Finite element modeling of the consolidation behavior of multi-column supported road embankment

A multi-column composite foundation is a new concept utilizing different column types with varying lengths and diameters to support the embankment fill and to mobilize the strength and stiffness of the soil at shallow depths. This study presents the results of finite element analyses using the finite element software PLAXIS to investigate the consolidation behavior of a road embankment constructed on a multi-column composite foundation. The finite element results are calibrated for a period of 200 days. The settlement, horizontal displacement, differential settlement, column axial force, and the development and dissipation of excess pore pressure are presented and discussed in detail. It is concluded that a multi-column composite foundation allows a fast rate of consolidation and significantly increases the embankment stability. A multi-column composite foundation formed by CFG–lime columns is more effective than one formed by SC–lime columns. The CFG–lime columns improve the long-term stability of the embankment because the compression modulus of CFG columns is significantly greater than that of SC columns.     

水泥土搅拌桩施工扰动评价的一种方法

水泥土搅拌桩广泛应用于高速公路软土处理工程中,但是在工程实践中发现水泥土搅拌桩施工会对桩周土产生扰动使地基土强度降低从而导致路基路面产生开裂。水泥土搅拌桩这类非挤土桩其施工扰动方面研究比较少,为此展开水泥土搅拌桩施工对桩周土的扰动研究。围绕水泥土搅拌桩施工性状,采用圆孔扩张理论分析了施工过程中桩周土受力,采用Sekiguchi和Ohta（关口-太田）模型计算了桩周土体积应变,根据Hong和Onitsuka扰动度的定义得到了搅拌桩施工桩周土的扰动度变化。结果表明水泥土搅拌桩施工对桩周土的扰动随深度的增大和距桩边距离增大而减小,该分析结果与静力触探现场测试得到的锥尖阻力变化规律一致。     

Stability analysis of an embankment resting upon a column‐reinforced soil

The design against failure of an embankment resting upon a soft soil improved by a group of columns is investigated with the help of the yield design homogenization approach. Assuming that both constituents of the reinforced ground are purely cohesive materials (‘lime column’ technique), an upper bound estimate for the macroscopic strength condition of the reinforced soil as a homogenized medium is first obtained, providing definite evidence of a shear strength anisotropy associated with the reinforcement preferential orientation. The kinematic method of yield design is then performed on the basis of such a criterion, making use of rotational failure mechanisms involving slip circles in the reinforced ground. Upper bound estimates are finally obtained for the embankment stability factor, as functions of the degree of reinforcement and relative thickness of the soil layer. These results are compared with those derived from a simplified analysis, where the reinforced soil is assumed to exhibit an averaged isotropic cohesion. This comparison clearly indicates that the latter simplified analysis may produce quite unsafe estimates for the embankment stability, which can be attributed to the fact that it fails to capture the inherent strength anisotropy of the reinforced soil. Copyright © 2010 John Wiley & Sons, Ltd.     

短芯劲芯水泥土桩承载路堤失稳破坏模型试验

在劲芯水泥土桩承载路堤的工程应用中,芯桩比外桩短的情况比较普遍。但是,人们对短芯劲芯水泥土桩承载路堤的桩体破坏模式和失稳破坏机理的认识较为欠缺,无法合理评价路堤的稳定性。鉴于此,本文通过室内模型试验,开展短芯劲芯水泥土桩承载路堤失稳破坏模式研究。通过监测芯桩导电通路变化、芯桩桩身应变和桩土竖向应力,并结合PIV技术,综合分析桩体渐进式破坏模式和路堤整体失稳规律。研究结果表明,在路堤顶面下,荷载主要由芯桩承担,并随着超载增加,芯桩外桩荷载分担作用减小,但路堤失稳后芯桩仍具有一定荷载分担作用。在路堤失稳破坏过程中,路堤下桩体表现出受压破坏且芯桩底部局部鼓胀破坏,坡面下桩体表现出压弯和拉弯破坏模式,地基滑动面并不完全穿过桩体破坏位置。     

