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1.
A series of model tests were performed on steel- and Perspex-made suction caissons in saturated dense marine sand to explore installation and extraction behaviors. The extractions of the caisson were conducted by applying monotonic loading or by pumping water into the caisson. Responses of suction caissons to pullout rates, aspect ratios, and extraction manners were examined. Test results show that a cone-shaped subsidence region occurs around the suction caisson during the suction-assisted installation. The pullout bearing capacity of the suction caisson in sand is dominated by the loading rate and the loading manner. For the suction caisson subjected to monotonic loading, the maximum bearing capacity is reached at the pullout rate of about 20.0?mm/s. The mobilized vertical displacement corresponding to the pullout capacity increases with increasing the pullout rate. The passive suction beneath the suction caisson lid reaches the maximum value when the pullout bearing capacity is mobilized. In addition, during the suction caisson extracted by pumping water into the caisson, the maximum pore water pressure in the caisson is obtained under the displacement of approximately 0.04 times the caisson diameter. The absolute values of the maximum pore water pressures for the suction caissons approximately equal those of the maximum vertical resistances at the monotonic pullout rate of 5 mm/s. When the vertical displacements of the suction caissons with the aspect ratio of 1.0 and 2.0 reach 0.92 and 1.77 times the caisson diameter, respectively, the seepage failure occurs around the caissons. Using a scaling method, the test results can be used to predict the time length required for the prototype suction caisson to be extracted from the seabed. 相似文献
2.
Response of skirted suction caissons to monotonic lateral loading in saturated medium sand 总被引：2，自引：1，他引：1
Monotonic lateral load model tests were carried out on steel skirted suction caissons embedded in the saturated medium sand to study the bearing capacity. A three-dimensional continuum finite element model was developed with Z_SOIL software. The numerical model was calibrated against experimental results. Soil deformation and earth pressures on skirted caissons were investigated by using the finite element model to extend the model tests. It shows that the ＂skirted＂ structure can significantly increase the lateral capacity and limit the deflection, especially suitable for offshore wind turbines, compared with regular suction caissons without the ＂skirted＂ at the same load level. In addition, appropriate determination of rotation centers plays a crucial role in calculating the lateral capacity by using the analytical method. It was also found that the rotation center is related to dimensions of skirted suction caissons and loading process, i.e. the rotation center moves upwards with the increase of the ＂skirted＂ width and length; moreover, the rotation center moves downwards with the increase of loading and keeps constant when all the sand along the caisson＇s wall yields. It is so complex that we cannot simply determine its position like the regular suction caisson commonly with a specified position to the length ratio of the caisson. 相似文献
3.
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
4.
A series of model tests were conducted on Perspex-made suction caissons in saturated dense marine sand to study the sand plug formation during extraction. Suction caissons were extracted by pullout loading or by pumping air into the suction caisson. Effects of the pullout rates, aspect ratios and loading ways (monotonic or sustained) on the pullout capacity, and plug formation were investigated. It was found that the ultimate pullout capacity of the suction caisson increases with increasing the pullout rate. The sand plug formation under the pullout loading is significantly influenced by the pullout rate and the loading way. When the suction caisson is extracted at a relatively slow rate, the general sand boiling through the sand plug along the inner caisson wall occurs. On the contrary, the local sand boiling will occur at the bottom of the suction caisson subjected to a rapid monotonic loading or a sustained loading. Test results of the suction caisson extracted by pumping air into the caisson show that the pressure in the suction caisson almost follows a linear relationship with the upward displacement. The maximum pressures for suction caissons with aspect ratios of 1.0 and 2.0 during extraction by pumping air into the caisson are 1.70 and 2.27 times the maximum suction required to penetrate the suction caisson into sand. It was found that the sand plug moves downward during extraction by pumping air into the caisson and the variation in the sand plug height is mainly caused by the outflow of the sand particles from the inside of the suction caisson to the outside. When the suction caisson model is extracted under the pullout rate of 2?mm/s (0.28?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases to the maximum value with increasing the penetration depth and then reduces to zero. On the contrary, when extracted under the pullout rate of 10?mm/s (1.4?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases with increasing the pullout displacement. When extracted by pumping air into the caisson, the hydraulic gradient reaches the critical value, and at the same time, the seepage failure occurs around the suction caisson tip. 相似文献
5.
