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。     

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。     

非共面复合加载条件下桶形基础稳定性研究

采用荷载-位移联合搜索方法,对于饱和软黏土地基上裙板式桶形基础在水平荷载H、力矩M与扭矩T的非共面复合加载条件下的稳定性进行了比较系统的三维有限元分析,主要探讨了在不同荷载空间内桶形基础的破坏包络轨迹。在分析中,地基土服从基于Tresca屈服准则的理想弹塑性本构关系。计算结果表明,与传统共面复合加载情况相比,非共面复合加载条件下桶形基础的破坏包络面形状有较大差异。非共面复合加载情况下,包络面形状不依赖于基础埋深比,可用简单的圆或椭圆方程进行描述。根据有限元计算结果,将已有的三自由度破坏包络面方程扩展到六自由度复合加载空间内,进而初步建议了一个六自由度破坏包络面方程,并对于实际工程中常用的桶形基础埋深比,给出了方程中偏心度参数取值的确定方法。该方程可用来合理评价复杂荷载条件下软黏土地基上桶形基础承载力。     

软黏土强度非均质性对复合加载模式下桶形基础破坏包络面的作用分析

针对非均质软黏土地基上复合加载模式下桶形基础的破坏包络面,运用大型有限元分析软件ABAQUS进行了三维非线性数值分析。通过比较系统的数值计算与分析,考察了软黏土非均质性对复合加载模式下桶形基础破坏包络面形状的作用及影响。结果表明：软黏土地基的不排水抗剪强度对于力矩为0的平面内,其应力归一化的破坏包络面影响不大,而在水平荷载与竖向荷载为0的平面内,其应力归一化的破坏包络面具有一定影响。通过与有关模型试验的对比分析,在一定程度上验证了有限元计算模型与数值分析结果的合理性。     

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。     

复合加载模式下地基失效机制研究

在土体极限平衡定理的基础上,利用有限元数值分析软件ABAQUS对复合加载模式下地基失效机制进行了详细地研究。结果表明,竖向荷载V控制着地基破坏滑裂面的深浅程度,水平荷载H和力矩荷载M造成了地基破坏模式的非对称性;文中提出的前倾勺形破坏模式和后仰勺形破坏模式,合理地解释了H-M荷载空间中破坏包络面的非对称性,并给出了H-M荷载空间中不同的地基失效模式与荷载分量间的关系和地基土体中出现极限力矩荷载时的经验判断公式。     

复合加载情况下双层地基极限承载力研究

在复合加载情况下精确求解层状非均质地基的极限承载力,具有很强的工程实用与理论参考价值。基于土体极限平衡理论与通用有限元软件ABAQUS,针对复合加载情况下上硬下软的双层不排水饱和软黏土地基的极限承载力,进行了大量的数值计算,得出了上层土临界深度Hcr的计算公式、竖向极限承载力Pv的计算公式以及复合加载情况下地基破坏时的破坏包络面方程。研究结果表明：上层土临界深度Hcr取决于土层间强度比Su1/Su2;竖向极限承载力Pv与破坏包络面取决于土层间强度比Su1/Su2、上层土深度H1与基础型式。     

复合加载情况下非均质地基上条形基础的极限承载力研究

复合加载情况下精确求解非均质地基上条形基础的极限承载力以及评价影响极限承载力的相关因素,具有很强的工程实用与理论参考价值。基于极限平衡原理,在Mohr-Coulomb破坏准则的基础上,将非均质地基上条形基础极限承载力问题等价为一个边界待定的泛函极值问题。利用变分原理得到与平衡方程相等价的积分约束条件以及相应的欧拉方程与横截条件。引入问题边界条件,利用VC++6.0编制了数值计算程序,求得了复合加载情况下非均质地基破坏时的滑裂面函数与破坏包络曲线。从理论上研究了土体内摩擦角、土体黏聚力、土层强度比与地下水位变化等因素对地基破坏包络曲线的影响。研究结果表明,其解答是地基极限承载力真实解的某一最小上限。     

