A NEW METHOD OF DEFINING CRITICAL WATER LEVEL IN LAND SUBSIDENCE

摘要

The critical water-level was firstly put forward by Niu Xiujun according to over-consolidation pressure in the study of land subsidence in Tianjin. It is objectively existent as a limit water-level which will cause land subsidence. The definition has important value in forecasting and controlling of land subsidence. Recently, Ranxinglong's study showed that under Jacob assumptions, aquifer's skeleton elastic specific storage is a constant, if each aquifer's drawdown is synchronous vertically , the comprehensive skeleton elastic storage coefficient will also be a constant. That is, the relationship between land subsidence and ground-water drawdown should be linear. On the basis of Ran Xinglong's study, this paper advanced a new method to define critical water-level and compared it with the method of Niu Xiujun through the following aspects: (1) Concept of critical water-level; (2) The mechanism expressions defining critical water-level; (3) The calculated results of critical water-level for Tianjin. Results showed: (1) Two concepts have identically physical meaning, and they all described the limit water-level which can lead to land subsidence. (2) There is a marvelous consistence between two mechanism expressions; (3) Because of the complexity of pre-consolidation pressure which is usually obtained by soil compression experiment or estimated by geological experience, there are some potential errors which prevent us from obtaining a precise value but a variation range for critical water-level. Taking Tianjin for example, the critical water-level value in the second aquifer was between 30-40m according to Niu Xiujun's method, compared with 30.2 m obtained by the new method. Obviously the new method can calculate a more precise critical water-level, and has significant role in sustainable ground-water resource development as well as land subsidence forecasting and controlling.