连云港徐圩地面沉降BOTDR监测与评价

连云港市位于苏北沿海地区,地面沉降灾害面积较大,多地沉降速率超过20 mm/a,徐圩的沉降现状尤为严重。为了能够对徐圩地区的地面沉降进行精细化观测,文章采用BOTDR分布式光纤感测技术,对徐圩镇127 m深的钻孔地层进行了两年多的全断面精细化监测。结果表明:徐圩镇共有四个承压含水层组,I-1隔水层和I-2隔水层土体沉降量分别占总沉降量的70.29 %、24.59 %,抽水层的土体最大沉降量仅占比1.38 %。I-1隔水层和I-2隔水层的地层岩性包括淤泥质黏土（L2）、亚黏土（L3）、亚砂土夹粉砂（L4）,总厚度为44 m,由于抽水过程中隔水层向含水层失水,导致该隔水层土体固结压缩。同时,工程建设附加荷载对地面沉降的影响也不可忽视。徐圩地区现阶段的沉降仍在继续发生,但沉降速率有减小的趋势。BOTDR技术可有效获取地面沉降钻孔全断面的土层变形分布信息,为地面沉降评价提供了一种精细化的分布式监测手段。     

基于弱光栅技术的地面沉降自动化监测系统

光纤监测技术具有分布式、精度高等特点,在地面沉降监测中具有独特优势。但受限于监测成本较高与监测环境复杂,目前地面沉降光纤监测多通过人工采集数据,限制了在特殊环境变化情况下地面沉降的实时信息获取。文章在地面沉降钻孔全断面光纤监测技术的基础上,设计并建立了基于弱光栅技术的地面沉降自动化监测系统。该监测系统利用弱反射光栅、时分复用、物联网和数据库等技术,通过4G无线通信手段实现了地面沉降在线自动化监测和远程数据实时采集,并通过客户端系统软件实现数据的存储、查询和分析。将其应用到衡水地区地面沉降监测中,结果表明:钻孔内土层压缩变形主要发生在以黏性土为主的隔水层（Ad2、Ad3、Ad4）;受季节性地下水开采的影响,钻孔100～400 m深度范围内砂土含水层存在波动变化,在监测期内,冬季略回弹,随后春季地下水开采量增大,地下水位下降,土层呈现压缩趋势。监测结果验证了该系统的可行性与准确性,使得整个地面沉降监测流程趋于自动化、规范化和低成本化,具有广泛的应用前景。     

基于DFOS的排灌水条件下土体变形响应模型试验研究

黏土层和砂土层交替变化的多层土体在强烈开采地下水作用下极易产生压密固结而引发地面沉降灾害。本文针对含水层释水引起地面沉降问题,研制了地面沉降试验装置,进行了排灌水条件下含水层系统的沉降及回弹试验。采用分布式光纤感测技术对土体内部应变分布及含水率变化进行耦合监测,并分析了各分层对水位变化的响应特征。结果表明:黏土层和砂土层均表现出了排水压缩和灌水回弹特点,黏土层变形较砂土层明显。各层变形与含水率变化具有良好的对应关系,表现为砂土层变形和含水率变化基本同步,而黏土层变形略微滞后于含水率变化。黏性土压缩曲线具有明显的分段特征,排水时当含水率低于液限后迅速减小,黏土层压缩速率明显加快;回灌时当含水率高于液限后,回弹速率明显加快。试验结果对研究地面沉降机理、评价地面沉降潜力及地下水利用具有重要意义。     

分布式光纤监测技术在三峡库区马家沟滑坡中的应用

针对传统感测技术的不足,本文开展了滑坡分布式光纤感测技术(DFOS)研究。对当前主要的几种DFOS技术及其相关原理做了简单的介绍,重点阐述了分布式传感光纤在三峡马家沟滑坡的布设方式。根据马家沟滑坡的工程地质环境特点,在抗滑桩内布设应变和温度感测光纤;沿坡体走向,采用直埋和定点相结合的方式开槽植入感测光纤;沿坡体纵向,在不同高程设置光纤综合监测孔,将分布式感测光纤粘贴于测斜管外壁和直埋于测斜孔内,同时在孔内布设FBG准分布传感器。监测结果表明:各类应变传感光纤均有效识别和定位了边坡表面变形的异常位置,证明了DFOS技术应用于滑坡监测的优越性。     

