Study On Uplift Mechanism Of Grouting Spiral Anchor In Gravel Soil

With China’s emphasis on the western region,the development of the western region is particularly important.Due to its harsh environment,the construction of the power grid in some western regions has been greatly challenged.Therefore,a more stable foundation form is needed to anchor the upper transmission tower.Traditional pile foundation is widely used throughout the country,but it has the disadvantages of high cost,high labor intensity,slow construction speed,poor bearing capacity and so on,which make it low cost performance.In order to solve this problem,it is urgent to replace a better foundation form.Spiral anchor emerges with the advantages of low cost,fast construction speed,environmental protection and strong bearing capacity.There are a large number of gravel soil areas in the west,where the terrain is high and the wind is large,the wind erosion is serious,and the bearing capacity of the foundation is higher.For gravel soil with large soil cracks,pressure grouting is an effective measure to improve the bearing capacity.At present,a large number of researches on spiral anchor are focused on clay,soft soil and sand.For China with complex terrain,this is obviously limited,and the pressure grouting of gravel soil outside the spiral anchor is rare.In this paper,ABAQUS finite element software is used to take the spiral anchor as the main research object.The spiral anchor is placed in the gravel soil and analyzed under two conditions.One is the pressure grouting of the external gravel soil of the spiral anchor,and the hollow spiral anchor is internally grouted.The grouting material uses micro-expansive geopolymer mortar.The mechanical properties and durability of the material are stronger than ordinary Portland cement mortar,and the second is not grouted.The two conditions are divided into three working conditions for analysis,namely,different anchor diameter,different anchor diameter,and different anchor spacing.The variation law of bearing capacity of spiral anchor under three working conditions under vertical uplift load is explored,and the internal force analysis is carried out.Comparing the two conditions of grouting and without grouting,the increase of bearing capacity of grouting is analyzed.The formulas for calculating the bearing capacity of spiral anchor grouting and without grouting are proposed,and the calculation results are compared with the simulation results to obtain the error.The specific research conclusions are as follows:(1)Without grouting,under different anchor diameter conditions,the ultimate bearing capacity of single anchor and double anchor does not appear large deviation.The ultimate bearing capacity of double anchor is about 1.8 times that of single anchor,and the increase is large.When grouting,with the change of different anchor diameter,the bearing capacity of single anchor and double anchor changes significantly,and the change rule is that the ultimate bearing capacity increases with the increase of anchor diameter.The ultimate bearing capacity of double anchors is about 50-100 kN higher than that of single anchor,and the effect of double anchors is better.Compared with without grouting,the ultimate bearing capacity of grouting is increased by about 4-5 times,and the critical point of failure displacement of grouting is significantly lower than that of without grouting.(2)Without grouting,under different anchor plate diameter conditions,the larger the diameter of single anchor and double anchor plate,the greater the ultimate bearing capacity,and the ultimate bearing capacity of double anchor is about 2 times that of single anchor.During grouting,the diameter of the anchor plate is an important factor affecting the bearing capacity,and the ultimate bearing capacity increases with the increase of the diameter of the anchor plate.The increase of the ultimate bearing capacity of the double anchor is smaller than that of the single anchor,which is roughly increased by 10-40 kN.Under different anchor plate diameters,the single anchor is more cost-effective than the double anchor.For single anchor,the ultimate bearing capacity of grouting is about 7 times that of without grouting.For double anchors,the ultimate bearing capacity of grouting is nearly 4 times higher than that of without grouting.For spiral anchors of two sizes,grouting is significantly higher than without grouting,and the failure displacement during grouting is the same as that of different anchor diameters.(3)For the influence of different anchor plate spacing,the load-displacement curves of single anchor with six different anchor plate diameters are approximately the same,while for double anchors,with the increase of anchor plate spacing,the ultimate bearing capacity increases,and the efficiency of each anchor plate is improved.However,when the anchor plate spacing S=4.5D,the ultimate bearing capacity is close to S=6.0D,and the anchor plate efficiency is also close,up to 94%,S=4.5D is identified as the critical anchor plate spacing;when grouting,the ultimate bearing capacity of single anchor and double anchors under six anchor disc spacing is similar,but for double anchors with different spacing,the critical point of failure displacement is different;when grouting,the maximum efficiency of the anchor plate spacing of the double anchors is only 55%.In the internal force analysis,axial force is the key influencing factor of bearing capacity change under grouting and without grouting conditions,and bending moment and shear force can be ignored.(4)Since the practical effect of single anchor is worse than that of double anchors,the uplift failure mode is only studied for double anchors.When there is without grouting,the failure mode of the double anchor is the cylindrical shear failure mode when the anchor plate spacing is less than or equal to 3.0D,and the failure mode is the bearing capacity method when the spacing is greater than 3.0D;as the anchor plate spacing increases,the stress of the spiral anchor increases and gradually distributes evenly between the two anchor plates.During grouting,the failure mode is the failure of the cohesive force between the mortar and the soil,which leads to the overall upward movement of the mortar driven by the spiral anchor.At the same time,the contact surface between the spiral anchor and the mortar has a small failure surface.In addition to the distribution on the anchor plate,the bolt stress is also distributed at the upper part of the bolt.(5)The calculation formula of uplift bearing capacity of spiral anchor under without grouting and grouting conditions is proposed and compared with the simulation results.Without grouting,the error between the simulation results and the calculation results is within 10%,and most of them are within 5%.When grouting,the error between the simulation results and the calculation results is also within 10%,which confirms the accuracy and reliability of the calculation formula without grouting and grouting.