Precise identification of land-subsiding layers and reconstruction of subsid-ence process in Tianjin Binhai New Area

Land subsidence of Tanggu results from multi-layers subsidence of Cenozoic unconsolidated sediments, so accurate identification of each layer's contribution is s key problem for the prevention and control of land subsidence. In this paper, based on the land subsidence monitoring data of Tanggu G2 borehole extensometer during 2011~2014, the authors identified land subsidence of different layers of Cenozoic sediments precisely. Combined with previous data, the land subsidence process since 1960s was reconstructed, and the result shows that during the early stage groundwater mining of 1960~1970, land subsidence resulted mainly from shallow clayey soil layers of natural consolidation and second aquifer groundwater exploitation; during deep groundwater mining peak of 1970~1980, the main subsidence layers gradually deepened from two groups into three or four groups; after the beginning of the land subsidence control measures in 1985, land subsidence caused by Quaternary groundwater exploitation decreased; from 2000 to the present, with the establishment of Tianjin Binhai New Area and large-scale urban construction, the land subsidence caused by building foundation of urban construction has gradually become a major subsidence layer. In this paper, a new thinking for precise land subsidence prevention and control is proposed on the basis of the analysis of major land subsidence layers during different periods.