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1.
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
2.
Vikram Singh 《Marine Georesources & Geotechnology》2018,36(5):505-514
Although the uplift behavior of offshore plate anchors under undrained conditions has been investigated well in the past, studies on the behavior of anchors under long-term sustained loading are in relatively few numbers. The time required for consolidation under sustained load is important because the shear strength of soil changes after dissipation of excess pore pressure. In this paper, small strain finite-element analyses have been performed to investigate the consolidation time history above and beneath strip anchors. The modified cam clay plasticity constitutive model is used for modeling coupled pore fluid stress analysis. The effects of magnitude of preloading with embedment level have been studied. As expected, the FE results have shown that excess pore pressure dissipation time for soil above the anchor increased with the increase in embedment depth and the magnitude of preload. Rapid dissipation of negative excess pore pressure beneath the anchor was observed with increasing embedment depth, if the preload magnitude is equal to or more than 60% of the undrained capacity. Observed consolidation responses are presented as nondimensional design charts and simplified equations for ease of practice. 相似文献
3.
AbstractPile foundation is the most popular option for the foundation of offshore wind turbines. The degradation of stiffness and bearing capacity of pile foundation induced by cyclic loading will be harmful for structure safety. In this article, a modified undrained elastic–plastic model considering the cyclic degradation of clay soil is proposed, and a simplified calculation method (SCM) based on shear displacement method is presented to calculate the axial degradated capacity of a single pile foundation for offshore wind turbines resisting cyclic loadings. The conception of plastic zone thickness Rp is introduced to obtain the function between accumulated plastic strain and displacement of soil around pile side. The axial ultimate capacity of single piles under axial cyclic loading calculated by this simplified analysis have a good consistency with the results from the finite element analysis, which verifies the accuracy and reliability of this method. As an instance, the behavior of pile foundation of an offshore wind farm under cyclic load is studied using the proposed numerical method and SCM. This simplified method may provide valuable reference for engineering design. 相似文献
4.
Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load 总被引：3，自引：0，他引：3
The degradation strength of soils under cyclic loading is studied and a method for deter-mining the cyclic degradation strength with cyclic triaxial tests is given in the paper.Furthermore,a dum-my static method for estimating the undrained bearing capacity for offshore soft foundation under waveloads is developed.It can consider the effect of the difference of cyclic stress for different parts of the foun-dation on both the degradation strength of the foundation soil and the bearing capacity so that the esti-mated result can better reflect the real condition of foundation under cyclic loading.The method can be ap-plied to plane and space problem. 相似文献
5.
Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil. 相似文献
6.
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
7.
Havvanur Kılıç Saadet Arzu Berilgen Perihan Biçer Mustafa Yildirim 《Marine Georesources & Geotechnology》2013,31(4):303-323
Istanbul, the largest city in Turkey and one of the major metropolitan areas in the world, cleaned one of its environmentally polluted areas—Golden Horn—by dredging 5 million m3 of the bottom sediments and pumping the resulting sludge to a storage area behind a dam built at an abandoned rock quarry site in Alibey district. The reclamation of the land that formed over the storage area of Golden Horn dredged material is socially and economically very desirable. In this paper, results from experimental studies that are focused on determining the shear strength behavior of the dredge material and undisturbed soil are presented. Slurry consolidometer test, large model tests and small model tests are used to consolidate the dredged soil samples from Halic to simulate the natural consolidation behavior of these soils. Shear strength parameters are determined by laboratory vane tests; unconfined compression tests; undrained-unconsolidated (UU) and consolidated-undrained (CU) triaxial tests on samples that are obtained through in situ undisturbed samples and laboratory model tank and slurry consolidation. Moreover, the effects of fly ash and lime additives on the undrained shear strength were determined by mixing the materials with the dredged clay from Golden Horn during the model experiments conducted in the laboratory. Based on these findings, equations are proposed that govern the relationships between undrained shear strength and water content value. 相似文献
8.
Suction buckets are a promising foundation solution for offshore wind energy systems. The bearing behavior of monopod buckets under drained monotonic loading in very dense and medium dense sand is investigated in this study by means of numerical simulation with the finite element method. Special focus is given to the ultimate capacity and the initial stiffness of the bucket-soil foundation system. The numerical model is validated by comparison with field test results. The bearing behavior of the structure is explained through an evaluation of a reference system. It is shown that the bucket experiences a heave during horizontal loading, which leads to the formation of a gap between the bucket lid and the soil with increasing load. At large loads and rotations close to failure of the system there is no contact between lid and soil, and the whole load is transferred to the soil via the bucket skirt. A parametric study shows how the ultimate capacity and initial stiffness of the system depend on the bucket dimensions and loading conditions, i.e. load eccentricity. Normalized equations for ultimate capacity and initial stiffness are derived from the numerical simulation results, which can be used in the scope of a preliminary design for buckets in sand. 相似文献
9.
