砂土地基上圆形浅基础三维破坏包络面的理论研究

对砂土地基上圆形浅基础在竖向荷载V 、水平荷载 H 及力矩 M 复合加载条件下的承载力进行了系统的三维有限元分析。在分析中,砂土假定为纯摩擦材料,遵循基于 Mohr-Coulomb 破坏准则的理想弹塑性本构关系。首先,对圆形浅基础的竖向承载力进行了有限元计算,并与滑移线解法进行了对比,两种方法所得结果比较吻合。进而探讨了砂土内摩擦角对于基础在 V-H 、V-M荷载平面与V-H-M 三维荷载空间内的破坏包络轨迹的影响。计算结果表明,与不排水情况下软黏土地基上基础破坏包络面相比,砂土地基上圆形浅基础的破坏包络面形状有较大差异,但V-H 和V-M 平面内的破坏包络面形状仍具有较好的归一化特性。基于有限元计算结果,建立了圆形浅基础在V-H-M 三维荷载空间内的破坏包络面方程,该方程可用来合理评价复合加载条件下砂土地基上圆形浅基础的整体稳定性。     

偏心荷载下双层饱和黏土地基承载特性

在海洋与近海工程中,海洋建筑物地基受到水平荷载、竖向荷载和弯矩荷载的共同作用,地基土体非均质成层分布。因而,复合加载模式下层状地基的极限承载力与破坏机理研究是海洋工程设计的关键问题。针对竖向荷载(V)和弯矩荷载(M)共同作用下双层饱和黏土地基的极限承载力与破坏机理进行有限元分析。土体采用理想弹塑性模型,屈服准则为Mohr-Coulomb准则;基础与地基间假定切向完全黏结、竖向可分离。通过有限元计算,得到双层地基在V-M荷载空间的破坏(极限)荷载包络线,归纳得出地基的破坏模式。计算结果表明,双层地基的破坏包络线随着上层土体的厚度与基础宽度比值、上下土层的抗剪强度指标比值的增大而不断扩大,最终达到一个稳定值;在V-M复合加载条件下,地基可能发生深层整体剪切破坏或浅层局部剪切破坏。     

复合加载条件下筒形单基础极限承载力分析

筒形基础广泛应用于海洋工程,由于海洋风力、波浪等共同作用,使得传统的地基承载力理论难以分析复杂荷载条件下筒形基础的极限承载力。借助ABAQUS有限元数值软件,基于等比例位移加载模式,对饱和软黏土地基上筒形基础在水平荷载H、弯矩荷载M复合加载条件下的极限承载力进行了分析。计算分析结果表明,筒形基础在H-M荷载空间中的承载力包络面呈非对称性,水平荷载和弯矩荷载发生耦合作用,两者同向时承载力包络面增大,异向时承载力包络面减小,筒形基础埋深比的增大使得承载力包络面的非对称性越强;通过对荷载作用点的位置修正,有效减少水平荷载和弯矩荷载的耦合作用,使不同埋深比的筒形基础在H-M荷载空间中的承载力包络面对称分布,归一化后承载力包络面形状一致,采用曲线拟合得到承载力包络面的表达式,该公式可方便计算复杂荷载作用下筒形基础的承载力。     

复合加载模式的单桩复合桶型基础极限承载特性和包络线分析

运用数值分析法或试验手段建立不同加载模式下的地基承载力包络线是评价地基稳定性的重要途径。根据工程地质条件，采用有限元法建立吸力桶-地层的三维大型数值模型，进行桶型基础在水平荷载、竖向荷载、弯矩复合加载下的变形和承载力计算。结果表明，随着荷载的逐渐增大，复合桶型基础的位移初期呈线性增大，后期呈指数型增大、突变、失稳的变化规律，且适当的竖向荷载有利于提高复合桶型基础的水平和弯矩承载能力，过大的竖向荷载则会不利于基础的稳定。桶型基础的破坏包络线，对结构设计、基础稳定性的判定和整体安全的评价有重要作用。     

