上海粉砂土弹塑性应力-应变模型的探讨

本文对上海某地粉砂土进行三轴固结排水试验,参照拉德-邓肯（Lade-Duncan）模型,试图确定上海粉砂土弹塑性应力-应变模型及其参数,分析试验参数A,α,β的某些规律性,提出塑性功W_p的计算方法。阐述弹性功W_e和塑性功W_p的关系,竖向弹性应变ε1^е和竖向塑性应变ε₁^р的关系;对竖向应变ε₁的计算值与试验值进行比较,得到满意的结果,为提供计算砂土地基变形研究创造有利条件,至于采用有限单元弹塑性增量分析的具体应用不在本文论述。     

正常压密土在偏压固结、等应力比加荷条件下的变形特性

<正> 在充分固结的地基上修建建筑物所产生的附加应力和变形是人们所关心的基本课题之一。工程师们常常采用的一个有效的手段是,通过室内试验,并模拟地基的受力过程,探寻土单元的附加应力和附加应变的基本规律,并由试验得到的本构关系和现有的计算方法求解地基的应力和变形。七十年代初,邓肯（Duncan）等人由常规三轴试验结果建立了主应力差（σ₁-σ₃）与轴应变ε₁,轴应变ε₁与侧应变ε₃均为双曲线关系的双曲线模型。     

软土地基刚性桩复合地基动力离心模型试验

采用离心机-振动台系统对饱和软土地基中连续地震作用下上部结构-性桩复合地基体系的抗震问题开展试验研究。试验分析了结构和基础模型在水平输入地震作用下的加速度、位移以及桩身应变等响应规律。结果表明,基础板与桩顶之间设置砂垫层利于削弱传递到上部结构的水平地震力作用,发生较大地震时能有效减小上部结构的加速度响应;地震结束时基础瞬时沉降随地震强度增加而增大,但震后长期再固结沉降随地震强度变化不大;受周围土体地震软化行为影响,群桩荷载分担比例在震后有所降低;桩身峰值弯矩沿桩长分布形式明显不同于传统桩基础,且弯矩峰值较常规桩基减小不少。     

不同应力路径对砂土应力-应变关系影响

<正> 一、前言 目前广泛应用于土坝和地基应力-变形分析中的本构关系数学模型,多建立在常规三轴试验的基础上,并认为模型参数与应力路径无关。实际工程中填方工程和一些地基中的土体常经历等主应力比的应力路径,地基中的固结情况则更符合于K₀固结应力状态。工程中应力条件千差万别,而建立在常规三轴试验基础上的数学模型对工程的适用性如何,即如何验证模型的问题,越来越被重视。国外已有许多人对不同应力路径的应力-应变关系进行了研究,如     

偏压固结下软黏土循环强度弱化分析及数值模拟

在波浪等循环荷载的作用下,近海工程地基中的软黏土会发生强度弱化,影响上部结构的稳定性。以烟台港的淤泥质粉质黏土为研究对象,进行了动、静三轴试验,探讨不同固结静偏应力和不同循环动应力作用下淤泥质粉质黏土的循环强度弱化规律以及土体循环后不排水剪有效应力路径发展规律。通过对土体循环后不排水剪切过程中有效应力路径的分析,发现当动应力比达到0.4后,土体强度弱化趋势加快,临界有效应力比(M)减小,结构发生一定的破坏,并据此探讨了土体强度弱化机理。此外,基于等效超固结理论,在考虑偏压固结使土体初始有效应力提高的前提下,经过理论推导,得出了考虑初始固结静偏应力和循环效应的土体强度弱化公式,并通过试验数据对其进行验证。最后,通过对ABAQUS的二次开发,利用拟静力算法将该公式应用于烟台港数值模型的有限元计算中。     

吹填软土植物垫层真空预压现场试验研究

 针对崇明岛吹填软土地基砂源紧缺、人机进场与施工困难、无法满足农业用地需求等状况,提出采用现场芦苇代替砂垫层进行真空预压地基处理试验研究,并与无垫层真空预压进行对比。通过试验研究发现：由于有垫层与无垫层导致真空预压法的加载规律不同,使得地基土的固结特性与效果也不同。无芦苇垫层时真空度经过主管、塑料排水板,短时间内即加载到土体上,地基前期固结速率比较快,但速率降低也很快,地基沉降达到稳定的时间短、所完成的沉降量小;有芦苇垫层时真空度需经过芦苇垫层的扩散缓慢地加载到土体上,负压荷载加到土体上的历时长,地基前期固结速率比较慢,但速率降低也慢,沉降速率在经历约8 d、沉降量在经历约33 d后才与前者相当,但最终所完成的沉降量却比无垫层时高出约10.4%,地基承载力也略高于无垫层情况。研究结果表明,植物垫层真空预压处理吹填软土地基是可行的,且由于其效果好、成本低,值得在类似工程中推广应用。     

