Research On The Uplift Bearing Mechanism And Capacity Calculation Method Of Helical Anchors In Sand Foundation

Helical anchor has the advantages of strong bearing capacity,simple construction and good economy,etc.,and is widely used in the engineering practice of drawing load,such as the foundation of towering structures,the fixing of anti-floating members and the anti-sliding reinforcement of slope.Determined by the use function,the pull-out bearing capacity of the helical anchor is a basic index that must be specified in its design,and its calculation method has always been one of the most concerned directions in related research fields.For vertical drawing of helical anchor,its bearing capacity mainly depends on the anchor plate,so the interaction mechanism between anchor and soil and anchor is the key to analyze the mechanical model of helical anchor against drawing,and the mechanical model is the prerequisite to establish the theoretical calculation method of bearing capacity.However,the existing research results often artificially cut the continuous evolution mechanism of anchor-soil and anchor-anchor interaction into extreme cases for simplified treatment by introducing the concepts of critical burial depth ratio and critical spacing ratio,and there is no consensus on the definition criteria.This will not only fail to uniformly describe the pull-out bearing mechanism of the helical anchor,but also bring confusion to the engineering design of the helical anchor and cause hidden safety hazards.Therefore,it is necessary to carry out more in-depth research on the pull-out bearing mechanism and bearing capacity calculation method of helical anchor from the perspective of continuous evolution.In this paper,the mechanism and calculation method of uplift bearing capacity of helical anchor are studied based on the view that the failure of sliding surface around anchor is a continuous evolution process with the change of buried depth ratio and space ratio.Firstly,the self-made visual drawing test device was used to simulate the single/double helical anchor in sand and carry out the vertical drawing half-mode test of single/double plate round anchor under 50 working conditions.The results of the test load displacement curve analysis show that the ultimate bearing capacity of the helical anchor increases with the increase of the buried depth,and the growth rate slows down from fast,with an asymptotic value under different buried depth ratios.Under the same buried depth ratio,the ultimate bearing capacity and ultimate displacement increase obviously with the increase of anchor diameter,while the updraft bearing capacity coefficient N_rdecreases with the increase of anchor diameter.For double helical anchors,the ultimate bearing capacity increases with the increase of buried depth ratio and spacing ratio,and the growth rate slows down from fast,and there is also a gradual value.Photo Infor and Post Viewer were used to process the digital image of the soil around the anchor during the drawing process,and the corresponding displacement field and maximum shear strain field were obtained to identify the sliding surface.When the burial depth is relatively small,the single anchor sliding surface extends to the surface,presenting an"inverted trapezoid"overall failure.When the buried depth is relatively large,the sliding surface completely shrinks to the interior of the soil,showing a"balloon"local failure.For double helical anchors,the soil displacement and deformation field around the anchor is only affected by the depth ratio of the anchor,but has nothing to do with the position of the lower anchor.The displacement field corresponding to the lower anchor segments is only affected by the spacing ratio of anchor segments,and has little relation with the true buried depth ratio.Further study shows that the variation of soil sliding surface around a single anchor with the ratio of buried depth can be characterized by a partial circle of ellipse with decreasing ratio of long and short axes.Therefore,based on the elliptic function,a unified morphological function was proposed to reflect the continuous change of sliding surface,and a unified bearing mechanical model was established for the analysis of shallow and deep buried single helical anchor,and then a unified calculation method for vertical drawing capacity of single helical anchor was established.In the comparative calculation of 4 test cases,the unified method ranks first in the comprehensive score of calculation effect among 7 methods,showing good applicability.On the basis of the single helical anchor limit bearing capacity calculation method,combining with double helical anchor the change law of the sliding surface,drawing test is based on the elliptic function,constructed the lower anchor plate anchor weeks slip surface of soil with distance than the continuous evolution of the form function,set up considering depth ratio,spacing ratio continuous variation of spiral double anchor bearing uniform analysis model,The calculation method of vertical drawing capacity of spiral double anchors is established.The method does not need to introduce the concepts of critical depth ratio and critical spacing ratio,and the calculated results are in good agreement with the test values.