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1.
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. 相似文献
2.
S. Rezazadeh 《Marine Georesources & Geotechnology》2018,36(6):625-639
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. 相似文献
3.
AbstractComposite 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. 相似文献
4.
Drag anchor is a widely used anchor type in offshore engineering for the mooring system. The prediction of the anchor trajectory installation and the final position is important for anchor selection in design. The existing method using yield envelope method for trajectory prediction ignored the shallow anchor behavior but applied the deep yield envelope from a deeply embedded horizontal fluke in uniform clay for the whole drag-in installation process. However, the anchor fluke embedment depth and inclination angle change continually during installation in clay with linearly increasing shear strength soil profile in practice. Studies on the effect of fluke inclination angle on the anchor behavior in clay with such non-uniform soil profile under unidirectional and combined loading are important and necessary for the improvement of the yield envelope method to ensure a reasonable prediction. The current 2D finite element studies investigate the anchor behavior for inclined fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then the effects of the fluke inclination angle, soil non-homogeneity and embedment depth ratio on the shallow yield envelopes are investigated. It is found that the effect of fluke inclination angle on the vertical capacity factors for anchor in clay with non-uniform and uniform soil profile is largely different. The resultant large impact on the yield envelopes shown here illustrates the importance of considering the fluke inclination angle and soil non-homogeneity in the prediction of anchor trajectory using yield envelope method. 相似文献
5.
由于预载下土体固结，海底浅基础的承载力会随作业时间的增加而改变，其时变效应评估困难。基于修正剑桥模型，采用水土耦合有限元方法研究了预载作用下浅基础在正常固结黏土海床中承载力破坏包络面的时变规律。在验证数值模型准确性后，通过位移探针测试获取复合加载模式下浅基础的破坏包络面，揭示了预载和固结程度对基础承载力和破坏包络面的影响，给出了预载作用下浅基础承载力包络面计算方法。结果表明：随着预载比增加，固结单轴承载力呈现线性增长，固结承载力增幅在水平向最大；部分固结承载力相对增幅与预载比无关，而随固结度变化；破坏包络面形状由预载比控制，而包络面大小由预载比和固结度共同控制。研究结果可为海洋浅基础的时变承载力评估提供参考依据。 相似文献
6.
Jack-up platforms of the Ocean engineering structures always withstand the vertical gravity loads which are applied to the seabed by spudcan, so it is important to determine the bearing capacity and the penetration depth of the spudcan for its geometry. In fact, it is up to the deformation law and the failure modes of soil surrounding the spudcan which can calculate the ultimate bearing capacity of the spudcan foundation on the soil seabed. By using the finite element analysis software Abaqus, the deformation law of soil around the spudcan is analyzed in detail, and the failure modes of soil surrounding the spudcan foundation are achieved. At the same time, based on the limit equilibrium theory, by use of static permissible slip-line field, the ultimate bearing capacity of the spudcan foundation is analyzed and the lower limit solution is derived theoretically, and the effect of the spudcan angle on the ultimate bearing capacity is investigated. The numerical results are compared with those obtained by the theoretical formulas deduced in this paper. On the basis of the lower limit solutions in this paper, the effect of the spudcan angle on the ultimate bearing capacity is revealed, and a practical bearing capacity formula is given to take the effect of the spudcan angle into consideration. 相似文献
7.
复合加载模式作用下地基承载性能数值分析 总被引：11，自引：3，他引：11
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。 相似文献
8.
ABSTRACTBucket foundations have been widely used for a variety of offshore applications. The effects of skirt length on ultimate bearing capacity of bucket foundation have been studied and reported in published scientific papers. However, few studies have addressed the behavior of bucket foundations in loose saturated sand. In this paper, a series of experimental investigations were performed to determine the bearing capacity of bucket foundation under uniaxial loading. The experiments were conducted on small-scale foundations under vertical loading in loose saturated sand. It was found that increasing the skirt length would enhance the bearing capacity of bucket foundation. As reflected in the present study, bearing strength might be enhanced more than 5 times in loose saturated sand in comparison to surface footing with equivalent diameter. Based on the experimental investigation, a depth factor was proposed to approximate bearing capacity of bucket foundations in terms of those for surface footing and embedment ratio. Moreover, the corresponding settlement of foundation at the failure load was found to increase with skirt length. 相似文献
9.
Drag anchor is a widely used economical anchor option for offshore floating structures. The anchor behavior under unidirectional loading and combined loading is important for anchor selection. The anchor behavior under combined loading, characterized by the yield envelope, can also be used for the prediction of anchor installation, which is still an issue in anchor design. However, most existing studies on anchor capacity are for plate anchors which focused only on the anchor pullout capacity in soil with uniform shear strength. The behavior of drag anchor under unidirectional and combined loading in soil with linearly increasing shear strength profile is seldom investigated. The current 2D finite element studies investigate the anchor behavior for a horizontal anchor fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then based on the results of anchor unidirectional loading behavior, the yield envelopes for anchor under combined loading for both shallow and deep embedded flukes are studied. The effect of anchor embedment depth, soil non- homogeneity, soil overburden pressure and the soil/anchor interface breakaway conditions are studied to provide insight for drag anchor design. 相似文献
10.
