Undrained Capacity of a Bucket Foundation with a Limited-Tension Interface for Offshore Wind Turbines

In this paper,the influence of the limited-tension interface between lid and soil on the undrained bearing capacity of the wide-shallow bucket foundation is examined by finite element(FE)analysis.The interface between the lid and the soil is modeled using a simplified approach called the surface-based cohesive behavior,with the aim of simulating the limited-tension interface.Initially,the interaction between the lid and the soil is explored under the zero-and unlimited-tension conditions by small-scale experiments.Afterward,the effects of the embedment ratio,soil strength heterogeneity,and lid-soil interface on the bearing capacity are outlined,and the failure mechanisms are explained by FE analysis.A modified closed-form formula is given to compute the moment bearing capacity with the limited-tension interface between the lid and the soil for different embedment ratios and soil strength heterogeneities.The numerical results reveal that the existing approximating solutions,which assume fully bonded interaction,accurately exhibit the shape of the normalized failure envelopes in hm and vh load space for the limited-tension interface.However,the shape of the vm envelopes differs,requiring a novel solution to estimate the combined bearing capacity of the bucket foundation based on the embedment ratio and soil strength heterogeneity with a zero-tension interface between the lid and the soil.

Influence of Incomplete Soil Plugs on Bearing Capacities of Bucket Foundations in Clay

Due to the uneven seabed and heaving of soil during pumping,incomplete soil plugs may occur during the installation of bucket foundations,and the impacts on the bearing capacities of bucket foundations need to be evaluated.In this paper,the contact ratio(the ratio of the top diameter of the soil plug to the diameter of the bucket)and the soil plug ratio(the ratio of the soil heave height to the skirt height)are defined to describe the shape and size of the incomplete soil plug.Then,finite element models are established to investigate the bearing capacities of bucket foundations with incomplete soil plugs and the influences of the contact ratios,and the soil plug ratios on the bearing capacities are analyzed.The results show that the vertical bearing capacity of bucket foundations in homogeneous soil continuously improves with the increase of the contact ratio.However,in normally consolidated soil,the vertical bearing capacity barely changes when the contact ratio is smaller than 0.75,while the bearing capacity suddenly increases when the contact ratio increases to 1 due to the change of failure mode.The contact ratio hardly affects the horizontal bearing capacity of bucket foundations.Moreover,the moment bearing capacity improves with the increase of the contact ratio for small aspect ratios,but hardly varies with increasing contact ratio for aspect ratios larger than 0.5.Consequently,the reduction coefficient method is proposed based on this analysis to calculate the bearing capacities of bucket foundations considering the influence of incomplete soil plugs.The comparison results show that the proposed reduction coefficient method can be used to evaluate the influences of incomplete soil plug on the bearing capacities of bucket foundations.

Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load

The degradation strength of soils under cyclic loading is studied and a method for determining 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 wave loads is developed. It can consider the effect of the difference of cyclic stress for different parts of the foundation on both the degradation strength of the foundation soil and the bearing capacity so that the estimated result can better reflect the real condition of foundation under cyclic loading. The method can be applied to plane and space problem.

Bearing Capacity and Technical Advantages of Composite Bucket Foundation of Offshore Wind Turbines

Based on mechanical characteristics such as large vertical load, large horizontal load, large bending moment and complex geological conditions, a large scale composite bucket foundation （CBF） is put forward. Both the theoretical analysis and numerical simulation are employed to study the bearing capacity of CBF and the relationship between loads and ground deformation. Furthermore, monopile, high-rise pile cap, tripod and CBF designs are compared to analyze the bearing capacity and ground deformation, with a 3-MW wind generator as an example. The resuits indicate that CBF can effectively bear horizontal load and large bending moment resulting from upper structures and environmental load.

Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy load imposed by high-rise structures, due to the low settlement and high bearing capacity. In the optimum design of semi-deep foundations, prediction of the shaft bearing capacity, rs, of foundations socketed in rocks is thus critically important. In this study, the unconfined compressive strength(UCS), qu, has been applied in order to investigate the shaft bearing capacity. For this, a database of 106 full-scale load tests is compiled with UCS values of surrounding rocks, in which 34 tests with rock quality designation(RQD), and 5 tests with rock mass rating(RMR). The bearing rocks for semi-deep foundations include limestone, mudstone, siltstone, shale, granite, tuff, granodiorite, claystone, sandstone, phyllite, schist, and greywacke. Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS based on the types of rocks. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data. Since rock-socketed shafts are supported by rock mass(not intact rock), a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, qu(modified), based upon RMR and RQD in order to take into account the effect of the rock mass properties.

