【作者】 王志云；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2008， 博士

【摘要】 随着石油、天然气和金属矿物等海洋资源开采活动由浅海向深海(水深大于500m)的推进,重力式、桩基导管架平台等常见的浅海平台形式不再适用而开始推广采用大型浮式结构,因此大型浮式平台结构在深海海底的锚泊问题对岩土工程提出了新的挑战。与浅海平台相比,深水海洋平台基础的系锚竖向荷载不再是向下的压力,而是上拔荷载,并存在着水平荷载和力矩荷载分量,所以传统的以受压为主的桩基等基础型式已不再适用,这就需要人们开发新的海洋平台基础形式。近年来国内外的研究学者们正研究和推广一种新型的海洋平台基础形式——吸力式沉箱基础,其比较适宜于软土地基和恶劣的海洋环境,具有运输与安装方便、工期短、造价低、可重复使用等优点,并可提供全方位锚固能力,抗拉力发挥稳定,更适合浮式结构作业要求。由于吸力式沉箱基础在工作机理上不同于传统的重力式基础和桩基基础,目前尚缺乏较为统一的认识与成熟的设计理论,而且工程中不可避免地要遇到大量的深厚软黏土等不良海洋土地基,因此必须发展和完善软土地基上吸力式沉箱基础的有关设计理论体系与计算方法。为此,本文围绕波浪循环荷载作用下软土地基上吸力式沉箱基础稳定性研究中所存在的主要问题及沉箱基础的失稳模式等方面进行了比较系统而深入的探索研究,论文的主要研究工作包括下列方面:1.基于大型通用有限元分析软件ABAOUS平台,建立了单调加载条件下软土地基上吸力式沉箱基础的有限元计算模型。根据有限元分析,软土地基上吸力式沉箱基础的失稳破坏机制明显不同于传统的重力式基础或者桩基基础。当缓慢施加荷载分量时,吸力式沉箱基础的竖直上拔破坏模式是沿着沉箱内外筒壁所产生的局部剪切破坏,沉箱基础被单独拔出,土塞留在沉箱外;水平破坏模式为“单面破坏机制”,沉箱筒后界面与主动侧土体之间形成裂缝,筒前土体被挤压隆起形成被动侧破坏楔体,筒底部形成了以筒体中轴线上某点为中心点的圆弧形旋转破坏面;力矩破坏模式也形成单面破坏机制,转动破坏中心与水平破坏模式相比有所提升,从而筒前土体被挤压隆起所形成的被动侧破坏楔体变小,简体周围形成近似于圆球形的破坏区域。当快速施加荷载分量时,吸力式沉箱基础的竖直上拔破坏模式为基础的整体破坏,沉箱内部产生土塞,筒体底部的土体会随着沉箱向上一起拔起;水平破坏模式为“双面破坏机制”,不仅在筒前形成被动侧破坏楔体,沉箱筒后界面与主动侧土体之间不形成裂缝,在筒后形成了主动侧破坏楔体,筒底部形成明显的圆弧形旋转破坏面;力矩破坏模式也为双面破坏机制,转动破坏中心与水平破坏机制相比有所提升,筒体周围也形成近似于圆球形的破坏区域。通过变动参数的对比计算,探讨了单调加载条件下沉箱长径比、强度折减系数、土的抗剪强度、土的变形模量等因素对吸力式沉箱基础极限抗拔承载力的影响。计算还表明,随着水平荷载作用点与沉箱基础底部距离的增加,沉箱基础水平承载力先增加后减少而呈现抛物线形状。同时不同的水平荷载位置会使吸力式沉箱基础发生不同机制的失稳破坏,产生转动或水平移动破坏模式。2.针对软土地基上吸力式沉箱基础的抗拔破坏机制及极限承载力特性,基于“反向承载力”的观点建立了吸力式沉箱基础抗拔极限承载力的三维极限分析上限解法。假定沉箱基础地基土体的破坏模式是“反向地基承载力”破坏机理,即基础为整体破坏,在沉箱内部产生土塞,少量土体环绕粘附在沉箱外壁随沉箱基础向上拔出,沉箱底部的反向地基承载力计算采用典型的Prandtl破坏机制。吸力式沉箱基础地基的破坏区分为基础下锥体主动区、对数螺线变形区、Rankine被动区、基础内的土塞及粘附在筒体外壁的薄层土体五部分。为了验证所建议的基于“反向承载力”观点的极限分析上限解法的可行性和有效性,针对饱和均质软黏土地基,将其计算所得的长径比L/D=2.0的吸力式沉箱基础抗拔承载力结果与有限元解及简化极限平衡解进行了对比,发现三种计算结果比较吻合,最大误差不超过10%。进而通过变动参数的对比分析,进一步探讨了极限分析上限解与有限元解、极限平衡解的差异。结果表明:在给定土体参数等条件下,变动沉箱长径比L/D所得到的吸力式沉箱基础抗拔承载力上限解与有限元解比较一致,而当沉箱长径比相对较大时,极限分析上限解略大于极限平衡解。在土体各向同性情况下,按照极限分析上限解法计算所得到的吸力式沉箱基础归一化抗拔承载力系数对于土体重度、土体不排水抗剪强度的依赖性与有限元分析所得到的规律是一致的。3.基于Andersen对于软黏土所提出的考虑波浪荷载导致的软土地基循环软化效应的循环强度,建议将循环强度与Mohr-Coulomb屈服准则相结合,并在大型通用有限元软件平台上通过二次开发,建立了循环荷载作用下吸力式沉箱基础的三维有限元计算模型。通过拟静力数值计算确定了不同循环荷载分量作用下吸力式沉箱基础的循环承载力,并与单调加载作用下的吸力式沉箱基础的极限承载力进行对比,为工程设计提供了参考依据。通过计算表明:当考虑荷载的变值特征和土体的循环软化效应时,吸力式沉箱基础的承载力会大大降低,而沉箱基础的失稳机制没有太大变化。吸力式沉箱基础的循环承载力与沉箱长径比、荷载循环破坏次数、荷载作用点位置等诸多因素密切相关。随着循环破坏振次的增加,吸力式沉箱基础的循环承载力降低,降低程度趋于平缓。4.数值实现Swipe试验方法、固定位移比法及荷载—位移控制方法以搜寻吸力式沉箱基础的承载力破坏包络面,以此来探讨复合加载模式下吸力式沉箱基础的承载特性。通过有限元数值计算表明,在不同长径比和强度折减系数情况下,吸力式沉箱基础的承载力破坏包络面形状基本不变,但其大小发生变化。在V-M-H三维空间中,吸力式沉箱基础的承载力破坏包络面形成一个关于M轴的非对称的近似1/4椭球体空间曲面。随着力矩荷载分量的增大,V-H平面上的吸力式沉箱基础承载力破坏包络面的形状和大小均发生了明显的变化,当力矩荷载分量为0时,破坏包络面关于V轴是对称的,而随着力矩荷载分量的增加,破坏包络面关于V轴呈现明显的非对称性,同时这种非对称性随着力矩荷载分量的增加而逐渐增强。与单调复合加载下极限承载力相比,循环复合加载下吸力式沉箱基础的循环承载力大约降低30%左右,且循环承载力破坏包络面始终位于极限承载力破坏包络面之内,两者的变化趋势基本相似。更多还原

【Abstract】 As the exploitation depth of marine resources is progressively deepening from shallow sea toward the deep sea (depth of water is lager than 500 meter), the mooring problem of large floating constructions in deep-sea submarine presents some new challenges. In engineering practice, a lot of thick soft and weak ground is inevitable encountered with. Complex load combination and feasibility and economy of execution and so on must be considered. The traditional pile foundation is no longer applicable in design, so the newly type of foundation for deep-sea platform is required. To compare with platforms of shallow sea, vertical loading of deep-sea platform foundations is no longer the pressure downward, but the drawing force upward. Moreover, there exit horizontal load component and moment component. As a