A simplified method for prediction of embankment settlement in clays简

The prediction of embankment settlement is a critically important issue for the serviceability of subgrade projects, especially the post-construction settlement. A number of methods have been proposed to predict embankment settlement; however, all of these methods are based on a parameter, i.e. the initial time point. The difference of the initial time point determined by different designers can definitely induce errors in prediction of embankment settlement. This paper proposed a concept named “potential settlement” and a simplified method based on the in situ data. The key parameter “b” in the proposed method was verified using theoretical method and field data. Finally, an example was used to demonstrate the advantages of the proposed method by comparing with other methods and the observation data.     

非饱和土地区高速铁路路基沉降预测模型

准确、合理地预测线路工后沉降是高速铁路建设的关键,现场沉降观测数据表明不同饱和度地区路基沉降曲线线型变化较大,基于饱和理论的沉降预测方法在工后沉降预测中存在不能准确描述沉降规律,且存在预测值偏小的风险。基于实测沉降规律,提出了一个适用于非饱和土地区路基的沉降预测曲线模型。分析了预测曲线模型的特点,并基于最小二乘法给出模型参数的求解方法。结合兰新铁路第二双线LXS-15标段沉降数据,提出相关系数、偏差度、稳定度为模型有效性检验指标,结合施工完成2 a的实测资料,进行了在施工完成3个月及6个月所提模型与规范要求3种预测模型的对比研究;对3条不同饱和程度高铁路基沉降预测结果表明所提出的非饱和土预测模型具有较好的精度和广泛的适用性,为非饱和土地区高速铁路建设合理判断工后沉降提供参考。     

基于最优组合模型的路基沉降预测方法

通过监测获取的沉降——时间数据进行拟合曲线分析,在高速铁路路基沉降预测中得到了广泛应用。路基工程沉降——时间曲线呈现反S形,但采用常规的反S形成长曲线模型预测路基沉降时,由于受到数据样本和模型自身的限制,预测的沉降曲线并不能完全反映实际沉降的发展过程。基于此,提出了基于不同类型的反S形成长曲线的最优组合预测模型,通过赋予各个反S形曲线模型权重系数,采用最优化方法求解权重系数,得到最终的路基沉降预测公式。实际工程数据分析表明,该方法精度较高、适应性较好,可适用于高速铁路路基沉降量预测。     

路基沉降不确定性预测研究

路基沉降计算和预测是工程建设中一个重要的课题,但是很多情况下理论计算的沉降量与实际沉降量有较大的出入。所以必须对路基沉降进行监测,根据实测数据控制填土速率,保证路堤在施工中的安全与稳定;根据实测沉降曲线预测工后沉降,使工后沉降控制在设计允许范围之内。但沉降预测模型的参数是不确定的,用概率和概率分布去描述更加合适,将模型参数视为随机变量,基于贝叶斯理论和MCMC方法,借助WinBUGS软件,建立了贝叶斯沉降时间序列不确定性预测模型。实例分析表明,该方法所得结果合理可信,将其应用于路基沉降预测是可行和值得研究的。     

附加应力法计算刚性桩复合地基路基沉降

部分刚性桩复合地基路基实际沉降超过计算沉降的重要原因之一是现有沉降计算方法存在严重缺陷。在分析桩土沉降关系和桩土作用的基础上,提出了路堤下刚性桩复合地基沉降计算新方法——附加应力法。首先根据桩土作用计算桩土附加应力,然后采用分层总和法计算复合地基沉降。经工程实例验证后,利用附加应力法研究了桩长、桩间距、扩底、桩帽等因素对路基沉降的影响,并与现行方法计算的沉降进行对比。分析表明:附加应力法可以考虑单桩竖向承载力、桩帽转移荷载能力、桩土相互作用等因素的影响,计算沉降与实测沉降接近;利用桩帽将路堤大部分荷载转移到桩顶可以有效减小路基沉降;扩底比桩长加大更经济合理;按"强桩、大间距、大桩帽"原则设计的复合地基比密桩复合地基更经济合理。     