自由段先注浆对预应力锚索抗拔试验结果影响研究

为了解决无法采用已注浆锚索进行抗拔试验问题,基于界面脱黏剪切模型,利用荷载传递和弹性变形理论,考虑锚索锚固段与自由段长度比Lb /La的影响,建立了采用自由段已注浆锚索抗拔试验结果评价锚索抗拔性能的方法。结果表明：自由段注浆对锚索抗拔力有较大幅度的提高,尤其是当界面剪切阶段由弹性向脱黏过渡时,提高比例快速增加,并在脱黏后逐渐趋于稳定。其次,锚索自由段与锚固段交界面处剪切位移和轴力最大,并向两端逐渐降低。随着拉拔力的提高,界面剪切位移和轴力都不断增大,同时向锚索两端的衰减速度也加快。此外,弹性阶段时剪应力曲线为山峰型,而脱黏阶段为马鞍型,并随抗拔力的提高,马鞍开口逐渐扩大。最后,锚固段与自由段长度比Lb /La对锚索极限抗拔力的影响较小,其极限抗拔力折减系数?Q取值介于0.81～0.83之间,且Lb /La =2.5时,?Q计算值仅比实测结果小6.02%,验证了本文方法的可行性。     

强夯碎石墩处治“山地型”软土路基的变形特征研究

为明确填方路堤荷载下强夯碎石墩的变形特征与其路堤稳定性的内在关联,针对“山地型”软土地区某高速铁路车站高填方路堤的失稳破坏现象,基于现场实测数据和三维数值分析模型,着重研究了不同位置强夯碎石墩墩体的鼓胀变形与侧向弯曲变形规律,并定义墩体鼓胀率和最大侧向弯曲值对其变形特征予以描述。研究结果表明：路堤荷载作用下不同位置的强夯碎石墩发生不同程度的鼓胀变形和侧向弯曲变形,其中鼓胀变形主要由上覆路堤荷载所产生的压缩鼓胀和路堤侧滑所产生的滑动剪切鼓胀所组成,且其最大鼓胀变形区域集中在距桩端1～2倍桩直径范围内;位于强夯碎石墩复合地基潜在滑裂面剪入口附近墩体的鼓胀率最大(0.75%),约为路堤中心处墩体鼓胀率的1.5倍,表明该处以滑动剪切鼓胀变形为主。沿路堤中心线向外不同位置的强夯碎石墩墩体的最大侧向弯曲变形的发展规律近似呈三段线模式,其中潜在滑裂面剪入口附近的17^#墩体为其侧向弯曲变形开始陡增的拐点,表明17^#与其附近墩体为控制路堤侧向变形与稳定的关键构件。路堤填筑过程中所出现数条裂缝的原因可能在于强夯碎石墩的剪切破坏而致使路堤出现整体滑移所引起的,建议...     

三灶湾海堤地基水上原位试验

本文介绍在珠江口三灶湾水上进行的十字板剪切和静力触探试验,获得了室内试验未能得到的土的十字板强度 C_u 和比贯入阻力 P_s 随深度增加而增大的有规律的结果。由这些试验结果,获得了两个近似公式 C_u=0.0069h 和 C_u≈0.024P_s,为筑堤提供了设计依据。另外,根据现场试验条件,因地制宜,采用简易的水上试验方法,为类似工程提供了方便和经济的离岸试验技术。     

劲芯水泥土桩承载路堤渐进式失稳破坏机制

劲芯水泥土桩是一种软土地基处理的新方法,近年来已成功应用于公路、铁路路基处理工程中。然而由于人们对其承载路堤的失稳破坏机制认识不足,无法正确指导设计。采用反映桩体材料破坏后特征的应变软化模型,模拟劲芯水泥土桩承载路堤失稳破坏1g模型试验,通过分析路堤失稳破坏过程中桩体塑性区的开展和桩身受力的变化情况,研究了桩体的破坏顺序及其破坏模式。结果表明：路堤失稳过程中,劲芯水泥土桩并非同时发生破坏,路面正下方桩体首先发生受压破坏,坡面下方桩体自坡脚至坡肩依次发生弯剪破坏;由于桩体的存在,路基中滑动面并非完全穿过桩体破坏位置。基于桩体破坏顺序、破坏模式、受力情况变化,以及荷载传递规律,阐释了劲芯水泥土桩承载路堤渐进式失稳破坏机制。采用现行规范中基于残余强度的柔性桩处理方法,计算的安全系数与试验结果较为接近,但其适用性还需做进一步研究。     

