土石混合体填筑路堤中的非线性蠕变模型探析

针对土石混合体填筑路堤工后沉降的非线性蠕变特性,提出了一种能考虑材料非线性黏弹性变形的蠕变模型。基于目前对土石混合体蠕变机理研究,将路堤工后的蠕变沉降部分考虑为非线性的衰减蠕变过程,建立了修正的Kelvin模型、广义修正模型,且得到了模型参数的识别方法;通过对修正模型分析表明,非线性常数取值对蠕变曲线变化有较大影响,修正模型的蠕变长期应变值比Kelvin模型要小;结合相关文献中对土石体填料的蠕变试验研究资料,用修正Kelvin模型对试验结果进行拟合,并对参数进行了反演,反演效果良好。修正模型的建立可为编制非线性黏弹性数值计算软件提供理论基础。     

桥头粉喷桩复合地基工后沉降原因分析

为了研究引起搅拌桩复合地基工后沉降的原因,针对粉喷桩加固桥头软基沉降进行Biot固结有限元正反分析。有限元分析时的土体本构模型采用Duncan-Chang非线性弹性模型,反分析时的土体本构模型分别采用Duncan-Chang非线性 弹性模型和Merchant黏弹性模型。计算分析表明,引起粉喷桩加固地基工后沉降的主要因素：早期时是交通荷载引,而后期土体蠕变的影响则占主导地位。     

基于西原模型的变参数蠕变方程

岩石材料的蠕变具有明显的非线性特征,在蠕变模型中考虑蠕变参数随应力、时间等因素而变化是解决非线性蠕变问题的方法之一。试验已经证实,应力、时间及渗流等外部条件会引起岩石某些力学参数的改变,因此,将蠕变参数视为变量符合岩石材料的本质属性。以西原蠕变模型为基础,研究了各蠕变参数与应力及时间的关系。根据试验结果,通过非线性最小二乘法从蠕变试验数据中求出不同应力作用下不同时刻的各个蠕变参数,再根据参数随应力、时间的分布规律经非线性拟合得到各蠕变参数与应力及时间的表达式,进而可得到变参数的蠕变方程。蠕变参数的变化是岩石非线性特征的充分体现,从损伤力学的角度来讲,蠕变参数的变化反映了岩石内部损伤不断加剧、累积,材料特性不断劣化的演变过程。     

预压作用下软土微观结构试验及弹黏塑性模型研究

为了揭示软土固结蠕变机制,计算软基“工后沉降”,开展了室内模拟真空、堆载以及真空-堆载联合预压加固软土的固结蠕变三轴试验及三轴试验前、后的微观结构试验。建立了能考虑微观结构改变的弹黏塑性(EVP)软土蠕变模型。研究结果表明：原状软土样在真空预压、堆载预压、联合预压3种预压荷载作用下,固结过程和蠕变过程具有耦合效应;土体微观结构参数的改变与宏观工程性质的改变具有相辅相成的关系;EVP模型弱化了具体应力路径的变化,其反映弹性的参数和反映黏塑性的参数可由微观结构参数确定;改进的EVP模型既能计算土体的弹黏塑性变形,也考虑了土体微观结构的变化,能更全面地反映土变形的实质。     

基于变渗透系数的深基坑单井抽水沉降研究

地下水抽采是造成地表沉降的重要原因。基于弹性多孔隙介质Konzeny-Carman渗透模型,考虑地层在抽水过程中变形导致土层内部物理力学性质的改变,建立了基于土体应变的变渗透系数模型,并引入了渗透影响系数概念。运用有限元多物理场耦合软件COMSOL建立模型,采用Darcy渗流模式与弹性多孔隙介质完全耦合方法,研究了抽水过程中地层沉降量与渗透影响系数以及渗透系数的变化规律,并与实测沉降值进行对比。研究表明,采用考虑土体变形的变渗透系数模型所计算的沉降量与实测沉降变形较好地吻合,该模型能更好地反映抽水过程中沉降动态过程。     

