理论计算地基极限承载力的研究

对迈耶霍夫和汉纳公式计算成层土地基承载力的步骤细化,推导出计算承载力的一般计算模式;运用现场载荷试验结果对计算成层土地基承载力的迈耶霍夫和汉纳公式进行验证,同时对迈耶霍夫和汉纳公式进行适当的修正。计算结果表明,成层土的迈耶霍夫和汉纳计算公式计算步骤的关键在于确定破坏面的极限深度,极限深度的判断对其计算的结果有重要影响。根据大型荷载板现场试验的结果,当土层超过两层时的成层土地基的极限承载力按迈耶霍夫和汉纳理论公式进行计算,其计算结果与实测值基本吻合。     

条形基础加筋砂土地基极限承载力计算

基于传统的极限平衡条分法,利用临界滑动场法计算了条形基础的加筋地基极限承载力。假定土体处于极限平衡状态时,土体与筋材间存在均匀的摩擦力,通过建立土体条块极限平衡方程,推导了地基承载力的递推关系式。首先,设定计算土体范围,并划分条块和离散状态点;其次,根据递推公式计算各个状态点的参数,并搜索临界滑面;最后,根据搜索出的滑面计算地基承载力。通过实例比较进一步验证了计算结果的可靠性,并分析了首层筋带埋深、铺设层数和长度对地基承载力和滑面位置的影响。研究结果表明：地基承载力随着筋带埋深的增加先增大后减小;随着层数和长度的增加先逐渐增大,最后趋于稳定;滑面位置的变化规律主要是垂直影响深度和水平影响范围增大或减小。该方法原理简单、易于编程,为条形基础加筋地基承载力的计算提供了一种新思路,是临界滑动场法在地基承载力计算中的推广应用。     

砂-黏土双层地基极限承载力的数值研究

在实际工程中,常见天然的或通过人工换填形成的上部砂土、下部黏土的层状地基,目前关于这种双层地基极限承载力和破坏机制研究还不够深入。通过有限差分法建立双层地基数值模型,分析基底粗糙程度、砂土剪胀角和超载对地基破坏模式及极限承载力的影响,并根据有限差分法计算结果对强度加权平均法、应力扩散法和冲剪破坏法等现有实用计算方法的估算正确性进行评价。研究结果表明,基底粗糙程度对极限承载力的影响随着砂土内摩擦角的增大而减小;当剪胀角较小时,剪胀角变化对承载力的影响更为明显;当下层黏土强度较小时,超载的作用更明显。强度加权平均法由于低估破坏面影响深度导致砂土层权重较大,计算结果偏大;应力扩散法忽略了砂土剪切强度,在砂土层较厚时出现低估;冲剪破坏法由于可较为精确地计算砂土破坏面上抗剪强度和被动土压力,是3种实用方法中计算最准确的,当砂土厚度和黏土强度较大时,建议按太沙基经验公式对下卧黏土进行局部剪切破坏修正后确定极限承载力。     

Bearing Capacity of Strip Footings Near Slopes

In the last decades a great attention was given by many authors to the evaluation of the static and seismic bearing capacity of footings near slopes. In this paper a model has been developed based on the limit equilibrium method, considering a circular surface propagates towards the slope until the sloping ground is reached. The bearing capacity is investigated considering either the distance of the footing from the edge of the slope and/or the effect of the footing embedment. A validation of the proposed model was made by a comparison with solutions taken from literature regarding the evaluation of the bearing capacity for a footing adjacent to a slope and for an inclined load. The loading conditions consist in vertical and horizontal stress on the footing and on the soil below the footing. Both the inertial and kinematic effects of the seismic loading have been analyzed, and a simple equation has been derived for the evaluation of the seismic bearing capacity. The static and seismic bearing capacity has been investigated as a function of the soil friction angle, of the seismic coefficient, of the sloping ground. Finally, the influence of the distance of the footing from the edge of the slope was taken into consideration in the evaluation of the bearing capacity, and a threshold distance at which the reduction of the bearing capacity due to the sloping ground vanishes has been defined.     

Effective width rule in calculations of bearing capacity of shallow footings

The classical solution to the bearing capacity problem predicts the limit load on symmetrically loaded shallow strip footings. A useful hypothesis was suggested by Meyerhof to account for eccentricity of loading, in which the footing width is reduced by twice-the-eccentricity to its ‘effective’ size. This hypothesis sometimes has been criticized as being overconservative. This paper examines Meyerhof’s suggestion and presents the bearing capacity of eccentrically loaded footings calculated using the kinematic approach of limit analysis. It is found that the effective width rule yields a bearing capacity equivalent to that calculated based on the assumption that the footing is smooth. For more realistic footing models and for cohesive soils the effective width rule is a reasonable account of eccentricity in bearing capacity calculations. Only for significant bonding at the soil-footing interface and for large eccentricities does the effective width rule become overly conservative. For cohesionless soils, however, the effective width rule may overestimate the best upper bound. This overestimation increases with an increase in eccentricity. ©