Bucket Foundations In Sand Research Articles

Experimental study of local scour around bucket foundation in sand

Laboratory flume experiments were conducted to study scour characteristics around bucket foundation under current only condition and current-wave condition. Scour topography from the experiments shows that the evolution of scour can be divided into three stages, rapid scouring stage, slow scouring stage, and scouring equilibrium stage. Semicircular scour pit occurs in front of the bucket and herringbone siltation appears behind the bucket. Scour depth of the mono-bucket was compared with that of the unsubmerged monopile and submerged monopile. Slope of the scour pit is found to be smaller than the predicted value of the monopile. Maximum scour depth of the tripod-bucket foundation is shown to be affected by the factors including the ratio of exposed height to diameter H/D, the relative bucket spacing G/D, and the incoming flow angle α. The maximum scour depth increases with the increase of H/D. Scour depth of the inner side of the tripod-bucket foundation is shown to be significantly affected by G/D due to the jet effect. The rear bucket may be influenced by shielding effect or vortex shedding. And the incoming angle has influence on the scour development around tripod-bucket foundation.

Bearing characteristics of mono-column composite bucket foundation in sand for offshore wind turbines

Using a new steel compartment structure, the mono-column composite bucket foundation applied in the South China Sea deep-water project for the first time is based on the optimization of the steel-concrete composite bucket and is suitable for wind farms with a water depth of more than 30m. However, at this stage, there are few studies on large-diameter mono-tube multi-compartment bucket foundations, especially those related to the bearing characteristics of mono-column composite bucket foundations after long-term service. In this study, a cyclic load test is carried out on a mono-column composite bucket foundation in sand, and the influence of cyclic load factors on the cumulative rotation angle, cyclic stiffness, and ultimate bearing capacity after cyclic loading of the foundation is analyzed, and the bearing characteristics of the foundation are analyzed by finite element analysis. The results show that after the cyclic loading, the bearing capacity and stiffness have a certain increase compared with before the cyclic loading in sand. The ultimate bearing capacity calculation formula based on the Mohr-Coulomb strength criterion derived from the limit state of the foundation failure has higher reliability compared with the test results and can be used to predict the bearing capacity of the mono-column composite bucket foundation.

Suction penetration characteristics and resistance calculation of bucket foundation in sand

• Based on the suction penetration test of bucket foundation, the distribution regularity of the excess porewater pressure of soil and soil pressure on both sides of bucket wall and bucket end are revealed. • Based on finite element method, the seepage characteristics of the bucket foundation during the penetration process are analyzed, and the change regularity of the effective stress and resistance of soil during the penetration process of bucket foundation is obtained. • Considering the effect of effective stress on seepage, a calculation method for required suction of bucket foundation with few easy-obtained parameters is established. The accurate estimation of penetration resistance of the bucket foundation is the key step for its installation, and the core part of the estimation of the penetration resistance is the exact prediction of the drag reduction effect due to suction. Therefore, in the present paper, the change regularity of the excess porewater pressure along the bucket wall and at the tip is analyzed by combining the model test and finite element analysis method, along with the soil pressure of the bucket foundation during the suction penetration process in sand. The results showed that, during the suction penetration process of the bucket foundation, the soil pressure and excess porewater pressure change approximately linearly with the depth of the bucket wall, and the reduction degree of lateral resistance due to seepage increases in a power function growth trend. Based on the effective stress principle of soil and basic theory of seepage and considering the effect of seepage on penetration resistance, the change regularity of the effective stress of soil along the bucket wall during the penetration process is obtained. Besides, the calculation method for penetration resistance and required suction of bucket foundation in sand soil is established, and the effectiveness of the present method is verified by comparing with the model test and engineering case, which can provide theoretical basis for engineering design and construction of bucket foundation.

Effect of drainage condition on the pullout characteristics of bucket foundations in sand

A series of 1 g experimental tests was conducted to investigate the effect of drainage condition on the pullout characteristics of bucket foundations in sand. Model buckets with a constant diameter of 150 mm and length-to-diameter ratios of 1.5, 1.0 and 0.5 were employed with pullout rates ranging from 0.05 mm/s to 20 mm/s. The phenomena of heaving, gapping below the lid, and suction pressure have been investigated and discussed. The peak pullout load, peak suction pressure and heave increased with the increase in the pullout rate and L/D. The drainage factor was proposed to classify the drainage conditions during bucket pullout. The drainage factor is a function of the pullout rate and the inflow velocity. The inflow velocity converged to a nearly constant value defined as the critical inflow velocity. The pullout rates beyond the critical inflow velocity induced undrained conditions. The drainage conditions were classified as drained, partially drained, and undrained based on the proposed drainage factor. The drainage factor criterion reasonably classified the drainage conditions induced at peak pullout load.