novel type of offshore foundation, suction caisson foundation is being researched and popularized in recent years. Suction caisson foundation suits for soft ground and formidable ocean environment and has advantages of low cost, short construction period and repeatable application. It can supply omnidirectional anchoring ability and play stable tensile force. However, because the work mechanism of suction caisson foundation is different with that of traditional gravity foundation and pile foundation, there lacks sophisticated design code and unified analysis method for this type of foundation at present. Therefore it will be theoretically important and practically significant to examine the working bearing capacity of suction caisson foundation in soft ground under complex conditions. In this dissertation, the studies are emphasized on numerical methods of evaluating the bearing capacity and failure mechanism of suction caisson foundation in soft ground under wave loading. The main investigations consist of the following parts.1. Based on general- purpose finite element software ABAQUS, the finite element computational model for suction caisson foundation in soft ground under monotonic loading is established. According to finite element analysis, the failure mechanism of stability for suction caisson foundation in soft ground is different with that of traditional gravity foundation and pile foundation. When the component of load is applied slowly, the vertical uplift failure mechanism of suction caisson foundation may be local shear failure mode. The caisson is only pulled out of the subsoil and the soil plug remains at the same place. The horizontal failure mechanism of suction caisson foundation may be a single-sided failure mode. The potential crack between the posterior wall of caisson and neighboring soil in the active zone is constructed. At the same time, only an individual wedge near the mudline exists in neighboring soil of the passive zone, and a truncate spherical slip surface exists in the base of caisson obviously. The moment failure mechanism of suction caisson foundation may be also a single-sided failure mode, but the rotation center of the caisson is higher than that under horizon and the weight soil in the passive zone is smaller. When the component of load is applied rapidly, the vertical uplift failure mechanism of suction caisson foundation may be the overall instability. A soil plug exists in the caisson and the subsoil under the bottom of the caisson is extracted accompanying the uplift of the caisson. The horizontal failure mechanism of suction caisson foundation may be a double-sided failure mode. No crack between the posterior wall of caisson and neighboring soil in the driving side is constructed, and two wedges in both active and passive zones near the mudline exist. At the mean time, the moment failure mechanism of suction caisson foundation may be also a double-sided failure mode, but the rotation center of the caisson is lifting. Moreover, by parametric computation and comparative analysis the ultimate bearing capacity of suction caisson foundation under monotonic load is evaluated for different aspect radio of caisson, loading speed, shear strength of subsoil, deformation modulus of subsoil and strength reduction factor. When the action point of horizontal load is about z/L=0.6, the horizontal bearing capacity is maximal. Moreover, the different damage mechanisms occur for