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1.
The suction caisson (or called suction anchor) which is considered as a relatively new type of foundation of offshore structures, has been extensively studied and applied for offshore wind turbines and oil platforms. The installation of the suction caisson is of great importance in the design and construction because it can bring about several issues and further influence the performance of holding capacity in safety service. In this paper, large deformation finite element (FE) analyses are performed to model the installation of suction caisson (SC) by suction and jacking in normally consolidated clay. The penetration of the suction caisson is modeled using an axisymmetric FE approach with the help of the Arbitrary Lagrangian–Eulerian (ALE) formulation which can satisfactorily solve the large deformation problem. The undrained shear strength of the clay and elastic modulus are varied with depth of soil through the subroutine VUFIELD. The numerical results allow quantification of the penetration resistance and its dependence on the installation method. The centrifuge test and theoretical solution are used for the FE model validation. After the validation, the penetration resistance, the soil plug heave, and the caisson wall friction have been examined through the FE model. Based on the numerical results, it is shown that the ALE technique can simulate the entire suction caisson penetration without mesh distortion problem. The installation method can play an important role on the penetration resistance, namely, the suction installation reduces the penetration resistance significantly compared to the purely jacked installation. With a further study on the suction case, it is found that as the final applied suction pressure increases, the soil plug heave increases, while the penetration resistance reduces with increase of the final suction pressure. The effect of the friction of internal caisson walls has been also investigated and a conclusion is drawn that internal wall friction has a significant contribution to the penetration resistance and it can be implicitly represented by varying coefficient of internal wall friction. As for the penetration resistance, both jacked and suction installation have great dependency on the internal wall friction. 相似文献
2.
Hiroyoshi Hirai 《Marine Georesources & Geotechnology》2018,36(7):805-817
An investigation was made to present analytical solutions of cyclic response to suction caisson subjected to inclined cyclic loadings in clay using a three-dimensional displacement approach. A model representing the relationship between vertical load and vertical displacement and that between lateral load and lateral displacement along the skirt of suction caisson subjected to cyclic loadings is proposed for overconsolidated clay. For the effect of vertical load on cyclic load capacity of suction caisson, using the Mindlin solution in the case of a vertical point load, the vertical stress of soil under the base of suction caisson is presented. For the stress state of soil beneath the base of suction caisson subjected to cyclic loading, the Mohr–Coulomb failure line and critical state line are presented and the relationship between total stress, effective mean principal stress, stress difference, and pore-pressure is elucidated. The comparison of results predicted by the present method for a suction caisson subjected to cyclic loadings in clay has shown good agreement with those obtained from field tests. Cyclic behavior of clay up to failure is made clear from the relationship between cyclic tensile load, vertical and lateral displacements, and rotation and that between depth, vertical, and lateral pressures. 相似文献
3.
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
4.
深水井口导管在黏土中的贯入阻力与导管土体界面的黏性摩擦系数密切相关。在导管安装过程中,管土界面会产生大位移剪切,使得导管-黏土界面剪切力产生退化。本文针对传统剪切试验装置小位移剪切的限制,无法正确描述井口导管贯入时导管-黏土界面大位移剪切时的退化机理,采用GDS多功能界面剪切试验仪在恒应力条件下进行大位移单调剪切试验,研究法向应力对导管管土界面摩擦退化机理的影响规律。试验结果表明,大位移单调剪切时,界面剪应力随剪切位移先增大后减小,最终趋于稳定,界面表现出剪应力退化现象,随着法向应力的增大界面峰值应力与残余应力均增大;土样法向位移随着剪切位移、法向应力的增加而增大最终趋于稳定。最后在对试验数据分析的基础上,基于剪应力随无量纲位移的退化规律,提出了界面剪应力随累积位移退化的计算方法。并结合算例对比分析了井口导管在贯入80 m深度处,由于界面摩擦退化效应导致导管贯入阻力降低了28.48%。 相似文献
5.
