Bearing Behavior Of Cone-shaped Hollow Flexible Reinforced Concrete Foundation For Mountain Wind Turbines

Shortage of energy,environmental pollution and greenhouse effect are three serious issues.To solve these problems,the people have to seek clean energy and reusable energy.Developing onshore wind power becomes one of effective ways to tackle such problems.The wind turbine construction in mountainous areas exhibiting no occupation of the farmland and low investment attracts more attention.Gravity-based foundation has been the most popular for mountain wind turbines.However,it has several disadvantages:large amount of the reinforced concrete is needed,and the mass concrete construction requirements are relatively complex and more expensive.Besides,a large amount of rubbles and soils will be excavated from the foundation pit,destroying the vegetation around the wind turbine.This dissertation presents an innovative type of a mountain wind turbine:the cone-shaped hollow flexible reinforced concrete foundation(CHFRF),consisting of a top plate,a base plate and the side wall that are made of reinforced concrete.The inner cavity of the CHFRF is filled with gravel and soil directly from the excavation for the CHFRF,which means that the CHFRF can reduce the dosage of cement content and steel.Furthermore,a thin rubber flexible layer is placed between the foundation and the surrounding soil to increase the foundation flexibility to resist cyclic or dynamic loading.In this dissertation,model tests and numerical simulations were conducted to study the monotonic and cyclic lateral bearing capacity of the CHFRF in sand or strongly-weathered layer.Besides,the bearing behavior of the CHFRF under combined loading was analyzed.Test and simulation results help to guide the CHFRF design.The main work and conclusions are obtained as follows:(1)A series of small-scale model tests was conducted to investigate the lateral bearing capacity of the CHFRF under various load eccentricity ratios,foundation sizes,loading rates and rubber flexible layers,respectively.It is concluded that the rotation center of CHFRF is located at a depth of about 0.45-0.66 length of CHFRF and is about 0.28-0.45 diameter of the foundation away from its center line in the loading direction.The CHFRF can provide 25%more bearing capacity than the corresponding conventional circular gravity-based foundation under their same self-weight.Furthermore,the CHFRF significantly reduces the lateral deflection and increases the rebound compared with these of the regular circular gravity-based foundation.Results also indicate that the rubber flexible layer can effectively decrease the earth pressure acting on the soil and limit the accumulated rotation of the CHFRF under lateral loading.In addition,based on the failure mechanism and the distribution of earth pressure along the outside wall of the CHFRF,an expression to estimate the lateral capacity of the CHFRF was proposed.(2)Effects of the cyclic loading level,the loading cycles,rubber flexible layer and vertical pressure on the accumulated deformation and the cyclic secant stiffness of the CHFRF were obtained in terms of the results of the cyclic loading tests.It is found that,under the horizontal cyclic loading,the accumulated displacement is about 28%smaller than the circular gravity-based foundation.With the cyclic load ratio and the loading frequency increasing,horizontal and vertical displacements of the CHFRF increase significantly,resulting in the CHFRF moving upwards.On the other hand,the rubber flexible layer placed between the foundation and the surrounding soil can decrease the deformation and the cyclic secant stiffness of the CHFRF,and significantly reduce the earth pressure acting on the soil.The results indicate that the stability of the CHFRF is enhanced compared with the regular circular gravity-based foundation during the cyclic loading.(3)A series ot three-dimensional numerical models of the CHFRF were constructed to study the bearing behavior of the CHFRF under various combined loading conditions.Furthermore,effects of the aspect ratio,cone angle and embedded depth of the CHFRF,and the rubber flexible layer on the bearing behavior of CHFRF were explored.In addition,the failure envelope in a three-dimensional load space of three components was established,and then an expression to estimate the failure envelope of the CHFRF was proposed.Besides,the cyclic bearing capacity of the CHFRF was investigated.The results show that the cyclic bearing capacity of the CHFRF decreases about 35%compared with that of bearing capacity under combined loading.Besides,the rubber flexible layer decreases the lateral bearing capacity of the CHFRF,while the effect of rubber flexible layer on the failure envelope of the CHFRF under combined loading is not obvious.(4)Effects of soil strength parameters,the thickness and the hardness of the rubber flexible layer on the lateral bearing capacity of the CHFRF were studied by using the numerical simulation.The results indicate that the internal friction angle and cohesion of soil have a strong influence on the lateral bearing capacity of the CHFRF.In addition,it was found that,when the hardness of rubber flexible layer is 43?60,and the thickness of rubber flexible layer is 0.008?0.016 times diameter of the CHFRF,the maximun energy dissipation percentage of the rubber flexible layer is approximately 180%.Moreover,the energy dissipation of the rubber flexible layer increases with increasing the ratio of soil elastic modulus to the rubber elastic modu us.(5)Based on the simulation results,an expression to estimate the required diameter of the CHFRF was proposed,and the isolated area of the CHFRF under extreme wind loading was also formulated.Furthermore,the maximum settlement and the tilting of the CHFRF were calculated using three-dimensional finite element models.Results from this dissertation provide strong theoretical supports for practical engineering in design and construction.We definitely assure that this novel foundation must have a prosperous future in wind farms.