竖向荷载下加筋碎石桩复合地基数值分析

采用三维有限元程序建立了一长为6 m、直径为0.8 m的加筋碎石桩复合地基流固耦合数值模型,分析了其在堆载和孔压消散过程中的荷载传递和变形特性。较传统碎石桩,加筋碎石桩复合地基桩土应力比显著增大,超孔压、沉降和桩身侧向变形显著减小,且随筋材刚度的增大,其性能进一步改善。加筋碎石桩复合地基在桩间土固结过程中产生明显的桩土差异沉降,形成土拱效应,使得堆载结束后桩土应力比变化很小。筋材长度对加筋碎石桩复合地基桩土应力比和沉降影响显著,应对其全长加筋才能保证桩体刚度和有效减少沉降。     

包裹碎石桩和碎石桩复合地基抗震性能对比试验研究

包裹碎石桩是在碎石桩外包裹一层土工材料制成的一种新型桩体,由于包裹作用,碎石桩的刚度和抗剪强度得到了明显提高。然而,目前有关包裹碎石桩的研究较少,特别是其抗震性能方面。鉴于此,通过开展包裹碎石桩复合地基和碎石桩复合地基振动台模型试验,在考虑不同类型、幅值的地震波作用下,研究两种复合地基的加速度、桩土应力、破坏现象和位移,进而对比分析这两种复合地基的动力响应规律和抗震性能。试验表明:包裹碎石桩桩顶和桩间土水平方向加速度放大系数均约为碎石桩的2倍;相同地震波条件下,包裹碎石桩复合地基的峰值桩-土应力比约为碎石桩复合地基的3倍;与碎石桩复合地基相比,0.9g人工波加载后包裹碎石桩复合地基在较大区域内产生较窄裂缝;包裹碎石桩复合地基产生的总沉降较碎石桩复合地基减小了51%。因此,包裹碎石桩复合地基的抗震性能优于碎石桩复合地基。     

基于圆孔扩张理论的碎石桩承载力计算方法

本文深入研究碎石桩复合地基的受力变形机理,利用摩尔库伦弹塑性材料的剪胀特性,同时引入应力跌落三折线模型,视筋箍碎石桩承受上部荷载时的径向鼓胀为圆孔扩张,将产生变形的桩周土体分为弹性区和塑性区两部分,运用Vesic圆孔扩张理论分别建立了弹性区和塑性区桩周土体的应力场和位移场表达式,进而利用桩体影响半径处土压力为静止土压力的假设,求出了桩周土对碎石桩的径向围限力,再结合被动土压力公式导得了碎石桩的单桩承载力计算式。为验证该方法的可行性,本文结合实际工程数据对所推导的承载力计算式进行了验证分析,结果表明计算值与实测值吻合。     

夯实水泥土桩复合地基有效桩长与桩身强度关系试验研究

通过室内模型试验,实测得到水泥土材料的破坏形式、夯实水泥土桩复合地基桩土荷载分担比、桩土应力比、桩间土深层变形等承载及变形性状。给出了文中试验条件下,给定桩身强度的夯实水泥土桩复合地基的有效桩长或有效复合土层厚度,认为设计夯实水泥土桩复合地基时,应考虑桩身强度对承载力的影响桩体,设计应以桩身强度控制,使由桩身强度确定的单桩承载力大于（或等于）由桩周土和桩端土的抗力所提供的单桩承载力。     

碎石桩复合地基承载力性状分析

按等置换率原则建立碎石桩复合地基有限元计算模型，在考虑基础刚度、荷载强度变化条件下对比分析了天然地基与复合地基附加应力分布特征、塑性区扩展规律；根据有限元计算结果分析了桩身荷载传递规律并与实测结果对比确定桩身应力随桩身呈指数衰减。计算结果还表明：桩土应力比不仅与桩土模量比有关，也与荷载水平及置换率有关；在鼓胀破坏情形下桩体侧向变形性质取决于桩周土而与置换率、模量比、桩长无关。     