分布式光纤监测在阜阳地面沉降监测中的应用

分布式光纤监测是利用光纤作为传感介质,采用先进的OTDR技术,探测出沿光纤不同位置土层的应变情况,实现真正分布式测量。阜阳地面沉降是由于过量抽汲阜阳城区地下水引起水位下降,在正常固结和超固结[1]土层分布区,由土层固结压密而造成的大面积地面下沉现象。省地质测绘院、阜阳市测绘院、安徽省地质调查研究院在该区域进行大量的高程测量工作,仅查明了地面沉降范围及垂直沉降量,但对地面沉降的形成机理,各压缩层的变化量缺少统一认识。采用分布式光纤监测技术能准确测出各压缩层的变化关系,为阜阳地面沉降防治研究提供科学依据。     

基于层间位移协调的承压水降压引起土层变形分析

承压水降压引起的地面沉降由含水层、弱透水层和潜水层的变形组成。当承压层降压时间短、弱透水层固结变形较小时,可以假设弱透水层为严格的隔水层。采用层状弹性体系理论,基于位移协调条件分别建立了单井抽水以及第三类基坑工程降水（隔水帷幕插入降水含水层）引起的土层变形分析模型,与数值模拟和现场抽水试验结果的对比,验证了文中方法的正确性。研究结果表明,上覆土层弹性参数变化对地表变形的影响可以忽略;抽水井附近的土层变形呈现“上小下大”特点,一定距离以外含水层与地面沉降大致相等,根据承压含水层降深要求,可估算出基坑外的水位降深。     

武汉一级阶地基坑降水引起土层水位变化及压缩变形研究

武汉一级阶地地层具有典型的二元结构特征,从浅层至深层土体渗透性逐渐增加,在基坑工程开挖降水过程中,随着疏干含水层水位的下降,各层土体水位变化特点各不相同,水位的变化影响着土体固结的压缩变形。采用现场群井抽水试验的方法,通过在抽水试验期间对不同深度含水层水位的观测及不同深度土体沉降的监测,结合各层土体性质及相关理论对武汉一级阶地基坑降水引起的土层水位变化及压缩变形规律进行研究。结果表明,武汉一级阶地基坑降水水位变化存在着明显的时空效应,沉降与水位变化密切相关,且沉降过程中土层中出现架空效应。     

常州地区地面沉降及地层压缩性研究

系统分析了常州地区地下水开采动态和地面沉降发生发展历史,概述了研究区地面沉降的框架,在此基础上,系统研究了常州市分层标从1984年至2002年的分层沉降资料。根据常州地区地下水含水层系统及土层特性,将研究区松散土层垂向划分为四个不同层次,分别研究了它们的压缩变形历时特性及其与累计地下水开采量的关系,研究了各自的应变特性。由于土层结构及物理力学性质的不同、地下水开采层次及强度的差异、土层不同应力历史的影响等诸多因素的综合效应,导致了地面沉降及分层压缩特性的显著差别。常州地区的主要压缩层为第II承压含水层的顶板弱透水层,与含水层距离近的土层变形量及应变量均较大,其次是第II承压含水层本身及其与第III承压含水层之间的弱透水层。地面沉降及地层压缩与地下水开采之间的滞后效应在常州地区表现得并不明显,这一点至少在月或年的时间尺度上是正确的。     

吴江盛泽地区建筑荷载和地下水开采对地面沉降的影响

为了准确分析建筑荷载和地下水开采对地面沉降的影响,为吴江盛泽地区科学防控地面沉降提供决策依据,基于比奥固结和地下水渗流理论,建立了建筑荷载和地下水开采与地面沉降三维全耦合有限元数值模型,分别模拟预测了在建筑荷载单独作用、地下水开采单独作用及建筑荷载和地下水开采叠加作用三种情况下,2015-09-01—2030-08-31盛泽地区逐年地面沉降变化趋势。结果表明,建筑荷载对盛泽地区地面沉降的影响大于地下水开采。第Ⅰ黏性土弱含水层和第I承压含水层分别为建筑荷载和地下水开采单独作用下的主压缩层,单层压缩量占比分别为43.04%和54.06%;第Ⅰ承压含水层及其上覆第Ⅰ黏性土弱含水层是二者叠加作用引发土体变形的主压缩层,其压缩量之和占总压缩量的71.30%。建筑荷载和地下水开采单独作用下引发的地面沉降量的线性叠加之和大于二者叠加作用下引发的地面沉降量,建筑荷载和地下水开采叠加作用引发的地面沉降具有耦合效应。     

