Abstract: The conception of the groundwater level is described; and the characters of the groundwater under the normal atmosphere pressure and vacuum pressure are analyzed; thus coming to the conclusion that the groundwater level is the top surface of the gravity water or the interface between the gravity and capillary water; and the zero pressure line is the ground water level when under the normal atmospheric pressure but not when under the vacuum pressure. Based on the force equilibrium equation and gas state equation, the existing problem for the conventional technique is analyzed. It is concluded that the conventional technique can’t measure the true groundwater level. The water height measured is really the zero pressure line and the water-level fluctuation essentially reflect the average dissipation of pore water pressure in filter zone when the mouth of pipe is always open; the big error will occur due to the difference of the vacuity between in the sighting pipe and under the sealed film when the mouth of pipe is always sealed and it will become bigger and bigger as the reading times increase. So a new measuring technique that the true groundwater level can be calculated according to the force equilibrium equation by using the readings of the pore pressure gauge and vacuum meter in the sighting pipe is presented. The new measurement device can continually measure the variation of the groundwater level but not influence the results.

Key words: vacuum preloading, groundwater level, measuring technique, zero pressure line, top surface of gravity water