Study On Lateral Bearing Mechanism And Calculation Method Of Rigid Single Pile And Two-piles On Sand Slope

Pile foundations play an irreplaceable role in large-scale structures such as subways,railways,bridges,wind power generation,transmission towers,and marine infrastructure.These structures are not only subjected to the axial loads but also lateral loads caused by winds,wave action,impacts,etc.Due to the complex geological conditions,piles are often built on slopes or near slopes.Different from piles placed on level ground,the influence of slope effect needs to be considered in the calculation method and design process for the pile located on slope,and the bearing deformation mechanism is more complex for the pile located on slope.At present,there is no calculation method for bearing deformation of such pile foundation in China ’s current norms and standards.This paper relies on the National Natural Science Foundation Project “Study on mechanism and design calculation method of large diameter pile foundation of wind power under coupling effect of slope and load direction”.The model tests,theoretical analysis and finite element analysis are used to study the bearing deformation mechanism and deformation characteristics of single pile and two-piles under lateral load on slope,which provides reference for the design and internal forces calculation of pile foundation on slope.(1)Based on the indoor model test,the bearing characteristics of the laterally load pile on sand slope are discussed.The results show that the deformation of soil around single pile on slope generally presents several stages,such as crack arising-crack developing-crack increasing and soil uplifting.The larger the slope angle is,the larger the extension range and size of crack are;The load-displacement curve is divided into three stages: elastic,elastic-plastic deformation and plastic failure;The cracks on the slope surface of the two-pile foundation develop from scratch,and gradually increase and widen,and the cracks on both sides of the pile foundation occur earlier than the cracks on the slope surface in front of the pile foundation.When the pile spacing is large,the cracks on slope are mainly gathered in front of each pile,while the cracks between the two piles are few.The earth pressure in front of the pile increases with the increase of the lateral load,and increases first and then decreases along the pile body.The “negative” earth pressure appears below the zero point and reaches the maximum at the pile end.The large slope angle and small pile spacing weaken the earth pressure in front of the pile obviously.The soil resistance in front of the pile,especially in shallow depths,is obviously weakened by the slope.The increase of the slope angle and the decrease of the pile spacing weaken the initial subgrade reaction modulus of the soil in front of the pile,resulting in the decrease of the lateral soil resistance in front of the pile.(2)With the help of finite element simulation and indoor model tests phenomena,the passive wedge theory for the failure of single pile under lateral load considering pile-soil interface and slope is established,based on which,the calculation method of ultimate soil resistance of single pile under horizontal load on slope is put forward.Then combined with the hyperbolic p-y curve criterion,the deflection differential equation and the finite difference matrix equations,the finite difference decomposition for the internal force and deformation of single pile under lateral load is obtained.The analysis of the influencing factors of the passive wedge model shows that the diffusion angle is related to the physical properties of the foundation soil,and the bottom angleβ is related to the effective friction angle φ,wedge depth and slope.(3)Combined with the model test results and previous research results,the rationality of the calculation method of slope passive wedge model is verified.Finally,the influence of slope on the bearing capacity of single pile under lateral load is discussed.The results show that the lateral load of pile top decreases with the increase of slope angle.The bending moment of pile body decreases with the increase of slope angle,the maximum bending moment point of pile body moves down along the pile body with the increase of slope angle,and the slope angle weakens the ultimate soil resistance of soil obviously.(4)The passive wedge model of single pile on slope is extended to two-pile model.The failure passive wedge theory of two-pile on slope considering pile-soil interaction is established.The calculation method of ultimate soil resistance of two-pile side under lateral load on slope is proposed.The model considers that the passive wedges formed in front of two piles are shadowing in a certain range,which leads to the loss of soil resistance.The interaction between piles decreases with the increase of depth,and the supporting effect of soil on pile group foundation decreases with the increase of passive wedge overlap.The modulus reduction factor of foundation is introduced to consider the influence of pile spacing on the soil resistance around two piles on slope.And the mechanical model and calculation method suitable for two piles on slope are proposed.(5)Combined with the indoor model results,it is proved that extending the passive wedge theory of single pile to two-pile foundations on slope in feasible.Parameter analysis shows that the slope effect increases with the increase of lateral displacement,and the point of the maximum bending moment of pile and the turning point of soil resistance appear in the middle and lower part of the pile,and move down with the increase of slope angle.With the increase of pile spacing,the lateral bearing capacity,pile bending moment and soil resistance of two pile foundations on slope are improved,and the influence depth moves downward along the pile.When the pile spacing is greater than a certain value,the pile group effect is reduced.(6)Finally,with the help of software visualization module and passive wedge theory,the depth of passive wedge is obtained.Based on this,the influence of internal friction angle,slope angle and foundation elastic modulus on the pile group effect of lateral load piles on slope is analyzed.The results show that the passive wedge depth of rigid pile under ultimate load is about 0.8l,and the influence range of horizontal loaded pile is about 2d ～ 4d.The increase of internal friction angle improves the pile group effect of slope pile foundation.The increase of the slope angle causes the reverse bending point of the pile body to move down,and the overlapping area formed in front of the pile gradually decreases,which reduces the pile group effect of the two-pile foundation to a certain extent.The reduction of the elastic modulus of the foundation directly affects the relative stiffness of the pile-soil,which in turn affects the bearing capacity of the pile foundation.The influence on the p-y curve is mainly reflected in the initial stiffness,which reduces the ability of the soil resistance around the pile,and reduces the pile group effect in disguised form.139 pictures,29 tables and 231 references in this paper.