桩体长径比对桩基动力特性的影响研究

弯曲破坏是地震中桩基失效的一类常见模式,其破坏的一个重要原因就是桩体的抗弯刚度不足。对于一定长度的桩体,桩的抗弯刚度与桩的长径比直接相关,但桩基规范中对摩擦桩的长径比并没有像端承桩那样作出限制。在构建的结构性饱和黄土动力本构模型的基础上,针对黄土地区的单桩基础,结合大型有限元程序建立了摩擦桩-土-结构体系、端承桩-土-结构体系的有限元-无限元模型,探讨了不同桩体长径比对两者动力反应特性的影响,得出如下规律：(1) 其他条件相同时,不管是端承桩还是摩擦桩,长径比对桩身截面剪应力和水平加速度的分布形态没有影响。但随着长径比的增大,桩身各截面对应的的剪应力值减小,而加速度值反而增大;(2) 随着长径比的增大,桩身变形形态从正弯型逐渐过渡到反弯型,桩体的破坏模式也相应由剪切破坏过渡到弯曲破坏;(3) 端承桩体系比摩擦桩体系更容易出现反弯型,但当长径比增大到一定程度时,摩擦桩也会呈现反弯型。从抗震角度来说,对摩擦桩长径比的限制应以桩体变形模式表现为正弯型为宜。所得结论对工程实践具有一定的指导意义。     

悬臂式抗滑桩模型试验研究

作为治理滑坡的重要手段之一的抗滑桩,由于岩土体介质的特殊性,桩后滑坡推力、土体抗力及桩身变形破坏模式与理论计算存在较大差异。通过悬臂式抗滑桩加固滑坡的模型试验,对滑体进行逐级加载,测得桩后滑坡推力、桩前土体抗力和桩体的应变,研究滑坡推力分布、土体抗力的变化情况、桩身变形破坏模式。试验结果表明,对于悬臂式抗滑桩可分为分离段和接触段两部分,滑坡推力逐渐向接触段集中;桩前土体抗力主要在桩前25 cm以上,随着深度增加,抗力逐渐减小;悬臂式抗滑桩为折断破坏形式,破坏点的位置在滑面以下25 cm处。模型破坏主要是由于桩前土体发生屈服,从而使桩顶部位移过大,致使桩身因折断破坏而失效,最终滑坡模型失稳。其结果可为实际工程提供借鉴。     

基桩水平静载试验及内力和变形分析

桩基作为一种重要的基础形式,在岩土工程中应用广泛,对于安全等级要求较高的重大工程项目,往往需要通过原位水平静载试验确定基桩的水平承载力,分析桩身的内力及变形。结合实际工程详细地介绍了利用钢筋计测试水平荷载作用下桩身弯矩、挠度和转角分布的方法,根据桩顶位移和钢筋内力测试结果判定试验桩的水平临界荷载为120 kN;根据钢筋计测试结果可知,在水平荷载作用下,桩身最大弯矩截面位于在地面以下2～3 m处,且随着荷载的增大最大弯矩截面逐渐向深部转移;发生弯曲变形的部分主要是桩长1/3以上的桩体,而其下的桩体几乎不发生变形。     

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。     

水泥粉喷桩荷载传递规律的试验研究

通过水泥粉喷桩的现场足尺试验,测试了极体的应力-应变关系,推算出极侧摩阻力、轴力变化曲线。结果表明,荷载沿桩身的传递有一定范围,桩体的变形、应力、侧摩阻力主要集中在临界桩长范围内,而各量变化梯度在浅部较大。桩侧摩阻力的发挥与极土相对位移有关。桩体破坏也发生在浅层,破坏形式为环向拉裂和桩体横向压碎。     

Three-dimensional DEM analysis of single geogrid-encased stone columns under unconfined compression: a parametric study

Three-dimensional discrete element method (DEM) was employed in this study to analyze the behavior of single geogrid-encased stone columns under unconfined compression. Four important parameters were investigated to understand and evaluate their effects on the behavior of the encased columns by seven DEM models. The biaxial geogrid used as an encasement material for stone columns was simulated using parallel-bonded particles, and the aggregate in the stone column was simulated using graded particles. Both the macroscopic responses (e.g., vertical pressure–strain curves) and the microscopic interactions (e.g., contact force, coordination number, and sliding fraction) of the columns under unconfined compression were analyzed and are presented in this paper. The numerical results show that the geogrid encasement with high tensile stiffness could provide high confining stresses and then effectively increased the bearing capacity of the column. The short column yielded quickly even though its column modulus at a small deformation was relatively high. The modulus of the column slightly decreased with an increase in the column diameter due to high circumferential strains mobilized in the geogrid encasement. The column with large aggregate was stiffer and deformed less than the column with small aggregate. Selecting aggregate with a size larger than the geogrid aperture size was an effective way to achieve better interlocking between the aggregate and the geogrid and to minimize mass loss for the geogrid-encased stone column under loading. Due to limited deformation allowed by the geogrid encasement, a coefficient of radial stress equal to half of the coefficient of passive earth pressure was suggested to estimate the ultimate bearing capacity of the geosynthetic-encased stone column.     

