邻近地铁基坑后插型钢MJS工法桩试桩研究

Field Tests on MJS Piles of Post-inserted H-shaped Steel in Soil Excavation Adjacent to the Subway

MJS工法桩在邻近地铁基坑工程中得到广泛应用,但对不同土质及工序条件下MJS工法桩的施工参数和成桩质量的认识仍不足。本文依托地铁联络通道基坑工程,针对MJS工法桩进行三组试桩,研究护壁方法、喷浆时长和型钢插入准备时长对杭州典型粉土粉砂地层中MJS工法桩成桩质量的影响;通过地表沉降与水平位移监测,分析施工对周边环境的影响。成桩结果表明:膨润土护壁可减小扭转阻力;采用喷浆压力40 MPa、水灰比1∶1、水泥掺量≥35%等施工参数,能实现成桩连续性,使桩身抗压强度达到1.5 MPa以上,渗透系数小于10^(-7)cm/s;控制喷浆时长为6 h 40 min,缩短型钢插入准备时长至48 min,能使型钢插入深度达到13 m。监测结果表明:成桩对周边环境影响小,最大地表沉降为0.69 mm,最大地表水平位移为0.68 mm。本研究最终确定的成桩参数,可为类似基坑工程的设计和施工提供依据。

Metro Jet System(MJS)pile is widely used in soil excavation projects adjacent to the subway.However,the construction parameters of MJS pile and the corresponding quality of MJS pile are not fully understood,especially for different soil profiles and different construction procedures.In this study,based on a foundation pit project of subway connectional passage,three MJS piles were constructed and tested in the field,in order to investigate the effects of borehole protection method,grouting duration and preparation time of H-shaped steel on the quality of the MJS piles in silt stratum of Hangzhou.Through field monitoring of the vertical and horizontal displacements of surrounding ground,the effects of MJS pile construction process on surrounding environment were analyzed.The testing results indicated that:(i)the addition of bentonite in borehole protection could effectively reduce the torsional resistance during borehole drilling;(ii)setting grouting pressure of 40 MPa,water-cement ratio of 1:1,cement content≥35%etc.could achieve good continuity,compressive strength above 1.5 MPa and permeability smaller than 10^(-7)cm/s for the pile body;and(iii)the control of grouting duration within 6 h 40 min and preparation time of H-shaped steel within 48 min could achieve a successful insertion of H-shaped steel into a depth of 13 m.The monitoring results showed that the construction process of MJS pile had little influence on the surrounding environment,and the maximum settlement and horizontal displacement of ground surface were 0.69 mm and 0.68 mm,respectively.The parameters of MJS piles finally determined in this study can provide references for the design and construction of similar foundation pit projects.