南水北调前后北京平原区地下水和地面沉降演变特征

超量开采地下水引发的地面沉降已成为制约北京区域社会经济可持续发展的重要因素之一。2014年12月，南水北调中线工程正式通水，每年向北京输水超过10×108 m3，改变了北京供水格局，也为地下水压采、涵养及控制地面沉降创造了条件。本文利用多种监测数据，分析南水进京前后，北京平原区地下水和地面沉降的变化；研究不同水位变化模式下不同岩性及深度土层的变形特征；计算土层不同变形阶段的弹性和非弹性储水率；并对黏性土层产生较大残余变形和滞后变形的原因进行了探讨。结果表明：① 2015～2020年，平原区大部分地区第一至第四含水层组地下水位逐渐上升，地面沉降呈减缓的趋势。② 第二和第三压缩层组是沉降主要贡献层，除平各庄和榆垡站外，其余各站第三压缩层组沉降占比逐渐增大，沉降主控层有向深部转移的规律。③ 平原区北部和东部，第二和第三压缩层组对应的地下水位由降转升。在水位下降阶段，土层呈塑性和蠕变变形；水位上升阶段，土层以塑性变形为主，部分时间出现弹性变形，具有黏弹塑性。平原区南部，地下水位始终持续下降，土层变形始终呈塑性和蠕变变形。含水砂层则主要呈弹性变形。④ 土层变形的不同阶段，弹性和非弹性储水率并不是恒定的，随着地下水位下降，储水率呈减小的趋势。⑤ 黏性土层存在较大残余变形和变形滞后的原因，一是非弹性储水率大于弹性储水率，二是黏性土层的弱渗透性。

Characteristics of groundwater and land subsidence evolution before and after the South- to- North Water Diversion Project in Beijing, China

Land subsidence caused by over- exploitation of groundwater has become one of the important factors restricting the sustainable development of the society and economy of Beijing. In December 2014, the middle route of the South- to- North Water Diversion Project was officially opened, and more than 1 billion m3 of water was delivered to Beijing every year. As a result, the pattern of water supply in Beijing has changed, and conditions have been created to reduce the exploitation of groundwater, conserve groundwater, and control land subsidence in Beijing. This paper uses a variety of monitoring data to analyze the changes in groundwater and land subsidence in the Beijing Plain before and after the South- to- North Water Diversion Project. The deformation characteristics of soil layers with different lithologies and depths under different groundwater level variation patterns are studied. The elastic and inelastic skeletal- specific storage rates of the soil layers at different deformation stages are calculated. The reasons for the large residual deformation and hysteresis deformation of the cohesive soil layer are discussed. The results show that: ① From 2015 to 2020, the groundwater level of the first to fourth aquifer groups in most areas of the plain gradually increased, and the land subsidence gradually slowed down. ② The second and third compression layer groups in the plain are the main contributing layers to the subsidence. Except for Pinggezhuang and Yufa station, the subsidence proportion of the third compression layer group in the other stations gradually increased. The main subsidence layer is gradually transferred to the deep formation. ③ In the north and east of the plain, the groundwater levels corresponding to the second and third compression layer groups changed from falling to rising. During the drop stage of the groundwater level, the soil layer exhibits plastic and creep deformation. During the rise stage of groundwater level, the soil layer exhibits plastic deformation and elastic deformation for part of the time. which is viscoelastic plastic. The soil layer exhibits the characteristics of viscoelastic plastic. In the southern part of the plain, the groundwater levels continued to decline, and the deformation of the soil layer was always plastic and creep deformation. The sand layer mainly exhibits elastic deformation. ④ For different stages of soil deformation, the change in elastic and inelastic skeletal- specific storage rates is not constant. As the groundwater level decreases, the soil skeletal specific storage rate shows a decreasing trend. ⑤ The reasons for the large residual deformation and deformation hysteresis of the cohesive soil layer mainly include two aspects. Firstly, the inelastic skeletal specific storage rate is greater than the elastic skeletal specific storage rate. Secondly, the weak permeability of the cohesive soil layer.