Finite Element Analysis of Geogrid Encased Stone Columns

Stone columns in soft soil improve bearing capacity because they are stiffer than the material which they replace, and compacted stone columns produce shearing resistances which provide vertical support for overlying structures or embankments. Also stone columns accelerate the consolidation in the native surrounding soil and improve the load settlement characteristics of foundation. In this paper, the finite element method is utilized as a tool for carrying out analyses of stone column–soil systems under different conditions. A trial is made to improve the behaviour of stone column by encasing the stone column with geogrid as reinforcement material. The program CRISP-2D is used in the analysis of problems. The program allows prediction to be made of soil deformations considering Mohr-Coulomb failure criterion for elastic–plastic soil behaviour. A parametric study is carried out to investigate the behaviour of standard and encased floating stone columns in different conditions. Different parameters were studied to show their effect on the bearing improvement and settlement reduction of the stone column. These include the length to diameter ratio (L/d), shear strength of the surrounding soil and, the area replacement ratio (a_s) and others. It was found that the maximum effective length to diameter (L/d) ratio is between (7–8) for Cu, between (20–40) kPa and between (10–11) for Cu?=?10?kPa for ordinary floating stone columns while the effective (L/d) ratio is between (7–8) for encased floating stone columns. The increase in the area replacement ratio increases the bearing improvement ratio for encased floating stone columns especially when the area replacement ratio is greater than (0.25). The geogrid encasement of stone column greatly decreases the lateral displacement compared with ordinary stone column.     

双向土工格栅加筋碎石桩单轴压缩试验

对拉伸塑料、焊接聚酯和经编涤纶等3种双向土工格栅加筋碎石桩桩体进行单轴压缩试验,以研究不同双向土工格栅加筋碎石桩桩体的变形和强度特性。研究结果表明:拉伸塑料和经编涤纶格栅加筋碎石桩桩体强度与筋材强度之间呈较好的线性关系,而焊接聚酯格栅加筋碎石桩桩体强度与筋材强度之间的线性关系较差;拉伸塑料格栅加筋碎石桩桩体破坏时其格栅本身拉伸强度能充分发挥,而焊接聚酯格栅和经编涤纶格栅加筋碎石桩桩体,均在这两种格栅远未达到其本身拉伸强度前,因格栅焊接点脱开或横肋从纵肋中抽离而导致破坏,拉伸塑料格栅加筋碎石桩桩体强度要显著高于经编涤纶和焊接聚酯格栅加筋碎石桩桩体强度;在实际工程中可采用双向拉伸塑料格栅来加筋碎石桩桩体,其桩体强度可采用聚丙烯土工编织布加筋碎石桩桩体强度修正公式计算。     

Improvement of soft clay using installation of geosynthetic-encased stone columns: numerical study

The use of geosynthetic-encased stone columns as a method for soft soil treatment is extensively used to increase the bearing capacity and reduce the settlement of raft foundations and the foundation of structures like embankments. Pre-strain is an effect occurring in the encasement during stone column installation due to the compaction of the stone material. The present study uses the finite element program Plaxis to perform a numerical analysis of the soft clay bed reinforced by geosynthetic-encased stone columns. An idealization is proposed for simulation of installation of geosynthetic-encased stone columns in soft clay based on the unit-cell concept. In the analyses, initially, the validity of the analysis of the single column-reinforced soil in the unit-cell model was performed through comparison with the group columns. Then, by considering a unit-cell model, the finite element analyses were carried out to evaluate the stiffness of the reinforced ground to estimate the settlement. The results of the analyses show that the improved stiffness of the encased stone column is not only due to the confining pressure offered by the geosynthetic after loading, but the initial strain of the geosynthetic that occurred during installation also contributes to the enhancement of the stiffness of the stone column and the reduction of the settlement.     

