【作者】 肖国强；

【导师】 徐恒力；

【作者基本信息】 中国地质大学 ， 水文学与水资源， 2014， 博士

【摘要】 地面沉降现象自19世纪30年代在墨西哥、英国等地发现以来,已经遍及了世界各地,受到国际社会的广泛关注,已成为环境地质学科的重要研究内容和研究方向。国内外学者对地面沉降的成因、机理、监测、预测、防治等进行了长期多学科的科学研究,在理论、方法和实践上都取得了较大的进展。但随着社会经济的快速发展和资源的持续开发,地面沉降问题在一些国家和地区呈逐渐加重趋势,地面沉降的成因也逐渐多元化和复杂化。我国自60年代上海、天津发现地面沉降以来,陆续在长江三角洲、华北平原及汾渭断陷盆地的50多个城市发现地面沉降。经过多年的调查研究,在地面沉降成因机理研究、监测技术、模拟预测、防治对策等方面取得了许多成果。但随着沉降范围的不断扩展和沉降深度的不断增大,在地面沉降调查、深部粘性土压缩变形特征、分层标监测技术、地面沉降模拟预测、地面沉降防治对策等方面还有很多问题需要深入研究。天津市是我国地面沉降最严重的地区之一,塘沽地面沉降中心最大地面沉降量达3.44米。天津地面沉降的成因复杂,包括构造沉降、软土沉降、自然固结沉降、城市浅层沉降、地下水开采沉降、地热开采沉降多种影响因素的复合叠加。随着地下水开采深度的增加,深部粘性土高压固结机理研究成为天津市地面沉降研究的焦点,而随着各影响因素的彼此消长,不同时期地面沉降层位的时空演化过程研究也成为地面沉降防控对策制定的关键。本文针对天津市塘沽地区地面沉降的特点和存在的主要问题,利用位于天津塘沽的1228米深全取芯钻孔G2孔,进行了不同深度粘性土高压固结试验、次固结试验、反复加卸荷试验,研究了粘性土高压固结特征,模拟了不同水位变化条件下粘性土压密固结过程,深入分析了粘性土高压固结机理。利用地面沉降分层标监测技术,分层观测了不同层位地面沉降量、地下水水位、孔隙水压力的变化过程,恢复了塘沽地区地面沉降层位的时空演化过程。计算了各粘性土层潜在最大沉降量,对不同层位粘性土地面沉降潜在风险进行了评价,提出了有针对性的地面沉降防控对策和地下水开采、城市建设、地热开发政策的调整建议。主要研究成果如下：1、首次在G2孔开展了650-1200米深部粘性土高压固结试验,获取了深部粘性土高压固结参数。试验表明：深部粘性土高压固结特征与浅部粘性土高压固结特征有显著差别,粘性土压缩性自上而下宏观上具有逐渐减小的趋势。浅部晚更新世以来粘性土一般为欠固结土和正常固结土,且分布三层淤泥质海相软土层,因此压缩性较高：第四系早、中更新世由于地下水长期开采,粘性土经过反复压缩后固结程度已转化为正常固结和超固结土,压缩性明显降低；新近系明化镇组上部由于尚未大规模开采地下水,粘性土压缩尚处于初期阶段,但存在数层欠固结粘性土,其压缩性较高,与当时粘性土沉积环境及沉积速度有关；明化镇组下部及馆陶组粘性土一般为正常固结和超固结,由于地热水开采已产生一定的压缩,但其压缩量与浅部相比要小。2、开展了三种不同压力模式下的粘性土反复加卸荷试验,模拟了在不同水位变化条件下粘性土的压密固结过程,深入分析了粘性土压密固结机理。试验表明：(1)浅部粘性土和深部粘性土的压密固结过程具有相似性,在地下水水位第一期下降过程中,粘性土压缩性较高,压缩量较大,在后期的第二、第三次地下水水位反复升降过程中,粘性土压缩性呈明显的逐渐衰减趋势。(2)粘性土压缩量在地下水水位初次下降过程中占80-90%,其后的水位反复升降所引起的粘性土压缩量占20%以下,说明在地下水水位初次下降过程中引起的地面沉降较大,而其后的水位反复升降过程中,只要后期最低水位不大于前期最低水位,地下水开采所引起的地面沉降量将逐渐减小。(3)地下水水位在一定范围内反复升降条件下,粘性土在水位初次下降压密后,其压密过程表现为压缩量逐渐减小的趋势；地下水水位在持续下降条件下,粘性土的压密过程表现为随着水位的每次下降,粘性土压缩量较大,衰减趋势不明显；地下水水位在持续回升条件下,粘性土的压密过程表现为反复压缩和回弹,压缩量逐渐减小,如果压缩量大于回弹量,地面仍表现为沉降,如压缩量小于回弹量,地面则表现为回升。(4)本次的粘性土反复压缩实验表明：粘性土不仅具有塑性变形的特性,而且还有一定的弹性变形特性：粘性土弱透水层在外部荷载作用下(如地下水水位下降),粘土层的顶底部开始首先释水压密,孔隙比逐渐减小,渗透性逐渐减弱,对粘土层内部的孔隙水释放形成阻碍,中间部分的粘土孔隙水压力逐渐增大,形成类似“气囊”的效应,粘性土压缩速率逐渐减小。当粘性土层外部荷载开始减小后(如地下水水位回升),粘性土层已压缩的顶底部由于应力释放而膨胀回弹,中间未压缩“气囊层”部分孔隙水压力释放而膨胀回弹。这是粘性土压缩性逐渐衰减,粘性土回弹的重要原因。3、利用地面沉降分层标观测技术,分别观测城市建设影响层、咸水层、第四系地下水开采层、新近系地下水开采层、地热开采层等八个不同层位地面沉降量。观测结果表明：(1)城市建设引起的地面沉降已成为塘沽地区主要的沉降来源,沉降量86.1mmm,占总沉降量的68%；(2)咸水层位由于地下水不开采,地面沉降较小,沉降量4.69mm,占总沉降量的3.7%；(3)第四系地下水由于长期开采,粘性土固结程度增大,地面沉降量逐渐衰减,再加上长期的限采措施使得水位有所回升,地面沉降量已大大减小甚至回弹,沉降量5.93mm,占总沉降量的4.7%;(4)明化镇组上部深层地下水由于尚未大规模开采,地面沉降处于初期阶段,沉降量6.51mm,占总沉降量的5.14%：(5)首次设置了650-1218米深度地面沉降分层标,获取了塘沽地区明化镇组地热开采层馆陶组地热开采层地面沉降量实际观测数据,地热开采引起的地面沉降量25.57mm,占总沉降量的20.2%。4、系统分析了塘沽地区不同时期的地下水水位动态、地面沉降层位演化过程,结合分层标观测资料,揭示了地面沉降在时间和空问上的多层叠加效应,分析了不同时期的主沉降层位,有助于制定有针对性的地面沉降防控对策。研究结果表明：在60-70年代地下水开发初期,地面沉降主要层位是浅部粘性土自然固结和第二含水组地下水开采；70-80年代深层地下水高峰期,主沉降层位由二组逐渐加深到三四组；1985年以后实施了地面沉降控制措施后,第四系地下水开采引起的地面沉降逐渐减小；2000年之后,随着滨海新区的成立和大规模的城市建设,城市建设引起的浅层沉降逐渐成为主要的沉降层。5、地面沉降大小取决于粘性土的压缩固结特性与压缩阶段,地面沉降风险取决于沉降层位及该层位最大沉降量和沉降阶段。本文利用试验获取的不同阶段粘性土压缩参数,计算了不同层位地面沉降潜在最大沉降量,结合不同层位地面沉降历史过程,对G2孔不同层位地面沉降风险进行了评估。提出了不同于现有地面沉降防控措施的分层地面沉降防控对策和地下水、地热资源合理开发利用建议。研究表明：(1)浅层地面沉降潜在风险最大,危害性更高,应做为主要控沉层位；咸水含水层由于基本不开采,因此地面沉降潜在风险较大,应限制咸水开采；第四系地下水开采层由于经过长期开采,地层反复压缩后固结程度增大,地面沉降已处于后期阶段,地面沉降风险已显著减小,可以在一定水位控制前提下适度开采：地热开采层由于深部粘性土压缩固结程度较浅部高,地面沉降风险中等,随着深部地层的逐渐压实,地面沉降风险将逐渐减小,应在加强回灌的前提下适度开发。(2)塘沽地区由于地面沉降的影响因素和沉降层位的不断变化,初期的限采和禁采措施目前已不适用,效果也不再明显。地面沉降防控措施应针对不同时期、不同层位地面沉降时空演化而进行有针对性的调整。(3)避免不同层位地面沉降在时间上和空间的的叠加是地面沉降防控的有效对策。由于塘沽地区地面沉降影响因素多,沉降层位多,因此应避免多因素、多层位地面沉降在同一地区特别是城市建设区叠加；根据各沉降层位首次沉降量大的特点,应采取措施控制各层位地面沉降的发生时间,避免多层位首次地面沉降在时间上同时出现。更多还原

