Proposal of a Simple Method for Assessing the Susceptibility of Naturally Deposited Clay Grounds to Large Long-Term Settlement due to Embankment Loading

A simple method that utilizes the results of laboratory tests has been proposed for determining the susceptibility of soft clay grounds to large residual consolidation settlement due to embankment loading. It was found that there is a possibility of large long-term settlement if the sensitivity and compression index ratios of the clay material that constitutes the ground are equal to or more than 8.0 and 1.5, respectively. The compression index ratio is defined in this paper as the ratio (Cc/Ccr) of the steepest gradient of the compression curve of an undisturbed sample to that of the remolded sample. Through the SYS Cam-clay model, an elasto-plastic constitutive model that describes the actions of the soil skeleton structure, it was found that clays with large sensitivity and compression index ratios are characterized by initially highly structured soils and that decay/upgradation of the structure can easily occur due to plastic deformation. In addition, by following Schmertmann's graphic method for in-situ compression curve (1953), this paper proposes a method of deducing the in-situ initial conditions from the results of laboratory consolidation tests on undisturbed samples. These investigations revealed not only that large delayed settlement is facilitated in clays, which have higher degrees of structure and faster rates of structural decay, but also that the Δe method and other simple methods of predicting settlement may underestimate the amount of settlement.     

软土地区超大吨位基桩堆载试验地基沉降特性研究

对于超大吨位堆载静载试验,地基土承载力、检测设备能力和施工安全三者缺一不可。采用头顶手提悬挑的方式,可以有效减小码垛高度,减小因地基不均匀沉降造成的平台倾斜、倾覆现象发生。通过实时监测,支墩下经过打桩处理的地基虽然产生可观的沉降,但沉降均匀,且基本集中在堆载过程中,堆载完成静置一段时间后地基沉降基本趋于稳定。测试过程中基准桩沉降不足5mm,对桩沉降数据影响不大,测试得到的桩沉降是可信的。对于超大吨位堆载试验,试桩、压重平台支墩边和基准桩之间的中心距离很难达到或超过3D,对经充分处理后的地基,该距离值得深入研究。     

Predictive Simulation of Deformation and Failure of Peat-Calcareous Soil Layered Ground Due to Multistage Test Embankment Loading

The continuous behaviour of a ground-embankment system from the stage of deformation up to failure was predicted with respect to an actual test embankment that had been constructed in stages on a soft ground made up of peat and calcareous soil. The behaviour prediction was carried out by employing soil-water coupled finite deformation analysis, which also included simulation of the embankment construction process. The information used in the analysis was limited to such things as the results of soil tests on soil materials (peat and calcareous soil) sampled from the ground, including their sensitivity ratios, and the embankment's construction history. The SYS Cam-clay model was used in the constitutive equations of the soils to determine the material constants of the soils and the initial conditions of the ground, and the computations were performed under plane strain conditions. As a result, the computed profiles of W-type ground settlement and of slip surfaces running through the embankment were found to be in good overall agreement with the actual profiles measured at the site. Furthermore, we found that this slippage is attributable to the undrained shear response of the soil elements in the calcareous soil layer, where slippage begins to occur during embankment loading. In other words, the slippage is caused by the rapid softening behaviour caused by the degradation of structure after the effective stress ratio reaches the vicinity of the critical state line.     

基坑开挖工程降水引起的沉降预测

以上海某基坑工程为例,建立三维有限元模型分析基坑地下水位变化以及因抽取地下水引起的地面沉降。分析结果与现场实测数据基本吻合,表明所用方法可以较好地模拟实际工程。     

地铁荷载作用下饱和软粘土的长期沉降分析

针对地铁荷载作用下饱和软粘土地基出现的长期沉降问题提出了一个采用拟静力有限元计算和经验拟合计算模型相结合的预测方法,易于在工程中应用。将沉降分为不排水循环荷载作用下土体中的累积变形引起的沉降和由于动荷载引起的累积孔压消散产生的固结沉降,这两部分沉降的计算选用较为合理的基于临界状态理论的经验拟舍公式,土中应力分量的计算采用有限元方法计算并从中提取,最后采用分层总和法计算迭加得到总沉降,实例的计算结果得到了较好的验证。     

预压加固软土路基沉降补方系数研究

通过对软土路基在预压荷载作用下产生的附加压力的非线性计算与分析,提出在计算软土路基工程补方量时,应结合地基的路堤断面尺寸与填土荷载大小(即填土高度)等因素综合分析考虑,以得出预压加固软土地基的沉降补方系数。     