十字形内隔板对桶形基础各单向承载力和破坏模式的影响分析

为了研究十字形内隔板对吸力式桶形基础各单向承载力和破坏模式的影响,分别建立了软土地基上带十字形内隔板和不带内隔板的吸力式桶形基础及实心圆形基础的有限元模型,并利用位移加载法对3种基础在不同基础埋深比和土体剪切强度时的各单向承载力进行了大量的有限元计算和分析。计算结果表明,当软土地基的剪切强度均一时,吸力式桶形基础的各单向承载力基本不受内隔板的影响,3种基础的破坏模式基本相同;当软土地基的剪切强度不均一时,在深径比较小时,十字形内隔板可明显提高桶形基础承受力矩和水平荷载的能力(尤其是力矩承载力),其提高效果随着土体剪切强度不均一度指标的增大而增大,且基本等同于实心圆形基础,但对竖向承载力提高不大,剪切强度不均一的软基上埋深比较小的不带内隔板的桶形基础在转角或水平位移加载下的破坏模式和另外两种基础有显著不同。给出了带十字形内隔板的吸力式桶形基础各单向承载力的简化计算公式。     

复合加载模式下单桩复合筒型基础地基承载力包络线研究

海上风力发电机属于高耸结构物,其地基基础不仅要承受上部结构传递的竖向荷载V、水平荷载H,更要抵抗巨大的弯矩荷载M。单桩复合筒型基础是新型的海上风力发电机基础型式,建立复合加载模式下单桩复合筒型基础地基的破坏包络线,并据此评价其稳定性是地基基础设计的重要手段。在Swipe加载方法和固定位移比加载方法的基础上,通过数值分析研究了不排水饱和软黏土中单桩复合筒型基础在V-H、V-M、H-M和V-H-M加载模式下的地基承载力包络线,开展了室内模型试验,验证了数值分析结果的可靠性。研究表明,V-H和V-M加载模式下地基承载力包络线具有对称性,而H-M加载模式下呈非对称性;V-H-M加载模式下地基承载力包络线的形状受弯矩荷载M的影响显著,表现为随着弯矩荷载的增加,V-H平面内的破坏包络线逐渐缩小。实际工程中,可根据受力状态与破坏包络面的相对关系,评价单桩复合筒型基础的稳定性。     

Numerical modelling of a hybrid skirted foundation under combined loading

Offshore foundations are subject to combinations of vertical, horizontal and moment (VHM) loading. Different foundation solutions are appropriate, depending amongst others, on the expected load combinations. This paper investigates in detail the soil failure mechanisms and resulting foundation capacity under undrained VHM loading of a hybrid foundation concept. Comprising of a skirted mat combined with an internal caisson compartment, this foundation design is found to provide significantly increased horizontal capacity that is often critical in offshore applications, with key influences for increased moment capacity identified as well. The numerical study investigated both variations in the geometry of the internal caisson compartment and the soil shear strength profile. The paper concludes by proposing expressions that allow slightly conservative prediction of the combined capacity of a hybrid foundation.     

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。     

Analysis of pile‐raft foundations under complex loads in layered soils

Considering there is hardly any concerted effort to analyze the pile‐raft foundations under complex loads (combined with vertical loads, horizontal loads and moments), an analysis method is proposed in this paper to estimate the responses of pile‐raft foundations which are subjected to vertical loads, horizontal loads and moments in layered soils based on solutions for stresses and displacements in layered elastic half space. Pile to pile, pile to soil surface, soil surface to pile and soil surface to soil surface interactions are key ingredients for calculating the responses of pile‐raft foundations accurately. Those interactions are fully taken into account to estimate the responses of pile‐raft foundations subject to vertical loads, horizontal loads and moments in layered soils. The constraints of the raft on vertical movements, horizontal movements and rotations of the piles as well as the constraints of the raft on vertical movements and horizontal movements of the soils are considered to reflect the coupled effect on the raft. The method is verified through comparisons with the published methods and FEM. Then, the method is adopted to investigate the influence of soil stratigraphy on pile responses. The study shows that it is necessary to consider the soil non‐homogeneity when estimating the responses of pile‐raft foundations in layered soils, especially when estimating the horizontal responses of pile‐raft foundations. The horizontal loads and the moments have a significant impact on vertical responses of piles in pile‐raft foundations, while vertical loads have little influence on horizontal responses of piles in pile‐raft foundations in the cases of small deformations. The proposed method can provide a simple and useful tool for engineering design. Copyright © 2013 John Wiley & Sons, Ltd.     