Research on sinking and leveling control technology of tripod bucket foundation in sand

High capacity wind-turbines are increasingly installed offshore and further away from the coast, where strong foundations are required, and their installation is challenging. The so-called multi-bucket foundation is a promising candidate to be used for such scenarios. However, the sinking control technology, which is commonly used to ease the installation, is not very mature. In this paper, a series of theoretical studies and tests are carried out to investigate the sinking process of a wide-shallow tripod bucket foundation (WTBF). First, the mechanism of seepage-failure of a WTBF at different depths is investigated using both empirical formulas and experiments. In addition, the optimal method to calculate/predict seepage-failure of a WTBF in the sand and the value range of the kfc coefficient are determined. Then, an experiment to determine the maximum adjustable initial tilt-angle (based on seepage failure control) is conducted. This enables the improvement/determination of the control index and control methods for seepage failure as well as the tilt-angle during the sinking process. Finally, an automatic control system for the sinking and leveling of the multi-bucket foundation, which uses a dual-index control method (seepage failure, leveling angle) is developed. The feasibility of this approach is verified through experiments.

Effects of embedment depth on the pullout capacity of bucket foundations in sand

A series of 1 g experimental tests was conducted to investigate the effects of embedment depth on the pullout capacity of bucket foundations in sand. Polycarbonate model buckets with a constant diameter of 150 mm and length-to-diameter (L/D) ratios of 0.5, 1.0 and 1.5 were employed varying loading rates from 0.002 N/s to 4.1 N/s. The slow loading rate of 0.002 N/s induced drained conditions characterised with substantially small suction force and high skirt resistance. The fast loading rate of 4.1 N/s induced nearly undrained conditions characterised by large suction force and almost full uplift of the soil plug. The pullout capacity increased almost linearly with the L/D ratio. A 50% increase in embedment depth resulted in an average increase in pullout capacity of 40%. The pullout capacity and suction force inside the bucket rapidly increased with the loading rate in the partially drained condition, and converged to nearly constant values in the undrained condition. In the nearly undrained condition, the suction force at the tip was approximately half of that under the lid. The simplified approach reasonably predicted the capacities of buckets with various L/D ratios.

Cyclic lateral response of OWT bucket foundations in sand: 3D coupled effective stress analysis with Ta-Ger model

Bucket foundations have been increasingly used to support offshore wind turbines as alternatives to monopiles. Despite the substantial research effort on the bearing capacity and stiffness of such foundations in recent year, there is still a lack of knowledge regarding their cyclic response in saturated sand. In this paper, the multiaxial sand constitutive model Ta-Ger implemented in the finite deference code FLAC3D is employed in the analysis of the lateral response of bucket (skirted) foundations subjected to wind/wave loading. The model is reformulated to reproduce the cyclic response of sand for undrained, fully drained and partially drained conditions, using a unique set of calibration parameters. Having been calibrated against laboratory data available in literature, it is then used to predict the long-term cyclic lateral response of a bucket foundation in dry medium dense sand from a centrifuge experiment. After building confidence in the numerical approach the drainage effects are investigated. To gain qualitative insights into the effect of drainage conditions, the 3D numerical model, used to simulate the centrifuge test, was analyzed under saturated conditions and a range of soil permeabilities. It was shown that when flow is allowed the response up to a number-of-cycles threshold resembles that of fully drained conditions. Above this threshold, significant and abrupt increase of excess pore water pressures occurs causing liquefaction. Increasing the permeability delays the occurrence of liquefaction and the associated development of large deformations

Effect of loading rate on the pullout capacity of offshore bucket foundations in sand

A series of 1 g model tests was performed to investigate the effect of loading rate on the vertical pullout capacity of offshore bucket foundations in sand. The bucket model had a length-to-diameter ratio of 1.0. Loading rates varied from 0.004 N/s to 2.05 N/s. The pullout load increased with bucket displacement and loading rate. The loading rate of 0.004 N/s induced drained conditions with negligible suction pressure and high skin resistance. Increasing the loading rate induced high suction pressures inside the bucket. The bucket pullout capacity increased with bucket displacement and loading rate. Suction pressure increased with bucket displacement and loading rate and contributed considerably to the pullout capacity. The bucket resistance to pullout load increased with the increase in loading rate. Equations were proposed to evaluate the drained and undrained pullout capacities of the bucket.

