下卧基岩双层地基上刚性圆板的扭转振动

运用积分变换的方法研究了下卧基岩双层地基上刚性圆板在简谐扭矩作用下的动力响应问题。假设地下水位以上地基为单相弹性层,以下为饱和两相介质,结合上表面为混合边界条件、下边界为刚性基岩以及弹性层与饱和土层接触面处位移和应力连续等条件,得出了刚性圆板扭转时的对偶积分方程。通过数学方法化对偶积分方程为第二类Fredholm积分方程,求解了相应的动力响应问题。数值分析结果表明,层厚和土层的剪切模量比对扭转振动影响较大。     

黏弹性地基中埋置刚性圆柱状基础水平振动问题

考虑埋置基础侧面土体作用,对黏弹性半空间中埋置刚性圆柱状基础在水平简谐荷载作用下的动力响应特性进行了理论研究。基于弹性动力学理论,建立黏弹性土和基础的动力控制方程,并结合混合边值条件,使用Hankel积分变换得到基础水平动力放大系数和动力刚度表达式。将所得理论解与已知经典解对比以验证其正确性,并通过数值计算分析刚性基础和地基土的主要参数对动力放大系数和动力刚度的影响。分析结果表明：基础埋深比及侧面土体相对刚度可显著影响基础水平动力刚度和动力放大系数,且基础侧面土体作用对基础动力刚度的贡献占比随之增大而愈加突出。地基土体阻尼比和泊松比对基础刚度系数的影响较大,阻尼比对其影响随激振频率增大更趋明显,泊松比对其影响在低频和高频范围内得到相反的规律。     

层状饱和地基上刚性条形基础的动力刚度

利用饱和土层的精确动力刚度矩阵求解了动力Green函数,将基础和场地交界面离散,根据交界面上的位移边界条件得到层状饱和场地上刚性条形基础的动力刚度。对工程中常用的四种场地(饱和均匀半空间、基岩上单一饱和土层、饱和半空间上单一饱和土层、层状饱和场地)上刚性条形基础的动力刚度进行了对比和分析。研究表明:层状场地结果与其他场地结果有着明显的不同;当上覆土层较厚时,其他三种模型结果基本一致。本文方法可进一步推广到层状饱和场地中任意埋置基础动力刚度的计算中。     

饱和地基上刚性条形基础的竖向振动分析

基于Biot动力方程,研究了饱和均质弹性半空间上刚性条形基础的竖向振动问题。通过Fourier积分变换求解了饱和土的动力控制方程,并结合上边界为混合边界的条件得到了刚性条形基础的竖向振动对偶积分方程,利用正交多项式对偶积分方程转化为求解一组线性代数方程组,得到了动力柔度系数随无量纲频率的关系曲线;同时,将其退化到单相弹性半空间,得到了单相弹性半空间上刚性条形基础竖向振动的动力柔度系数,并进行了对比。数值分析结果表明:对于饱和半空间上刚性条形基础的竖向振动,动力渗透系数较大时,动力渗透系数对动力柔度系数的影响较大,而当动力渗透系数较小时,则可以忽略动力渗透系数对动力柔度系数的影响。此外,动力柔度系数基本上不受土体参数泊松比的影响,且实部和虚部的绝对值均小于退化到单相弹性半空间上的值。     

层状横观各向同性地基上刚性条形基础动力刚度系数

采用间接边界元方法（IBEM）研究了层状横观各向同性（TI）地基上明置条形基础平面内动力刚度系数。首先,在波数域中求解TI介质动力平衡方程,建立了TI土层和TI半空间的精确动力刚度矩阵,并通过集整土层和半空间刚度矩阵,求得层状TI地基整体动力刚度矩阵。然后,采用刚度矩阵方法求得层状TI地基表面均布荷载动力格林函数。最后,由基础与地基表面的混合边界条件求得明置条形基础的平面内动力刚度系数。均布荷载动力格林函数的引入克服了传统边界元方法的奇异性问题,同时,精确动力刚度矩阵的引入使得方法不受土层厚度的限制。通过与已有结果比较验证了方法的正确性,并以均匀TI半空间地基、单一TI土层地基和多TI土层地基上明置基础为例进行了数值计算分析,探讨了TI参数、振动频率和土层对刚度系数的影响。研究表明,土体TI参数对刚度系数有着显著的影响,尤其是层状TI地基中刚度系数的峰值频率和峰值十分依赖于TI参数的变化;逆序地基与正常序列地基上基础刚度系数差异明显,逆序地基对应刚度系数随频率振荡剧烈且数值较大。     

饱和地基上刚性基础的竖向振动分析

基于第一作者提出的饱和土弹性波动方程,研究了圆柱形刚性基础在饱和地基上的垂直振动,即首先应用Hankel 变换求解动力基本方程,然后按混合边值条件建立饱和地基上刚性基础垂直振动的对偶积分方程,将其化为易于数值计算的第二类Fred holm 积分方程,最后给出了地基表面动力柔度系数Cv 和基础振幅随无量纲频率a0 的变化曲线     