Combined loading of caisson foundations in cohesive soil: Finite element versus Winkler modeling

The undrained response of massive caisson foundations to combined horizontal, vertical and moment loading is parametrically investigated through a series of 3D finite element analyses. The parameters are: (a) the embedment ratio (D/B), (b) the factor of safety against initial vertical loading (FS_V) and (c) the ratio of the overturning moment to the horizontal force applied at the top of the caisson (M/Q). Emphasis is given on: (i) the identification of all possible failure mechanisms in M–Q–N space, (ii) the developed stress distributions along the caisson walls for various load levels up to complete failure conditions. The results are then used as a feedback for calibrating the parameters of a generalized four-type spring model, originally proposed by Gerolymos and Gazetas (2006), through a genetic algorithm-based optimization procedure. The predictions of the Winkler model compare very well with the FE results, not only at the local response level (in terms of stress distributions along the caisson shafts), but at a global response level (in terms of force–displacement curves and M–Q–N failure envelopes at the top of the caisson) as well. Contrary to established lateral soil resistance theories, it is shown that both the ultimate horizontal soil reaction and resisting moment per unit depth do not solely depend on the strength properties of soil and geometry of the caisson but are also functions of the applied load ratio M/Q and initial soil yielding due to vertical loading. Interesting conclusions are also drawn regarding the transition from the elastic to the ultimate limit state (hardening). Quantifying through analytical expressions the contribution of each of the two basic lateral resisting mechanisms to the response of the caisson, a classification method for embedded foundations is then proposed. The capabilities of the Winkler model are further demonstrated through comparison with FE analysis of the caisson cyclic lateral response.     

Experimental research on horizontal bearing behavior of caisson-pile composite foundationEI北大核心CSCD

In this study, four groups of different types of bridge foundation model are tested to research the horizontal bearing behavior of caisson-pile composite foundation in lab based on the Qiongzhou Strait bridge project. The Q-s curve and horizontal ultimate bearing capacity of these four groups of foundations in sandy soil layer are obtained. Pile bending moment and shear force of pile shaft are analyzed in detail. At the same time, the load sharing ratio of caisson and piles is discussed. The results show that the horizontal ultimate bearing capacities of a single caisson foundation are increased by 1.2 times, 1.6 times and 2 times respectively with adding skirts, steel pipe pile, or steel pipe pile and the skirt. The maximum bending moment point is in the middle of the pile shaft, i.e. at about 0.5 m under the mud surface. The horizontal load is borne mainly by the upper caisson foundation and soil layers above the middle upper part of pile. The maximum shear force is found at the joint of pile top of steel pipe and caisson pile, where the reinforcement measures should be taken in practical engineering. The research achievements could provide a better reference for design or construction of caisson-pile composite foundation.     

带桩沉箱复合基础水平向承载性状模型试验研究

以琼州海峡跨海大桥工程为背景,通过4组不同型式深水桥梁基础模型试验,对新型带桩沉箱复合基础的水平向承载性能进行了初步研究。试验得到了4组模型在砂性土层中的Q-s曲线及水平向极限承载力,并对复合基础在各级水平荷载下的桩身弯矩和剪力进行了详细分析,同时对沉箱-桩荷载分担比展开了讨论。试验结果表明：当增加裙边、钢管桩、或同时增加钢管桩和裙边后能够使单体沉箱基础的水平向极限承载力分别提高1.2倍、1.6倍和2.0倍;桩身最大弯矩点均出现在桩身中部即泥面下约0.5 m处。水平荷载主要由上部沉箱基础及桩身中上部土体承担,钢管桩桩顶与沉箱连接部位出现最大剪力,在实际工程中应在此处采取加固措施。研究成果可为这一新型深水桥梁基础的推广应用提供诸多有益参考。     

桶间距对四桶吸力式基础各单向承载力的影响及最优间距的确定

四桶吸力式基础由4个按正方形排列的吸力式桶形基础组成,并通过上部结构连成整体共同承受外部荷载,相比于单桶基础,四桶基础能承受更恶劣的海洋环境荷载,可作为海上风机和海洋平台等结构物的基础,具有广阔的应用前景。为探究饱和软土地基上四桶基础和单桶基础在承载力以及破坏模式方面的关系,优化四桶基础的设计,建立了大量的四桶和单桶基础的三维有限元模型,系统研究了不同基础埋深和土体剪切强度时桶间距对四桶基础各单向承载能力和对应的破坏模式的影响。结果表明,竖向承载力系数受桶间距影响较小,而水平承载力系数受其影响较大,力矩承载力系数受其影响尤为显著;在同一桶间距比时,四桶吸力式基础的各单向承载力系数随基础埋深比的增加逐渐增大,随土体剪切强度不均匀度指标的增大逐渐减小。通过分析四桶基础的各单向承载力、对应的破坏模式和群效应系数,定义了四桶基础的最优桶间距,可为四桶基础桶间距的优化设计提供依据。