盾构施工引起地面长期沉降的理论计算研究

 对盾构施工引起的隧道轴线上方地面工后沉降进行研究,结果表明,土体开挖卸荷引起应力释放,产生初始超孔隙水压力,其分布呈三角形。假定衬砌不排水、土体为单面排水、压缩层厚度为隧道覆土厚度,采用太沙基一维固结理论,得到地面工后固结沉降的理论计算公式。假定地面长期沉降主要由施工期间沉降和工后固结沉降组成,进而得到地面长期沉降的理论计算公式。算例分析结果表明：该方法的预测值与实测值非常吻合;上海软土地区盾构隧道施工引起的地面长期沉降相当显著,最终地面沉降量在80 mm以上,固结沉降占总沉降量的80%～90%;按最小覆土深度5 m计算,需要2 a以上地面沉降才能最终稳定。     

塑料排水板堆载法处理软基的沉降分析

东莞虎门港区E号路软基采用塑料排水板堆载法处理,通过对该软基沉降的监测,整理出了监测时间内沉降随时间变化的曲线图,并用多种方法推测该处的最终沉降.依据实测曲线推算出的沉降值与用ABAQUS分析软件计算的沉降值结果接近,表明了依据实测资料求解最终沉降的可靠性和用ABAQUS计算塑料排水板堆载预压法沉降问题的可行性.     

软土层未打穿的桩承式路堤现场实测研究

 以申苏浙皖高速公路为工程背景,对软土层未打穿的桩承式路堤进行现场实测研究。共选取2个管桩断面作为试验观测断面,在填筑时及填筑完成后对桩托板和桩间土上的土压力、路基沉降及下卧层沉降进行监测,并与软土层打穿时的实测结果作比较。研究表明：桩间距越大,桩体荷载分担比越小;软土层未打穿时荷载分担比为61.4%～75.5%,比打穿时小一些;软土层打穿与未打穿时的土拱高度都为桩托板净间距的1.0～1.4倍;软土层未打穿时桩土不均匀沉降比打穿时大;路堤填筑完成后的沉降约占总沉降的30.0%;下卧层沉降约占路堤总沉降的60.0%。将现场试验结果与4种理论方法的计算结果进行比较,评价各种方法的适用性。     

Modelling the Effects of Surcharge to Reduce Long Term Settlement of Reclamations over Soft Clays: A Numerical Case Study

Long-term settlement of reclamations constructed over soft soils may be reduced by the use of temporary pre-loading, although there is often uncertainty over how much surcharge to use, and how long it should be maintained. An elastic visco-plastic constitutive model is described and its application in assessing surcharging strategy for reclamation schemes is discussed. The model is incorporated in a one-dimensional finite difference consolidation analysis with vertical and radial drainage of a multi-layer soil profile. The analysis allows the inclusion of vertical drain resistance, a zone of peripheral smear around the drain, permeabilities that vary with void ratio, and creep both during and after primary consolidation. Drawing on data from the Bothkennar soft clay research site, the model is used to predict settlements beneath a hypothetical test fill, including the creep settlement behaviour when a surcharge is removed after different time periods. The behaviour during primary consolidation is back-analysed using Asaoka’s and the hyperbolic observational methods, and both are found to under-predict the magnitude of the settlement.     

沿海滩涂软土路基堆载预压沉降变形特征与卸载时机研究

沉降变形与卸载标准是铁路路基堆载预压的主要问题。针对处于沿海滩涂软土路基之上的铁路专用线,采用插塑板与堆载预压相结合的方案进行地基处理,开展了大量的表层沉降、孔隙水压力、分层沉降等观测项目。据实测资料,总结分析了路基表面的沉降规律,并考虑了路基分层沉降与孔隙水压力的变化规律,全面地揭示了整个路基地层的沉降变化特征,运用双曲线合理地预测了工后沉降,且推算出路基整体的固结度,采用工后沉降与地基固结度建立卸载时机判别标准。为堆载预压的卸载时机判别标准研究提供了一种新的思路。     