软土地区地铁道床沉降特征及其诱发因素分析

以上海4条地铁线路道床长期沉降监测资料为基础,分析了地铁隧道的纵向沉降特征;结合该地区地质环境调查资料,深入分析地铁隧道沉降与下卧层地层结构、浅部地下水位变化以及区域地面沉降三个地质环境因素之间的相互作用关系。结果表明,地铁隧道下卧地层结构差异是地铁隧道沉降差异性的重要地质环境因素,深基坑降排水引起降水目的含水层地下水位下降,使得降水目的层以及相邻软黏性土层发生较大的压缩变形,导致以降水目的层上覆软黏性土层为承载体的地铁隧道也随之发生沉降,区域地面沉降也是地铁隧道沉降的重要影响因素。     

上海市地下水位大幅抬升条件下土层变形特征分析

过去对地下水位持续下降条件的地面沉降研究较多,但对水位大幅持续抬升过程中的地面沉降研究较少。本文根据 上海大量地面沉降、水位观测和钻孔资料,系统分析上海市90年代末以来地下水位大幅抬升条件下各土层的变形特征。自 1998年以来,上海市通过大幅压缩开采量、回灌地下水等措施使第二、三、四和五承压含水层水位分别平均抬升2.1 m, 3.6 m, 12.4 m, 12.7 m。水位的抬升使上海市地面沉降平均速率由1998年的12.2 mm/a减小到2011年的1.83 mm/a,减少85%。 通过对27组分层标数据分析发现：现阶段主要压缩层位在第一、二软土层,年沉降速率为2~4 mm/a;而第二含水层以下土 层已经有少量回弹。在水位持续大幅抬升过程中,本文总结了两种变形特征：1） 变形和水位变化基本同步,残余变形量非 常小,变形可概化为线弹性变形,这种变形主要发生在第一、二、三和五承压含水层、第五和六弱透水层;2） 压缩速率逐 渐减小,无明显持续回弹趋势,有较大残余压缩量且存在变形滞后现象,变形可概化为弹塑性变形,这种变形主要发生在 地第二、三和四弱透水层。第四承压含水层变形较复杂,两种变形特征都有。其中较大残余变形量主要由塑性贮水率比弹 性贮水率大2个数量级引起;变形滞后主要由弱透水层中超孔隙水压力消散较慢引起。本文研究成果对于掌握水位抬升过 程中土层变形方式、发生发展机理、预测未来地面沉降及地下水科学管理和资源评价具有重要意义。     

Investigation into subsidence hazards due to groundwater pumping from Aquifer II in Changzhou,China

This paper presents an investigation into increased deformation of Aquifer II caused by groundwater pumping from the aquifer in Changzhou, China. As groundwater levels of aquifers have been decreasing in recent decades due to uncontrolled water pumping, land subsidence is becoming a serious geohazard in Changzhou. Based on recently reported field data, the compression of aquitards has not increased compared to that of aquifers with the same scale of layer thickness. The Cosserat continuum model was adopted to analyse the observed phenomenon in this study. A classic Cauchy continuum model is also used for comparison. The comparison between these two models indicates that the proposed approach can interpret the increased deformation well, and the classic Cauchy continuum model underestimates the aquifer deformation as it does not consider shear displacement and macro-rotation. A discussion on the relationship between the groundwater level in the aquifer and subsidence is then undertaken. The results show that the severity of the annual subsidence is correlated with the variation in groundwater level in Aquifer II. To mitigate the subsidence hazards, countermeasures should be adopted to avoid the shear stress in aquifers which results from the high hydraulic gradient, by the appropriate allocation of pumping wells and by restricting groundwater withdrawal volume from each pumping operation.     

