地铁列车振动荷载对穿越泥水盾构泥膜渗透系数的影响

Change of hydraulic conductivity of filter cake caused by train vibration load of a running subway

泥水盾构穿越饱和砂土地层中的运营地铁时,在地铁列车振动荷载的作用下,开挖面前方土体中会产生超孔隙水压力,从而降低泥膜承受的压力差,使泥膜回弹,渗透系数增大,进而影响开挖面稳定。通过动三轴试验,研究了不同类型砂土在地铁列车振动荷载的作用下的超孔隙水压力增长规律,并利用改进的Kozeny-Carman公式分析了其对泥膜渗透系数的影响。研究表明:地铁列车振动荷载振幅与在其作用下产生的砂土最大超孔隙水压力之间的关系可用指数函数表示;砂土中粗颗粒间的孔隙起控制作用时的最大动孔压比和泥膜最大渗透系数增大比约为细颗粒间的孔隙起控制作用时的最大动孔压比和泥膜最大渗透系数增大比的50%和60%;当动应力比不大于0.15时,振动荷载振幅的增大对最大动孔压比的增加影响并不明显;相对密实度对最大动孔压比的影响并不明显;卸荷程度相同时,轴向卸荷比水平卸荷对最大动孔压比的影响约大一倍;一般情况下,地铁列车振动荷载最大可使泥膜渗透系数增大约15倍;当泥膜渗透系数不大于7.73×10-8 cm/s时,可不必考虑运营地铁列车振动荷载对泥膜渗透系数的影响。

The pressure of filter cake declines because of the generation of the excess pore pressure caused by the train vibration load of a running subway when a slurry shield crosses the subway. The hydraulic conductivity amplification of the filter cake on account of its swelling due to the reduction of the pressure brings negative impact on the stability of excavation face. The effect of the running subway train vibration load on the generation of excess pore pressure in three categories of sand specimens is investigated through dynamic triaxial tests. The findings from these tests are used to predict the change of the hydraulic conductivity of filter cake via the improved Kozeny-Carman formula. It is concluded that the relationship between the maximum excess pore pressure and the amplitude of train vibration load can be described as an exponential function, and the maximum dynamic excess pore pressure ratio when the sand void is determined by the void between coarse particles is 50% of that when it is determined by the void between fine particles. The amplitude of train vibration load has no significant influence on the maximum dynamic excess pore pressure ratio when the dynamic stress ratio is not more than 0.15. Moreover, the effect of sand relative density on the maximum excess pore pressure is not evident. The rise of the maximum excess pore pressure in sand is the same when the decline of vertical stress is half of that of horizontal stress. The vibration load makes the hydraulic conductivity of filter cake increase 15 times in most cases, however, it will not rise to be too large and permeable if the hydraulic conductivity of filter cake is less than 7.73×10-8 cm/s.