11.
在自升式平台的预压载过程中，桩靴在层状地基上较易发生“穿刺”现象，很大程度上影响着平台的安全运行。准确地分析桩靴峰值阻力，避免平台桩靴发生“穿刺”是非常重要的。采用极限分析上限定理，合理构建运动许可速度场，从理论上推导了层状地基上桩靴峰值阻力的上限解答。为了进一步验证峰值阻力理论解答的准确性，采用ABAQUS有限元软件构建了“桩靴—弹塑性海床”的三维数值模型，对桩靴贯入海床的过程进行了数值模拟，分析桩靴周围土体的塑性变形演变规律，研究土体的破坏机理。研究结果表明：推导的桩靴峰值阻力上限解答，能够较好地计算层状地基极限承载能力，通过与离心机试验和数值结果对比，计算误差在18%以内；给出的运动许可速度场能够较好地反映桩靴周围土体破坏模式；桩靴阻力达到峰值时，下层软土中的土塞高度约为桩靴直径的0.2倍。 相似文献
12.
13.
Suction caissons are widely used to support offshore fixed platforms in coastal areas. The loadings transferred to suction caissons include the eccentric lateral force induced by waves and self weight of the platform structure. However, under this kind of combined loading conditions, the failure mechanism of caissons with shallow embedment depths is quite different from conventional deep foundations or onshore shallow footings. The behaviour of caissons subjected to combined loadings may be described with the "failure locus" in force resultant spaces. Here the failure loci of smooth caissons are studied by use of finite element approach, with the embedment ratio of caissons varying in the range of 0.25～1.0 and eccentricity ratio of horizontal loadings in 0～10. The platform settlement and tilt limits are involved into determination of failure loci, thus the platforms can avoid significant displacements for the combined loadings located inside the failure locus. Three families of loading paths are used to map out the locus. It is found that the shape of failure loci depends on 3 non-dimensional parameters, and the failure locus of a given caisson changes gradually from the elliptical curve to hooked curve with increasing shear strength of soil. The lateral capacity of short caissons may be enhanced by vertical forces, compared with the maximum lateral capacity of long caissons occurring at the vertical force being zero. The critical embedment ratios partitioning elliptical and hooked loci are proposed. 相似文献
14.
The passive suction of suction foundations plays a significant role in pull-out resistance. The factors influencing the uplift capacity include stress state, embedment ratio, and loading rate. This article investigates the effect of embedment ratio and loading rate on the bearing behavior of suction foundations using centrifuge testing. A series of uplift tests on a suction foundation in clay were performed using a beam centrifuge. During the tests, uplift displacement, suction, and loading rate were monitored. The suction was obtained by measurement of water pressure. To compare the influence of different factors on uplift capacity due to passive suction, two types of uplift tests were conducted; the first was on the closed caisson and the second was on the vented caisson. The results show that the pull-out resistance increased with an increase of the uplift loading rate, which was induced by the suction. The maximum resistance occurred when the upward displacements reached 14%D under a ratio of skirt length (L) to diameter (D) (L/D) of 0.5 and 17%D under an L/D ratio of 2. These findings provide a way for suction caissons to resist pull-out load or for structures to be removed from the seabed. 相似文献
15.
复合加载下桶形基础循环承载性能数值分析 总被引：1，自引：0，他引：1
作为一种新型基础形式,吸力式桶形基础除了承受海洋平台结构及自身重量等竖向荷载的长期作用之外,往往还遭受波浪等所产生的水平荷载及其力矩等其它荷载分量的瞬时或循环作用。对复合加载模式下软土地基中桶形基础及其结构的循环承载性能尚缺乏合理的分析与计算方法。应用Andersen等对重力式平台基础及地基所建议的分析方法,基于软黏土的循环强度概念,在大型通用有限元分析软件ABAQUS平台上,通过二次开发,将软土的循环强度与Mises屈服准则结合,针对吸力式桶形基础,基于拟静力分析建立了复合加载模式下循环承载性能的计算模型,并与复合加载作用下极限承载性能进行了对比。由此表明,与极限承载力相比,桶形基础的循环承载力显著降低。 相似文献
16.