Large-scale laboratory tests of a new type of bucket foundation in sand subjected to vertical loading

As a new breakwater foundation form, a bucket foundation with two skirted side semicircles and a skirted middle rectangle was applied to the breakwater project at Xuyu Port, China. A series of laboratory tests were performed on the bucket foundation to investigate the vertical bearing capacity and the bucket-sand interaction mechanisms during the load bearing process. The main failure mode of the bucket foundation under vertical loading was general shear failure. The vertical loads increased with penetration displacement, ultimately reaching 330~404 kN with a displacement criterion of 0.06De (equivalent diameter of the foundation) at different incremental load steps. The inner soil pressures were clearly greater than the outer soil pressures induced by the skirt constraint effect. Regarding the bearing capacity, the outer skirt friction decreasing from approximately 0.05 to 0 with increasing penetration displacement, contributed only slightly, while the foundation base provided the majority. The cover plate resistance increased rapidly after the penetration displacement exceeded 25 mm, and its sharing ratio ultimately reached approximately 0.82. The tip resistance was estimated via the assistance of the cone resistance, and its sharing ratio decreased with penetration displacement. The inner friction and corresponding friction coefficient first increased and then decreased with penetration displacement, reaching a final sharing ratio of approximately 0.14.

Model Test Study on Horizontal Static Loading of Suction Bucket Foundation under Different Scour Conditions

Abstract Under a scouring action, scouring holes occur around a suction bucket foundation and have different depths and widths. The loss of soil in scouring holes causes changes in the stress history of the remaining soil. In this study, considering the stress history, scouring depth, scouring width, and length-diameter ratio, we conducted a horizontal static loading model test of a suction bucket foundation in sand. We obtained a variation law for the ultimate bearing capacity of the foundation considering three aspects: displacement change, soil pressure distribution, and turning point position. In addition, we summarized the relationship between horizontal loads and displacements at different aspect ratios. We also demonstrated the influence of scouring factors on the distribution of soil pressure and the rotation point and revealed the mechanism influencing the scouring depth, scouring width, and history of stress on the horizontal bearing capacity of a suction bucket foundation.

Jacking penetration resistance and mechanical characteristics of bucket foundations in sand

Model tests have been performed to investigate the penetration and tip resistance and inner and outer earth pressure of bucket foundations embedded in sand with the jacking installation method. The soil plug, the soil-skirt interaction, and the penetration resistance of the bucket are determined by these tests. Based on the soil arch theory, analytical expressions of the inner soil pressure and the penetration resistance for the bucket are presented and verified by the test results. The results show that the soil plug begins to form when the penetration displacement reaches 1 times the width of the bucket, and the incremental filling ratios of each bucket are 0.65 to 0.7 and 0.8 at the end of penetration. The inner earth pressure of the bucket is in the form of a double broken line, which is same as the open-ended piles. The theoretical formula of the penetration force considering the soil arching effect can predict the penetration force well, and the ratio of the soil arch resistance to the total resistance during the penetration is up to 85%, which indicates that the soil arch resistance plays an important role in the total penetration resistance during the jacking installation process.

Analysis of laterally loaded bucket foundation with external skirt in sand using a Winkler model approach

An investigation is made to present analytical solutions provided by a Winkler model approach for analysis of a bucket foundation with external skirt subjected to lateral loads in sand. The bucket foundation with external skirt consists of an internal compartment and an external short skirted compartment. The lateral stiffness coefficient along the skirt of the bucket foundation in nonhomogeneous soils employing the Mindlin solution for lateral loads is presented. The relationship between horizontal displacement, rotation, moment and shear force of the bucket foundation subjected to lateral loads in nonhomogeneous soils is obtainable in the form of the recurrence equation. The vertical, lateral and rocking stiffness coefficients on the base of the bucket foundation are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of a soil. An envelope of the combined ultimate horizontal and moment load capacities of the bucket foundation in sand is shown. The relationship between ultimate lateral and moment load capacities and aspect ratios (internal and external skirt-lengths to diameter) is presented. Behavior of the bucket foundation with external skirt in sand up to failure is clarified from the relationship between lateral loads and displacement and that between depth and lateral pressure.

Experimental investigation of the vertical pullout cyclic response of bucket foundations in sand

A series of 1g model tests was conducted to investigate the accumulated vertical pullout displacement and unloading stiffness of bucket foundations embedded in dry and saturated sands. The foundations were subjected to vertical pullout cyclic loading with different load amplitudes. Cyclic load was applied up to 104 cycles. Test results showed that the accumulated vertical pullout displacement increased with the increase in the number of load cycles and cyclic load amplitudes. The unloading stiffness of the bucket foundations decreased with the increase in load amplitude and number of cycles. Empirical equations were proposed based on the test results to evaluate the accumulated vertical pullout displacement and unloading stiffness of the bucket foundations in saturated sand. These equations can be used for the preliminary design of single or tripod bucket foundations.