软土地区基坑对周边环境影响空间效应分析

软土地区基坑开挖引起的变形具有明显的空间效应,为了进一步研究开挖引起周边地面及建构筑物变形的特性,结合对温州某大型深基坑工程的监测,从垂直于基坑围护墙以及平行于围护墙两个方向,对基坑外地面道路以及建筑物的沉降和裂缝情况进行了监测分析,总结了开挖引起基坑外地表沉降的分布特性,分析发现：基坑边角对围护墙变形以及地表沉降具有明显限制作用;在垂直于基坑方向上,处在沉降主要影响区2H_e范围内,建筑物均具有明显的差异沉降,最大角变量达到1/1300;平行基坑方向距离边角（0.4~1.2）H_e（最大开挖深度）范围观测到纵向裂缝,沉降变化率较大,可能破坏道路或建构筑物;此外,桩基础的建筑物总沉降以及差异沉降均明显小于条形基础建筑。     

明挖基坑对周边地表及建筑影响的实测分析

对长沙某二元结构土层中一个地铁车站明挖基坑对周边高层建筑的影响进行分析,首先,介绍工程地质条件、周边建筑情况、基坑施工及监测方案;然后,分析连续墙的侧移和坑外沉降,并重点分析高层建筑的沉降、变形的历程和特点。与软土地区的多层建筑的沉降相比,高层建筑的沉降受周边连续墙的施工和坑内降水的影响很小;建筑物的沉降小于邻近地表沉降,且出现了转动和隆起;建筑物的沉降、变形与建筑的长宽比、距基坑的距离呈一定的正比关系。     

Consideration of new results on the proneness of soils to slump-type settlement during surveys,design, and construction

This paper addresses new results of investigation of laws governing the development of soil deformations and, among other things, the change, and, as a rule, the increase in the stress state in a soil mass during construction, the influence exerted by the filling and the cutting of soils during the grading of sites on the nonuniformity of their slump-type settlements, additional settlements of the soils underlying the stratum prone to slump-type settlement when loads on these soils are increased, the effect of different factors on the size of the stratum prone to slump-type settlement, and residual proneness of soils to slump-type settlement when the ground water table rises. Proposals are given for consideration of these results during geological-engineering surveys, design, and construction of buildings and structures on soils prone to slump-type settlement. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 15–19, January–February, 1995.     

Consolidation of soft clay foundation improved by geosynthetic-reinforced granular columns: Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.     

Influence of geotextile encasement on the behaviour of stone columns: Laboratory study

This paper presents a study of the influence of the geotextile encasement on the behaviour of soft soils improved with fully penetrating encased columns. This influence is analysed by means of measuring soil-column stress distribution, pore pressures and soil deformation during the consolidation process. For this purpose, a horizontal slice of a representative “unit cell” has been analysed by means of small-scale laboratory tests. The tests were carried out in a large instrumented Rowe-Barden oedometric cell. Results showed that the vertical stress supported by encased columns is about 1.7 times that sustained by the non-encased ones. The stress concentration factor for encased columns is between 11 and 25, which is clearly higher than that obtained in tests with non-encased columns, which are between 3 and 6. Finally, the improvement in relation to settlements is presented by the ratio of settlement in soils reinforced with ordinary or encased columns and the settlement of non-treated soft soil. This settlement reduction factor is around 0.6 when the soil is treated with encased columns and 0.8 for soil with non-encased columns.     

公路软基超载预压卸荷沉降速率标准研究

软土地基上的路基施工常采用沉降速率法确定超载卸荷时间。现有的沉降速率法主要依据地方经验沉降速率标准,无法与获得的工后沉降相联系。基于分级加荷条件下地基固结度计算的改进太沙基公式,推导出超载沉降速率与等载工后沉降间的关系式,据此建立按容许工后沉降控制的超载预压卸荷沉降速率标准。有限元数值模拟分析表明,相对于地方经验沉降速率标准过于严格,该沉降速率标准卸荷控制的工后沉降满足规范要求。通过应用于实际工程,给出了超载卸荷时间。     

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。     

基于指数法的分级填筑路堤沉降预测方法研究

在传统指数曲线拟合法基础上,提出一种软土地基上路堤分级施工情况下沉降预测新模型。该模型考虑了土体变形的非线性和固结性质随荷载的变化,将沉降拟合方程中的待定参数在两个不同荷载级中分别确定:利用有较长预压期的前一级实测沉降确定与土的固结性质有关的参数,其它参数则由需预测沉降荷载级中较短预压期内实测沉降确定。工程实例表明,本文模型预测的沉降与实测值有很好的一致性,可实现沉降的及早预测。     