我国地面沉降模拟现状及需要解决的问题

我国现有模型有下列特点:水流模型都是准三维模型,模型中的参数都是常数,沉降模型采用线弹性模型,水流模型和沉降模型的耦合分二步进行。因此,模型还存在难以精确刻画由弱透水层释水引起的地面沉降,不符合准三维模型的应用条件。水流方程应是一个变系数的动态水流方程,渗透系数、贮水率不应视为常数。土层压缩非线性,有蠕变、塑性变形存在,沉降模型采用线弹性模型不适合,对土层厚度变化大的地区沉降模型必须是三维的,滞后现象必须克服。如何描述引起地裂缝的水平位移,分两步进行的耦合不合适,要做到水流模型与沉降模型的真正耦合等八个方面的步骤。本文提出分三步进行的解决方案:首先采用三维水流模型,以便不仅给弱透水层建立方程,还能考虑它的各向异性,参数值能随着土层的变形沉降而不断改变。沉降模型为能考虑蠕变、塑性变形的非线性模型;水流模型和沉降模型要做到能真正耦合。其次,解决滞后问题。最后,建立三维沉降模型,考虑水平位移和地裂缝问题。     

滨海软土非线性流变模型及其工程应用研究

基于室内一维固结蠕变试验,研究了天津滨海淤泥质软黏土的蠕变变形规律,建立了反映滨海软黏土的非线性流变本构模型,确定了模型中的各个参数。通过此本构模型推导出天津软黏土的沉降计算公式,利用该公式对天津滨海地区北塘水库大坝软土堤基进行了沉降计算和分析,计算值与实测值比较吻合,证明了流变模型的合理性,同时也说明在软土地基沉降计算中,考虑非线性流变性状的影响是必要的。并对天津滨海新区软土长期蠕变沉降作出预测。     

考虑循环变温的软土热固结模型

在能源岩土工程发展需求背景下,土体工程特性的温度效应研究受到了国内外学者的广泛关注。针对现有关于循环变温热固结理论研究的不足,提出考虑循环变温条件下土体先期固结压力随温度循环次数变化的关系式,计算多次循环变温后软土的固结压缩量。在此基础上,推导考虑循环变温的太沙基固结修正公式,分析整个循环变温过程中孔压和固结沉降的变化规律。最后通过与单元体测试结果进行对比,验证该计算模型的可靠性。研究结果表明,在循环变温和瞬时荷载的耦合作用下,土体的沉降变形和孔压随温度循环次数的变化而变化,且影响程度与循环次数相关;新型热固结模型为涉及循环变温的岩土工程问题分析提供了一定的理论支撑。     

考虑应力应变时间效应的桩基长期沉降计算方法

土体的应力应变时间效应对桩基长期沉降有重要影响。在长期荷载作用下桩基的沉降主要包括两个部分：桩尖刺入量和桩底土体的蠕变压缩沉降。采用Mesri蠕变模型描述土体的蠕变行为，用桩的Mindlin应力公式计算桩端和桩侧荷载在桩端平面以下产生的附加应力，用Boussinesq应力公式计算承台分担荷载产生的应力。分别给出了桩尖刺入量和桩底土体蠕变压缩量的计算方法，从而提出了一种考虑土体蠕变效应的桩基长期沉降计算方法。计算结果与桩基长期监测结果较为吻合，表明在获得可靠的计算参数后可用提出的方法估算桩基长期沉降。     

饱和软土大变形非线性自重固结模型

基于分段线性方法,建立了饱和软土一维自重固结模型（简称SWC模型）。该模型能考虑自重固结过程中土体的大变形效应和材料参数的非线性变化。将该模型的计算结果与相关解析解、现场试验及室内试验结果进行了对比验证,证明了SWC模型能准确计算出大变形和非线性条件下饱和软土的自重固结过程,包括沉降量、平均固结度、孔隙比分布和超孔隙水压力分布等参数随时间的变化过程。随后,以现场试验为基础,采用SWC模型对饱和软土自重固结的4个主要影响因素（即土体初始高度、边界排水条件、初始孔隙比和土粒相对密度）进行了参数分析。结果表明,上述4个参数对软土自重固结过程均具有重要影响：土体初始高度越高,则自重固结沉降量和最终平均应变值越大;边界排水条件对土体自重固结的速度有重要影响,但不影响自重固结的最终沉降量;初始孔隙比越大,则自重固结沉降量越大,其完成自重固结所需时间越短;土粒相对密度越大,则土体的最终沉降量越大,完成自重固结所需时间越短。     