【Abstract】 Tianjin is one of the most serious land subsidence areas of China, and subsidence center of Tanggu in the east of Tianjin, the largest land subsidence is 3.44 m. The causes of land subsidence is complex in this area, which include tectonic subsidence, soft soil subsidence, natural consolidation subsidence, city building foundation subsidence, groundwater mining subsidence, geothermal mining subsidence, and complex superposition of multiple factors. With the increase of mining depth of groundwater, deep clayey soil consolidation mechanism by high pressure has become the focus of subsidence research in Tianjin, and with the changing of the various factors, spatial and temporal evolution of land subsidence layers during different periods also become ground subsidence key strategies for developing prevention and control.In this paper, based on the characteristics of land subsidence and the main problems in Tanggu, the G2 hole (1228m depth) taken from Tanggu was studied, clayey soil high pressure consolidation test in different depths, sub-consolidation test, repeated plus unloading experiments were carried out in order to study the characteristics of clayey soil consolidation by high pressure, simulated the clayey soil consolidation process under different groundwater level conditions, and deeply analyzed the clayey soil consolidation mechanism by high pressure. The standard land subsidence monitoring techniques was used in order to monitoring the different layer’s land subsidence, groundwater level, the process of pore water pressure changes, and reconstructed the temporal and spatial evolution process of different layer’s subsidence of Tanggu. At the same time, the potential maximum subsidence were calculated, land subsidence potential risks of different layer’s clayey soil were assessed, and proposed targeted prevention and control of land subsidence, suggested that the groundwater mining, urban construction and geothermal development policies should be adjusted.The main results include:1. It is the first high pressure clayey soil consolidation test in-depth of 650-1200m in G2 core, and gets deep clayey soil high pressure consolidation parameters. The result show that (1) the clayey soiley soil compressibility show a decreasing trend from top to bottom, compression is high in shallow clayey soiley soil, clayey soil compressibility decreases in deep layer; (2) Clayey soil compressibility show segmentation rules, in a certain depth it’s showing similarity, while it is showing a significantly different from other layers; (3) Clayey soil compressibility is special in certain layer, where have high pressure unconsolidated clayey soil, which is controlled by particle size, void ratio, water content and other relevant physical index, also influenced by depositional environment, deposition rate and the force of history.2. Three different models of clayey soil repeated loading and unloading pressure test were launched, in order to simulate clayey soil compaction consolidation process under different water level, and deeply analysis the clayey soil compaction consolidation mechanism. Tests showed that:(1) clayey soil compaction consolidation process in line with Tergaghi one-dimensional consolidation theory, repeated process show multi-cycle decay law. (2) Clayey soil show high compaction, and accounted for 80% during the first decline in water level process. Then the compaction show decay trend, and accounted for 20% in the following compaction process, which means that land subsidence by ground water mining will reduced if the groundwater level is not lower than the previous lowest level.3. Based on the standard land subsidence monitoring techniques, quantity observed the land subsidence controlled by urban construction layers, saline aquifers layers, Quaternary groundwater extraction layer, Tertiary groundwater extraction layer, geothermal exploitation layer. The observe results during July 10,2011-July 10,2014 show