粉喷桩复合地基沉降预测的改进灰色模型

高速公路粉喷桩复合地基的沉降预测不仅可以反映路基的沉降发展趋势,而且可以通过预测的沉降量来反映出路基的加固效果及稳定性,为后续工序的开展提供指导。以某高速公路粉喷桩复合地基预压期的监测结果为例,运用改进的灰色模型建立非等时距的GM(1,1)预测模型,并用后验差法对其预测精度进行检验。结果表明,改进的灰色模型的预测精度较高,能作为粉喷桩复合地基沉降预测的一种方法。     

路堤荷载下双向增强复合地基荷载分担比及沉降计算

针对路堤荷载下双向增强复合地基受力变形特性,以单桩有效影响范围内的路堤与复合地基为分析对象,引入大挠度环形薄板考虑加筋垫层的“柔性筏板效应”与“拉膜效应”,同时通过假定桩土相对位移模式,考虑地基成层性,从而建立了路堤、水平加筋体、桩体、桩间土协调变形三维模型,获得了路堤荷载作用下双向增强复合地基的荷载分担比及沉降计算方法。采用某工程试验数据对该计算方法进行验证,同时分析了路堤高度、桩帽宽度、筋材抗拉模量对中性点位置、桩土差异沉降以及复合薄板中面最大拉应力的影响,结果表明该方法所求得的荷载分担比及沉降与实测值较为接近,证明了其合理性。     

地基沉降预测模型研究综述

地基沉降包括初始沉降、固结沉降和次固结沉降。对于软黏土地区,地基固结沉降和次固结沉降占很大比例。目前,计算沉降量与时间关系的方法主要有两大类。第一类为依据固结理论并结合各种土本构模型计算沉降量的有限元法;第二类为根据实测资料推算沉降量与时间关系的预测方法。综述已有的各种地基沉降预测模型,将其分为六大类,同时分析其各自的优缺点,并提出将地基沉降预测模型应用于高层建筑地基基础设计的概念,这对于研究上部结构-基础-地基共同作用的时间效应具有重要意义。     

基于双曲线拟合法的等载预压卸荷沉降速率标准

基于双曲线拟合法,推导出恒载下地基沉降速率与剩余沉降之间的关系式、剩余沉降与工后沉降之间的关系式,据此建立了等载预压沉降速率标准和工程实用的卸荷时间预报方法。公式表明,沉降速率与剩余沉降的平方呈正比,工后沉降占剩余沉降的比例不仅与地基固结速率有关,而且与路面设计年限和预压时间有关,但预压时间的影响很小,沉降速率标准是所控制的工后沉降的幂函数。工程实例表明,规范和地方经验沉降速率标准适用于桥头路堤,用于一般路堤和结构物与路堤连接处过于严格。     

路堤下钉形搅拌桩复合地基沉降计算方法——广义桩体法

针对钉形搅拌桩这种变截面搅拌桩,通过一定的假设,将其简化为上、下直径相等而模量不等的常规等截面桩,提出了钉形搅拌桩复合地基沉降计算方法--广义桩体法。根据位移、应力连续条件,将路堤填土、加固区和下卧层作为统一的整体,提出了考虑桩顶上刺、桩土相互作用和桩端下刺的加固区计算模型,并建立了模型的有限差分解法。基于三维数值模拟结果,提出考虑路堤下钉形搅拌桩复合地基下卧层应力集中的附加应力计算方法,从而建立路堤荷载下钉形搅拌桩复合地基的沉降计算方法。计算实例表明:复合模量法计算结果偏小,而应力修正法偏大,本文方法最接近实测结果,说明其具有一定的适用性。     

高填方路堤粘弹性参数反演与工后沉降预测分析

路面的工后沉降问题是高填方路堤工程的主要问题之一。用广义Kelvin模型来表征填筑体的本构关系，在现场沉降监测资料的基础上，运用三维有限元正交数值试验、回归分析和优化相结合的方法，反演了计算模型的材料参数，并在此基础上对高填方路堤的丁后沉降作出预测，得到了较高的计算精度，并和实际监测结果比较．致。本文的方法可为今后高填方路堤的工后沉降计算提供参考。     