长期往复荷载作用下近海饱和软黏土强度和刚度的弱化特性

长期往复荷载作用下,饱和软黏土强度和刚度的衰减是近海桩基础设计必须考虑的,利用带弯曲元的循环三轴仪对原状饱和软黏土进行了一系列等向固结和非等向固结条件下不排水单调和循环加载三轴试验,并在循环加载过程中通过弯曲元测试土体剪切波速。试验结果表明,在相同的固结条件下,随循环次数的增加和动应力比的提高土体的强度和刚度衰减显著;在相同动应力比作用下较小的有效固结应力和偏压固结减缓了土体刚度的弱化。已有的相对偏应力水平参数不能反映不同固结条件下近海饱和软黏土强度和刚度循环弱化特性,为此引入了动偏应力水平参数,考虑了有效固结应力、固结静偏应力、动偏应力的相互影响,利用动偏应力水平对饱和软黏土的强度和刚度循环弱化特性进行描述。     

考虑堆载区水位变化的固结沉降计算方法

以往研究的基础上,推导砂垫层残余水头公式,并进行简化;综合考虑地面沉降、砂垫层残余水头对外加荷载的影响,推导地基附加应力公式;建立考虑地面沉降、砂垫层残余水头的砂井地基径向排水固结方程,并给出相应的定解条件;在等应变假设的基础上,推导地基平均固结度公式,由得到地基沉降公式。由深圳机场扩建工程实例得出的理论计算沉降过程曲线与实测曲线相符,验证了计算方法的正确性。     

基于三轴试验和联合强度理论预测盾构施工引起的地表变形

现有盾构施工引起地表变形研究中,将土体视为线弹性材料而采用弹性模型来预测地表沉降,具有很大的近似性。引入联合强度理论,分析盾构掘进中、离开后上部土体加荷–卸荷应力状态下的稳定性;通过模拟盾构掘进中、离开后土体的应力路径,开展三轴固结不排水试验,依据试验结果的应力–应变双曲线形态,推导出盾构上部土体竖向加、卸荷条件下的非线性切线弹性模量公式,并确定各参数值。研究结果表明:相同围压或相同固结应力比时,加荷路径土体强度大于卸荷路径土体强度;同一围压下,轴向加荷、卸荷土体破坏时主应力强度值均会随固结应力比增加而增加。预测地表变形时,应力路径法运用了三轴试验成果中轴向应变数据,弹性理论法运用了加、卸荷非线性切线模量及试验参数。以武汉过江隧道江南盾构段为例,应力路径和弹性理论法隆起和沉降量预测值分别相差4.2%和13.0%;两者隆起量预测值与实测值分别相差7.5%和10.7%,沉降量预测值与总沉降实测值趋势一致。     

深埋暗道CFG桩及碎石垫层复合地基变形特性

将CFG桩复合地基技术应用于深埋暗道这类特种结构的地基处理问题,通过对具体试验工程进行现场测试以及不同厚度碎石垫层的室内模型试验研究,对深埋暗道CFG桩复合地基沉降变形进行了分析,指出褥垫层的压缩量是应该被考虑到的,沉降变形计算不能仅仅考虑针对复合地基的修正,并应同时考虑褥垫层变形的修正。利用ABAQUS有限单元法分析了不同厚度褥垫层对CFG桩复合地基桩土应力比、总沉降量、垫层压缩量和桩身轴向应力的影响规律。介绍了一种输煤暗道沉降变形监测方法,通过现场测试、室内模型试验以及有限元分析,确定了深埋暗道CFG桩复合地基褥垫层的选用原则。     

Stiffness and strength parameters for hardening soil model of soft and stiff Bangkok clays

A comprehensive set of experimental data on Bangkok subsoils from oedometer and triaxial tests are analysed in this paper in order to determine the stiffness and strength parameters for Hardening Soil Model. The parameters determined are the Mohr–Coulomb effective stress strength parameters together with the stiffness parameters; tangent stiffness for primary oedometer loading, secant stiffness in undrained and drained triaxial tests, unloading/reloading stiffness and the power for stress level dependency of stiffness. The oedometer data are obtained from three different Bangkok soil layers: soft clay at 6–8 m depths; medium clay at 12–14 m depths; and stiff clay at 15.5–18 m depths. The triaxial tests data are carried out for soft and stiff clays at depths of 5.5–6 m and of 16–18 m under both undrained and drained conditions, respectively. Finally, two sets of parameters for soft and stiff Bangkok clays are numerically calibrated against undrained and drained triaxial results using PLAXIS finite element software.     