Evaluation of horizontal and moment bearing capacities of tripod bucket foundations in sand

Bearing capacities of tripod bucket foundations differ from those of single bucket foundations because of the interaction among individual buckets of the tripod. This study analyzed the bearing capacities of tripod bucket foundations in medium and dense sands by performing a series of 3D finite element analyses. The sandy soil was modeled by using an elasto-plastic model following the Mohr-Coulomb failure criterion. The Young's modulus of the sands increased with depth because of the stress-dependency of the modulus. Parametric studies were performed, in which the spacing between each bucket, the embedded depth of the bucket, the bucket diameter, and the magnitude of vertical loading were varied. Results showed that the horizontal bearing capacities of tripod bucket foundations reached the maximum values at a spacing ratio of S/D=1.5 to 3.5 for the foundation with an embedment ratio of L/D=0.5 to 1 (where S is the bucket spacing, D is the bucket diameter, and L is the skirt length of the tripod bucket). However, the moment bearing capacities increased linearly as the S/D ratios increased. Finally, bearing capacity equations were proposed in consideration of bucket spacing, embedment depth, effect of foundation diameter and vertical load, and soil density.

Centrifuge modeling of lateral bearing behavior of offshore wind turbine with suction bucket foundation in sand

Suction bucket foundation is a promising alternative for offshore wind turbine foundations. The lateral bearing behavior and failure mechanism are significant topics for optimizing its design criteria. In this study, a group of centrifuge tests were performed to investigate the lateral bearing capacity of suction bucket foundation with three aspect ratios. Force-controlled lateral static and cyclic tests were performed at a centrifuge acceleration of 50g. Four soil conditions were considered in centrifuge tests with a combination of loose/dense and dry/saturated sand. The load-displacement and the stiffness-displacement relationships are presented in the paper, and it is concluded that the lateral bearing capacity can be reached when the normalized lateral displacement (displacement divided by the bucket diameter) reached 3% as the first method. Displacement rates are plotted against the lateral load to give the second method for defining the ultimate lateral capacity. In cyclic tests, the lateral displacement and stiffness are correlated to the cycle numbers in the first 5 cycles, but the change is less apparent in the rest. The results demonstrated the behavior of bucket foundation in service conditions. Finally, a simplified calculation method is used as the third method and checked with tests results.

Vertical bearing capacity of bucket foundation in sand overlying clay

We perform a suite of axisymmetric finite element analyses to study the load transfer mechanism and predict the vertical bearing capacity of bucket foundations in sand overlying clay profiles. We use elasto-plastic soil models following Mohr-Coulomb and Tresca failure criteria for sands and clays, respectively. A non-associated flow rule is used with the Mohr-Coulomb model to realistically simulate the dilatant behavior of sands. A broad range of strength parameters for both sands and clays are used. The aspect ratio and bucket tip-to-clay depth (d) are also varied. The traces of plastic shear strain increments are monitored to observe how the vertical load is transmitted to underlying sand and clay layers, and to study the associated failure surface. The shape of the failure surface is shown to be not unique and display complex pattern conditional on strength parameters, aspect ratio, and d. We present a suite of equations to predict the vertical base capacity. The proposed set of equations provide enhanced estimate of the base capacity compared with theoretical models. We also calculate the shaft resistance of the bucket foundations on multi-layered profiles. The shaft resistance can reliably estimated from an empirical equation developed for bucket foundations in uniform sand.

Stiffness of Bucket Foundation in Sand

Stiffness of Bucket Foundation in Sand

Vertical bearing capacity of bucket foundations in sand

Bucket foundations are widely used to support offshore structures. The vertical capacity of bucket foundations in clays has been extensively studied. However, the unique load transfer characteristics of bucket foundations in sands are not well documented. We perform a series of two-dimensional axisymmetric finite element analyses using the Mohr-Coulomb model following a non-associated flow rule. Because the depth-to-diameter ratio of a bucket foundation is much lower compared with that of a pile, a higher level of arching is shown to occur in the soil. A pronounced increase in the horizontal stress and a corresponding increase in the shaft resistance (Qs) are observed. The arching also causes the failure surface to widen due to additional vertical stress imposed on the soil. Based on numerical results, predictive equations for Qs and a combined shape and depth factor for the base capacity (Qb) are proposed for bucket foundations in sands that follow a non-associated flow rule.

Load-bearing behavior of suction bucket foundations in sand

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.