条形基础下刚-柔性桩复合地基设计方法研究

刚-柔性桩复合地基是近年才发展起来的一种新型地基处理方式,其理论的发展明显地落后于工程实践,尚未形成比较完善的设计理论和计算方法。应用条形基础下刚-柔性桩复合地基实用简化的设计方法对工程实例进行优化设计,结果表明,刚-柔性桩复合地基能合理发挥刚性桩、柔性桩以及土体的力学性能,有显著的经济效益。     

与黏弹性地基相互作用的梁的自由振动分析

以黏弹性力学理论为基础,首先推导出黏弹性地基上黏弹性梁振动的一般方程;然后忽略梁的黏性,但考虑地基的黏性,提出分析该地基梁的自由振动的方法,该方法适用于具有N个标准线性体通过串联或并联而构成的一般线性体地基,且具有通用性,可进一步用于分析地铁区间隧道类复杂结构由于列车行进引起的受迫振动等问题。在此基础上,推导出三参数标准线性体地基梁满足任意边界条件和任意初始条件的结果;最后给出数值算例,并对数值结果进行分析。算例表明,采用分离变量的级数解法,级数收敛很快,一般只要取前两项即足够满足工程要求。     

墙下条形基础与柱下条形基础的对比分析

通过某工程基础设计实例,对工程中的墙下条形基础和柱下条形基础进行了对比,同时从受力和配筋方面加以分析计算,从而使两种基础在工程中的应用和研究更加清晰、明确。     

带桩条形基础的计算分析

通过对带桩条形基础的计算方法的分析 ,提出在带桩条形基础的计算过程中 ,应考虑桩与桩之间的相互作用。基于此方法 ,可以正确描述此类基础的性状。最后通过一简单算例分析此方法与传统方法存在的差异。     

Stability of strip footings above concrete-lined soft ground tunnels

There are cases that footings are underlain by shallow tunnels. The presence of underground tunnels may adversely affect the stability of the overlying footings. Presently, there is no widely accepted analysis or design methodology available to ensure the stability of such footing-soft ground tunnel systems. As a step toward the development of such a methodology, this study was undertaken to investigate the effect of underground tunnel on the bearing capacity of strip footings. In this study, a two-dimensional plane-strain elasto-plastic finite element computer program was used for analysis. Using this computer program, the stability of strip footings was analyzed for a wide range of soil type and tunnel conditions, including tunnel size, tunnel location, and lining thickness. Based on the results of analysis, the mechanistic behavior of the footing-tunnel system was evaluated. Furthermore, bearing capacity equations for strip footings above concrete-lined soft ground tunnels were formulated. The developed equations are shown to be capable of predicting the bearing capacity of footings located above concrete-lined tunnels reasonably well     

考虑桩底土层波动效应的饱和黏弹性半空间中摩擦桩竖向振动

为深化对饱和黏弹性半空间地基中摩擦桩竖向振动特性的认识,基于Boer多孔介质动力控制方程组,考虑桩底土层波动效应,采用Hankel积分变换和微分算子分解理论求解相关方程得到桩底、桩侧反力,进而建立饱和土中桩基竖向振动偏微分方程,结合桩土接触面混合边值条件推导得出了饱和黏弹性半空间地基中摩擦桩桩顶竖向动力阻抗解析表达式。并在此基础上进一步通过数值算例对比分析探讨了液固耦合系数、桩长径比、桩土模量比、地基土黏滞阻尼系数这些重要因素对所得桩顶动力阻抗的影响规律,得到了一些对工程实践有意义的结论。     

Combined loading of strip footings on sand-over-clay with layers of varying extents

This study describes the numerical analyses performed to investigate the bearing capacity of strip footings placed on granular (frictional) material overlying soft clay subjected to combined vertical-horizontal and vertical-moment loading. A plane-strain finite element limit analysis is used to estimate the limiting load combinations for two layer soil geometries where the top layer is either fully extended in the horizontal direction or it is of limited horizontal extent, representative of rock or gravel berms commonly used in offshore practice.Bearing capacity envelopes for combined vertical, horizontal and/or moment loading are well-documented in the literature for cases of footings resting on single sand or clay soil. For two-layer sand-over-clay soil with a horizontally extensive top layer, the vertical-horizontal and vertical-moment envelopes initially coincide with the envelope for a single sand layer at low vertical loads, but show an abrupt reduction in the horizontal or moment capacity as the vertical load increases beyond a certain critical vertical load. The critical vertical load is found to vary as a function of the thickness and the lateral extent of the upper layer. Relationships are presented to enable the vertical-horizontal and vertical-moment envelopes to be estimated based on the problem geometry and material properties.The findings in this study provide insight into the response of subsea foundations placed on rock or gravel of limited extent overlying a clay seabed as well as the general response of shallow foundations on two-layer profiles.     