Numerical Analyses of Caisson Breakwaters on Soft Foundations Under Wave Cyclic Loading 总被引：1，自引：0，他引：1 
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下载免费PDF全文 A caisson breakwater is built on soft foundations after replacing the upper soft layer with sand. This paper presents a dynamic finite element method to investigate the strength degradation and associated pore pressure development of the intercalated soft layer under wave cyclic loading. By combining the undrained shear strength with the empirical formula of overconsolidation clay produced by unloading and the development model of pore pressure, the dynamic degradation law that describes the undrained shear strength as a function of cycle number and stress level is derived. Based on the proposed dynamic degradation law and M-C yield criterion, a dynamic finite element method is numerically implemented to predict changes in undrained shear strength of the intercalated soft layer by using the general-purpose FEM software ABAQUS, and the accuracy of the method is verified. The effects of cycle number and amplitude of the wave force on the degradation of the undrained shear strength of the intercalated soft layer and the associated excess pore pressure response are investigated by analyzing an overall distribution and three typical sections underneath the breakwater. By comparing the undrained shear strength distributions obtained by the static method and the quasi-static method with the undrained shear strength distributions obtained by the dynamic finite element method in the three typical sections, the superiority of the dynamic finite element method in predicting changes in undrained shear strength is demonstrated. 相似文献
6.
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
7.
The influence of cyclic loading on the strength and deformation behavior of cemented marine clay has been studied. This marine clay is of recent Pleistocene origin and deposited in a shallow water marine environment. Open pits were dug in sheeted enclosures and from these pits, undisturbed samples were taken for strength testing. A series of standard triaxial shear tests and stress controlled one-way cyclic load tests were conducted at consolidation stress ranges below and above the preconsolidation pressure. For the stress levels below the preconsolidation pressure, the cyclic loading has brought about the collapse of the cementation bond through an increase in strains, and at higher pressure ranges, the soil behaves like typical soft clay. This experiment studied the rate of development of strain and pore water pressure and shows that rate is a function of number of cycles, applied stress, and stress history. In addition, soil degradation during cyclic loading is studied in terms of Degradation Index. Attempt has been made to predict stain, pore water pressure, and degradation index through an empirical model. 相似文献
8.
In the field of ocean engineering, pile foundations are extensively used in supporting several structures. In many cases, piles are subjected to significant lateral loads. The environment prevalent in the ocean necessitates the piles to be designed for cyclic wave loading. In this investigation, the behaviour of rigid piles under cyclic lateral loading has been studied through an experimental programme carried out on model piles embedded in a soft marine clay. Static tests were also conducted on piles embedded in a clay bed prepared at different consistencies suitable to field situations. Cyclic load was applied by using a specially designed pneumatic controlled loading system. Tests were conducted on model piles made of mild steel (MS), aluminium and PVC with wide variation in pile soil relative stiffness. For cyclic load levels less than 50% of static lateral capacity, the deflections are observed to increase with number of cycles and cyclic load level and stabilise after a certain number of cycles. For cyclic load levels greater than 50% of static lateral capacity, the deflections are observed to increase enormously with number of cycles. The results of post-cyclic load tests indicate that the behaviour under static load can improve for cyclic load levels less than 40% of the static lateral capacity. The variations in the load capacity due to cyclic loading are explained in terms of the changes in strength behaviour of soil. 相似文献
9.
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
10.
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
11.
Shuwang Yan Jingjing Zhang Jian Chu Zhiliang Huo 《Marine Georesources & Geotechnology》2017,35(3):435-440
Concrete suction caissons have been successfully used as breakwaters or seawalls in recent years. The relative large wall thickness-to-diameter ratio of a concrete caisson can lead to the formation of a full soil heave plug that may cause difficulties in the installation of concrete caisson in clay. One way to overcome this limitation is to use a tampered tip for the caisson wall. An analytical method is proposed in this article to calculate the minimum suction pressure required to penetrate a caisson and the maximum allowable suction pressure that can be applied to avoid too much soil heave plug during the installation of the suction caisson. Four model tests were conducted in normally consolidated clay to study the installation process of a concrete suction caisson with tampered tip and to verify the proposed analytical method. The height of the soil heave plug in the caisson with a tampered tip is observed to be about half of that in the caisson with a flat tip. 相似文献
12.
