海底管道沉箱基础的复合承载力包络面研究

当负压沉箱被用作深水管汇或管道终端基础时,其长径比常介于1～2之间,而目前的沉箱复合承载力包络面表达式大都针对长径比不超过1的情况,少数覆盖长径比大于1的研究又不适用于土体表层强度非零的情况。采用有限元方法,模拟竖向力、水平力和弯矩共同作用下沉箱基础的响应,采用Probe加载模式获得沉箱的复合承载力包络面。进行大量变动参数分析,针对长径比为1～2的沉箱,讨论了长径比和土体强度分布对单向承载力和包络面的影响,并给出了预测沉箱复合承载力的归一化表达式。     

组合荷载作用下伞式吸力锚基础承载特性数值分析

针对用于海上风力发电机的伞式吸力锚基础(USAF)实际条件下的受力特点,采用数值模拟方法,基于大型通用有限元软件ABAQUS构建数值计算模型,对伞式吸力锚基础在H-V、H-T、V-T荷载平面内以及H-V-T非共面复合加载模式下的承载特性进行分析,进而推导其破坏包络面数学表达式。分析中采用固定位移比加载法进行复合加载,并将桶顶位移作为失效破坏标准。结果表明：(1)拟定的应力归一化复合加载破坏包络面椭圆曲线方程可以较好地模拟不同主筒长径比USAF在不同荷载空间内的破坏包络面形式;(2)H-T空间内USAF复合承载性能随主筒长径比(L/D)的增大而提高,而H-V、V-T空间内变化不明显;(3)绘制了H-V-T空间内USAF三维破坏包络面,可根据实际受荷状态与包络面之间的相对位置关系,评价实际工况下伞式吸力锚基础的稳定性。     

软土中单桶基础竖向循环承载力的模型试验

利用真空预压法制备了一个大尺寸软土模型试验土池,针对两种不同直径桶型基础,进行了多组竖向静荷载与循环荷载共同作用下单桶基础承载力的模型试验,研究了软土中单桶基础循环承载力的变化。研究结果表明,如果基础受到的竖向总荷载比小于0.85,则循环荷载作用下基础的竖向累积沉降随循环次数的变化趋势是安定的;如果竖向总荷载比大于0.9,那么基础的竖向循环累积沉降将逐渐增加,并最终导致基础破坏;如果竖向总荷载比接近或大于1.0,基础的循环累积沉降发展加快,而且此时竖向静荷载越大,竖向循环累积沉降发展就越快。依据模型试验结果可以得出,竖向静荷载与循环荷载共同作用下,软土中单桶基础的循环承载力大约为静承载力的80%左右。     

V-M荷载作用下双层黏土地基破坏包络线研究

地基破坏包络线的研究是复合加载模式下地基承载力设计的关键。根据建立的数值计算模型,用Abaqus商业软件分析了V-M荷载作用下,位于双层黏土地基上条形基础的破坏包络线随上层土厚度比、土层强度比的变化规律,以及地基破坏模式随荷载偏心率的变化趋势。用有效面积概念将传统竖向极限承载力换算为V-M荷载平面的破坏包络线,与数值计算结果进行对比。结果表明,公式换算方法适用于均质黏土地基,而对双层黏土地基偏于保守。     

端承型桩承载力离心模型试验中的粒径效应研究

利用modeling of models的方法研究端承型桩承载力离心模型试验中的粒径效应。在模拟同一原型时,不同桩径的模型桩,桩身压缩性及桩长均不同,导致侧摩阻力发挥机理及程度不同,本文分别探讨了桩端阻力,侧摩阻力及承载力(桩顶荷载)的粒径效应对承载机理和承载特性的影响。结果表明,桩端阻力的粒径效应作用规律与浅基础一致,可以借用浅基础的粒径效应定量评价方法评价端承桩承载力离心模型试验中的粒径效应。侧摩阻力的粒径效应比桩端阻力的粒径效应显著。由于侧摩阻力的影响,相同条件下承载力的粒径效应比桩端阻力有所增强。对于极限桩端阻力和极限承载力,粒径效应均随长径比的增加而减弱。     

波浪作用下黄河口沉积物液化导致的灾害现象

选择黄河口浅滩不同沉积历史的4个典型研究区,取样进行室内物理力学参教测试,现场利用普氏贯入仪测量1 m深度内土体强度在二维空间上的分布.然后,建立波浪作用下土体破坏的计算模型,计算50年一遇情况下土体破坏状况,并与调查的浅滩灾害比较.通过研究发现,黄河口浅滩沉积物处于超固结状态,且超固结比随深度增加而减小;浅滩沉积物强...     