孔压静探试验在珠江三角洲软土地区中的应用研究

孔压静探试验（简称ＣＰＴＵ）在国内是一项尚待推广的土体原位测试新技术,着重介绍了珠江三角洲高速公中工程建设中的孔压静探试验研究成果;提出了新的土分类图及不排水抗剪强度的估算公式;试验结果表明,孔压静探试验估算的固结系数Ｃｈ较室内试验估算的固结系数大１￣２个数量级,而与沉降观测资料反算的固结系数接近。     

土工格栅与土界面作用特性试验研究

土工格栅与土的界面摩擦特性指标是加筋土工程设计的关键。通过分析土工格栅与土的界面摩擦作用和进行了直剪摩擦试验和拉拔摩擦试验,测试了两种试验条件的界面摩擦特性。在两种试验条件下,土工格栅加筋土复合体的抗剪强度均有界面摩擦角φsq和界面凝聚力csq,且土工格栅与土相对位移量的不同,其复合体的强度机理有区别。在拉拔摩擦试验中,剪应力峰值强度对应的剪切变形值高于直剪摩擦试验中剪应力峰值强度的剪切变形值5～10倍以上。两种试验均有其适用性,而土与土工格栅的相对位移较小时直剪摩擦试验较能反映实际;土与土工格栅相对位移较大时土与格栅双面均发生相对位移,拉拔摩擦试验更为合适。随法向应力的增大,直剪摩擦和拉拔摩擦试验的剪应力峰值以及剪应力峰值对应的位移均提高。直剪摩擦的剪切速度小,剪应力峰值强度高,且达到峰值强度的剪切位移大;增加剪切速度,剪应力峰值强度降低,且对应的位移也减少,其原因是界面上的孔隙水压力消散和筋材的应力松弛。应根据具体工程的需要选择直剪摩擦试验和拉拔摩擦试验确定设计参数。     

Statistical analysis of the effective stress method and modifications for prediction of ultimate bond strength of soil nails

Uncertainty in the predicted ultimate pullout strength of soil nails can be significant due to the complexity of nail–soil interactions, inherent variability in soil properties and the effects of nail installation. The paper first presents a statistical evaluation of the accuracy of ultimate bond strength of soil nails using the effective stress method (ESM) equation that has been adopted in Hong Kong. A total of 113 ultimate nail capacity data points from field pullout tests were collected from the literature and used to estimate the accuracy of the current ESM. Based on the available data, the current ESM default pullout model is found to be excessively conservative (on average) by at least a factor of three. The spread in prediction accuracy measured by the coefficient of variation (COV) of bias is in the range of 36–43 % after removing anomalous test data. Here, bias is the ratio of measured to predicted pullout load capacity. In addition, the accuracy of the current ESM equation for prediction of nail bond strength is shown to be dependent on the magnitude of predicted ultimate bond strength and magnitude of nominal vertical effective stress which is undesirable. The paper examines four candidate-modified bond strength equations with empirical coefficients that have been back-fitted to measured bond strengths to improve the overall accuracy of the equation and to reduce or remove the undesirable dependencies noted above. One equation with an empirically corrected stress-dependent term is judged to be the best candidate model based on the mean of bias values, spread (COV) of bias values, lack of dependencies and simplicity. Finally, the relative contributions of random variability in soil shear strength to measurement bias in bond strength (prediction accuracy) for each soil type are computed for the best bond strength model. Analysis of the contribution of soil shear strength to prediction accuracy showed that the combination of variability due to factors other than soil shear strength was greater than the variability in soil shear strength alone, where the latter is defined by the soil secant friction coefficient.     

