Monitoring method of water level outside pit: electric water level meter is adopted, which is composed of probe, cable disc and receiver. The probe of the instrument is lowered along the water level pipe. When it comes into contact with water, the upper receiver will beep. The distance between the underground water level and the pipe mouth can be directly measured through the dimension scale of the signal line. Orifice elevation is regularly measured with a precision level in conjunction with the reference level. The reading accuracy of the electric water gauge is ±1mm