基坑中土的应力路径与强度指标以及关于水的一些问题

 基坑开挖是在原状土层中进行的,其地基土的应力路径既不同于常规挡土墙中土的应力路径,也不同于室内常规三轴压缩试验中试样的应力路径。基坑工程中,支挡结构物前、后土体的平均主应力或者某些方向的主应力常常是减少的,对于饱和黏性土的固结不排水三轴试验,可能产生负的超静孔隙水压力,从而会影响土的抗剪强度指标。本文指出,对于黏性土中的基坑,在近期施加的墙后地面超载q,以及欠固结土地基的情况下,使用固结不排水(或固结快剪)强度指标计算土压力与进行稳定分析是偏于不安全的;同时指出,重力式水泥土墙的抗滑移和抗倾覆稳定验算,以及用瑞典圆弧法进行整体稳定验算时,对于饱和黏性土,如使用固结不排水强度指标,其抗力部分中的自重应按浮重度计算。结合对《建筑基坑支护技术规程》(JGJ 120-201?)报批稿进行的一些讨论,分析在基坑支挡结构计算中水压力的作用,提出地基土为粉土时,水土压力分算还是合算取决于其下土层的性质。     

基坑支挡结构设计若干问题探讨

根据基坑开挖应力路径的室内试验和现场实测数据,从土工试验方法、土体强度指标、地基土水平反力系数等方面分析了现在基坑工程支挡结构设计存在的一些问题.研究成果如下:支挡结构设计参数的获取应考虑主动区及被动区地基土的应力路径,并恢复地基土的固结状态.基坑开挖后主动区和浅部被动区土体抵抗剪切破坏的能力大于以常规CU强度指标的计...     

对与基坑工程有关的一些规范的讨论（2）

本文是对与基坑有关的规范系列讨论的第二部分,结合基坑工程土的应力路径问题讨论了不同强度指标与计算方法的适用性。针对目前勘察报告对于一层土只给出一个或一组强度指标的情况,指出了应重视取样及现场测试深度的问题;认为对于支挡结构前后土体应进行不同围压的三轴试验;讨论了基坑工程中墙前后土体的应力路径与室内试验应力路径的区别,讨论了应力路径对于不同强度指标以及水土压力计算的影响。并特别指出,支挡结构物前、后土体的平均主应力或者某些方向的主应力常常是减少的,对于饱和黏性土的固结不排水三轴试验,可能产生负的超静孔隙水压力,从而会影响土的抗剪强度指标。     

考虑坑侧堆载基坑开挖土体应力路径试验研究

基坑开挖过程中,基坑坑侧各区域土体受到不同程度的卸荷或堆载作用,从而呈现出复杂的应力路径。因此,开展了常规三轴固结不排水剪切试验、K₀固结不排水剪切试验、K₀固结侧向卸荷试验和K₀固结侧向卸荷轴向加荷试验等不同的应力路径试验,来研究基坑开挖前后基坑坑侧土体的不同应力路径变化情况,比较分析了各应力路径条件下土体的抗剪强度、应力应变的变化情况,归纳了基坑开挖前后复杂应力路径条件下土体的应力变形及强度特征。研究表明,在不同的应力路径条件下,土体所对应的应力变形及强度参数,具有明显的差异性;软黏土在不同的侧向卸荷应力路径情况下,应力应变曲线均呈现应变软化型,且侧向卸荷对土体强度影响较大,坑侧堆载引起的轴向加荷对土体强度变化也有一定影响。     

天津滨海软土不同应力路径下抗剪强度对比试验研究

现有基坑支护设计参数试验条件与实际不符。选取天津滨海新区海相软土为研究对象,进行三轴固结不排水试验、三轴不固结不排水试验和减p路径试验并对比分析了几种试验条件下土的强度参数。得到:滨海软土在不同应力路径下的强度与初期固结方式有关。减p路径对总应力状态下抗剪强度有影响,但对有效应力状态下抗剪强度基本无影响。在滨海软土基坑支护设计中,选用CU试验参数进行计算,设计偏于安全。     