The bearing behavior of suction caissons supporting offshore wind turbines under two-way cyclic lateral loading and dead load in clay was investigated with consideration of soil strength degradation and adhesive interface friction between caisson walls and heterogeneous clay using the finite-element package ABAQUS.An ABAQUS built-in user subroutine was programmed to calculate the adhesive interface friction between clay and caisson walls.The results of parametric studies showed that the degradation of bearing capacity could be aggravated by the decrease of the aspect ratio.The offset between the rotation point of the soil inside the caisson and the central axis of the caisson increased with the increasing vertical load and number of cycles.The linearly increasing strength profile and adhesive interface led to the formation of an inverted spoon failure zone inside the caisson.The settlement-rotation curves in each cycle moved downwards with increasing number of cycles due to the soil strength degradation. 相似文献
6.
Suction caisson foundation derives most of their uplift resistance from passive suction developed during the pullout movement. It was observed that the passive suction generated in soil at the bottom of the caisson and the failure mode of suction caisson foundation subjecting pullout loading behaves as a reverse compression failure mechanism.The upper bound theorems have been proved to be a powerful method to find the critical failure mechanism and critical load associated with foundations, buried caissons and other geotechnical structures. However, limited attempts have been reported to estimate the uplift bearing capacity of the suction caisson foundation using the upper bound solution. In this paper, both reverse failure mechanisms from Prandtl and Hill were adopted as the failure mechanisms for the computation of the uplift bearing capacity of the suction caisson. New equations were proposed based on both failure mechanisms to estimate the pullout capacity of the suction caisson. The proposed equations were verified by the test results and experimental data from published literature. And the two solutions agree reasonably well with the other test results. It can be proved that both failure mechanisms are reasonably and more consistent with the actual force condition. 相似文献
7.
1 .IntroductionSuctioncaissons have been widely usedfor offshore oil exploration duetothe advantages of econo-my and simple installation over traditional piles (Huanget al .,2003) .For tensionleg platforms andspar platforms in deep ocean,suction caissons … 相似文献
8.
A series of centrifuge model tests have been conducted on a model suction pile embedded in sand to evaluate its inclined pull-out loading capacity. This paper describes the centrifuge model tests, the analytical solution, and comparisons between the centrifuge model test results and the analytical predictions of the pull-out capacities of the suction pile under inclined loads. The main variables of the study are the load inclination angle and the point of mooring line attachment which varies from the top to the bottom of the suction pile’s side surface. Effects of these two parameters on the suction pile inclined pull-out loading capacity are described. 相似文献
9.
AbstractSince the pull-out response of upwind caissons governs the design of multi-caisson foundations, it is worthwhile to study scour effect on the tensile capacity of suction caissons. The tensile capacity of suction caissons after scour is relevant to the scour depth and the pressure under the caisson lid: the tensile capacity decreases dramatically with increasing scour depth; the smaller the pressure, the stronger the weakening effect of scour. Moreover, the scour effect is investigated in two cases: ignoring stress history and considering stress history. The results show that tensile capacity after scour is larger when the stress history is considered, so ignoring the stress history leads to a conservative design. In order to quantitatively evaluate the effect of scour depth and pressure, an empirical formula for the tensile capacity of suction caissons after scour is proposed based on multiple regression analysis. 相似文献
10.
Suction caisson foundations are often subjected to vertical uplift loads, but there are still no wide and spread engineering specifications on design and calculation method for uplift bearing capacity of suction caisson foundation.So it is important to establish an uplift failure criterion. In order to study the uplift bearing mechanism and failure mode of suction caisson foundation, a series of model tests were carried out considering the effects of aspect ratio,soil permeability and loading mode. Test results indicate that the residual negative pressure at the top of caisson is beneficial to enhance uplift bearing capacity. The smaller the permeability coefficient is, the higher the residual negative pressure will be. And the residual negative pressure is approximately equal to the water head that causes seepage in the caisson. When the load reaches the ultimate bearing capacity, both the top and bottom negative pressures are smaller than Su and both the top and bottom reverse bearing capacity factors are smaller than 1.0 in soft clay. Combined the uplift bearing characteristics of caisson in sandy soil and soft clay, the bearing capacity composition and the calculation method are proposed. It can provide a reference for the engineering design of suction caisson foundation under vertical load. 相似文献
11.
Hiroyoshi Hirai 《Marine Georesources & Geotechnology》2018,36(4):425-437
A study was made to present analytical solutions of pullout load capacity for a suction caisson subjected to inclined tension in clay. The inclined tension on the skirt of the suction caisson is transformed into an equivalent system comprised of the vertical, horizontal, and moment load applied on the center of the lid. The vertical and horizontal stiffness coefficients along the skirt of the suction caisson in clay are presented by three-dimensional elastic solutions considering the nonhomogeneous and nonlinear property of clay. The vertical, horizontal, and rocking stiffness coefficient of the suction caisson on the base are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of clay. The envelopes of the horizontal and vertical ultimate load capacity for clay are presented. The yield, pullout, and failure for clay are taken into consideration. The effects of load inclination, loading depth, and aspect ratio on the pullout load capacity are shown. Behavior of the suction caisson in clay up to failure is investigated using the relationship between tensile load and displacement and that between depth, vertical, and horizontal pressure. 相似文献
12.