路堤荷载下软土侧向挤出沉降分析

路堤荷载下软土瞬时沉降并不是瞬时完成的 ,由此提出了侧向挤出沉降的概念以描述这种现象。侧向挤出沉降量可占总沉降量 1 4以上 ,目前通用的利用侧斜管测量侧向挤出量的方法是不适用的。本文根据实际资料 ,对侧向挤出进行了计算分析 ,结果表明侧向挤出沉降在软土总沉降中占有较大比例。最后讨论了影响侧向挤出的主要因素     

路基软粘土动力反应及震陷变形研究

国内外对软粘土的震陷与计算分析还不多 ,我国建筑抗震设计规范也尚未评价软粘土的震陷问题及规定其抗震措施。根据某高速公路建设的需要 ,研究了软粘土在循环荷载作用下的动力特性和计算模拟方法 ,进行了震陷估算及抗震措施评价 ,得到了一些新的认识与结论 ,供地震区软粘土震陷评价参考     

深厚软黏土层上路堤堆载的数值模拟

在我国广泛分布着深厚海相、湖相软土层,其表现为高含水量、高孔隙比、高压缩性和低强度,故在深厚软土层上修建路堤面临着巨大的挑战。由此,结合修正剑桥模型,对软黏土层上的路堤堆载进行了数值模拟,研究了堆载作用下不同填土高度时黏土层内的位移场及应力场,坡脚处土体水平位移的发展规律,并以此作为最大填土高度的一个判断依据。数值模拟结果表明:坡趾处最大水平位移发生在地下2 m处,最大填土高度与模型路堤结果接近。     

考虑盾尾注浆隆起的盾构掘进地面位移预测

通过对3个盾构隧道工程实例中盾尾注浆引起的地面隆起的分析,对比了Mindlin解、Sagaseta法、Verruijt-Booker法、Loganathan-Poulos法、Chi法和Park法6种解析方法及高斯公式预测盾构掘进盾尾注浆引起的地面隆起的适用性,并结合工程实例给出了考虑注浆地面隆起后盾构掘进地面位移计算的修正Peck公式。分析表明：盾尾注浆引起的横断面地面隆起曲线,可以用高斯公式和Chi法较好地拟合,在确定注浆隆起宽度参数Kh和沉降影响角β的经验取值后,可预测盾构注浆隆起。考虑注浆隆起的修正Peck公式,可合理地预测包含注浆隆起在内的盾构掘进引起的横断面地面位移。     

高填方路基沉降预测的半数值半解析方法

根据高填方路基施工过程和沉降的特点,基于对北京某工程的天然地基、藕塘抛石挤密、CFG桩、夯扩载体桩和天然地基与CFG桩的混合地基等典型地基的7个典型断面实测数据的分析,提出一种适用于工程应用的半数值半解析方法。本方法充分利用数值模拟方法能够模拟复杂地基类型,传统解析方法能够考虑时间和加载条件的优点,预测高填方路基施工期间和工后的沉降。该方法应用简便,可用于分析各种型式的地基填方工程,具有较强的实用性,能够为工程施工、建设和维护提供可靠的依据和建议。     

承德机场高填方地基工后沉降预测

目前我国机场建设正处于迅猛发展阶段,机场高填方工程屡见不鲜,然而对于高填方工程的地基沉降计算研究却不够完善。河北省承德机场具有以下工程特点:(1)原地基无特殊性岩土且进行地基处理,(2)填筑体以硬质岩为主。以高填方工程工后沉降现场监测资料为基础,研究了不同回归参数模型对高填方地基工后沉降数据拟合的合理性,在指数模型、幂函数模型、对数模型、双曲线模型、平方根模型中,指数模型能够真实的反映此类高填方工程工后地基沉降规律。由预测结果及沉降影响因素可知,此类工程工后沉降组成以填筑体压缩固结为主,填筑体厚度为工后沉降的主要影响因素,填筑体厚度变化较大处工后差异沉降相对显著。     

基坑开挖地表沉降影响因素及关联度研究

城市地铁基坑工程开挖,打破了土体原始平衡,影响已建建筑物安全,地表沉降是反应施工影响的重要而又直观的因素。论文对基坑地表沉降的影响因素的作用机理进行了分析,在此基础上,应用灰色关联度分析理论,构建某地铁基坑工程地表沉降量与桩顶水平位移、深层位移、地下水位、支撑轴力、锚索拉力等监测项目之间的灰色关联度模型,得出地表沉降与各因素的关联度。结果表明:影响地表沉降的因素关联度大小顺序是桩顶水平位移、锚索拉力、地下水位、深层位移、支撑轴力。论文为以后研究地表沉降规律及类似工程监测方案优化提供理论依据。     

Consideration of Additional Pile Loads Due to Forces of Loading Friction in Soils Prone to Slump-Type Settlement

Basic factors affecting additional pile loadings due to forces of loading friction that develop during the slump-type settlement of soils under their own weight are described. Suggestions for determination of design loads on piles are given with consideration of these factors.     