Lateral Bearing Capacity of Multi-Bucket Foundation in Soft Ground

The bucket foundation is a new type of foundation for offshore application to intermediate-depth waters. It has advantages over conventional ones. However, there is no consensus in the analysis and design of this type of foundation. In this paper, the lateral bearing capacity and the failure mechanism of multi-bucket foundations are studied with different connection stiffness and bucket spacing by use of a three-dimensional finite element method. Based on the numerical analysis results, a limit analysis method of plasticity for evaluating the lateral bearing capacity of large-spacing multi- bucket foundation with rigid connection in soft ground is proposed. This method provides a simple procedure that gives results comparable to those from the finite element analyses.

Probabilistic analysis of ultimate seismic bearing capacity of strip foundations

This paper presents a reliability analysis of the pseudo-static seismic bearing capacity of a strip foundation using the limit equilibrium theory. The first-order reliability method(FORM) is employed to calculate the reliability index. The response surface methodology(RSM) is used to assess the Hasofer e Lind reliability index and then it is optimized using a genetic algorithm(GA). The random variables used are the soil shear strength parameters and the seismic coefficients(khand kv). Two assumptions(normal and non-normal distribution) are used for the random variables. The assumption of uncorrelated variables was found to be conservative in comparison to that of negatively correlated soil shear strength parameters. The assumption of non-normal distribution for the random variables can induce a negative effect on the reliability index of the practical range of the seismic bearing capacity.

ULTIMATE LOAD-BEARING CAPACITY OF CYLINDER DERIVED FROM AUTOFRETTAGE UNDER IDEAL CONDITION

According to the basic theory on autofrettage and according to the 4th strength theory, several parameters and their relations are studied under ideal condition, including σej/σy, the equivalent stress of total stresses at elastoplastic juncture; σei/σy, the equivalent stress of total stresses at inside surface; σej＇/σy, the equivalent stress of residual stresses at elastoplastic juncture; σei＇/σy, the equivalent stress of residual stresses at inside surface; and p/σy, load-bearing capacity of an autofrettaged cylinder. By theoretical study on relations between the parameters, noticeable results and laws are achieved： to satisfy ｜σei＇｜=σy. the relation between kj and k is, k^2lnkj^2-k^2-kj^2＋2=0, when k→∞, kj = √e = 1.648 72, as based on the 3rd strength theory, where k is the outside/inside radius ratio of a cylinder, kj is the ratio of elastoplastic juncture radius to inside radius of a cylinder; If the plastic region covers the whole wall of a cylinder, for compressive yield not to occur after removing autofrettage pressure, the ultimate k is k=-2.218 46 as based on the 3rd strength theory; With k=2.218 46, a cylinder＇s ultimate load-bearing capacity equals its entire yield pressure, or p/σy=21nk/√3; The maximum and optimum load-bearing capacity of an autofrettaged cylinder is just 2 times the loading which an unautofrettaged cylinder can bear elastically, or p/σy=2（k^2-1）/√3 k^2, and the limit of the load-bearing capacity of an autofrettaged cylinder is also just 2 times that of an unautofrettaged cylinder. The conclusions are the same as based on the 3rd strength theory, but some equations are different from each other.

New design equations for estimation of ultimate bearing capacity of shallow foundations resting on rock masses

Rock masses are commonly used as the underlying layer of important structures such as bridges, dams and transportation constructions. The success of a foundation design for such structures mainly depends on the accuracy of estimating the bearing capacity of rock beneath them. Several traditional numerical approaches are proposed for the estimation of the bearing capacity of foundations resting on rock masses to avoid performing elaborate and expensive experimental studies. Despite this fact, there still exists a serious need to develop more robust predictive models. This paper proposes new nonlinear prediction models for the ultimate bearing capacity of shallow foundations resting on non-fractured rock masses using a novel evolutionary computational approach, called linear genetic programming. A comprehensive set of rock socket, centrifuge rock socket, plate load and large-scaled footing load test results is used to develop the models. In order to verify the validity of the models, the sensitivity analysis is conducted and discussed. The results indicate that the proposed models accurately characterize the bearing capacity of shallow foundations. The correlation coefficients between the experimental and predicted bearing capacity values are equal to 0.95 and 0.96 for the best LGP models. Moreover, the derived models reach a notably better prediction performance than the traditional equations.