Hiroyoshi Hirai 《Marine Georesources & Geotechnology》2017,35(8):1121-1134
An investigation was conducted to obtain analytical solutions for the pullout behavior of a suction caisson undergoing inclined loads in sand. The inclined load is transformed into an equivalent load system in which the vertical, horizontal, and moment loads are applied on the center of the lid of the suction caisson. The vertical and lateral stiffness coefficients along the skirt of the suction caisson in sands are presented using the new three-dimensional elastic solutions taking into account the nonhomogeneous and nonlinear properties of the sand. The vertical, lateral, and rocking stiffness coefficients on the base of the suction caisson are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of a soil. The yield, pullout, and failure for sands with the nonhomogeneous and nonlinear characteristics are taken into consideration. The effects of the load inclination, the loading depth, and the aspect ratio on the pullout load capacity of the suction caisson are presented. Behaviour of the suction caisson in sand prior to failure is clarified from the relationship between tensile load, displacement, and rotation and that between depth, vertical pressure, and lateral pressure. 相似文献
13.
吸力基础具有施工速度快、安装过程中受海况天气影响小且易于回收重复利用等优点,被广泛应用于海洋工程。当吸力基础作为海上风电塔架的基础时,常常承受较大的水平荷载,因此其水平承载力是设计的主控因素。介绍了海上风机基础的设计要求,分析了影响基础水平承载性状的因素,总结了吸力基础受水平单调荷载、水平循环荷载和不同荷载组合三个方面的研究现状。讨论了水平荷载的大小、水平加载的高度(偏心率)、循环荷载的频率、循环荷载的次数、循环荷载的幅值、循环荷载的方向性、竖向荷载对吸力基础水平承载性状的影响,考虑了水平荷载的非共线性,指出了目前研究的不足,明确了吸力基础水平承载性状进一步研究的方向,提出了供工程实践参考的建议。 相似文献
14.
This paper presents an incremental elastoplastic finite element method (FEM) to simulate the undrained deformation process of suction caisson foundations subjected to cyclic loads in soft clays. The method is developed by encoding the total-stress-based bounding surface model proposed by the authors in the ABAQUS software package. According to the model characteristics, elastoplastic stress states associated with the incremental strains of each iteration are determined using the sub-incremental explicit Euler algorithm, and the state parameters describing the cyclic accumulative rates of strains are updated by setting state variables during the calculations. The radial fallback method is also proposed to modify the stress states outside the bounding surface to the surface during determination of the elastoplastic stress states. The stress reversals of soil elements are judged by the angle between the incremental deviatoric stress and the exterior normal vector at the image stress point on the bounding surface to update the mapping centre and state variables during cyclic loading. To assess the general validity of the method, the reduced scale model tests and centrifuge tests of suction caissons subjected to cyclic loads are simulated using the method. Predictions are in relative good agreement with test results. Compared with the limit equilibrium and quasi-static methods, the method can not only determine the cyclic bearing capacity, but can also analyse the deformation process and the failure mechanisms of suction caisson under cyclic loads in soft clays. 相似文献
15.
Lateral Bearing Capacity of Modified Suction Caissons Determined by Using the Limit Equilibrium Method 总被引：1，自引：0，他引：1
The modified suction caisson (MSC) adds a short-skirted structure around the regular suction caissons to increase the lateral bearing capacity and limit the deflection. The MSC is suitable for acting as the offshore wind turbine foundation subjected to larger lateral loads compared with the imposed vertical loads. Determination of the lateral bearing capacity is a key issue for the MSC design. The formula estimating the lateral bearing capacity of the MSC was proposed in terms of the limit equilibrium method and was verified by the test results. Parametric studies on the lateral bearing capacity were also carried out. It was found that the lateral bearing capacity of the MSC increases with the increasing length and radius of the external skirt, and the lateral bearing capacity increases linearly with the increasing coefficient of subgrade reaction. The maximum lateral bearing capacity of the MSC is attained when the ratio of the radii of the internal compartment to the external skirt equals 0.82 and the ratio of the lengths of the external skirt to the internal compartment equals 0.48, provided that the steel usage of the MSC is kept constant. 相似文献
16.
