Abstract: Xi'an region is a geothermal anomaly zone. The geothermal exploitation in this region began in 1990, and has sped up since 1993. At present, there are 120 geothermal water wells with a depth of greater than 1 000m in this region, and this number is still increasing at a rate of 5 to 10 wells per year. Ground subsidence and ground fissures are two natural calamities peculiar to Xi'an City. They are famous for long standing formation time, a vast involved range and causing the serious damage of a great number of buildings. Most people considered that the ground subsidence and ground fissures in the city are induced by the overexploitation of the confined water. The overexploitation of confined water may induce ground subsidence and ground fissures; does the speeding up of exploitation of geothermal water aggravate the afore mentioned geological hazard? In this paper the development history, nature and formation mechanism of ground subsidence and ground fissures in Xi'an region, as well as the relationship between the two calamities are demonstrated in detail. Moreover, the variations of geothermal water level, the quantity of geothermal water exploitation and the amount of subsidence for individual stratum in the past decade are summarized and analyzed. All the results show that the spatial distribution of the ground subsidence coincides well with that of the ground fissures in Xi'an City, while their development processes and variations of intensity display a good synchronism. The main cause of the induced surface subsidence is the overexploitation of confined water. The drop of the confined water level may give rise to the decrease of pore pressure of the medium within the aquifer. The compaction of gummy formations due to the release of water leads to the permanent surface subsidence, while the compaction is greater at shallow level than at deep level. The inhomogeneous surface subsidence then induces the ground fissures. Either the data of the variation of geothermal water level and the quantity of geothermal water exploitation, or the data of the subsidence of various strata show that the geothermal exploitation in Xi'an City at present does not intensify the surface subsidence and the activity of ground fissures. It is suggested that the most effective approach to minimizing ground subsidence and ground fissures is to control and decrease the exploitation of confined water in Xi'an City.

Key words: Geothermal water exploitation, Confined water, Ground subsidence, Ground fissures

摘要： 西安市地面沉降和地裂缝在空间分布上有明显的一致性,它们的发展过程和强度变化在时间上也有明显的同步性.诱发地面沉降的主因是过量开采承压水导致承压水位下降,含水层介质的孔隙压力减小,黏性土层释水压密导致地面永久性的沉降,而且浅层的释水压密量大于深层;不均匀的地面沉降又诱发了地裂缝.无论是地热井水位或热水开采量的变化,还是分层沉降量的观测资料都表明,目前西安市深层热水开采还没有加剧西安市的地面沉降和地裂缝活动.控制并减少承压水的开采量,是减弱西安市地面沉降和地裂缝活动的最有效的方法.

关键词: 热水, 承压水, 开采, 地面沉降, 地裂缝