different action point of horizontal loading.2. According to the uplift damage mechanism and ultimate bearing capacity feature of suction caisson foundation in soft ground, a three-dimensional upper bound limit analysis method for uplift ultimate bearing capacity is proposed based on the viewpoint of "reverse bearing capacity". The failure mechanism is assumed to be overall instability, a soil plug exists in the caisson and a little soil around caisson is extracted accompanying the uplift of caisson. The reverse bearing capacity calculation uses typical Prandtl failure mechanism. For the aspect ratio L/D =2.0, the uplift ultimate bearing capacity by upper bound limit analysis method is validated by finite element numerical computations and limit equilibrium method. Three results are consistent with each other and maximum error is less than 10%. For the same soil parameters and different aspect ratio of caisson, upper bound limit analysis results are consistent with finite element numerical results, but little more than limit equilibrium results.3. Based on the concept of cyclic strength proposed by Andersen et.al. to consider cyclic softening behavior of soft clay seabed induced possible by wave loading, a three-dimensional finite element model is developed to evaluating the cyclic bearing capacity of suction caisson foundation by combining cyclic strength and Mohr-Coulomb yield criterion. The quasi-static numerical computation determines the cyclic bearing capacity of suction caisson foundation under different load component, and these results are compared with the ultimate bearing capacity of suction caisson foundation under monotonically loading pattern. The computational results indicate that non-uniform distribution of static stress ratio and cyclic strength in subsoil induces the bearing capacity of suction caisson foundation reduced greatly, but the aspect ratio of caisson has no significant effects on the decreased degree of bearing capability induced by cyclic softening behavior of soft clay. when considering variate characteristic of load and cyclic softening behavior of soft clay, the cyclic bearing capacity of suction caisson foundation is decreasing with increasing of cyclic loading number to failure, and the decreased degree is drive to flat.4. The failure envelopes of suction caisson foundation are searched by Swipe test procedure, fixed displacement ratio procedure and load-displacement controlling procedure, furthermore the bearing capacity behavior of suction caisson foundation under combined loading mode is developed. According to finite element analysis, for the different aspect ratio of caisson and strength reduction factor of subsoil the failure envelope shape of ultimate capacity of suction caisson foundation is similar, but the dimension may change. The V-M-H three-dimensional failure envelope of suction caisson foundation is a closed asymmetrically approximate 1/4 ellipsoid forms. When moment component is zero, the failure envelope is symmetric to V axis. With the increasing of moment component, the failure envelope is obviously asymmetrical to V axis. At the meantime, this asymmetry is gradual enhanced with increasing moment component. Then comparative numerical computations indicate the failure envelope of cyclically combined loading pattern locates in that of monotonically combined loading pattern all along. The former is significantly reduced compared with the latter, about 30%.更多还原