Characterization of land subsidence induced by groundwater withdrawals in Su-Xi-Chang area, China

Su-Xi-Chang area is one of the typical regions in China which suffers from severe land subsidence. Various tools of field monitoring were integrated to study the characteristics and mechanisms of land subsidence in this region. The occurrence and the development of the land subsidence in this region are strongly related to the groundwater pumping both in time and space. The main consolidation layers are the soft mud layers; however, the compressibility of the confined sandy layers should not be ignored. The second and third confined aquifers contributed more than 30% of total subsidence. Meanwhile, irrecoverable deformations were also observed in the sandy layers. Different sandy layers deform diversely under different stress conditions. Some have the elastic feature. But the soil strata, including both sandy layers and clayey layers, located in the center of the groundwater level depression cone exhibited obvious viscous mechanical behavior which caused the common lag phenomenon. The sand composition (mingled with small clay particles or interbeds) and sand rheology are the two main reasons for the lag phenomena in sandy layers. A series of laboratory tests for modeling the effective stress changes due to groundwater withdrawals, were conducted to investigate the mechanism of the lag phenomenon. Based on the test results, the relationship of stress–strain–time for saturated sands is obtained; and it could be expressed as power functions. The results also showed that the compression of the sandy layers was time dependent, and its deformation could be remarkable. When establishing land subsidence model, the deformation for the similar soil formation could be elastic, visco-elastic and even visco-elastic–plastic, because of the different groundwater level fluctuation experienced.     

The development and control of the land subsidence in the Yangtze Delta,China

Land subsidence in China occurs predominantly in 17 provinces (cities) situated in the eastern and middle regions of the country, including Shanghai, Tianjin and Jiangsu, and Hebei provinces. It is primarily caused by groundwater overpumping. One of the areas most severely affected by land subsidence is the Yangtze Delta, most of which consists of Shanghai City, the Su-Xi-Chang area (Suzhou, Wuxi and Changzhou cities) of Jiangsu Province, and the Hang-Jia-Hu area (Hangzhou, Jiaxing and Huzhou cities) of Zhejiang Province. The excessive exploitation of groundwater forms in a large regional cone of depression and, consequently, land subsidence is also regional, currently centered in the Shanghai and Su-Xi-Chang areas. In 2002, the maximum cumulative subsidence of Shanghai, Su-Xi-Chang and Hang-Jia-Hu were 2.63 m, 2.00 and 1.06 m, respectively. The land subsidence area is continuing to expand throughout the Yangtze Delta. To study the characteristics and the pattern of this land subsidence, the government has implemented a monitoring system involving the placement of 37 groups of extensometers (layers marks) and drilling of more than 1000 observation wells. These provide an invaluable historical record of deformation and pore water pressure and facilitate studies on the special features of soil deformation when the groundwater level changes due to pumping. Several measures have been taken in recent years to control the development of the land subsidence in the different areas; these include groundwater injection, prohibition of pumping deep confined groundwater, and an adjustment of the pumping depth and magnitude of the groundwater withdrawn. At present, although the subsidence area is still increasing slowly, the subsidence rate is controlled.     

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.     