各向异性软黏土地基上浅基础破坏包络面研究

 根据已有的关于土体强度各向异性的表达式,针对基于Tresca屈服准则的理想弹塑性模型进行改进,使之可以考虑不排水条件下软黏土强度的各向异性,并在大型通用有限元软件ABAQUS中予以数值实施。联合应用swipe加载模式和固定位移比加载模式,对于条形浅埋基础在竖向荷载V、水平荷载H与力矩荷载M的复合加载条件下的承载性能进行比较系统的数值模拟,主要探讨地基土不排水强度的各向异性、竖向荷载水平对于浅基础在H-M荷载空间内的破坏包络面的影响。首先进行浅基础分别在竖向、水平与力矩荷载单独作用下极限承载力的计算,并与已有结果进行对比。进而对于不同竖向荷载水平下浅基础在H-M荷载空间内的破坏包络面特性进行分析。计算结果表明：当浅基础埋深与宽度之比较大或基础承受三维自由度加载时,swipe加载模式得到的加载路径比真实的破坏包络面明显偏小;对于给定埋深的浅基础,其包络面的大小与地基土强度的各向异性与竖向荷载水平都有关系,而在归一化荷载平面内,包络面形状主要受到竖向荷载水平的影响,而与地基土强度的各向异性无关。     

水平-竖向-扭转荷载下吸力沉箱基础承载特性数值研究

海上风机吸力沉箱基础承受结构自重及环境因素引起的竖向、水平、扭转荷载,处于复合加载状态。以位于饱和黏土地基上长径比为0.25～1.0的单桶沉箱基础为对象,ABAQUS为计算工具,研究沉箱基础的承载特性,并考虑沉箱与地基土的摩擦接触作用。首先分析沉箱基础在单一的竖向荷载、水平荷载及扭转荷载作用下的极限承载力,并给出沉箱基础在这些荷载组合下的破坏包络面,其中倾覆力矩通过施加偏心水平荷载实现。结果表明,沉箱基础的竖向承载力可用改进的经典承载力公式计算,而水平承载力系数在给定的接触条件下随沉箱长径比增加而减少,扭转承载力与理论预测结果非常吻合;扭转荷载对沉箱的竖向承载力影响较大,但对水平承载力影响较小;不同扭转荷载下的V-H包络面可用椭圆曲线拟合。     

筒壁长度对筒型基础地基承载力影响研究

大直径筒型基础是为适应我国近海地质条件和风机荷载特点而研发的新型基础形式。本文针对单调加载模式下饱和软粘土地基所能达到的极限状态,应用数值计算方法,研究了筒型基础筒壁长度对地基承载力的影响。     

可折叠防沉板V-M加载模式下不排水承载力研究

可折叠防沉板可有效解决单块防沉板因面积过大而导致的巨额安装施工费用问题,是深海油气资源开采生产系统中的新型基础结构。在服役期间承受竖向、水平、弯矩及扭矩等荷载的复合作用,其承载力不仅与防沉板的尺寸和地基土强度及其分布特性有关,而且还与可折叠防沉板两翼间距有关,导致不能直接采用传统的防沉板承载力计算方法进行计算。采用有限元方法对可折叠防沉板的不排水承载力进行三维数值模拟,计算得到竖向V、弯矩M加载条件下及组合V-M加载条件下的极限承载力,系统分析了单向(V,M)及组合(V-M)加载条件下的地基破坏模式,提出了单向承载力计算方法并建立了组合加载条件下的破坏包络线,进而提出了复合加载条件下的承载力计算方法,为可折叠防沉板的承载力设计提供依据。     

宽浅式筒型基础竖向承载力研究

通过有限元计算,研究了不同长径比下筒型基础竖向压载作用下的承载特性,分析了竖向荷载作用下宽浅式筒型基础的失效模式,并与试验结果进行对照,数值计算与模型试验的荷载–位移曲线虽存在一定差距,但整体趋势相近。对筒型基础不同部位在承担竖向压载时所承担的比例变化进行分析,其中,筒顶接触反力、内侧摩阻力及端阻力承担90%以上的荷载比例,计算极限承载力时可将筒内土与筒作为一个整体。给出了筒壁土压力的分布形式,在汉森理论的基础上提出适用于宽浅式筒型基础竖向承载力的计算公式,并与有限元、试验结果进行比较,除长径比为1.0的模型外,相对误差均在10%以内。     

复合加载模式下桶形基础破坏包络面特性研究(英文)

