Field test and analysis of pore pressure in clay-limestone foundation

The key to anti-floating design of underground structures lies in clarifying the variation pattern of pore pressure. Relying on the subway project, a monitoring plan was formulated, with water level and meteorological monitoring points set up around the site, and pore pressure gauges installed at the base of the station. Real-time monitoring was conducted for a period of two years, obtaining the pore pressure in the clay-limestone foundation throughout the entire process from station construction to operation, and analyzing the factors affecting pore pressure. The test and analysis results indicate that the pore pressure at various points of the basement responds promptly to precipitation and rainfall. Precipitation within the monitoring area can effectively reduce the pore pressure, while precipitation outside the monitoring area will form a reduction in pore pressure due to infiltration loss, with an average reduction coefficient ranging from 0.14 to 0.70. After the cessation of precipitation, as the water level rose and hydraulic connectivity between various measuring points was established, the pore pressure approached hydrostatic pressure, with the mean value of the reduction factor varying between 0.87 and 0.98. Rainfall will increase the magnitude of pore pressure, and the increase is affected by the amount and duration of rainfall. A rainstorm can cause the pore pressure to reach up to 1.03 times the hydrostatic pressure, so it is necessary to set a certain anti-floating safety factor.