黄河三角洲粉质土海床临界起动切应力影响因素研究

在黄河口潮滩和室内环形土槽模拟新沉积的海床土体,观测固结过程中沉积物临界起动切应力的变化及对应的土体重度、含水率、贯入阻力、剪切强度指标值的变化,分析黄河三角洲粉质类土临界起动切应力的影响因素与变化规律。研究发现：黄河三角洲粉土的临界起动切应力与重度、贯入阻力和抗剪强度之间呈正相关关系,而与含水率呈负相关关系,并且与干密度、抗剪强度之间具有非常好的定量关系。将黄河三角洲粉土临界侵蚀切应力的试验结果与其他学者建立的计算公式进行了比较,发现在固结开始试验与计算结果间的吻合性较好,随着固结时间差异性变大,表现出黄河口粉质土的独特性。     

Strength and Compressibility Characteristics of a Soft Clay Subjected to Ground Treatment

The compressibility properties, undrained shear strength, and stress history are essential for reliable calculations of settlement and bearing capacity of soft soils. However, it is sometimes a challenge to determine representative parameters for very soft and high plasticity clays, which are often found in Brazilian coastal areas. In this study an extensive site investigation was planned aiming to interpret the behaviour of a trial embankment on stabilized soft ground. The site investigation was carried out in a test area located in the west of city of Rio de Janeiro, consisted of three clusters in which standard penetration tests, vane shear tests, and piezocone tests (CPTu) were performed. A number of correlations was developed and compared with empirical equations in order to verify their reliability. The results of these tests made it possible to define geotechnical parameters of the soft clay to use in the numerical and analytical computations of the embankment on reinforced ground.     

土工织物与吹填土界面作用特性试验研究

筋-土界面强度参数是加筋结构设计和稳定分析的关键技术指标。拉拔试验能较好模拟现场加筋行为而得到广泛的应用。基于拉拔和直剪试验研究了拉拔（剪切）速率对筋-土界面特性和吹填砂强度特性的影响规律及机制,同时探讨了不同填料界面、筋材类型的加筋效果。结果表明：随着拉拔（剪切）速率的增大,吹填砂-筋材-吹填砂界面的抗剪强度下降明显,而软土-筋材-吹填砂界面以及砂本身强度则变化不大。从强度指标来看,拉拔速率增大,筋-砂界面强度的降低主要表现为似黏聚力的降低,筋-软土界面抗剪强度的增加表现为内摩擦角增大,剪切速率对吹填砂则基本无影响。筋-土界面特性受拉拔速率和正应力的共同影响,与筋材类型和填料特性有关。筋-土界面内摩擦角小于填料摩擦角,但在一定正应力下低速剪切时（如<0.53 mm/min）可获得高于填料的抗剪强度。宜根据似黏聚力大小合理选择摩擦参数进行加筋结构的设计与评价。     

密度孔压静力触探的测试理论和工程应用综述

孔压静力触探试验(CPTU)是目前使用最为广泛的原位测试方法之一,但CPTU不能原位测量土体最基本的指标——密度。密度孔压静力触探试验(DCPTU)是把核子密度仪结合到孔压静力触探系统中,贯入过程中可以同时测量锥尖阻力、侧壁摩阻力、孔隙水压力和密度的一种新型原位测试方法。本文首先介绍了DCPTU的试验装置和密度测试理论;然后通过与室内试验结果对比评价了利用DCPTU测量砂土、黏土、填土地层以及海底淤泥的密度的可靠性,分析了非均质地层中DCPTU测量密度与真实密度的区别、并提出了真实密度剖面的推求方法;然后综述了DCPTU在判别软弱夹层、评价砂土的液化势、检验地基处理效果、检测块状填土中的空隙、调查浮泥层的厚度和密度分布等方面的工程应用;最后展望了DCPTU在工程勘察中的发展趋势和应用前景。     

Finite element modelling of laterally loaded lime–cement columns using a damage plasticity model

The behaviour of laterally loaded lime–cement columns in a shear box was studied. Laboratory tests are presented together with numerical analyses where the columns are simulated by a concrete damage plasticity model that considers stiffness degradation. Seven model tests were investigated where the columns were installed in a single column pattern and in rows with different column overlap in order to investigate the influence of the degree of overlapping of the columns in the rows. The results of the numerical evaluations showed good agreement with the experimental shear stress–displacement relation and a good accuracy with respect to the fractures developed.     