海床表层软土对宽浅式筒型基础承载特性影响研究

近海海床表层多为软黏土或淤泥质土,为探究海床表层软土对海上风电宽浅式筒型基础承载特性的影响,以中国广东某海域风电场为背景,通过有限元分析的方法,研究竖向、水平、弯矩荷载作用下软土层厚度和土体强度对基础极限承载力、破坏模式以及筒基土压力分布的影响。研究结果表明:当软土层厚度小于H/2(H为筒裙高度)时,单向荷载作用下宽浅式筒型基础极限承载力随软土层厚度的增加呈线性减小的趋势;当软土层厚度大于H/2后,承载力降低速率逐渐增大。表层软土的存在,使得塑性区范围缩小,软土层内土体塑性破坏更加明显。竖向荷载作用下,随软土层厚度的增大,筒顶承载先减小后增大,筒内侧摩阻力先增大后减小;水平荷载和弯矩作用下,筒侧被动土压力的降低是引起软土覆盖地基中基础承载能力降低的主要因素。     

基于椭圆型滑动面的砂土中条形锚板上拔承载力计算方法

对于砂土中的竖向拉拔锚板,锚周土体滑动面的形态会随埋深比的增大而发生变化。基于滑动面连续演化的观点,审视了既有统一模型中滑动面形态表征函数的不足,在此基础上,提出了一种新的椭圆形态函数,并建立了相应的统一力学分析模型,提出了新的承载力计算方法。结果表明：新的形态函数能更好地反映滑动面的实际形态,承载力计算新方法也表现出了更好的适应性。     

砂土中带裙板防沉板基础竖向承载力的上限解

防沉板是深海油气工程中水下生产系统的重要基础形式,因其埋深浅、水下施工方便等特点而具有广阔的应用前景。为了满足基础抗滑移的要求,可在防沉板底部设置裙板,裙板的存在增加了防沉板竖向承载力计算的难度。将防沉板基础简化为带裙板条形基础,考虑了内部土体与基础的相互作用,建立了竖向承载模式,即Terzaghi模式;运用极限分析法推导了该承载模式的上限解,引入了一个能够反映裙板与基础内部土体共同作用的经验参数—土体破坏率(η),其大小决定了地基中土体滑动面的范围;通过分析确定了在二维简化条件下带裙板防沉板基础的土体破坏率与基础高宽比的关系;并研究了土体破坏率不同时,地基承载力系数Nq和Nγ与土体内摩擦角之间的关系。     

复合加载下考虑接触作用的地基承载特性数值分析

港口、海洋工程结构物基础一般处于复合加载状态,其极限承载力通常采用近来引入的极限荷载图进行评价.对位于地基表面的重力式海洋基础,需要考虑基础与地基间的接触特性对极限承载力的影响.以大型通用有限元软件ABAQUS为计算平台,建立了复合加载模式的地基极限承载力数值分析方法;针对饱和黏土地基上的表面基础,利用在ABAQUS平台上开发的接触计算模块,模拟基础与地基间竖向可分离、切向完全粘结的接触作用;进而基于建立的分析方法,进行系统的有限元计算,分析地基的破坏模式随荷载条件的变化,给出地基的极限荷载包络图,并与经典承载力计算公式结果进行对比.研究结果表明,经典承载力计算公式低估了三维荷载条件下的地基极限承载力,有限元计算模型及数值分析方法,可以较好地分析研究地基的失稳机理及承载力特性,并可考虑基础与地基不同的接触条件对破坏模式及组合极限承载力的影响.     

Undrained bearing capacity of spudcan under combined loading

The bearing capacities of spudcan foundation under pure vertical (V),horizontal (H),moment (M) loading and the combined loading are studied based on a series of three-dimensional finite element analysis.The effects of embedment ratio and soil non-homogeneity on the bearing capacity are investigated in detail.The capacities of spudcan under different pure loading are expressed in non-dimensional bearing capacity factors,which are compared with published results.Ultimate limit states under combined loading are presented by failure envelopes,which are expressed in terms of dimensionless and normalized form in three-dimensional load space.The comparison between the presented failure envelopes and available published numerical results reveals that the size and shape of failure envelopes are dependent on the embedment ratio and the non-homogeneity of the soil.     

Unified numerical study of shallow foundation on structured soft clay under unconsolidated and consolidated-undrained loadings

Abstract

Based on a new elasto-plastic constitutive model, this paper presents a soil–water coupled numerical prediction of the bearing capacity for shallow foundation constructed on Ballina soft clay for unconsolidated undrained (UU) and consolidated undrained (CU) conditions. This elasto-plastic constitutive Shanghai model has an advantage of describing the mechanical behaviour of over-consolidated and structured soil under different loading and drainage conditions, by using one set of material parameter. In this paper, the Shanghai model used for both UU and CU conditions has the same initial parameters obtained from laboratory test results. The loading conditions and consolidation stages vary based on construction details. The predicted bearing pressure-settlement responses for UU and CU, approves the field observation. The phenomenon of gaining the bearing capacity due to consolidation is captured and explained by the use of soil–water coupled numerical analysis with a new elasto-plastic model. The stress strain behaviour, stress paths and the decay of the structure of elements at different depths presented in this study, reveal the mechanism for the difference between UU and CU conditions to understand the foundation behaviour. Effect of the initial degree of soil structure on the bearing capacity is also addressed. Overall, this approach provides the integrated solution for the shallow foundation design problems under short and long-term loadings.     

