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

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.