Experimental studies on sand plug formation in suction caisson during extraction

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.     

Theoretical studies on penetration resistance of suction caissons in clay

ABSTRACT

The suction caisson is commonly a top-closed cylindrical steel structure with large diameter, short length and much thinner skirt wall thickness. The resistance to penetrating is calculated as the sum of the tip bearing capacity and the adhesion on the both sides of the skirt wall. Since the thickness of the skirt wall is very small, the downward adhesion produced by the skirt wall will cause the additional vertical stress and shear stress in the soil at the skirt tip level, increasing the skirt tip resistance. However, the increase in skirt tip resistance caused by the additional vertical stress rather than shear stress in soil at the skirt tip level was only considered, this may lead to an inaccurate estimation for the tip bearing capacity and the suction required. Thus, a modified slip-line field is put forward in this study to estimate the tip resistance. The expression of obtaining the minimum suction to install the suction caisson in clay is derived in terms of the force equilibrium. Results from calculations of the minimum suction have been proved to be in a good agreement with the measured data.     

Evaluation of pullout load capacity of suction caissons in sand using a three-dimensional displacement approach

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.     

海上风电吸力基础在分层土中的沉贯特性研究综述

吸力基础与海洋工程大直径钢桩相比,具有成本低、安装周期短、对环境影响小、不受海况影响及可回收再利用等优点,近年来在海上风电工程中得到推广应用。吸力基础沉贯至海床预定位置,是其发挥承载力和确保服役稳定性的前提。海床地基土体常以分层土形式分布,且各层土体强度、压缩性和渗透性等存在显著差别,导致吸力基础吸力沉贯机理非常复杂。明确吸力基础在分层土中沉贯特性,有助于指导吸力基础在海上风电工程中的推广应用。对目前吸力基础在分层土中沉贯特性研究进行综述和总结,归纳了其沉贯机理研究进展,并对影响吸力基础在分层土中沉贯因素进行了分析;提出了分层土中吸力基础沉贯的研究方向和改进的沉贯方法。     

海洋平台吸力式基础的土工离心模拟研究与分析

综述了国内外已开展的海洋平台吸力式基础的离心模型试验研究,重点讨论了应用离心机模拟吸力式基础承载特性的研究现状。     

黏土中隔舱吸力式钢圆筒安装及水平受荷试验研究

近年来大直径钢圆筒结构在离岸人工岛工程中得到应用,如港珠澳大桥人工岛即采用振动下沉的方式安装钢圆筒,该方法对施工条件、装备以及施工控制技术要求较高。提出一种新型隔舱吸力式钢圆筒结构,在钢圆筒内部设置隔舱板,将结构分为上下两个隔舱,通过对下舱抽气实现隔舱吸力式钢圆筒在负压作用下的下沉安装。设计了隔舱吸力式钢圆筒安装及水平承载力模型试验,对比了负压贯入的隔舱吸力式钢圆筒和压力贯入的传统钢圆筒结构的贯入阻力及承载特性,分析了改变隔舱吸力式钢圆筒上下舱高度比L₁/L₂对其沉贯过程及承载特性的影响。结果表明,采用负压吸力沉贯的隔舱吸力式钢圆筒相比于采用压力贯入的传统钢圆筒结构的贯入阻力减小,水平极限承载力提高。在极限水平荷载作用下,随着隔舱吸力式钢圆筒的L₁/L₂从2.28减小到1.00、0.56,转动中心位置上移,水平极限承载力及弯矩承载力得到显著提高。     

Pullout capacity of embedded suction anchors in sand

An analytical solution has been developed to estimate the horizontal, vertical, and inclined loading pullout capacities of embedded suction anchors in sand. Validation of the analytical solution on pullout capacities has been made through comparisons with the centrifuge model test results. Primary variables for the centrifuge model tests are the depth to the loading point, the load inclination angle, and the addition of flanges. The results indicate that both the horizontal and vertical pullout capacities of the embedded suction anchor in sand increase, reach the peak and then start to decrease as the loading point moves downward. The inclined loading pullout capacity is very much dependent on the load inclination angle and the loading point. The effect of flanges on the pullout capacities is also found to be significant.     

Analysis of pullout load capacity of suction caissons in clay by a three-dimensional displacement approach

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.     

Large-scale experimental investigation of the installation of suction caissons in silt sand

Offshore wind power is a rapidly growing area of electricity in China. In the present paper, interaction mechanisms between the caisson for wind turbines and saturated silt sand are investigated with laboratory tests based on two different installation methods, jacking installation and suction installation. For the jacking installation process, the results indicate that the soil pressures inner and outer the skirt of the caisson vary with a similar feature and the magnitudes of the two are nearly balanced. The tip resistance plays a key role in the total jacking installation resistance. This paper examines the predictive performance of q_c method and API approach for jacking installation resistance. It is demonstrated that the q_c method provides better predictions. The resistance coefficients are recommended. For the case of suction installation, however, the changes of soil pressures inner and outer the skirt are contrasting. Specifically, the inner pressure and tip resistance fall dramatically, but the outer pressure increases when suction is applied. Seepage effect is found to be an important mechanism for the installation of suction caisson. The reduction ratios of the inner friction and tip resistance follow a power-function with the normalized suction. Based on the test results, a prediction method for the required suction has been developed and evaluated.     

Response of skirted suction caissons to monotonic lateral loading in saturated medium sand

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.     