Earth fissures in Jiangsu Province,China and geological investigation of Hetang earth fissure

Earth fissures are a geohazard in Jiangsu Province, China. They can be caused by earthquakes and active faults, underground mining, groundwater extraction and landslides. In order to establish a provincial rehabilitation plan in Jiangsu, a range of monitoring programs, surveys, geological investigations and modeling have been implemented or planned. One of the focuses of the project is the land subsidence and earth fissures caused by excessive groundwater withdrawal in Suzhou, Wuxi and Changzhou (Su-Xi-Chang) area, southern Jiangsu Province. Hetang earth fissure within the Su-Xi-Chang area was first reported in 1995 and a series of investigation has been conducted since then. The site investigations and geophysical survey in 1997 have recognized the causative factor as the excessive groundwater drawdown coupled by the underlying bedrock ridge. An open trench excavation in 2007 and a plane strain analysis suggest that Hetang earth fissures may have cracked from the bedrock ridge to ground surface. Geological drilling in 2007 has further confirmed the existence and configuration of the ridge and extracted soil samples for laboratory tests to obtain soil parameters for numerical analyses and modeling of land subsidence and earth fissures in the Su-Xi-Chang area, Jiangsu, China. The laboratory tests are currently in progress and the result of numerical analyses and modeling is expected to be presented in the near future.     

弹塑性变形条件下抽水引起的地面沉降三维数值模拟

基于部分耦合原理,采用TOUGH2和FLAC3D建立抽水引起的三维地面沉降弹塑性模型,模型中综合考虑土体的弹塑性变形特征、渗流-应力的双向耦合作用以及参数的非线性,探讨了持续抽水和脉冲抽水两种抽水过程中地面沉降发展演化过程。研究结果表明:（1）集中抽水停止后地面沉降会发生回弹,抽水中心沉降量不断减小。由于水平方向存在水力梯度,地下水继续向地下水位漏斗中心渗流从而导致沉降漏斗的范围仍继续扩大;（2）脉冲抽水导致土体的孔隙水压力、渗透系数以及沉降量均呈周期性波动变化,地面沉降会局部回弹,但总体仍随着抽水的持续,沉降量不断增加;（3）在抽水量相同前提下,对比持续抽水与脉冲抽水两种方式引发的塑形沉降量可知,抽水速率小、脉冲式多次开采导致的塑性沉降量较小,持续抽水的抽水速率越小、脉冲抽水间隔越短越有利于控制地面沉降。研究成果为地面沉降数值模拟提供了一种新方法,其中算例研究能为抽水条件下地面沉降的控制提供参考。     

Numerical simulation of land subsidence induced by groundwater overexploitation in Su-Xi-Chang area,China

Su-Xi-Chang area is one of the typical regions in China which suffers from severe land subsidence. Various field monitoring records were integrated to study the characteristics and mechanisms of land subsidence in this region. The development of the land subsidence in this region shows a tight spatial and temporal correlation with the groundwater pumping. Based on the analysis of the field data, it is found that the deformation patterns of the hydrogeologic units are greatly related to the hydrogeologic properties and groundwater level variations. Some have an elastic behavior, others may have an elastic–plastic rheology. Hence, a 3D finite element numerical model considering the rheological properties of the soil was developed to simulate the groundwater level and land subsidence. Both hydraulic conductivity and specific storage were expected to vary with the porosity during the process of consolidation. Multiscale finite element method (MsFEM) was applied to solve the model during the period from 1996 to 2004. After calibrating the model with the observed groundwater level and subsidence data, the parameters of the multi-layers system were estimated. The calibrated model outputs fit reasonably well with the observed data. Consequently the model can be applied to predict groundwater level and land subsidence in future pumping scenarios. The model predictive results show that land subsidence rate can be controlled and even rebound may occur after the implementation of the groundwater exploitation prohibition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sustainable development and utilization of groundwater resources considering land subsidence in Suzhou,China

