Study On Vertical Pullout Bearing Characteristics Of Screw Anchors In Permafrost Regions

With the promotion and implementation of the Belt and Road Initiative,the construction and operation of construction,transportation,energy,communication and other related infrastructure projects in permafrost regions have increased significantly.However,the high temperature instability of frozen soil seriously affects the safety and long-term stability of foundation and structures in permafrost regions.In recent years,in order to improve the long-term bearing performance of pile foundation in permafrost regions and ensure the safe operation of infrastructure such as construction,transportation,energy and communication,many scholars have proposed various new forms of pile foundation applied in permafrost regions,and conducted in-depth studies on their thermal and mechanical properties.Compared with the traditional pile foundation,screw anchor foundation has the advantages of easy installation,small disturbance to the frozen soil temperature field,can bear multi-directional load and so on.It is widely used in transmission lines,communication signal towers,underground pipeline foundation,photovoltaic support foundation and other fields.Most of the current studies on the bearing characteristics of screw anchors are confined to normal temperature soil,and there is a lack of systematic studies on the bearing characteristics of screw anchors in permafrost.Because the vertical pull-out capacity of screw anchor is of great significance to its application in permafrost areas,this paper studies the vertical pull-out bearing capacity of screw anchor in permafrost environment by means of laboratory model test,finite element numerical simulation and theoretical calculation,in order to provide technical reference and theoretical support for the design,construction and operation of screw anchor foundation in permafrost areas.The main conclusions are as follows:（1）In terms of laboratory model test,in order to study the influence of buried depth ratio H/D,blade spacing ratio S/D and other factors on the pull-out bearing performance of steel pipe screw anchor in frozen soil,single blade and double blade screw anchor models with different geometric parameters were prepared,and pull-up load tests were carried out.The test results show that for the single blade screw anchor in frozen soil,when H/D≤6,increasing the blade depth ratio can effectively improve the ultimate uplift capacity.When H/D≥6,the ultimate uplift capacity increases little with the increase of blade buried depth ratio.When S/D≤2,increasing blade spacing ratio can effectively improve ultimate pulling capacity of double-blade screw anchors in frozen soil.When S/D≥2,increasing blade spacing ratio leads to a small increase in ultimate uplift capacity.The critical depth ratio（H/D）cr of single blade screw anchor is between 4～6,and the critical distance ratio（S/D）cr of double blade screw anchor is between 1～2.（2）A finite element model was established according to the laboratory test conditions,and the load-pulling displacement curves of single blade screw anchors with different burial depth ratios and double blade screw anchors with different spacing ratios were obtained through numerical simulation.Compared with the results of the laboratory model test,the curve fitting results were better,which verified the effectiveness and reliability of the finite element numerical simulation method.The screw blade compressive stress distribution,screw anchor failure mode and load sharing law were analyzed.The end resistance load of single blade screw anchor accounted for 76%～94%of the ultimate pull-out force,and the end resistance load of double blade screw anchor accounted for 77%～83%of the ultimate pull-out force.The pull-out capacity of screw anchor in frozen soil is mainly provided by blade end resistance.Through the numerical simulation results,the critical depth ratio（H/D）_cr of single blade screw anchor was determined to be between 4 and 6,and the critical distance ratio（S/D）_cr of double blade screw anchor was determined to be approximately 2.（3）Based on the theory of the ultimate pulling capacity of screw anchors in normal temperature soil and considering frozen soil characteristics,the calculation formula of ultimate pulling capacity of screw anchors in frozen soil is proposed.According to the critical buried depth ratio（HD）_cr determined by laboratory model test,the single blade screw anchors in frozen soil are divided into"shallow buried"and"deep buried"working conditions,and the calculation formulas of the two working conditions are proposed.According to the critical interval ratio（S/D）_cr determined by laboratory model tests,the two-blade screw anchors in frozen soil were divided into two failure modes:single failure and cylindrical shear failure,and the bearing capacity method and cylindrical shear method were used to calculate the failure modes respectively.It provides a theoretical basis for the calculation of mechanical properties and ultimate bearing capacity of screw anchors under uplift load in permafrost areas.