Review: Regional land subsidence accompanying groundwater extraction

The extraction of groundwater can generate land subsidence by causing the compaction of susceptible aquifer systems, typically unconsolidated alluvial or basin-fill aquifer systems comprising aquifers and aquitards. Various ground-based and remotely sensed methods are used to measure and map subsidence. Many areas of subsidence caused by groundwater pumping have been identified and monitored, and corrective measures to slow or halt subsidence have been devised. Two principal means are used to mitigate subsidence caused by groundwater withdrawal??reduction of groundwater withdrawal, and artificial recharge. Analysis and simulation of aquifer-system compaction follow from the basic relations between head, stress, compressibility, and groundwater flow and are addressed primarily using two approaches??one based on conventional groundwater flow theory and one based on linear poroelasticity theory. Research and development to improve the assessment and analysis of aquifer-system compaction, the accompanying subsidence and potential ground ruptures are needed in the topic areas of the hydromechanical behavior of aquitards, the role of horizontal deformation, the application of differential synthetic aperture radar interferometry, and the regional-scale simulation of coupled groundwater flow and aquifer-system deformation to support resource management and hazard mitigation measures.     

Characteristics of aquifer system deformation in the Southern Yangtse Delta, China

Excessive groundwater pumpage has caused regional cones of depression and severe land subsidence in the Southern Yangtse Delta, China. The characteristics of aquifer system compaction are complex because of the difference in the types, compositions and structures of the soils that the hydrostratigraphic units are composed of, and in the histories of groundwater level change the hydrostratigraphic units have experienced. Based on the data measured from extensometer groups and observation wells, the characteristics of deformation for individual hydrostratigraphic units are analyzed. The results show that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at different sites of the cone of depression or in different periods. If the groundwater level rises continuously and remains constant later, the aquifers and the aquitard units that consist of hard clay primarily exhibit elastic behaviour, but the aquitard units that consist of soft clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour. If the groundwater level falls but is much higher than the previous lowest value, aquifers and aquitards consisting of hard clay exhibit elasto-plastic behaviour. If the groundwater level falls below the previous lowest value, aquifers and aquitards consisting of hard clay deform plastically and by creep and exhibit visco-elasto-plastic behaviour.     

Excessive groundwater withdrawal and resultant land subsidence in the Su-Xi-Chang area,China

Excessive groundwater withdrawal has caused severe land subsidence in the Su-Xi-Chang (SXC) area, China. The restriction and prohibition on groundwater pumping have been carried out since the late 1990s. Based on the latest updated field data, the changing pattern of groundwater level and the distribution of land subsidence are analyzed. The distribution of land subsidence in SXC is closely related to that of the cone of depression in the second confined aquifer in time and space. But land subsidence is not in synchronization with the changing groundwater level. Both aquitards and aquifers compacted continuously in the early period of groundwater level rising and behaved as creep materials. A series of laboratory tests were conducted on aquifer sands, which indicated that the creep deformation under virgin compression is much greater than that under recompression and unloading, and that the creep of sands decreases rapidly with the cycles of repeating load. The test results reveal the mechanism of sand creep under the condition of long-term groundwater pumpage. As a consequence of the restriction and prohibition on groundwater pumping, groundwater level has obviously recovered in the vast majority of the SXC area, and land subsidence has slowed down and even a little rebound has occurred in some sites in Suzhou and Changzhou. If the pumpage is strictly limited continuously, the groundwater level will not decline below the historical lowest value but fluctuate within a certain range. In such a case, land subsidence in SXC will no longer develop obviously.     

地面沉降对含水层参数及给水能力的影响研究

地面沉降主要由深层地下水开采造成,含水层的压缩释水是深层地下水开采量主要构成来源,同时随其压缩变形孔隙比减小造成储水系数、渗透系数的减小,对弱透水层非弹性释水量、越流补给量造成影响。地面沉降过程中的水文地质参数非线性变化及对承压含水层系统的反馈作用也成为水文地质领域的前沿问题。为对地下水开采量、沉降量、地质参数变化以及给水能力变化之间的关系做一个较为定量定性的探究,以含水层压缩过程中的物理机制为依据,并基于沧州地区深层承压含水层的地下水位变化过程和水文地质参数,采用数学手段构建储水系数和渗透系数变化的一维非线性沉降模型。模拟结果显示随承压水头下降,储水系数最高可减小77%,含水层系统的给水能力和储水能力会随压缩变形减小50%甚至更多。研究成果为深入认识水文地质参数与应力变化相关关系、科学评价承压含水层地下水储水调节能力有重要借鉴意义。     