Suzhou is located at the lower reaches of the Yangtze River in southeastern Jiangsu, China. It is part of the Su-Xi-Chang area including Suzhou, Wuxi and Changzhou. As one of the most developed areas in China, this region has suffered from severe land subsidence caused by extensive groundwater exploitation since 1980s. The land subsidence was controlled by prohibition of groundwater exploration in the past several years. However, the surface water pollution prompted a new task of how to sustainably utilize the groundwater resource, especially to satisfy the emergency demands of water supply. In this paper, we took Suzhou as a representative case to discuss how to develop groundwater resources while controlling the land subsidence. The relationship between the deformation and the groundwater level was analyzed, with focus on the deformation features after the period of groundwater exploitation ban. The results confirmed the conclusion by Shi et al. (2007, 2008a): even in the period of rising groundwater level, same units may manifest different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at different locations of the cone of depression. A land subsidence model that couples a 3-D groundwater model and a 1-D deformation model was developed to simulate the groundwater level and deformation. A high-resolution local grid (child model) for Suzhou was built based on the regional land subsidence model of Su-Xi-Chang area by Wu et al. (2009). The model was used for a number of predictive scenarios up to the year of 2012 to examine how to develop sustainable use of groundwater resources under the conditions of land subsidence control. Our results indicated that about 3.08 × 10⁷ m³/a groundwater could be provided as emergency and standby water source while meeting the land subsidence control target of 10 mm/a.     

土体自重固结压缩对地面沉降影响研究

以南通市为例,建立了土体自重固结压缩、地下水开采与地面沉降三维全耦合模型.在对模型进行识别、验证的基础上,模拟预测了南通市在地下水停止开采,仅在土体自重固结压缩影响下,自2010年12月31日-2025年12月31日各含水层组地下水流场变化和地面沉降的发展趋势.结果表明,在此期间,南通市由土体自重固结压缩引起的最大地面沉降量为2.42 mm,最大地面沉降速率为0.16 mm/a,土体白重固结压缩对南通市地面沉降的影响极为有限.     

地下水渗流与地面沉降耦合模拟

为了准确模拟由地下水开采导致渗流场和应力场发生变化而引起的地面沉降问题,根据Terzaghi有效应力原理,建立了地下水三维渗流与一维垂向固结的地下水渗流与地面沉降耦合数值模拟模型和以比奥固结理论为基础,并结合土体非线性流变理论,将土体本构关系推广到粘弹塑性,同时考虑土体力学参数及水力参数的动态变化关系的地下水渗流与地面沉降三维全耦合数值模拟模型.通过对比分析,结果表明:基于Terzaghi有效应力原理建立的地下水三维渗流与一维垂向固结地下水渗流与地面沉降耦合数值模拟模型模拟所得地面沉降与地下水位呈现出同步变化的趋势,并且当地下水位逐步回升至初始水位时,地面沉降也逐步回升到初始的零沉降状态.而以比奥固结为基础建立的地下水渗流与地面沉降三维全耦合数值模拟模型模拟所得的地面沉降变化趋势滞后于地下水位的变化趋势,并且当地下水位逐步回升至初始水位时,地面沉降虽也逐步得到回升,但回不到初始的零沉降状态,存在一个永久的残余沉降量.在土体参数变化方面,土体的孔隙度、渗透系数及泊松比均呈现先减小后增大的变化趋势,而弹性模量则呈现先增大后减小的变化趋势,与地面沉降的变化相对应.      

进化策略算法在地面沉降量计算参数反演中的应用

多数情况下,评估的研究区缺乏多年积累的地下水位及开采量资料。因此,通常采用土力学模型,根据太沙基固结理论,进行地面沉降量预测。在地面沉降量计算中,土层变形参数值的选取直接影响计算结果的精确性。深层含水层埋深通常大于100m。通过现场勘探,采取土样进行岩土物理力学性质试验获取参数值,成本过于昂贵。通过参考区域土层物理力学性质资料,结合工程经验获取参数值,预测结果的精度得不到保证。针对上述情况,作者提出用非线性优化算法-进化策略算法,根据实测地面沉降量反演土层变形参数值。通过这种方法确定参数值,既节约成本,又保证了计算结果的可靠性。进化策略算法通过模拟生物遗传及进化过程,利用转移概率来帮助指导搜索。搜索结果不依赖于初始点的选择,对于求解全局最优解有很强的鲁棒性。作者将进化策略算法用于某一工程实例土层变形参数的反演,结果表明了该算法的可行性及稳健性,值得在工程实践中推广应用。