上海地区深基坑周边地表变形性状实测统计分析

收集了上海软土地区 35 个具有墙后地表沉降实测资料的深基坑工程案例,从统计角度研究了深基坑的墙后地表变形性状。最大地表沉降随着开挖深度的增大而增大,其值介于 0.1 ％ H ～ 0.8 ％ H 之间,平均值为 0.38 ％ H ,其中 H 为基坑开挖深度。 影响因素分析表明最大地表沉降随着墙后软土层厚度的增大而增大,随着坑底抗隆起稳定系数的增加而减小,而与围护墙的插入比及支撑系统刚度的关系不大。 最大地表沉降与最大墙体侧移的比值基本介于 0.4 ～ 2.0 之间,其平均值约为 0.84 。统计了墙后地表沉降的分布模式,给出了墙后地表沉降的包络线。最大地表倾斜量介于 0.001 ～ 0.017 之间,给出了根据最大地表沉降量来预测最大地表倾斜量的统计关系。     

深基坑坑外降水引起土体沉降的有限元分析

深基坑开挖中的降水会引起周围土体沉降。为预测基坑开挖及降水过程中周围土体的沉降,本文采用基于比奥固结理论的有限单元法在大型有限元软件中建立轴对称模型进行了分析。结果表明:该方法可以反映抽水引起的孔隙水压力变化及土体沉降变化规律,是一种实用可行的方法。     

岩土地基自重应力相关问题的探讨

通过对容重、天然孔隙比和固结沉降量等岩土物理、力学性质指标随深度或自重应力变化而变化的规律的分析,探讨了岩土内部自重应力场的分布规律。结合工程现场荷载试验,并应用Hoek-Brown强度准则,讨论了砂砾软岩极限承载能力及自重应力对其极限承载能力的有利影响。结果表明,地壳浅层的岩土自重应力场分布复杂,并存在构造应力场。     

地铁隧道下穿历史风貌建筑影响的实测与分析

两座历史风貌建筑物紧邻地铁车站,并且盾构在其下方穿越,需对建筑物的变形进行严格控制。对建筑物的保护,针对性提出地面袖阀管注浆加固、车站基坑盖挖逆作法、地连墙截断承压水、冻结、车站内水平注浆加固、优化盾构机掘进参数等施工方案及措施。通过对基坑开挖与降水、端头加固、盾构掘进等阶段的监测数据进行分析,研究了不同施工阶段对建筑物变形影响的规律及特点,实测结果表明：合理的盾构掘进参数能够将建筑物的沉降控制在较小范围内;冻结孔施工、地下水通过冻结区盾尾间隙进入刀盘前方等均可引起建筑物的显著沉降;建筑物基础与隧道之间存在淤泥质土层时,在淤泥质土层下方进行注浆不能对自重较大的建筑物进行有效抬升,但对自重较小建筑物的抬升则有一定的效果,但注浆在淤泥质土层中引起孔压的消散可导致建筑物在后期产生沉降;盾构机进入冻结体前应做好全断面注浆止水,切断前后水力联系,盾构机掘进困难时,不应随意增大盾构推力。     

温岭软土地区建筑桩基沉降规律研究

将理论计算值和实测沉降值进行比较,得出适合温岭软土地基的沉降计算方法,并根据温岭地区具有代表性的己建建筑的实测沉降,统计得出桩基沉降计算公式,该公式适用于方案设计阶段和初步设计阶段桩基沉降量的预估.     

衬砌局部渗流对软土隧道地表长期沉降的影响研究

软土隧道地表长期沉降特性与隧道的渗透性以及周围土体的时效特性密切相关。在综合考虑隧道的局部渗透性以及土体时效特性的条件下,采用数值模拟的方法,对隧道长期性态发展过程中衬砌局部渗流对地表沉降、沉降槽及地层损失的影响进行了分析,在此基础上对上海地铁2号线某区间隧道长期沉降的发展进行了预测和比较。     

浅析珠海西区欠固结软土地基的沉降灾害及防治

通过对珠海西区沿海一带软土层的应力历史进行分析,论证了此类软土层欠固结状态产生的原因,应用考虑软土应力历史的固结沉降计算方法,对该区软土层固结沉降周期及沉降量进行了初步估算,并对其在工程建设中产生的灾害形式进行阐述和分析,结合近年当地工程建设经验,针对软土层的固结沉降对建(构)筑物的影响提出了相应的工程防治措施。