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    华北平原典型地段地面沉降演化特征与机理研究

    The evolution characteristics and mechanism of the land subsidence in typical areas of the North China Plain

    • 摘要: 华北平原是世界上最大的地下水系统之一,地面沉降问题突出。由于沉积环境变化多样、地质条件差异性显著和人类开采活动强烈,使得该地区地面沉降成因机理复杂。本文采用卫星对地观测技术与传统手段相结合,监测地面沉降灾变过程,分析华北平原地面沉降发展历史和现状。结合应力-应变图解法及土工实验研究地面沉降差异性特征及滞后变形成因机理。取得了以下关键认识:(1)华北平原地面沉降空间分布差异性明显,沉降主要分布在平原区第四纪沉积凹陷,呈现东西分带、南北分段特点。地面沉降空间发展部分受到北东向和北西向构造控制。在沧县隆起区,地面沉降也比较发育,主要原因是沧县隆起在第四纪时期构造运动相对不活跃,沉积了较厚的第四系;存在与构造走向一致的3期古河道,该地区赋存丰富的地下水资源并被大量开采。(2)地面沉降发生发展与地下水开采历史密切相关,沉降主要压缩贡献层随地下水开采层位变化而变化。北京平原100 m以深地层对地面沉降贡献呈增加趋势。天津平原目前地面沉降的主要贡献层来自300 m以下地层。(3)气候干旱导致地下水补给量减少,同时增加了地下水的开采,因而是引起地面沉降的重要间接驱动因素。高层建筑荷载、基坑降排水、地热开采对地面沉降的影响应引起足够重视。(4)地面沉降具有很强的滞后性,最大滞后时间可达25年。除了渗透固结成因以外,土体蠕变是另外一个重要原因。更新世地层在不同荷载下,蠕变特征明显。沧县隆起晚更新世地层次固结可达到总变形28.3%。(5)土的物理性质、地下水位变化模式对土层变形特征具有重要影响。不同埋深地层在地下水位变化条件下的变形特征存在较大的差异(弹性、黏弹性、黏弹塑性)。浅部含水组呈现以弹性为主的变形特征。

       

      Abstract: The North China Plain (NCP) is one of the biggest groundwater systems in the world, and land subsidence occurs commonly. Due to diverse sedimentary environments, different geological conditions, and intensive groundwater exploitation, the mechanism of land subsidence is complex. With the help of earth observing technique and traditional monitoring tools, the disaster process of land subsidence was monitored, and the developing history and current situation were analyzed. Combined with strainstress diagrams and soil mechanics tests, the difference features of land subsidence and the mechanism of hysteretic deformation were analyzed. Some conclusions have been reached:(1) The spatial distribution of land subsidence is significantly distinct. The land subsidence areas are mainly located in the areas of the Quaternary sedimentary depressions, having characteristics of west-east sub-zone and north-south subsection. The spatial development of land subsidence is partially controlled by NE-and NW-trending structures. The land subsidence in the Cangxian uplift is serious in that the tectogenesis is relatively inactive in Quaternary and the loose sediments are very thick. Also, there exist paleochannels of three layers with the same strike as the structures, whose groundwater resource is abundant and has been intensively exploited. (2) The development of subsidence is correlated with the local groundwater exploitation history, and the major contribution layers to the land subsidence have varied with the change of exploited layers. The contribution of the strata below 100m has increased in the Beijing plain. The land subsidence of the Tianjin plain mainly comes from the compression of the strata below 300 m. (3) The drought is an important and indirect factor leading to subsidence by reducing natural recharge and leading to the increase of groundwater exploitation for emergency water supply. Besides, in the groundwater exploitation, much attention should be paid to the effect of the heavy weight of densely constructed buildings, dewatering of foundation pits, and the exploitation of deep geothermal water.(4) The hysteresis of land subsidence is obvious, and the time of hysteresis can last for twenty-five years. Besides the consolidation, the creep is another important reason for the hysteresis. The creep is obvious for the Pleistocene strata, which can reach 28.3% of the total deformation for the Late Pleistocene strata in the Cangxian uplift. (5)The deformation characteristics of the soil layer bear strong relationship to the physical characteristics and change pattern of groundwater levels. The strata at different depths have distinct deformation characteristics such as elastic, visco-elastic, and visco-elastic-plastic deformations. Shallow aquifer groups indicate typical elastic deformation.

       

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