Spar平台吸力式基础极限承载特性数值分析 总被引：2，自引：0，他引：2
以国外某深海Spar平台工程为背景,针对其所采用的细长型吸力式基础的抗拔承载特性进行三维有限元数值分析.分析中充分考虑土体强度、加载位置和加载角度对吸力式基础极限抗拔承载力的影响,本构模型中钢筒基础采用弹塑性模型.分析结果表明,吸力式基础的极限抗拨承载力随着土体强度的增大而增大,倾斜加载时在基础插入土体部分中点左右加载可取得最大的极限承载力,极限抗拔承载力还随着加载角度的增大而增大.吸力式基础存在倾斜加载时桶基础与桶内外土体的共同塑性屈服破坏和垂直加载时桶外土体的局部剪切破坏等两种不同的破坏模式. 相似文献
17.
桶形基础越来越广泛应用于海洋油气平台、海上风机、输电塔、防波堤等构筑物,研究其循环承载特性对以上构筑物服役安全性具有重要意义。通过在软黏土中开展单桶循环上拔以及小间距群桶循环上拔和循环下压超重力离心模型试验,发现循环上拔地基破坏模式为整体破坏,裂隙均呈现圆弧形,循环下压呈现渐进式整体破坏模式,下压过程的挤压作用可明显减小桶周泥面高度,导致其承载力降低。模拟双向受荷工况的循环上拔试验在5次加载后荷载弱化系数开始趋于稳定,远早于单向受荷工况;单向和双向受荷工况循环上拔荷载弱化系数残余稳定值分别为0.31和0.32,循环下压荷载弱化系数最小值为0.35,表明不同加载方式竖向循环荷载作用下,此三者大小均可用软黏土地基灵敏度倒数预估。 相似文献
18.
The modified suction caisson(MSC) is a novel type of foundation for ocean engineering, consisting of a short external closed-top cylinder-shaped structure surrounding the upper part of the regular suction caisson(RSC). The MSC can provide larger lateral bearing capacity and limit the deflection compared with the RSC. Therefore, the MSC can be much more appropriate to use as an offshore wind turbine foundation. Model tests on the MSC in saturated sand subjected to monotonic lateral loading were carried out to investigate the effects of external structure sizes on the sand surface deformation and the earth pressure distribution along the embedded depth. Test results show that the deformation range of the sand surface increases with the increasing width and length of the external structure. The magnitude of sand upheaval around the MSC is smaller than that of the RSC and the sand upheaval value around the MSC in the loading direction decreases with the increasing external structure dimensions. The net earth pressure in the loading direction acting on the internal compartment of the MSC is smaller than that of the RSC at the same embedded depth. The maximum net earth pressure acting on the external structure outer wall in the loading direction is larger than that of the internal compartment, indicating that a considerable amount of the lateral load and moment is resisted by the external skirt structure. 相似文献
19.
Determining the ultimate capacity of suction caissons in response to combined vertical, horizontal, and moment loading is essential for their design as foundations for offshore wind turbines. However, the method implemented for stability analysis is quite limited. Numerical limit analysis has an advantage over traditional limit equilibrium methods and nonlinear finite element methods in this case because upper and lower bounds can be achieved to ensure that the exact ultimate capacity of the caisson falls within the appropriate range. This article presents theories related to numerical limit analysis. Simulations are conducted for centrifuge model tests, the findings of which reveal the ability of numerical limit analysis to deal with the inclined pullout capacity of suction caissons. Finally, this article proposes an estimation of the ultimate capacity of a 3.5 MW offshore wind turbine foundation on normally consolidated clay based on the typical environmental parameters of Bothkennar, Scotland. Undrained failure envelopes and safety factors are obtained for suction caissons with different embedment ratios. Failure mechanisms, plastic zones, clay stress distributions, and the influence of the skin friction coefficients of caissons are discussed in detail. 相似文献
20.
复合加载条件下吸力式沉箱基础承载特性数值分析 总被引：2，自引：0，他引：2
吸力式沉箱基础的承载特性是海洋工程结构设施建造与设计中的一个关键问题。这种新型的深水海洋基础型式,通常承受竖向上拔荷载与水平荷载的共同作用,其工作性能与设计理论远远不能满足工程实践的需要。本文采用有限元分析方法对吸力式沉箱基础的极限承载特性进行数值计算。以大型通用有限元分析软件ABAQUS为平台,通过二次开发,数值实现了Swipe试验加载方法和固定位移比分析方法,针对不同的沉箱长径比、土的强度折减系数,探讨了沉箱基础在垂直上拔荷载和水平荷载单调联合作用下的极限承载力,通过对不同荷载组合的数值计算构造了复合加载条件下沉箱基础破坏包络面。 相似文献