桶形基础结构是一种适宜于滩海软黏土地基上的新型海洋基础形式。研究复合加载模式下桶形基础的承载力特性,并进而依此评价海洋平台基础及地基的稳定性是桶形基础设计与施工中的关键问题。针对软黏土地基上桶形基础结构,利用大型通用有限元分析软件ABAQUS,采用基于Mohr-Coulomb屈服准则的理想弹塑性本构模型,对复合加载模式下桶形基础的承载特性进行了三维弹塑性数值分析。在V-H、V-M、V-M-H空间内建立了复合加载模式下桶形基础极限承载力破坏包络图,推导了破坏包络面的经验计算公式,由此可根据实际的复合加载状态与数值计算所得的破坏包络曲面之间的相对关系,评判实际荷载作用下桶形基础的工作状态,为工程设计提供参考依据。     

Failure surface for shallow foundations in sand using a classical bearing capacity

Much effort has been made to elucidate the ultimate capacity of shallow foundations under the general cases of vertical (V), lateral (H), and moment (M) loads in soils. The nature of the dependency of the ultimate capacity of shallow foundations on the combination of V, H, and M loads, the ratio of embedment to diameter, and soil properties has has not yet been analytically revealed. In this paper, an analytical investigation into failure effect of V, H, and M loads applied to shallow foundations in nonhomogeneous sand is made using a classical bearing capacity theory. Of the bearing capacity equations proposed thus far, the most appropriate bearing capacity equation which can predict experimental results of shallow foundations in sand is presented. A no-tension interface between a foundation base and a soil and an effective diameter of the foundation are used to analyze the base failure produced by the vertical stress of the soil below the foundation base when vertical and moment loads are applied. For shallow foundations with various embedment ratios in sand under the two different loading ways, the displacement-load curves and failure envelopes in the H-M, H-V, and M?V planes and in the H-M?V space are presented. For failure envelopes in the H-M plane under a constant vertical load for shallow foundations in sand, the results obtained from experiments are well predicted by the present method.     

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.     

淤泥质黏土中复合筒型基础水平承载力试验研究

复合筒型基础作为一种新型海上风力发电基础,与以往的筒型基础结构形式有较大差别,其极限承载能力及筒土作用机理需进一步深入研究。针对这一问题,设计了复合筒型基础大尺寸模型,开展了淤泥质黏土中的水平承载力试验,得到了复合筒型基础在水平荷载作用下的土压力分布规律、位移变化机制及极限承载能力等。试验结果表明,随着荷载的增加,被动区土压力有较大增长,主动区土压力呈现负压并逐渐恢复为初始土压力;旋转中心的位置随着荷载的增加从中轴线附近逐渐沿着加载方向移动,极限荷载时,旋转中心位于距顶面0.8倍筒壁高度处的分仓板附近。     

An Experimental Study of the Drained Capacity of Suction Caisson Foundations Under Monotonic Loading For Offshore Applications

In recent years there has been a surge in the development of new small scale offshore facilities, including minimum facility structures for oil and gas developments as well as offshore renewable energy devices such as offshore wind turbines. In these cases the loading applied to the structure and foundation is significantly different to that applied to more typical larger offshore structures. The weight of the structure is much lower, and the horizontal load and moment applied to the foundation is much higher as a proportion of the vertical load. Design guidance for the combined loading response of shallow foundations under low vertical loads is sparse, and in particular much guidance is drawn from work where the vertical load applied to the foundation is close to the vertical bearing capacity. This paper addresses this lack of data by presenting results of combined loading experiments at low vertical loads on novel shallow foundations known as suction caissons. The experiments are carried out on dry sand at a low relative density to explore the drained response of the foundation. The experiments are successfully interpreted within the framework of work hardening plasticity, and information on the shape of the yield surface and the post-yield behaviour of the foundation is deduced. One of the key results is that the foundation can sustain moments and horizontal loads even when a tension is applied to the foundation.     

复合加载情况下六自由度圆形基础失稳模式与 极限承载能力研究

海洋工程中结构物除了受到自身重力作用外,往往还受到海风、海浪、海流等的作用,使得海床土体中的基础一般受到集中力、弯矩和扭矩的联合作用,这一受力状态称为结构物基础的复合加载模式。详细研究复合加载模式下海床土体的失稳模式和极限承载能力,对海洋结构物的安全运行至关重要。以三维空间内圆形基础为研究对象,基于土体弹塑性极限平衡原理,利用非线性有限元分析软件ABAQUS,对空间六自由度圆形基础极限承载能力进行详细的数值分析。基于上述计算分析结果,揭示了单调加载情况下土体的失稳机理与极限承载能力,给出了复合加载模式下海床土体破坏包络面方程。给出的土体失稳模式和破坏包络面方程能够合理地评估复合加载模式下海床土体的极限承载能力。