Subsidence due to pumping from layered soil—a perturbation theory

Persistent pumping of groundwater is known to cause subsidence of the ground surface. When the soil is composed of alternating layers of sand and clay, subsidence is, in general, a consequence of pressure change in and deformation of all layers. Even with various simplifications, the mathematical task for the mechanics of a layered soil is complex and, in the past, many idealizations have been introduced in the literature. Among the existing approximations, a well-known approximation by Hantush¹ for well-hydraulics in leaky aquifers is intuitively attractive. He used the fact known to be strictly true for simple flows in a soil system with horizontal layers, that the flow must be mainly horizontal in a relatively porous aquifer and mainly vertical in a highly impervious aquitard, and invoked the same approximation for transient flows induced by well-pumping. In this paper we shall apply a perturbation analysis to give a mathematical confirmation of the hydrological approximation and to show its degree of accuracy. In particular, we treat a three-layered system, where the middle layer is a relatively soft aquitard, and the well withdraws water from the artesian aquifer at the bottom. The upper aquifer has a phreatic surface. Physically, we examine the often ignored effects of self-weight which is important for thick aquitards. The perturbation theory is worked out for sufficiently weak pumping rates and small soil deformation. Subsidence in the vertical direction due mainly to the deformation of the soft aquitard is studied. Results show that the common assumption of constant total stress with respect to depth is valid only when the storage coefficient is very small.     

Regional land subsidence simulation in Su-Xi-Chang area and Shanghai City, China

Su-Xi-Chang area and Shanghai City, located in the south of Yangtze Delta, China, has subsided due to groundwater overpumping. Because of the regional scale of the groundwater exploitation, cone of depression and land subsidence at present, Su-Xi-Chang area and Shanghai City are treated as a single area for land subsidence study to avoid the uncertainty of boundary condition due to the regionalism. The characteristics of aquifer system compaction are complex because of the difference in the types, compositions and structures of the soils that the hydrostratigraphic units are composed of, and in the histories of groundwater level change the hydrostratigraphic units have experienced. Considering the fact that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at different sites of the cone of depression or in different periods, corresponding constitutive laws have been adopted. This avoids the shortcomings of the previous research that the same constitutive law was adopted in all the hydrostratigraphic units during the entire time period. A coupled flow and subsidence model, which includes a three-dimensional flow model with variable coefficients and a one-dimensional (vertical) subsidence model, is built according to the complicated hydrological condition in the region. The simulation model is calibrated using observed data, which include compression of individual strata from groups of extensometers and groundwater levels from observation wells from 1995 to 2002. The model reproduced that the primary subsidence layer in Shanghai shifts from the shallow aquitard to the fourth confined aquifer because of the groundwater yield variations and the change of exploitation aquifers. However the third aquitard was the primary subsidence layer in Su-Xi-Chang area and the compaction deformation of the sandy aquifers was remarkable. The simulation results could provide some reasonable advice about groundwater exploitation in the future.     

应力历史对弱透水层参数影响试验研究

弱透水层储存的地下水是地下水资源的重要组成部分,弱透水层水文地质参数对地下水资源管理与评价以及地面沉降等具有重要意义。通过室内试验对定降深条件下弱透水层水流运移规律进行了研究,并探讨了弱透水层释水过程中的变形规律和释水规律。试验结果表明,在相邻含水层定降深条件下,弱透水层固结变形滞后于相邻含水层的降深变化,且变形速度逐渐变小并趋于零。基于室内试验,利用配线法求得弱透水层不同应力状态下的水文地质参数,对比分析,大变形和小变形试验中土层的渗透系数变化不大,但贮水率明显变小,土层的固结系数变大。因此,土层的应力历史对土层的渗透系数影响不大,而对贮水率有较大的影响。     

常州市地面沉降以及用水化学标志研究的探讨

常州市过量开采第Ⅱ承压含水层的水使水位大幅度下降,造成含水砂层压密和粘性土层固结而导致地面沉降。试验资料表明该区粘性土层的压缩性较低,它们的压密主要是由主固结阶段完成的。视压榨液,浸提液成分和含水层多年水质的变化,以及抽水时水质与氚量资料说明粘性土层释水在造成本区地面沉降中起着相当重要的作用。这种条件下,用对含水层水化学成分的检测来监视地面沉降的发生、发展是一个经济可靠的方法。