AbstractIn this article, the degradation of the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading is studied numerically. A modified kinematic hardening constitutive model is employed to simulate the degradation of soft clay after cyclic loading. The modified model is verified by comparing the numerical simulation results with the results of centrifuge model tests. Furthermore, the modified model is applied to numerical simulations for evaluating the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading. The degradation of the lateral bearing capacity of piles in soft clay after different cyclic displacement levels and different numbers of cycles is investigated. The study reveals that the modified kinematic hardening constitutive model can effectively estimate the cyclic degradation behavior of piles in soft clay subjected to cyclic lateral loading. The degradation of the ultimate lateral bearing capacity progresses slowly with increasing cyclic displacement level for fewer cycles, and the degradation develops significantly at higher levels of cyclic displacement after applying a larger number of cycles. 相似文献
13.
Bearing capacity of shallow foundations in clay with linear increase in strength and adhesion factor
Boonchai Ukritchon Sothoan Yoang Suraparb Keawsawasvong 《Marine Georesources & Geotechnology》2018,36(4):438-451
In this paper, the computational lower bound (LB) limit analysis using finite element with second-order cone programming was used to investigate the LB solutions of the undrained bearing capacity of continuous footing with a linear increase in the strength profile and an adhesion factor at the soil–footing interface. A full range of parametric studies of the dimensionless strength gradients and adhesion factors at the soil–footing interface were performed in the LB calculations. The results were verified by comparison with the available solution from the method of characteristics (slip-line analysis) for perfectly smooth and rough footings. The LB analyses were able to complete a prior solution of undrained bearing capacity with a linear increase in the strength profile by incorporating the influence of adhesion factor at the soil–footing interface. Based on the nonlinear regression to the computed LB solutions, an approximate expression of the LB solution regression was proposed, which is applicable to an accurate prediction of a safe load for offshore shallow foundations in clay with an arbitrary linear increase in strength and adhesion factor at soil–foundation interface in practice. 相似文献
14.
Coastal protection is proposed to be made out of a contiguous caisson type of wall. These caissons can be designed to resist both lateral static and cyclic loading. With adequate depth of embedment, the walls can be designed to offer significant lateral passive resistance to counteract the lateral static and cyclic loading arising out of wave action. This article describes a set of laboratory tests on model caissons embedded into soft marine clay with different embedment depths. Specially designed earth pressure cells are embedded into the caisson at different depths. A pneumatic system was used to apply lateral static and cyclic loading. Test beds were prepared conforming to soft clay conditions in a test tank of appropriate size. The test results reveal that with this type of arrangement the variation in earth pressure with depth can be conveniently established. The earth pressure developed is related to the lateral load applied. The depth at which the maximum earth pressure occurs is same for both static and cyclic loading. Further, under cyclic loading there is no failure encountered even under cyclic loading level corresponding to 0.9 times the ultimate static lateral capacity. 相似文献
15.
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
16.
Monopod caisson foundation is a viable alternative for supporting offshore wind turbines located at shallow water depths. This foundation system has to resist overturning moment generated due to resultant lateral load, arising from wind and water wave action, that can act at any loading height above the seabed. This paper presents results of a numerical investigation performed to determine the influence of loading height, caisson geometry and superstructure load on the ultimate lateral capacity, initial stiffness, and soil failure zone of the foundation, when installed in very dense sand. Both the ultimate and serviceable states of the caisson foundation obtained from the analyses are represented in terms of envelopes plotted between lateral load and overturning moment. Simplified expressions, which take into account the influence of caisson geometry, loading height, and soil properties, are also presented to serve as a preliminary base for design of the monopod caisson foundation. 相似文献
17.