高填路基沉降预测的两种方法对比研究

基于路基沉降实测资料,采用双曲线和泊松曲线模型对某高填路基的沉降变形进行了预测分析。实测资料和分析对比结果表明:双曲线模型简单易行,可对高填路基进行沉降预测(尤其是对中期的沉降预测),并得到较为满意的结果;泊松曲线模型的预测数据在沉降观测全过程中与实测数据吻合较好,因此更适合高填路基的全过程沉降预测。     

流变模型在建筑地基沉降预测中的应用

以一高层建筑为例,对其地基土进行了一维固结蠕变试验,分析试验结果可知土体变形包含弹性变形、粘弹性变形、粘性变形。因此用Burgers流变模型对试验数据进行拟合,拟合结果发现Burgers模型对地基土粘弹性变形阶段的描述较差,对Burgers模型进行修改,加入Kelvin体形成扩展的Burgers流变模型,采用扩展模型对试验数据进行拟合效果较好。最后将扩展的Burgers流变模型用于该建筑地基的沉降计算,结果显示:模型计算结果与实测结果有较好的一致性,说明在研究建筑地基沉降问题时考虑地基土的流变性质是必要的。     

双层地基沉降预测模型研究

准确预测地基沉降对于工程设计至关重要.根据上硬下软双层地基的实测沉降～荷载半对数曲线多呈两段直线形式的特点,以经典太沙基一维固结理论为基础,将土的室内压缩曲线用两段直线代替,推导建立了基于前期实测数据的沉降预测模型方程,并利用Crank-Nicholson差分法进行了求解.与两个工程实测数据的对比分析表明,该模型与实测数据吻合较好,能够很好地预测双层地基的沉降变形特性.该方法可为今后其他类似工程的沉降预测提供参考.     

Internal Stability of Group Column Type Deep Mixing Improved Ground Under Embankment Loading

The Deep Mixing Method (DMM), an in-situ soil stabilization technique using cement and/or lime as a binder, is often applied to improve soft soils. The group column type pattern is extensively applied to stabilize foundations of embankment or lightweight structures. An improved-ground design procedure in Japan assumes two failure patterns related to external and internal stabilities. For the external stability, a collapse failure pattern where the DM columns tilt like dominos could take place instead of a sliding failure pattern. For the internal stability, the DM columns show shear, bending and tensile failure mode, depending not only on the ground and loading conditions but also on the location of each column. However, the current design does not incorporate the effect of these failure modes, but only the shear failure mode. In this study, a series of centrifuge model tests was carried out to investigate the internal stability of group column type improved ground under embankment loading. This paper describes the failure modes of DM columns and a proposed simple calculation that takes into account the bending failure mode.     

External Stability of Group Column Type Deep Mixing Improved Ground Under Embankment Loading

The Deep Mixing Method (DMM), a deep in-situ soil stabilization technique using cement and/or lime as a binder has been often applied to improve soft soils. Group column type improvement has been extensively applied to foundations of embankment or lightweight structures. A design procedure for the group column type DM ground has been established in Japan mainly for application of embankment, in which two failure patterns are assumed: sliding failure in the external stability and rupture breaking failure in the internal stability. The internal stability of the improved ground has been investigated experimentally, and it was found that the DM columns show various failure modes: shear, bending and tensile failure, depending not only on the ground and loading conditions but also on the location of each column. However, the current design does not incorporate the effects of these failure modes, but only that of shear failure mode. For the external stability, it is known that a collapse failure pattern, in which the DM columns tilt like dominos, could take place instead of sliding failure. The current design method, which does not take into account this failure pattern, might overestimate the external stability. In this study, a series of centrifuge model tests and elasto-plastic FEM analyses were performed to investigate the external stability of group column type DM improved ground under embankment loading. The centrifuge model study has revealed that the improved ground does not fail with a sliding failure pattern but with a collapse failure pattern in the model test condition. The FEM analyses confirmed the model test results and showed that the improved ground could fail with sliding failure in a certain type of ground conditions such as a floating type improved ground. A simple calculation incorporating the collapse failure pattern gave reasonable estimation of the embankment pressure at ground failure. This paper demonstrates the importance of simulating appropriate failure pattern for evaluating the external stability accurately.