10.
A centrifugal model test was performed for an embankment backfilled with lime-stabilized soil on an undisturbed marine clay foundation. During the test, in-flight photographs were captured, settlements were measured by displacement sensors, and displacement contours were obtained from the markers installed on the front face of the model foundation. These test data were analyzed and discussed in this paper. The test results show that the embankment was stable at 2 m height but ruptured during the loading from 2 to 4 m height. The ratio of the maximum horizontal displacement increment to the ground settlement increment at the embankment centerline suddenly increased during the loading from 4 to 6 m height, indicating the failure of the foundation. This result is in agreement with the observation of the centrifugal test and the calculated Terzaghi ultimate bearing capacity under an undrained condition. Considering the brittle behavior and low tensile strength of the lime-stabilized soil, it is recommended that the lime-stabilized soil should only be used for a low embankment with a height less than 2 meters. 相似文献
11.
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
12.
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
13.
Suction buckets differ with their easy and cost-efficient installation technique from other foundation types for offshore wind turbines. For successful completion of their installation process, suction is essential, but the imposed seepage leads to the changes in states of the soil in and around the bucket. Especially, a loosening of soil inside the bucket affects the load carrying behaviour of bucket subjected to repetitive loading resulting from environmental conditions. In this study, the behaviour of buckets under cyclic axial compressive loads with considering a possible loosening and related changes in permeability of soil inside the bucket is investigated numerically. In the framework of finite element analysis, a fully coupled two-phase model and a hypoplastic constitutive model are used to describe the saturated sandy soil behaviour under repetitive loading. The porosity-permeability variation is taken into account by Kozeny–Carman relationship. Special attention is dedicated to load carrying behaviour of bucket top plate, inner and outer skirt as well as base and their changes resulting from a loosening of soil inside the bucket with variable aspect ratio. For this purpose, cyclic axial compressive loads which cause an attenuation and progressive failure of soil-bucket system response are considered. The main findings on the changes in load carrying behaviour of bucket are presented and discussed. 相似文献
14.
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
15.
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
16.
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
17.
土体在剪切变形过程中产生主应力方向的旋转时,主应变率方向与主应力方向之间存在着非共轴现象,然而,传统的弹塑性本构模型并不能考虑该现象的影响。通过在传统本构模型屈服面的切线方向增加一项非共轴塑性应变率,即可实现对非共轴现象的反映。利用有限元软件ABAQUS的材料子程序接口UMAT,通过显式积分算法和自动分步方法实现了非共轴模型在有限元分析中的应用。首先,对砂土的单剪试验进行数值模拟,预测了主应力方向和主应变率方向之间的关系,所得结论与试验结果较为吻合。然后,针对吸力桶与砂质地基间的相互作用问题进行弹塑性有限元计算,分析了土体主应力方向在剪切变形过程中的旋转规律,以及桶体的端部阻力、侧壁摩擦阻力和顶部阻力在变形过程中的变化规律。最后,检验了非共轴现象对地基承载力计算结果的影响。研究结果表明,所开发的非共轴模型对非共轴现象具有良好的预测能力。 相似文献
18.
The failure of marine structures is often attributed to liquefaction in loose sand deposits that are subjected to ocean waves. In this study, a two-dimensional integrated numerical model is developed to characterize the liquefaction behaviours of loosely deposited seabed foundations under various types of ocean waves. In the present model, Reynolds-Averaged Navier–Stokes (RANS) equations are used to simulate the surface wave motion, and Biot's consolidation equations are used to link the solid-pore fluid interactions in a porous medium. A poro-elasto-plastic solution is used to reproduce foundation behaviour under cyclic shearing. Unlike previous investigations, both oscillatory and residual soil responses were considered; they are coupled in an instantaneous approach. Verification of the model results to the previous centrifugal wave tests is carried out, obtaining fairly good agreement. Numerical examples show that foundation behaviour under various types of wave loading, particularly standing waves or a solitary wave, embodies a completely two-dimensional process in terms of residual pore pressure development. The parametric studies demonstrate that liquefaction caused by the build-up of pore pressures is more likely to occur in loosely deposited sand foundations with poor drainage and under large waves. 相似文献
19.
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