黄土路堤工后沉降预测新模型与方法

路堤的工后沉降是在黄土地区修建高标准铁路的主要问题之一,利用黄土路堤工后沉降的实测数据,找出其相应的工后沉降规律将有利于指导设计与施工。在分析了大量的兰武二线铁路黄土路堤工后沉降的实测数据的基础上,提出了一个新模型。为了检验新模型,运用常用的双曲线模型和指数模型分别对相同的实测数据进行了分析预测,并通过对比分析得出了不同的分析和预测结果。研究结果表明:新模型预测的误差小,与实测数据的吻合较好。新模型为工后沉降的分析预测提供了一种新的方法,可为今后黄土路堤工后沉降的预测计算提供参考。     

高填方夯实地基桩侧负摩阻力受力性状试验研究

对原为沟壑的场地,经回填全风化泥质粉砂岩形成高填方地基。对高填方地基采用3000kN·m能级强夯预处理后,打设钻孔灌注桩,通过在桩身钢筋笼主筋上安装应力计,在桩身截面和桩周土层分别埋设沉降杆、分层沉降仪,测试桩身轴力、桩身及桩周土层沉降变化情况,得到高填方夯实地基未处理填土层桩侧负摩阻力变化规律。试验结果表明,未处理填土层桩侧摩阻力沿深度呈现“负-正”变化的现象,随着固结时间的增加,端承桩负摩阻力区段大于摩擦桩。端承桩桩侧土层提供的最大负摩阻力约是摩擦桩的1.18~2.56倍,桩周土层密实度对桩侧最大负摩阻力有影响。采用一阶负指数函数拟合得到桩身下拉荷载预测模型,随着固结时间的增加,作用于桩身的下拉荷载趋于定值,作用于端承桩的下拉荷载比摩擦桩高41.2%~55.4%,从控制负摩阻力角度推导出高填方夯实地基摩擦桩桩长设计计算方法。桩身中性点位置均随固结时间增加而逐渐下移,端承桩中性点深度较摩擦桩平均大0.7m。     

公路软基超载预压卸荷时间确定的沉降速率法研究

推导出了超载沉降速率与等载工后沉降之间的关系式。该式表明,超载沉降速率控制标准不仅取决于允许的工后沉降,而且与土的固结性质、各级荷载增量的加荷速率和加荷起讫时间有关。基于分级加荷沉降预测理论,建立了超载卸荷时间快速预报方法。最后通过一工程实例,验证了方法的适用性。     

沉降控制复合桩基在桥头跳车问题中的应用

首次将基于沉降的桩基设计方法应用到桥梁桩基的设计实践中。分别通过调整桩长和桩数,使桩基沉降从桥梁主跨往边跨逐步增大,而在路桥连接处使边跨桥墩沉降与桥坡填土沉降基本一致,实现路桥沉降协同,从而开辟了解决桥头跳车问题的新途径。按此设计而成的某桥梁,建成后对桥梁桩墩及桥坡沉降进行了长期监测,结果表明最初的设计意图得以实现。这次设计实践表明桩基基于沉降设计对于简支桥梁是完全可行的,并能同时减少桥坡处理、桩基工程以及后期维修费用,因而具有较大的推广应用价值。     

灰色理论在海相软土路基沉降预测中的应用

通过对天津市滨海新区某道路K13＋840断面进行沉降观测资料分析,应用灰色理论建立等时距GM1．1路基沉降预测模型,并通过应用matlab程序语言编制模型计算程序,以实现对海相软土路基沉降变形特性研究,并运用预测分析结果指导路基施工,防止因路基沉降造成路面结构破坏。希望通过对归纳结果的分析,形成一套适应于滨海地区路基沉降预测方法,对于节约工程建设使用成本和延长公路使用寿命具有重要意义。     

Evaluation of ground settlement in response to shield penetration using numerical and statistical methods: a metro tunnel construction case