Hiroyoshi Hirai 《Marine Georesources & Geotechnology》2013,31(4):452-466
ABSTRACTAn investigation is made to present analytical solutions provided by a three-dimensional displacement approach for analysis of bucket foundations subjected to vertical and lateral loads in cohesive soils. The nonlinear vertical and lateral stiffness coefficients along the skirt of the bucket foundation in nonhomogeneous soil are presented using three-dimensional solutions for vertical and lateral loads and taking into account the dependence of stiffness coefficients on the shear strain. The vertical, lateral, and rocking stiffness coefficients on the base of the skirt of a bucket foundation are obtained from the solutions of hollow rigid cylindrical punch acting on the surface of a soil. The ultimate vertical stress of a soil under the base of a bucket foundation subjected to vertical and moment loads is presented analytically by considering only compression and ignoring tension on the base. The vertical and lateral yields along the skirt and the compression and shear failures on the base are taken into account in analysis of ultimate load capacities. Envelopes of the combined ultimate horizontal and moment load capacities of a bucket foundation in clay are shown. Relationships between ultimate lateral and moment load capacities and the embedment ratio (skirt length to diameter) are presented. 相似文献
17.
Study on Failure Mechanism and Bearing Capacity of Three-Dimensional Rectangular Footing Subjected to Combined Loading 总被引：2，自引：0，他引：2
This paper presents two kinematic failure mechanisms of threc-dimensional rectangular footing resting on homogeneous undrained clay foundation under uniaxial vertical loading and uniaxial moment loading. The failure mechanism under vertical loading comprises a plane strain Prandti-type mechanism over the central part of the longer side, and the size of the mechanism gradually reduces at the ends of the longer side and over the shorter side as the corner of rectangular footing is being approached where the direction of soil motion remains normal to each corresponding side respectively. The failure mechanism under moment loading comprises a plane strain scoop sliding mechanism over the central part of the longer side, and the radius of scoop sliding mechanism increases linearly at the ends of the longer side. On the basis of the kinematic failure mechanisms mentioned above, the vertical ultimate bearing capacity and the ultimate bearing capacity against moment or moment ultimate bearing capacity are obtained by use of upper bound limit analysis theory. At the same time, numerical analysis results, Skempton＇ s results and Salgado et al. ＇s results are compared with this upper bound solution. It shows that the presented failure mechanisms and plastic limit analysis predictions are validated. In order to investigate the behaviors of undrained clay foundation beneath the rectangular footing subjected to the combined loadings, numerical analysis is adopted by virtue of the general-purpose FEM software ABAQUS, where the clay is assumed to obey the Mohr-Coulomb yielding criterion. The failure envelope and the ultimate bearing capacity are achieved by the numerical analysis results with the varying aspect ratios from length L to breadth B of the rectangular footing. The failure mechanisms of rectangular footing which are subjected to the combined vertical loading V and horizontal loading H （Vertical loading V and moment loading M, and horizontal loading H and moment loading M respectively are observed in the finite e 相似文献
18.
Mohammed Y. Fattah Bushra S. Zbar Faris S. Mustafa 《Marine Georesources & Geotechnology》2017,35(8):1111-1120
In this study, the dynamic response of pile foundation in dry sandy soil excited by two opposite rotary machines was considered experimentally. A small scale physical model was manufactured to accomplish the experimental work in the laboratory. The physical model consists of two small motors supplied with eccentric mass (0.012?kg) and eccentric distance (20?mm) representing the two opposite rotary machines, an aluminum shaft as the pile, and a steel plate a pile cap. The experimental work was achieved taking the following parameters into considerations: pile embedment depth ratio (L/d, where L is the pile length and d is its diameter) and operating frequency of the rotary machines. All tests were conducted in medium dense fine sandy soil with 60% relative density. Twelve tests were performed to measure the change in load transferred through the pile’s tip to the underlying soil. To predict precisely the dynamic load that will be induced from the rotary machines, a mini load cell with a capacity of 100?kg was mounted between the aluminum plate (the machine base) and the steel plate (pile cap). The results revealed that, before machine operation, the pile tip load was approximately equal to the static load (machine and pile cap), whereas during machines’ operation, the pile tip load decreased for all embedment depth ratios and operating frequencies. This reduction was due to the action of skin friction that was mobilized along the pile during operation, and as a result the factor of safety against pile bearing failure increases. For all operating frequencies and pile lengths, the factor of safety against bearing failure increased during machines’ operation, where the pile tip load became less than its value before starting operation. During operation, the skin friction resistance mobilized along pile length led to decrease the bearing load. 相似文献
19.
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。 相似文献
20.
Numerical solutions have been obtained for the vertical uplift capacity of strip plate anchors embedded adjacent to sloping ground in fully cohesive soil under undrained condition. The analysis was performed using finite element lower bound limit analysis with second-order conic optimization technique. The effect of anchor edge distance from the crest of slope, angle and height of slope, normalized overburden pressure due to soil self-weight, and embedded depth of anchor on the uplift capacity has been examined. A nondimensional uplift factor defined as Fcγ owing to the combined contribution of soil cohesion (cu), and soil unit weight (γ) is used for expressing the uplift capacity. For an anchor buried near to a sloping ground, the ultimate uplift capacity is dependent on either pullout failure of anchor or overall slope failure. The magnitude of Fcγ has been found to increase with an increase in the normalized overburden pressure up to a certain maximum value, beyond which either the behavior of anchor transfers from shallow to deep anchor or overall slope failure occurs. 相似文献