剪切速率和材料特性对筋-土界面抗剪强度的影响

土工合成材料与填料之间的界面强度参数是加筋-土工程设计的关键技术指标,筋-土界面的直剪试验和拉拔试验在界面剪切特性试验研究中应用最为广泛。利用土工格栅、土工织物与砂土的直剪试验和拉拔试验,研究了剪切速率和筋材性质对筋-土界面强度的影响。研究结果表明,当剪切速率不超过一定界限（如7.0 mm/min）时,其对直剪试验结果的影响可以忽略;筋-土界面强度受加筋材料及砂土特性的影响,双向聚丙烯土工格栅和土工织物与砂土之间的内摩擦角与纯砂接近,界面强度较高,而玻纤格栅因其延伸率低和网格尺寸较小,与砂土的界面强度比较低。     

分级施工河堤下高灵敏性黏土的强度增加

软黏土地基由于其含水量高、孔隙比大、强度低、渗透性差,因此,在灵敏度很高的软黏土地基上建造高度较高的路堤,其稳定性很难得到保证。一般采用分级加载的方法使软土地基在路堤荷载的作用下发生固结,强度获得增长。为加速固结,常采用塑料排水板处理地基,使之能在较短的时间内完成固结,加速地基强度增加。针对日本佐贺县高灵敏度的有明黏土地基上修建的分级加载的6.5 m高河堤下软黏土的物理力学性质的变化,说明分级加荷的重要性。现场软土地基为结构性黏土,有很高的灵敏度。而且该软黏土地基的渗透系数极低。因此采用塑料排水板进行了加固处理,施工过程中对地基土的物理与力学性质随固结过程的变化进行了测试。分析了其发展规律,结果表明地基土的单轴强度在250 d内增长了近1倍。     

基于强度分区的膨胀土路堤稳定性及土工格栅处治效果分析

由于填筑密实度、含水率的差异,整个膨胀土路堤内存在以含水率和干密度为特征的强度分区,且路堤表层土体由于直接受大气、降雨作用,温度、湿度变化频繁,胀缩变形剧烈,裂隙发育,强度将剧烈衰减。但该实际存在的强度分区一般在稳定性计算中多被简化为均质体而不能反映实际情况。强度分区法则考虑大气和气候影响以及路堤内压实度差异,并综合考虑土工格栅的作用。基于强度分区的膨胀土路基边坡稳定性的分析结果表明,强度分区法更能反映路堤土体强度差异的实际情况,能真实地体现改性处治和铺设土工格栅对路堤稳定的贡献,具有显著的优点。对于素填土膨胀土路堤,边坡表层容易发生局部浅层滑动,进而诱发和牵引发生深部滑动破坏;石灰改性后虽能提高路堤稳定性,但表层局部稳定性安全储备并不宽裕,尚未彻底根除安全隐患。土工格栅虽对整体稳定性贡献不大,但可显著提高局部稳定性,并有效地消除膨胀土路堤边坡浅表滑塌的隐患。     

无法向荷载推剪试验的解析与应用

推剪试验常在滑坡现场勘探中采用,获取抗剪强度。工程地质手册中推剪试验的现有数据分析处理方法存在一些缺点,应加以改进。研究无法向荷载刚体模型的推剪试验数据处理方法,假设模型受压面在Y轴方向不受约束。推导出该模型推剪试验的微分方程,解析出求c、? 值与破坏面曲线方程组公式。根据原位试验发生的压缩现象,分析了试体受压面附近存在局部先期破坏。对新公式进行了算例与实例的分析,解释了影响破坏面曲线变形的原因是：推剪受压面必须满足Y方向不受约束,该面应安装能垂直滑动或转动的承压板。对受压面被约束的试验,可根据局部破坏面斜率推测破坏体长度,进行c、? 值计算。此外,还提出了推剪试验必须注意的几个问题。