基坑开挖卸荷应力路径地基土工程特性

基坑工程支挡结构分析计算是否能正确模拟地基土的工程行为,主要在于采用的应力-应变关系及计算参数的选取方法能否符合工程实际的应力路径。本次研究分析了现行基坑工程设计参数试验获取方法的不合理性,认为土体的物理力学试验应确定其初始应力和固结状态,且试验采用的应力路径应能反映基坑开挖引起土体应力状态的变化,所得参数应能将地基土的强度与变形有效地联系起来,应力-应变关系既能表达极限状态,又能描述过程状态。研究提出了一种针对基坑工程的土工试验方法,将基坑开挖应力路径简化为三种,并结合基坑足尺试验场地原状土样的土工试验结果,分析了基坑开挖卸荷路径地基土的强度、变形特征,得到了土体应力-应变关系及归一化方式。研究结果对改进基坑工程设计及计算分析方法具有重要意义。     

软土的卸荷模量

软土的室内应力路径试验研究表明，软土的卸荷模量不仅取决于土的物理性质，而且还和所经历的应力路径密切相关。本文根据大量的应力路径试验，特别是模拟基坑开挖和软土隧道施工过程中的应力路径，研究了上海地区的几种典型软土的卸荷变形模量与应力路径的关系表达式。对基坑开挖及支护结构设计计算、软土隧道施工研究及其它需要准确确定软土的卸荷模量的岩土工程问题的研究均有较大的实用价值     

基坑支挡结构设计若干问题探讨

根据基坑开挖应力路径的室内试验和现场实测数据,从土工试验方法、土体强度指标、地基土水平反力系数等方面分析了现在基坑工程支挡结构设计存在的一些问题。研究成果如下:支挡结构设计参数的获取应考虑主动区及被动区地基土的应力路径,并恢复地基土的固结状态。基坑开挖后主动区和浅部被动区土体抵抗剪切破坏的能力大于以常规CU强度指标的计算结果,这有利于工程安全;深部被动区土体抵抗剪切破坏的能力小于以常规CU强度指标的计算结果,对于地基土被动抗力发挥较大的工程,需验算嵌固深度,确保被动抗力小于极限被动土压力。定义了被动区基坑开挖"强影响区",并给出了该区域深度的计算方法。以相应应力路径下的土体割线模量值作为地基土水平反力系数,讨论推导了作用于支挡结构上的分布土反力与支挡结构水平位移的计算公式,其计算结果更接近工程实际。本次研究对以变形控制为主的基坑设计具有较好的指导意义。     

应力路径和强度指标对基坑支护结构上水土压力计算的影响

基坑支护结构上的水压力和土压力计算是基坑工程中的重大问题，针对一个假想的基坑模型，模拟基坑开挖的实际应力路径，通过室内三轴试验获得不同应力路径的总应力强度指标和有效应力强度指标，用6种不同的方法分别计算支护结构上的水土压力，并对各种方法的合理性和可行性进行了分析和比较，推荐了一种较为合理可靠的水土压力计算方法。     

考虑残余应力的基坑被动区土压力及强度计算

基坑开挖过程中坑内土体处于竖向卸荷状态,而土体在卸荷过程中是普遍存在残余应力的。为了分析土体卸荷引起残余应力对基坑被动区土压力及强度的影响,进行砂的静力模型加卸载试验,通过试验得到砂在卸荷过程中应力变化规律,即在卸荷初期砂中应力几乎不变,在荷载完全卸除后仍存在残余应力。通过分析影响土中残余应力的因素,建立归一化参数——残余应力系数ξ和卸荷比R之间的函数关系,结合坑内土体的受力特点,建立基坑被动区残余应力的计算公式。结合既有试验成果,提出利用考虑残余应力的竖向土压力和加载时的静止侧压力系数来计算基坑被动区土压力的方法。建立考虑残余应力的基坑被动区抗剪强度计算方法,并与室内试验结果进行对比,验证方法的正确性。     

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.     