Experimental Investigation on the Bearing Capacity of Stone Columns with Granular Blankets

Using stone columns is an efficient method to increase the bearing capacity of soft soils. This has led to an increased interest in further developing and improving the method. In addition, granular blankets are used to increase the bearing capacity of the stone columns. In this research, the bearing capacity of stone columns, granular blankets, and a combination of both methods in reinforced and unreinforced modes was examined using large-scale laboratory tests. A scale factor of 1–10 is used for the geometry of the models, and the stone columns are a floating type that are 60 mm in diameter and 350 mm in length. These columns are either reinforced with vertical encasement of a geotextile or they are unreinforced. The granular blankets are either reinforced by using a biaxial geogrid or they are unreinforced with 40 and 75 mm thicknesses. In general, 16 large experimental tests have been carried out. Results indicate that using all these variations (granular blankets, stone columns, and a combination of both) improves bearing capacity. Using geogrid as the reinforcement of granular blankets and geotextile as stone-column encasement increases the efficiency of granular blankets and stone columns significantly. The maximum bearing capacity was obtained when reinforced granular blankets and reinforced stone columns were combined. The stress-concentration ratio and bearing capacity increased as geotextile encasement was used in the stone columns.     

Bearing capacity of soft soil model treated with end-bearing bottom ash columns

Granular column technique is a soil improvement method used to increase the bearing capacity of a soft soil area by replacing the soil with a group of granular column materials. The by-product utilisation is a worldwide interest for sustainable infrastructure development. Bottom ash, which is a combustion deposit derived from coal burning, is a potential by-product that could be used alternatively to sand or aggregate as a green granular column material. This research is to study the potential use of the bottom ash column-improved soft clay by conducting a series of small-scale physical modelling test. The bearing capacity behaviour and failure mode of soft clay improved with end-bearing group of bottom ash columns with and without geotextile encasement are investigated. The bearing capacity of soft clay is significantly enhanced by the inclusion of bottom ash columns; that is, 239% of bearing capacity improvement is observed with only 13% of improvement area. The bulging of the bottom ash column is transferred to buckling failure with higher bearing capacity when the bottom ash column is encased by geotextile. The outcome of this research leads to the usage of bottom ash by-product as a granular column material in sustainable soil improvement technique.     

Stone Columns with Vertical Circumferential Nails: Laboratory Model Study

This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns. The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying capacity substantially.     

A State-of-the-Art Review of Stone/Sand-Column Reinforced Clay Systems

This paper presents a state-of-the-art review of published research papers and reports that focus on the modeling, testing, and analysis of soft clays that are reinforced with sand/stone columns in relation to bearing capacity and settlement considerations. The review is presented in chronological order to shed light on the development of this field of research in the last 40+ years. The objective of the study is to assemble published results from field, laboratory, and numerical investigations of sand/stone columns in clay in one resource to provide future researchers and designers with easy access to information and data. The majority of the reviewed papers include an experimental component that is based on field or laboratory scale tests (1-g, triaxial, or centrifuge) conducted on clay specimens reinforced with partially or fully penetrating, encased or ordinary, stone or sand columns that were installed as single columns or as column groups. Some papers included numerical experiments that were based on finite element models, while others presented analytical solutions for modeling the response of the composite system. A compilation of the important findings from physical, numerical, and analytical models in addition to a summary table that facilitates access to information from various research efforts are presented in the paper.     

沉管挤密抗拔防浮碎石桩的抗拔承载力计算分析

基于现场试验检测结果,分析了沉管干振挤密抗拔防浮碎石桩在砂土中的抗拔破坏模式和应力传递机理,运用极限平衡原理研究了抗拔承载力,得到了极限抗拔承载力计算公式,并利用公式对工程实测结果进行了分析比较,表明两者较为一致。现场试验表明,未达破坏荷载前,抗拔碎石桩的抗拔承载力与变形有较明显的线性关系;抗拔碎石桩破坏性状明显,能明确得到抗拔碎石桩的破坏荷载。     

Experimental Studies on Behaviour of Stone Columns in Layered Soils

Stone columns are found to be effective and economical ground improvement technique in soft grounds. Understanding its behaviour when they are installed in stratified soils, in particular when the upper layer consists of weak soil, will be of great practical significance. This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the behaviour of stone columns in layered soils, consisting of weak soft clay overlying a relatively stronger silty soil, for various thicknesses of the top layer. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. Laboratory tests were carried out on a column of 90 mm diameter surrounded by layered soil, for an area ratio of 15%. It is found that the depth of top weak layer thickness has a significant influence on the stiffness, load bearing capacity and bulging behavior of stone columns.