Vacuity and pore-water pressure are two different conceptions

the vacuity measured by vacuum meter in the soft soil is different from the difference of pore-water pressure measured by piezometer; but there is an immanent relation between them. The theoretical formula about vacuity in vacuum tube and pore-water pressure at the observation point was obtained in this paper by using equation of gas state and equilibrium equation. When the observation point is above the groundwater surface

the vacuity is the same as the difference of pore-water pressure. When the observation point is under the groundwater surface

the vacuity is less than the difference of pore-water pressure

but they can be calculated each other by using the theoretical formula. The test results explain the relations between the vacuity and the pore-water pressure and show that the theoretical formula is correct.