Effect of Optimum Plastic Depth on Stresses and Load-bearing Capacity of Autofrettaged Cylinder

Autofrettage is an effective measure to even distribution of stresses and raise load-bearing capacity for （ultra-）high pressure apparatus. Currently, the research on autofrettage has focused mostly on specific engineering problems, while general theoretical study is rarely done. To discover the general law contained in autofrettage theory, by the aid of the authors’ previous work and according to the third strength theory, theoretical problems about autofrettage are studied including residual stresses and their equivalent stress, total stresses and their equivalent stress, etc. Because of the equation of optimum depth of plastic zone which is presented in the authors’ previous work, the equations for the residual stresses and their equivalent stress as well as the total stress and their equivalent stress are simplified greatly. Thus the law of distribution of the residual stresses and their equivalent stress as well as the total stress and their equivalent stress and the varying tendency of these stresses are discovered. The relation among various parameters are revealed. The safe and optimum load-bearing conditions for cylinders are obtained. According to the results obtained by theoretical analysis, it is shown that if the two parameters, namely ratio of outside to inside radius, k, and depth of plastic zone, kj, meet the equation of optimum depth of plastic zone, when the pressure contained in an autofrettaged cylinder is lower than two times the initial yield pressure of the unautofrettaged cylinder, the equivalent residual stress and the equivalent total stress at the inside surface as well as the elastic-plastic juncture of a cylinder are lower than yield strength. When an autofrettaged cylinder is subjected to just two times the initial yield pressure of the unautofrettaged cylinder, the equivalent total stress within the whole plastic zone is just identically equal to the yield strength, or it is a constant. The proposed research theoretically depicts the stress state of ultra-）high pressure autofrettaged cylinder more accurately and more reasonably and provides the reference for design of （ultra-）high pressure apparatus.

Characteristic Test Study on Bearing Capacity of Suction Caisson Foundation Under Vertical Load

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.

Upper Bound Solutions for Uplift Ultimate Bearing Capacity of Suction Caisson Foundation

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.

Effect of Bearing Capacity on Designing Foundations in Iraq Using STAAD Pro-v8i

Most of Iraqi soil is classified as Quaternary deposits, especially in the Mesopotamian plain and tributaries of the River Tigris. Soil varies from north to south of Iraq. These differences in soil affected the process to select the suitable type of foundation. This research is to study the effect of bearing capacity on shallow foundations in different regions of Iraq. Seventy nine samples were collected from 23 boreholes at three different locations (Mosul at the North, Baghdad at the middle and Basrah at the south of Iraq). The samples were collected at varying depth between 1 to 24 m. They were subjected to the following testes: Atterberg limits, sieve and hydrometers, consolidation, direct shear, unconfined compression and the filed (SPT test). The values of the bearing capacity parameters ( and c) were obtained from the above tests. The results obtained were used in the application of the general equation of the bearing capacity. Then, the model of a building was designed (two floors, with mat foundation type) using STAAD Pro software. The average values of bearing capacity in each region were applied in the program (Mosul = 177 KPa, Baghdad = 125 KPa and Basrah = 84 KPa). In addition, the worst bearing capacity values were also used for the three regions (Mosul = 77 KPa, Baghdad = 68 KPa and Basrah = 24 KPa). The results obtained from the average and worst bearing capacity indicated that for Mosul, we could use shallow foundation (spread and mat used if there was basement) for different areas and for buildings with many stories. For Baghdad region, shallow foundation was more suitable for building not higher than five stories. Finally, for Basrah region, shallow foundations were an appropriate selection, but for most areas deep foundation was the right choice.

