Study On Bearing Characteristics Of Wind Turbine Cap-Pile Foundation In Loess Region

In the Loess Plateau,the wind turbine sets generally use pile foundation,the pile is worn over collapsible soil layer to non-collapsible old loess layer,the pile top load is transferred to the foundation soil,and the pile end is enlarged.With the increasing of wind turbine capacity,hub height and impeller diameter in the Loess Plateau,the design and optimization of the foundation become more and more important.Based on the example of Dingbian wind farm project in Northern Shaanxi,through site investigation,test pile test and indoor geotechnical test,relevant parameters such as pile foundation,foundation soil,backfill soil and so on are obtained.The wind load of 2MW wind turbine under cutting wind speed(the most unfavorable wind load)is calculated.Combined with the design data,the finite element model of upper wind power generation structure-cap-pile foundation is established by Abaqus.The reliability of the model is verified by comparing the numerical simulation results with the site settlement monitoring data.The wind load is applied to the finite element model and a series of studies are carried out.The main contents and results are as follows:Change the order on the cap pillars thickness,diameter and thickness of the pillar under the order,the diameter of pile settlement,and the variation of inner force of pile cap and pile of concrete backfill variation amount were studied.Results showed that:(1)a thickness on the order increases,the amount of concrete pile caps is decreasing,but will "change slope under the pressure of cap,variable slope thickness is insufficient,the leeward side of the punching failure will occur.With the decrease of the thickness of a order,the thickness of slope variable enough transfer force,but the overall tilt increases,increasing the uneven settlement.(2)the influence of the diameter of theupper stage column on the overall settlement of the cap is less than that of the uneven settlement..With the increase of the diameter of the upper column,the internal force of the superstructure can be diffused to the outer part of the cap,which is helpful to reduce the uneven stress difference on the bottom of the cap.With the increase of the diameter of the upper column,the backfill earthwork will be reduced,and the amount of concrete will increase,and the relative increase of the cost.(3)with the increase of the thickness of the lower step column,the cap will have enough stiffness to transfer internal force,which can significantly reduce the stress value and stress difference of the bottom surface.When the thickness of the next step column is too large,the overall stiffness of the foundation will increase,the settlement will increase,and the concrete consumption will increase significantly.In the design,careful consideration should be given to increasing the thickness of the lower step column.(4)increase the diameter of the lower step column.There are no obvious benefits to the settlement and the internal force distribution of the bearing platform,but it is not economical to increase the amount of concrete used and the amount of backfill soil.As the fan is located at the position of the loess ridge,the excavation size of the foundation pit is often limited,so the diameter of the lower order column should be reduced as much as possible.The minimum edge pick-out part of the lower order column of the bearing platform should meet the construction requirements,and the pile diameter value is reasonable.The settlement of pile cap and the change rule of pile top reaction force are studied by changing the layout of pile.The results show that more piles are arranged in the outer ring to reduce the uneven settlement of the cap foundation.After the internal ring pile is arranged,the distance between the piles decreases obviously,which can reduce the load sharing of soil under the cap,give full play to the bearing capacity of the pile,improve the integrity and stability of the fan foundation,and reduce the span of the cap relatively.The modulus reduction method is used to simulate the collapsibility of loess in the site.The distribution and variation of the axial force of the pile body and the friction on the side of the pile are studied.The results show that the negative friction is produced after the soil collapses and the axial force of the pile body increases obviously.The cloud map of the foundation deformation shows that the soil and the bottom surface ofthe cap are empty and the pile is exposed outside.In this example,the base pile is used to pass through the collapsible soil layer,and the slope ratio of the foundation can still meet the requirements of the specification,but it is very close to the standard value,and there are hidden dangers in safety.Therefore,the waterproof should be paid more attention to during the design,construction and operation.