Model tests on performance of offshore wind turbine with suction caisson foundation in sand

Abstract

The performance of steel caisson during and after installation with different penetration velocities in medium dense sand is presented. The applied jacking forces, the amount of formed soil heave and bearing capacity were measured in the model tests. The influence of penetration velocities on jacking forces, soil heave and bearing capacity were also discussed in detail. The results indicated that the jacking forces for caisson in medium dense sands were significantly affected by the penetration velocity. The larger the penetration velocity, the more soil flowed into the caisson cavity during installation. This will lead to larger inner shaft resistance and in turn more jacking forces required for the same penetration depth. The height of soil heave during installation increases with penetration velocity. The m value calculated by the ratio of the volumes of the soil heave to that of the penetrated caisson wall can be used to evaluate the soil heave. The larger the applied velocity, the larger the m value and larger bearing capacity of caisson after installation. The relationship between the m value and penetration velocity can be used to control the soil heave for a steel caisson with a wall thickness to external diameter ratio of 4.2% in medium dense sand by jacking method.     

水平荷载作用下裙式吸力基础承载性能研究

传统吸力基础是一个单桶结构,被广泛作为海洋平台、漂浮结构的基础,近年来也被推广到海上风电塔架。作为风电塔架基础,要充分提高其水平承载能力。为此,提出一种改进的基础形式—裙式吸力基础。采用Z_SOIL有限元软件,针对砂土地基,从水平单调加载和循环加载两个方面,对传统单桶吸力基础和裙式吸力基础进行了承载性能对比研究,得到了相应的荷载-位移曲线。研究结果表明,裙式吸力基础由于设置了"裙"结构,显著提高了其抵抗水平静载和循环水平动力荷载的能力,并能有效控制基础的水平位移,是值得推广应用的一种新型海洋工程基础形式。     

Analytical and experimental studies on installation of a suction caisson with tampered tip in clay

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.     

粉土中吸力桶沉贯阻力与土塞失稳机理研究

针对粉土中吸力桶在吸力沉贯中的两个关键性问题——沉贯阻力和内部土体稳定性,在自行研制的试验平台上进行了一系列吸力桶沉贯模型试验。试验结果表明,粉土中吸力沉贯时,Andersen所提供的基于CPT锥尖强度的阻力计算公式能较好预测阻力发展趋势,但计算结果偏小;而修正的承载力公式预测结果在沉贯前期与实测值较吻合,沉贯后期预测偏大。吸力沉贯前期,贯入阻力随深度稳步增长,直到贯入深度达到某个临界值后,阻力增长缓慢甚至保持不变。桶内粉土在吸力作用下的失稳机理与黏土或砂土不同,在渗透力作用下表现为从土层表面开始的自上而下管涌或渗流侵蚀。沉贯后期,端部土体在较高水头差下发生失稳并向桶内流动,造成了桶内土体密度降低,端阻力与内壁摩阻力降低,总贯入阻力主要由外壁摩阻力提供。并结合有限元模拟对上述实验现象的内在机理进行了解释。     

离岸裙式吸力基础在砂土地基中沉贯性研究

吸力基础是海洋工程中新型的一种基础型式,广泛应用于海洋平台、海洋浮动式结构等,近年来,也被作为浅海离岸风力发电工程的基础。吸力基础易遭受较大的水平动力荷载和弯矩,从而可能产生较大水平位移和转角;同时,由于海床冲刷,会降低其承载能力。为克服这些不足,提出了一种新型吸力基础———裙式吸力基础,把分析传统吸力基础砂土中的沉贯方法,拓广到裙式吸力基础中,研究该基础型式在砂土中的可沉贯性以及所需的吸力;并与同情况下的传统吸力基础进行了比较,证明了所提出的裙式吸力基础具有较好的沉贯性能,具有工程实践推广价值。     

Large deformation finite element analysis of the installation of suction caisson in clay

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.     

Investigation of scour effects on lateral behaviors of suction caisson

ABSTRACT

The use of suction caissons can reduce the development costs of offshore wind energy and has broad application prospects. However scour around marine foundations is inevitable, it gravely affects the stability of marine engineering. Therefore, there is an urgent need to study the weakening effects of scour on suction caisson. In this study, the variation trends of remaining soil parameters (the effective unit weight and the peak effective friction angle) after scour are examined with consideration of the dilatancy and stress history of sandy silt. It is found that the parameters of shallow soil change considerably after scour, and the larger the scour depth, the greater is the change in the parameters. However, the deep soil is less affected. On the basis of these findings, scour effects on the ultimate moment capacity of suction caisson are studied. The ultimate moment capacity is found to greatly reduce under scour, and its calculated value is larger than the actual value when the effects of dilatancy and stress history are ignored. To simplify calculation, it is feasible to replace the ultimate moment capacity when both dilatancy and stress history are considered with that when only dilatancy is considered.     

Investigation of scour effect on tensile capacity of suction caissons considering stress history of sand

Abstract

Since 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.     

吸力式沉箱基础长期水平荷载作用下承载特性试验研究

通过两组不同水平荷载作用下吸力式沉箱基础长期模型试验,对吸力式沉箱基础随时间的位移变化规律以及土压力分布规律进行了研究。试验结果表明:在长期模型试验中位移发展主要集中在试验前期,后期位移稳定需要更长时间,土体流变效应较为明显。土压力沿深度分布曲线呈抛物形状,表明沉箱基础在水平荷载作用下为转动模式,随时间增加被动区土压力变化呈增大趋势,主动区土压力呈减小趋势。土压力发展主要集中在试验前期,后期土压力变化相对较小,但土压力稳定所需时间较长,同时荷载值越大土压力稳定所需时间越长。