土体的小应变特性(英文)

岩土工程的设计与计算，常常需要研究土的力学性质，并以数学模型描述其基本特性。随着现场测试技术和土工试验的进步，对土在小应变下应力－应变性质，以及新近应力历史影响有了新的认识。表征这些特性的数学模型，如非线性的块串模型等，已经得到广泛应用。本文首先讨论土在小应变下的非线性特征，接着介绍表征小应变特性，特别是新近应力历史影响的块串模型，最后以一实例说明模型之应用。     

不同应力路径下结构性土的力学特性

天然土在受荷过程中会经历不同的应力路径，开展结构性土在不同的应力路径下的力学特性的试验研究可以为建立复杂应力路径下的合理结构性本构模型提供试验依据。对结构性土样在不同应力路径下的力学特性进行试验研究。所用土样是人工室内制备的结构性土试样，共进行不同的固结应力状态下常围压、减小围压和增大围压时施加竖向应力直至土样破坏的固结排水和固结不排水三轴试验，对结构性土样在不同的应力路径下的强度特性、破坏特性和变形特性进行探讨和分析。     

Understanding ground deformation mechanisms for multi-propped excavation in soft clay

Deep excavation works are carried out to construct underground infrastructures such as deep basements, subways, and service tunnels. The execution of these deep excavation works requires the use of retaining walls and bracing systems. Inadequate support systems have always been of major concern, as excessive ground movement induced during excavation could cause damage to neighboring structures, resulting in delays, disputes, and cost overruns. To gain a better understanding of the mechanisms involved in soil excavations, centrifuge model tests of deep excavations in slightly over-consolidated soft clay have been carried out using a newly developed testing system, in which the construction sequence of a multi-propped wall for deep excavations can be simulated in flight. Deformation mechanisms are observed using Particle Image Velocimetry. Settlements of the ground surface and changes in pore water pressure are monitored during the excavation. The effects of prop stiffness, wall rigidity, and excavation geometry on the characteristics of ground deformation and soil–structure interaction are demonstrated and discussed. The use of the conservation of energy within the framework of the mobilizable strength design in calculating ground movements is validated and shown to perform satisfactorily.     

Numerical simulation of the swelling behaviour around tunnels based on special triaxial tests

This paper is to contribute to the understanding of the behaviour of tunnels in swelling ground. An Italian case study of a tunnel, collapsed due to swelling of a stiff clay, is taken as an example. The stress paths during excavation of elements of ground around the opening are computed in order to evidence the significant difference to that reproduced by usual swelling tests in the laboratory. An innovative triaxial testing procedure is developed and the stiff-clay tested. A numerical simulation of the swelling phenomenon induced by the excavation of the tunnel, based on the experimental results obtained, is then compared to site observations.     

应力路径对重塑粘土本构关系影响的试验研究

为探讨应力路径对重塑粘土本构关系的影响,进行了增、等、减排水和常规三轴压缩不排水应力路径下的三轴压缩试验,获得了四种应力路径下重塑粘土的应力应变关系,采用数值建模方法通过可视化分别给出了四种应力路径下整个应力场中的剪切和体积三维变形曲面,并给出了四种应力路径下的剪切和体积屈服轨迹。对比这四种应力路径下的变形结果发现,无论在应力范围、应变峰值、应变曲面的形状和体积屈服轨迹的变化趋势上都存在显著差别,然而剪切屈服轨迹相似。这些结果证实了应力路径对粘土本构关系的影响是相当大的和不容忽视的,而有效平均正应力对应力路径相关性的影响是决定性的。     

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。     

Plane strain evaluation of stress paths for supported excavations under lateral loading and unloading

Excavations are generally analyzed as plane strain problems. The shear strength parameters determined from the triaxial conditions are not representative of the real field conditions. Therefore, plane strain tests on reconstructed silty clay were performed using a modified apparatus to measure the stress-strain curves directly. Three stress paths were tested, where the lower shear strength and the failure strain were measured for tests with unloading in the minor principal stress direction and compared to tests with loading. A set of hyperbolic constitutive models was developed to fit the experimental data at different stress paths. The stress-strain-strength characteristics and the deformation parameters were determined for these tests. A displacement-dependent earth pressure model was then proposed for retaining walls due to excavations, and then it was calibrated against the measured soil response. In the end, a case history of bored pile walls subjected to excavations was illustrated. Calculations using the proposed approach and the standard Mohr-Coulomb model were compared. The analysis using the standard Mohr-Coulomb model was found to underestimate the maximum wall deflection by 15%.