双层地基极限承载力的极限分析上限法

极限分析方法已广泛应用于岩土工程稳定性分析中,目前已有很多学者探讨了极限分析上限方法计算均质土地基极限承载力问题,事实证明该方法是卓有成效的。而实际中常遇到的是层状地基,特别是持力层范围内主要有两层土的情况,对于双层地基极限承载力的计算,目前的研究还相当不成熟。将一种新的多块体离散模式的上限方法应用于双层地基极限承载力的计算,详细探讨了该多块体离散模式应用时相容速度场的确定方法。并应用该方法对一些双层地基土极限承载力问题进行了计算,将计算结果与已有计算方法做了对比,通过对比可以验证该方法是正确和有效的。     

临近基坑条形浅基础地基承载力减损的下限估算

利用极限分析有限元方法和下限定理,分析计算了考虑土体自重的临近基坑(边坡)条形浅基础c-?土地基极限承载力下限解。作为一种简化手段,分别按土的粘聚力、基础埋深(超载)和土的自重这3项承载力影响系数表达临近基坑地基极限承载力,获得了基础位置、基坑开挖的边坡角度以及土的内摩擦角取不同值时,相应的地基承载力系数下限解答,并对计算结果进行了讨论。最后将计算结果与已有文献资料以及利用多滑块Prandtl破坏模式得到的极限分析上限解答进行了分析对比,从上、下限给出了临近基坑既有建筑物地基承载力系数取值范围及其承载力上、下限平均值时的减损因数,分析了影响计算结果的因素。     

Bearing capacity of strip footings on sand slopes reinforced with geogrid and grid-anchor

This paper presents the effect of a new type of geogrid inclusion on the bearing capacity of a rigid strip footing constructed on a sand slope. A broad series of conditions, including unreinforced cases, was tested by varying parameters such as geogrid type, number of geogrid layers, vertical spacing and depth to topmost layer of geogrid. The results were then analyzed to find both qualitative and quantitative relationships between the bearing capacity and the geogrid parameters. A series of finite element analyses was additionally carried out on a prototype slope and the results were compared with the findings from the laboratory model tests and to complete the results of the model tests. The results show that the bearing capacity of rigid strip footings on sloping ground can be intensively increased by the inclusion of grid-anchor layers in the ground, and that the magnitude of bearing capacity increase depends greatly on the geogrid distribution. It is also shown that the load-settlement behavior and bearing capacity of the rigid footing can be considerably improved by the inclusion of a reinforcing layer at the appropriate location in the fill slope. The agreement between observed and computed results is found to be reasonably good in terms of load-settlement behavior and optimum parameters.     

Ultimate loads for eccentrically loaded model shallow strip footings on geotextile-reinforced sand

A series of tests were carried out with an eccentrically loaded model surface shallow strip footing on reinforced dense sand to investigate the decrease of the ultimate loads with increasing eccentricity and to compare the experimental results with commonly used approaches such as Meyerhof's effective width concept and the customary analysis. An experimental system was produced and used to run the tests. The experimental system consists of a tank, model footing, sand, loading mechanism, etc. A single woven geotextile sheet was placed horizontally below the footing's base at a depth of half of the footing's width. Geotextile reinforcement increased ultimate loads when compared to the unreinforced cases. This contribution decreases with increasing eccentricity. The measured decreases in ultimate loads with increasing eccentricities in the unreinforced tests within the core (kern, middle third) are in good agreement with Meyerhof's approach, while customary analysis is a little on the conservative side. Outside the core, Meyerhof's approach is on the conservative side in this case. Decreasing ultimate loads with increasing eccentricity for the reinforced tests cases were in general agreement with customary analysis, although they are slightly greater.     

The ultimate bearing capacity of shallow strip footings using slip-line method

Based on the limit equilibrium theory, an accurate approach is proposed to solve the ultimate bearing capacity of shallow strip footings under general conditions. The foundation soil is considered to be an ideal elastic-plastic material, which obeys the Mohr-Coulomb yield criterion, and is assumed to be an ideal continuous medium which is isotropic, homogeneous and incompressible or non-expansive. Through analyzing the relative motion and interaction between the footing and soil, the problem of the ultimate bearing capacity of shallow strip footings is divided into two categories. A minimum model with the total vertical ultimate bearing capacity as its objective function is established to solve the ultimate bearing capacity using the slip-line method with no need to make any assumptions on the plastic zone and non-plastic wedge in advance. A convenient and practical simplified method is also proposed for practical engineering purposes. Furthermore, the first category of the problem in the case of the same uniform surcharges on both sides of footings is the focus of the study: the applicable conditions of Terzaghi’s ultimate bearing capacity equation as well as the theoretical exact solutions to its three bearing capacity factors are derived, and a new bearing capacity equation is put forward as a replacement for Terzaghi’s equation. The geometric and mechanical similarity principle is proposed by a dimensionless analysis. The results show that for perfectly smooth footings, the total vertical ultimate bearing capacity obtained by the present method is in good agreement with those by existing methods, whereas the existing methods underestimate the ultimate bearing capacity in the case of perfectly rough footings. The classic Prandtl mechanism is not the plastic failure mechanism of the ultimate bearing capacity problem of perfectly smooth footings on weightless soil.