黄土地区洞桩法地铁车站边桩和中柱的受力变形机理

Study on Stress and Deformation Mechanism of Side Pile and Middle Column for PBA Method Metro Station in Loess Area

[目的]为了确保黄土地区洞桩法车站施工过程中重要竖向承重构件的稳定性,需要对施工过程中边桩和中柱的受力变形机理进行研究,并对最不利受力阶段边桩和中柱提出变形控制措施。[方法]以西安地铁2号线何家营站为研究背景,通过数值模拟与现场监测的方法,对施工过程中边桩的水平位移、钢筋内力、水平土压力及中柱应变展开研究。[结果及结论]扣拱施作以及站厅层和站台层土体开挖是边桩水平位移最不利的三个阶段,在这些关键阶段施工时,应在结构内部架设钢支撑并适当加固已扰动的地层;在桩梁和扣拱施作中,边桩主筋承受轴向压力,而在站内剩余土方施工阶段,边桩背土侧的钢筋由轴向压力逐渐转化为轴向拉力,且随着边桩埋深的增大,钢筋的轴向拉压力增长变缓;在扣拱初期支护施作中,土压力在边桩埋深6.6 m处达到最大值34.5 kPa,且随着边桩埋置深度增大,边桩的桩侧压力增长速率显著,因此车站内侧的土方开挖是控制边桩迎土侧土压力大小的关键阶段,必要时对桩后土体进行注浆加固有利于提高桩基的承载力;中跨扣拱施工时,中柱的应变呈现出先减小、后增大的趋势。

[Objective]In order to ensure the stability of important vertical load-bearing components in the process of station construction with PBA(pile-beam-arch)method in loess area,it is necessary to study the stress and deformation mechanism of side piles and middle columns during construction,and put forward measures to control the deformation in the most unfavorable stress stage.[Method]With Hejiaying Station of Xi’an Metro Line 2 as the research background,the methods of numerical simulation and field monitoring are used to study the horizontal displacement,internal force of steel bar,horizontal earth pressure and strain of middle column of side piles during construction.[Result&Conclusion]Construction of buckle arch,excavation of station hall layer and platform layer are the three most unfavorable stages for horizontal displacement of side piles.During the critical stages of construction,steel support should be set up inside the structure and the disturbed stratum should be properly reinforced.During construction of pile beam and buckle arch,the main rebar of side pile bears axial pressure,while during construction of residual earthwork in the station,forces borne by the rebar of side pile back on the soil side gradually change from axial pressure to axial tension.With the increase of buried depth of side pile,the growth of axial tension pressure slows down.In the construction of the initial support of the buckle arch,the earth pressure reaches the maximum value of 34.5 kPa at the buried depth of 6.6 m of the side pile.As the aforementioned depth deepens,the growth rate of the pile side pressure is significant.Therefore,the excavation of the inner side of the station is the key stage to control the earth pressure of the side pile at the earth facing side.If necessary,grouting reinforcement of the soil behind the pile is helpful to improve the bearing capacity of the pile foundation.During the construction of the mid-span arch,the strain of the middle column shows a trend of decreasing first and then increasing.