that (1) Urban construction has become a major cause for land subsidence in Tanggu district, and 86.1mm have been observed, which is 68% of the total subsidence; (2) land subsidence of saltwater layers are small due to no exploitation in this layer, so the observed subsidence is only 4.69mm,3.7% of the total subsidence; (3) Quaternary groundwater due to the long-term exploitation, the clayey soil increases the degree of consolidation, ground subsidence gradually decline. The water level has become rise because the control of exploration, and land subsidence has been greatly reduced or even rebound, the observed subsidence is 5.93mm,4.7% of the total subsidence. (4) the upper Minghuazhen layer due to no large-scale exploitation of deep groundwater, land subsidence is in the early stages, the observed subsidence is 6.51mm,5.14% of total subsidence; (5) moreover, it is the first time to have chance to observe the land subsidence of 650-1218m depth in G2 core, and get the land subsidence data due to geothermal exploitation in Minghuazhenzu and Guantaozu layer. The observed subsidence is 25.57mm,20.2% of the total subsidence.4. Systematic analysis of the different periods of the groundwater level dynamic, stratigraphic vertical pressure distribution, the evolution of land subsidence layers in Tanggu district, combined with observe data, reveal land subsidence multi-layer stack effect of time and space, analyzed the different main subsidence layers during different period, which help develop a targeted land subsidence prevention and control measures. The results showed:during 1960-70s of the groundwater mining early stage, land subsidence is mainly shallow clayey soil layers of natural consolidation and second aquifer groundwater exploitation; during deep groundwater mining peak of 1970-80s, the main subsidence layers changed from the two groups gradually deepened into three and four groups; after 1985 beginning of the land subsidence control measures, Quaternary groundwater exploitation caused land subsidence decreased; from 2000 to present, with the establishment of Tianjin Binhai New Area and large-scale urban construction, urban construction building foundation settlement caused building foundation land subsidence gradually become a major subsidence layer.5. Based on comprehensive analysis of spatial and temporal evolution of land subsidence in Tanggu, calculate the maximum potential ground subsidence in different layers of G2 core, and the risk of land subsidence in different layers were assessed. Demonstrates the existing problem of land subsidence control measurements, proposed layered land subsidence prevention and control measures, groundwater and geothermal resources exploration and utilization suggestions. Studies show that:(1) in study area, the initial limit and control of ground water measurement on land subsidence has no significant effect currently due to changing of influencing factors and land subsidence layers. (2) After long-term mining on Quaternary groundwater extraction layer and geothermal exploitation layer, the degree of consolidation increased after repeated compression, and the risk of land subsidence in this layer has been largely released, and moderate exploitation in this layer will have little influence; However, the urban construction layer, Tertiary saline groundwater extraction layer is still in the early stage, so this layer have relatively large potential risks on land subsidence. (3) In Tanggu district, there are many factors influencing land subsidence, so control of multi-layers overlay is an effective countermeasure for land subsidence prevention and control.更多还原