软粘土地基基坑开挖性状的研究

本文发展了一个可用于分析以板桩为支挡结构的软粘土地基基坑开挖问题的有限元程序。用杭州友好饭店工地的饱和软粘土样进行了四种不同应力路径等向固结不排水剪(CIU)试验,结果表明,归一化初始切线模数Ei与本文提出的应力路径参数a成良好的直线关系。并根据试验结果建立了相应的模量方程。利用有限元程序对影响基坑性状及工程经济效益的几个重要因素(诸如板桩的插入深度、板桩的刚度以及土质条件等)进行了探讨,得出了一些有实用意义的结论。最后分析了杭州友好饭店基坑开挖工程实例,理论计算与实测值比较,是令人满意的。     

交通荷载作用下软粘土路基沉降有限元分析

通过应用Biot动力固结有限元程序对交通荷载作用下软土路基条件下的沉降进行了有限元模拟,揭示了沉降过程中软土的动力固结规律,得出了比较接近实测值的计算结果,对于指导路基施工具有一定的意义。     

利用弯曲元测量上海原状软黏土各向异性剪切模量的试验研究

在许多发达国家例如英国,土体的小应变特性已被广泛应用于预测地下建筑物施工引起的地表变形。尽管上海的城市地下基础、基坑与隧道开挖日益增多,但是关于上海原状软黏土小应变剪切模量的研究几近空白。利用装有霍尔局部应变传感器与弯曲元测试系统的三轴仪对上海软黏土剪切刚度的固有各向异性进行了研究。在等向应力状态下对方形原状上海软黏土试样进行了两组试验。试验结果表明：较之传统的初达波法和峰-峰法,互相关法一定程度上提高了弯曲元试验中确定剪切波速的客观性。由于土体水平向层理结构与颗粒间胶结作用的影响,上海软黏土呈现出明显的各向异性,实测最大剪切刚度的固有各向异性比（G_0(hh)/G_0(hv)）约为1.21。利用孔隙比函数F(e)=e^-1.3能够很好的描述不同平面内最大剪切模量与土体应力状态的相互关系。     

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.     

秦皇岛滨海饱和软土的试验研究

为保持饱和软土的天然结构特征，野外采用开挖探井，环刀取土的方法获得了大量Ⅰ级土试样，并在室内进行有关饱和软土的物理力学性质试验。结果表明：秦皇岛滨海饱和软土的天然含水量介于37％～44％，土性属淤泥质粉质粘土。基于浅层平板载荷试验、静力触探试验以及十字板剪切试验，对饱和软土的天然抗剪强度、灵敏度、承载力进行了试验研究。建立了十字板抗剪强度和试验深度、静力触探比贯入阻力的经验公式，为工程应用提供了便利条件。秦皇岛滨海饱和软土的天然不排水抗剪强度平均值为27kPa，灵敏度为2．4～12．5，具有中高灵敏性。其地基强度的使用应综合考虑建筑物的加载速率、地基容许变形等多种因素。     

Triaxial extension and tension tests on lime-cement-improved clay

This paper presents the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement-improved very soft clay. The main objective of these tests was to investigate the material strength and stiffness properties for stress conditions similar to those expected on the passive side of excavations where a retaining structure is supported by Deep Mixing columns. The different stress paths to failure were obtained by varying the directions of the major and minor principal stresses in a conventional triaxial test cell. The undrained tests conducted at low consolidation stresses, corresponding to depths of approximately 0–10 m below the ground surface, revealed significant differences in undrained strength depending on the directions of the major and minor principal stresses, indicating anisotropic material behavior. Based on the undrained triaxial test results, the relationship among the undrained strength, the effective consolidation stress and the over-consolidation ratio (OCR) is presented for different stress paths to failure.The experimental data from the drained tests show that a failure surface comprised of a shear failure function based on the Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement-stabilized clay.     

Numerical simulation and sensitivity analyses of full-scale test embankment with reinforced lightweight geomaterials on soft Bangkok clay

A full-scale test embankment was constructed on soft Bangkok clay using rubber tire chip–sand mixture as a lightweight geomaterial reinforced with geogrid under working stress conditions. The facing of the embankment was made of segmental concrete blocks with rock filled gabion boxes as the facing to the sloping sides. This paper attempts to simulate the behavior of the full-scale test embankment using PLAXIS finite element 2D program by means of undrained analysis in the construction stage and thereafter consolidation analysis was performed during the service stage. The settlement predictions of the soft clay foundation mostly depended on the assumed thickness of the weathered crust and the OCR values of the soft clay layer. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. The lateral wall movements were overpredicted by the analysis due to the partially drained consolidation process at the early stage of the construction. The FEM computed geogrid movements were smaller than the observed field data due to the use of lightweight tire chips-sand backfill. The maximum tension line agreed reasonably well with the coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying the weathered crust thickness, the OCR values of soft clay, the permeability values of the soft clay and the interface coefficient of the geogrid. The settlements and the excess pore water pressures changed significantly when the OCR and the permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements and tensions in the geogrids. The higher interface coefficient yielded less wall/geogrid movement and resulted in higher tensions in geogrids as expected. The results of analyses show that the FEM analysis using 2D plane strain conditions provided satisfactory predictions for the field performance.     

软土深基坑开挖的有限元分析

目前对开挖工程一般只作总应力法分析。本文结合某炮和软粘土地基深基坑开挖的工程实例,将Biot固结有限元法用于基坑性状的研究。分析表明,坑壁立体位移和支撑轴力的计算和实测值基本吻合;在土方停挖期,两者均表现出一定程度的固结效应。