The performance of traveling jet trenching in stiff clay was tested. All the key parameters varied in the experiment, including the jet velocity, the shear strength of clay, the diameter of the nozzle, and the translational velocity of the nozzle. The jet–soil interface was found to be an arc. An equation was given to describe the interface. On the basis of the erosion failure mechanism, a new method is presented for predicting jet trenching depth. We first calculated the trajectory of the jet–soil interface and then predicted the distribution of the boundary layer shear stress along the jet–soil interface. The point where the wall shear stress was equal to the critical shear strength of clay was the ultimate trenching depth. The predictive results were found to fit the experimental results well. 相似文献
18.
A series of model tests was conducted in sand to explore the anti-uplift behavior of suction caissons, considering the effects of aspect ratios, load inclination angles and loading positions. This paper emphasizes on analyzing the deformation characteristic and the mechanism of the suction caissons under various loading conditions. The movement modes of the suction caisson are different when the load inclination angle increases from 0° to 90° corresponding to various mooring positions. The pull-out bearing capacity decreases with load inclination angles increasing. When the load inclination angle changes from 0° to 60°, the bearing capacity reduces more significantly than that between inclination angle of 60° and 90°. While the load inclination angle is relatively small, the pull-out capacity of the suction caisson decreases after reaching the peak as the loading position moves downwards. Moreover, the optimum loading position locates between 2/3 and 3/4 of the caisson length. The optimum loading position is at the bottom of the caisson when the load inclination angle exceeds 60°. However, the influence of the loading position on the pull-out capacity of the caisson can be ignored while the load inclination angle equals to 90°. The pull-out bearing capacity increases as the aspect ratio increases but the aspect ratio has no effect on the deformation characteristic of the suction caisson. 相似文献
19.
针对现阶段深水软黏土地基防波堤建设的设计理论和稳定性分析方法尚不成熟,结合实际工程,采用三维弹塑性有限元数值分析方法,研究在水平或竖直单一方向荷载以及复合加载条件下软黏土地基上沉箱防波堤的失稳模式,提出破坏包络线的稳定性判别方法。在波浪水平荷载作用下,深水软基上沉箱防波堤发生倾覆失稳破坏,失稳转动点为沉箱底面以下中轴线偏右的某点,不同于规范中规定的岩石或砂质地基沉箱倾覆转动点为其后踵点;在重力等竖向荷载作用下,沉箱的失稳模式为结构整体下陷,抛石基床及地基形成连贯的塑性区域,呈现较明显地冲剪破坏形式;在水平、竖向复合荷载作用下,软基上沉箱防波堤的破坏包络线由结构倾覆破坏线和地基承载力破坏线组成,包络线将荷载组合区分成稳定区、仅发生水平承载力不足倾覆破坏区、仅发生地基竖向承载力不足破坏区、同时发生水平承载力和地基竖向承载力不足破坏区4个区域。研究成果为深水软基沉箱防波堤建设提供参考和借鉴。 相似文献
20.
Steel catenary risers (SCR) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep waters. Waves and currents induce motions of the structure and the risers. The repeated motions of the risers at the touchdown zone in turn induce loads on the seabed soil and might eventually cause fatigue damage to the risers. The analysis of riser fatigue damage is heavily dependent on the soil model. Soil behaviour at touchdown zone such as soil remoulding, stiffness degradation and deformation of the seabed at the touchdown zone further complicate the accurate assessment of riser fatigue damage, which is currently not appropriately quantified in existing design methods. This paper presents centrifuge model tests simulating the repeated vertical movement of a length of riser on clay seabed with increasing undrained shear strength with depth. During the tests, the pipe was subject to cyclic motion over fixed vertical displacement amplitude from an invert embedment of 0.5-3.5 pipe diameters into the soil. The test results show a significant progressive degradation of soil strength and diminution of excess pore water pressure with increasing number of riser penetration/uplift cycle. In view of the different types of environment loadings experienced by floating platforms and various soil conditions, tests were also conducted to investigate the effect of soil strength, riser displacement rate and loading mode on riser-soil interaction during repetitive penetration/uplift motion of the riser. 相似文献