Shield tunnelling can cause ground settlement, which poses significant risks to adjacent structures or facilities. To understand complex soil behaviour in response to shield penetration, a model that can establish the shield–ground relationship and accurately predict tunnelling-induced ground settlement is necessary. The aim of this paper is to combine numerical methods and statistical methods for settlement prediction in the Wuhan (China) metro project. During the pre-construction stage, due to the lack of instrumentation data, the numerical method was applied to simulate the tunnelling process. The relevant factors influencing ground settlements were identified by examining the model’s sensitivity to each parameter. After the shield launch, data of the relevant factors and field measurements were collected. Using these data, a statistical model based on an adaptive relevance vector machine (aRVM) was trained for real-time prediction of the ground settlement development. The simulation results show that a number of factors, including geometrical, geological and shield operational parameters, contribute to ground settlement, and the aRVM model can accurately and effectively predict settlement development. The example application demonstrates that the method is a practical tool for settlement prediction and can be widely used in metro projects.     

Effectiveness of CFG pile-slab structure on soft soil for supporting high-speed railway embankment

The use of cement-fly ash-gravel (CFG) pile-slab structures is a new effective technique for reducing the settlements of soft foundations in China. A comprehensive research was conducted to investigate the effectiveness of a CFG pile-slab structure-supported embankment of the Beijing-Shanghai high-speed railway in China. Firstly, an analytical method was formulated to calculate the settlements of the CFG pile-slab structure-supported embankment. Secondly, mechanical scaling laws were derived for the proposed testing plan at model scale. Model tests were conducted to quantify and to interpret the settlement distributions. Thirdly, the influence of three key factors (pile length, pile diameter, and slab thickness) for the structural form of the CFG pile-slab structure-supported embankment on the settlement distribution were studied using numerical simulations. The maximum settlements of the CFG pile-slab structure-supported embankment in optimizing the structural form were obtained using the above three research methods and then they were compared with each other. The results show that (1) the settlements obtained by the analytical method, the physical model tests, and the numerical simulations showed good concordance with each other; (2) the settlement-controlling effect of the CFG pile-slab structure was able to meet the requirements of high-speed railway construction; (3) the piles and the soil-bearing capacities of the CFG pile-slab structure-supporting embankment could be fully mobilized because of the “load re-distribution” function of the slab; and (4) the affected area of the engineering load had a depth of more than 18.75?m and a horizontal length of 7.5?m near the toe of the embankment slope.     

改进残差修正GM(1,1)模型在基础沉降预测中的应用

普通的灰色残差修正GM(1,1)模型利用差分代替微分,并用原始数据第一个点的值作为时间响应函数的初始值C0,从而给预测带来了一定的误差。本文用多项式逼近法对残差修正GM(1,1)模型进行了改进,并增加了一个初始值参数;还通过一个基础沉降预测的工程实例,对此模型与普通残差修正GM(1,1)模型进行对比。沉降预测结果表明,改进后的模型明显提高了精度,更加适合于基础沉降的预测,具有较好的准确性和工程应用价值。     

Prediction of Settlement in Natural Deposited Clay Ground with Risk of Large Residual Settlement Due to Embankment Loading

This study endeavors to predict the settlement that could occur in the future at a site where settlement due to embankment loading is already occurring at present. In the site studied here, large residual settlement of as much as 70 cm has already occurred in the 4 years since it entered into service. It is believed that settlement will also continue in the future because excess pore pressure is still present within the clay layers. Furthermore, according to the method of evaluation proposed previously by the authors, which is based on the sensitivity and compression index ratios, it can be judged that the ground includes clays that are sensitive to disturbance and have a strong possibility of large residual settlement. In this paper, simulation of the settlement observed up to now at the site as well as prediction of the settlement that could occur in the future was carried out by deducing the higher compressibilities possessed by the in-situ clays compared with the undisturbed clay specimens in the laboratory. The soil-water coupled finite deformation analysis was employed using the analysis program GEOASIA, in which the constitutive equation for the soil skeleton is mounted with the SYS Cam-clay model. In addition, the effect of modifying the vertical section of the embankment by overlaying in order to counter the settlement was also investigated by numerical analysis. The results showed that such a countermeasure cannot be expected to lead to faster consolidation and that it may require massive funding over a long period of time to cover the maintenance and management costs involved.