复合加载模式作用下地基承载性能数值分析

确定竖向荷载(V)、水平荷载(H)和力矩(M)共同作用下建筑物地基的破坏模式及在荷载空间(H,V,M)中的破坏包络面是地基设计中的关键问题。为提高Swipe试验方法计算精度,提出了改进方案,进而利用有限元方法分析了复合加载模式作用下均质粘性土地基上条形基础的破坏包络面。计算表明,基于改进Swipe试验方法的数值模拟结果明显好于常规Swipe试验方法。针对海洋工程中实际复合加载模式的特点,探讨了竖向荷载分量V对地基破坏模式和H-M荷载平面上的破坏包络线的影响,结果表明竖向荷载分量显著地改变了地基的破坏模式及包络线的形状。     

Evaluation of undrained failure envelopes of caisson foundations under combined loading

In this paper, results of a three-dimensional finite element study addressing the effect of embedment ratio (L/D) of caisson foundations on the undrained bearing capacity under uniaxial and combined loadings are discussed. The undrained response of caisson foundations under uniaxial vertical (V), horizontal (H) and moment (M) loading are investigated. A series of equations are proposed to predict the ultimate vertical, moment and maximum horizontal bearing capacity factors. The undrained response of caisson foundations under combined V-H and V-M load space is studied and presented using failure envelopes generated with side-swipe method. The kinematic mechanism accompanying failure under uniaxial loading is addressed and presented for different embedment ratios. Predictions of the uniaxial bearing capacities are compared with other models and it is confirmed that the proposed equations appropriately describe the capacity of caisson foundations under uniaxial vertical, horizontal and moment loading in homogenous undrained soils. The results of this paper can be used as a basis for standard design codes of off-shore skirted shallow foundations which will be the first of its kind.     

真空预压联合碎石桩加固港口吹填堆场地基的试验研究

在港口工程建设中,快速加固超软弱的吹填地基,采用真空预压法是一个经济有效的方法,再联合以碎石桩加固则能进一步提高地基的承载力、减少地基工后沉降.通过真空预压联合碎石桩加固地基的现场试验以及观测资料的分析,得出真空预压联合碎石桩加固后地基的承载力特征值由原天然地基的60 kPa左右提高到220 kPa,计算结果表明打碎石桩后的地基变形模量Esp与真空预压后的Ea相比提高了1倍,联合碎石桩加固后的地基沉降量比仅经过真空预压的地基减少1/3.     

Cumulative Deformation of Soft Clay Under Cyclic Loading

Reconstituted specimens are prepared by means of vacuum preloading. Both static and cyclic triaxial tests are carried out, with the specimens consolidated under different principal stress ratios. A finite element method is put forward for calculating the cumulative deformation of soft clay under cyclic loading.     

Bearing capacity of semi-deep skirted foundations on clay using stress characteristics and finite element analyses

Semi-deep skirted foundations are now considered to be a viable foundation option for a variety of onshore and offshore applications. The capacity under combined vertical, horizontal, and moment loadings must be found to ensure their capability and stability. In this study, undrained bearing capacity subjected to vertical loading, as part of combined loading is determined through stress characteristics and finite element analyses. Circular skirted foundations with different soil strength and geometries considering embedment depth effects have been studied. Stress field, kinematic mechanism accompanying failure, and bearing capacity factors for various embedment ratios are investigated. Acquired vertical failure mechanism has demonstrated the transition from a general shear to a punch shear failure. Comparisons with different research works including conventional methods, upper and lower bound, finite element analyses, physical modeling, experimental, and centrifuge tests have indicated the underestimation of conventional approaches and accuracy of proposed methods in determining bearing capacity. Furthermore, differences between predicted bearing capacities and the results of this study increased with D/B ratio due to ignoring the significant role of skin friction in larger embedment circumference.     

Scour effects on bearing capacity of composite bucket foundation for offshore wind turbines

Abstract

Composite bucket foundation (CBF) is a wide-shallow foundation for offshore wind turbines, which can be transported and installed with the turbine as one unit at a one-step operation. Compared with deep pile foundations, its structural stability is more sensitive to the scouring by waves and currents. In this paper, a three-dimensional finite element model with CBF and surrounding soil is established to estimate the failure mode at different given soil scour conditions. The loading on CBF for offshore wind turbines is characterized by relatively small vertical loading V, larger horizontal loading H, and bending moment M, and the effect of erosion on bearing capacity of CBF is determined by using the fixed displacement ratio method. In addition, the failure envelopes of the CBF applied in H–M and V–H–M loading modes are obtained. Results indicate that the bearing capacity of CBF under horizontal loading and bending moment will be significantly reduced by the decrease in the embedded depth of CBF due to the scouring depth and extent, as well as the H–M, and V–H–M failure envelopes. The structural stability safety factor of CBF under different scouring conditions can be obtained through the three-dimensional envelope surface with respect to scouring depth and extent.