Concept and measuring technique of groundwater level in vacuum preloading

Nowadays there are controversial viewpoints on variation rules of groundwater level in vacuum preloading technique due to insufficient analysis of concepts and shortcomings of measuring techniques. In this study, based on the status of the water in soil, the potential energy condition of the groundwater level and the reference datum, the groundwater level in vacuum preloading is redefined as the absolute elevation of a surface where the gravity field hydraulic pressure component of the gravity water in the soil is zero. In addition, two basic conditions, which are required to meet the accuracy criteria of groundwater level measurements in vacuum preloading, are proposed. Then, the water lifting function in filter pipe section and the confining effect in impermeable pipe section, which are two negative effects of the existing measuring techniques of groundwater level affected by the distribution of the filter pipe, are analyzed. Consequently, a sectional type buoy groundwater measuring technique is put forward, including the full outer filter tube installed in the foundation and the built-in partition sectional type buoy water level indicator with the special rubber concave-convex partition. It is a method which can keep sealing while measuring, overcoming the negative effects of the distribution of the filter pipe. This proposed technique is theoretically suitable for measuring groundwater level under negative pressure and sealing membrane in vacuum preloading practice.