Upper Bound Solution of the Resisting Moment Bearing Capacity of A Composite Bucket Shallow Foundation of An Offshore Wind Turbine in Sand

Bearing the large moment that is generated by the wind load that acts on the upper structure of offshore wind turbines is an important feature of their foundations that is different from other offshore structures.A composite bucket shallow foundation(CBSF)has been proposed by Tianjin University to address the soft geological conditions in the offshore regions of China for wind turbines.The CBSF is a new type of foundation and is effective against large moments.The soil deformation test of a CBSF and the numerical simulation study under the same working conditions are carried out to determine the failure mechanism of a CBSF under moment loading.The resisting soil compression rateηm is defined as a new empirical parameter that indicates the ability of the soil inside the bucket to resist moment loading.The upper limit of the resisting moment bearing capacity of the bucket foundation is derived through the upper bound theorem of classical plasticity theory based on the failure mechanism.The calculation method is validated by tests of bucket models with different height-diameter ratios in sand under moment loading.

Numerical analysis on bearing capacity of middle pylon caisson foundation of Taizhou Bridge

Because of the computation difficulty of the beating capacity of large underwater caisson foundation on thick overburden layer ground, the geotechnieal software FLAC3D was utilized in the 3D numerical analysis on the bearing capacity of middle pylon foundation. From the computational results, it is concluded that the caisson foundation has a good bearing capacity on thick overburden layer ground and the beating capacity can be improved assuming that the soil near the area of basal comer is reinforced.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Reply to Discussion on “Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks”

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy loads imposed by high-rise buildings and special structures, due to the low settlement and high bearing capacity. In this study, the unconfined compressive strength（UCS） and rock mass cuttability index（RMCI） have been applied to investigating the shaft bearing capacity. For this purpose, a comprehensive database of 178 full-scale load tests is compiled by adding a data set（n = 72）collected by Arioglu et al.（2007） to the data set（n = 106） presented in Rezazadeh and Eslami（2017）.Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS/RMCI. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data（series 3 in Rezazadeh and Eslami（2017））. Since rock-socketed shafts are supported by rock mass（not intact rock）,a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, based upon RMR and RQD to consider the effect of the rock mass properties.

Semi-Deep Skirted Foundations and Numerical Solution to Evaluate Bearing Capacity

Semi-deep foundations are a remarkable solution in conditions where the soil beneath the foundation is loose to a great depth and there is no possible way to use any way of soil improvement and applying piles would not be a logical way considering their cost and time of enforcing. Skirted foundations are a type of semi-deep foundations that can penetrate to the soil up to two times of their breadth. Estimating bearing capacity of these foundations is a long geotechnical problem for engineers whether under absolute or combined loading because of their usage in offshore and onshore projects. For estimating the vertical bearing capacity of these foundations, series of finite element analyses were performed for a range of embedment ratios to investigate the effect of the length of the skirt. The foundation has been modelled with two different types of soil and the results validated with previous analytical, numerical and experimental researches. In addition, the bearing capacity of a skirted foundation under combined loading in V-H space has been analyzed by this approach and the 2-dimentional failure envelope has been presented.

Methods for Estimating the Ultimate Bearing Capacity of Layered Foundations

The Meyerhof and Hanna′s(M-H) method to estimate the ultimate bearing capacity of layered foundations was improved. The experimental results of the load tests in Tianjin New Harbor were compared with predictions with the method recommended by the code for the foundations of harbor engineering, i.e. Hansen′s method and the improved M-H method. The results of the comparisons implied that the code and the improved M-H method could give a better prediction.

Comparison of seismic capacity/demand ratios of two kinds of foundation schemes of Taizhou Bridge

Pile group foundation and caisson foundation are two common foundation schemes of long-span bridges, and the seismic performances of the two kinds of foundations are different. Taking Taizhou Bridge as an example, which is the first kilometer level three-pylon two-span suspension bridge in the world, two foundation schemes are designed for the middle pylon, and two whole bridge models with two different foundation schemes of the middle pylon are established respectively in this paper. The effects of foundation-soil interaction are simulated by equivalent linear soil springs whose stiffnesses are calculated according to m method. Seismic capacity/demand ratios of the two models are calculated. The following conclusions can be drawn: the weak positions of the two schemes are not the same; if caisson foundation is adopted for the middle pylon, the weak position is the bearing capacity of the middle pylon foundation, while if pile group foundation is adopted for the middle pylon, the weak position is the bearing capacity of the side pylon foundation.