MECHANISM ANALYSIS OF LAND SUBSIDENCE IN KUNMING CITY AREA

摘要

Kunming city lies in the northern section of Kunming late Cenozoic tectonic fallen basin, which is widely piled up Quaternary loose deposits being composed of silty soil, clayey soils layers, several layers of silt, peat and lignite. The land subsidence occurred firstly in a certain area of the eastern urban districts in the early 1980s, and in the latest few years there are four land subsidence funnels in shape of funnel in the most gravely occurring central areas of Kunming city, namely Xiaobanqiao zone, Yuhucun zone, Datangzi zone and Yanjiashan zone separately, which are going to be linked together. The former two zones have come into being an area about 180 km~2, the average subsidence rate was about to 20.0mm/a, the maximum subsidence rate was up to 31.1 mm/a. Being one of the most serious subsidence centers, the cumulative subsidence value of Xiaobanqiao added up to more than 236.2 millimeter from 1985 to 1998, and the average subsidence rate of the Heweicun subsidence center was also up to 25.1mm/a recently. The land subsidence aggravated rapidly, its subsidence rage enlarged and subsidence velocity accelerated gradually presently. Meanwhile, there appears some new subsidence centers continually. The serious land subsidence has damaged building, lines and some others constructions, and became one of the most important potential geological hazards to the social and economic sustainable development of Kunming city. It is necessary to study on its spreading characteristics and forming mechanism throughly. Based on the observational level data from precise leveling survey of Kunming city area from 1985 to 1998, combining with the geological, hydrogeological and engineering geological conditions and the analysis of the distributing and evolutive rule of land subsidence in Kunming city area and its relations to several dominative factors, it is found that, besides the neotectonic movement having the characteristics of vary extent of ascend and descend, and the sedimentary compress and consolidation of the widely loosen and semi-consolidated Quaternary stratum especial the silt, peat and lignite layers whose physical mechanics properties are indirectly relative to the land subsidence, over-withdrawing deep groundwater and geothermal groundwater and long-playing, which lead to the hydrological conditions change and the water level drop quickly and the depression cone of groundwater table expanding rapidly and their interactions, are the vital factors to lead to the occurrence of land subsidence. In order to control and slow the acceleration of land subsidence and reduce the negative effects on the sustainable development of Kunming city, starting with the reasonable exploitation of groundwater and geothermal groundwater and city planning, adopting the comprehensive countermeasures, studying on the mechanics and modeling for land subsidence simulation and prediction,constructing the highly precise monitoring and measurement system and the hazards information forecasting and warning system, carrying out and strengthening the systematic study on the quantitative evaluation for the causal factors' respective contribution and reciprocity to the land subsidence, and so on, should be put in practice immediately.