Effects of restraining conditions on the bearing capacity of footings near slopes

In evaluating the ultimate bearing capacity (q_u) of a strip footing adjacent to a slope, conventional correction formulas for the effect of load eccentricity may not be applicable because these formulas were developed exclusively for footings situated on horizontal grounds, where loads eccentric to opposite sides of the footing yield identical results for q_u. In this study, loading tests and analyses are conducted on a strip footing placed adjacent to a model slope with various slope angles. The experimental evidence shows that a load eccentric toward the heel of a footing leads to an increase in bearing capacity, whereas the analytical results based on conventional formulas show the opposite trend. To address this discrepancy, an approach is proposed that uses a bearing capacity correction formula for a footing with a setback from the crest of the slope. Results of a comparative study show that the experimental values for bearing capacity factor N_γ(test), with full corrections for load inclination, load eccentricity, and footing setback are comparable to the theoretical solutions. Furthermore, fully corrected values for N_γ(test) for the fixed footing approximately follow the line of the upper boundary; those for the free-rotating footing follow the lower boundary of the theoretical solutions reported in the literature. This discrepancy is due to the different failure mechanisms induced by the restraining conditions of the footing which have yet to be considered in engineering practice.     

嵌岩桩承载性状的有限元分析

在有限元分析的基础上 ,采用Duncan非线性弹性E -B模型对嵌岩桩垂直荷载作用下的承载性状作了较为全面的探讨。指出嵌岩桩确实存在着深度效应 ,当嵌岩达到一定深度后 ,继续增加嵌岩深度 ,对桩的承载能力的提高已不明显 ,甚至无助于承载能力的提高。并得出了嵌岩桩垂直承载力与基岩强度成指数关系的结论 ,强调桩底沉渣不仅增加嵌岩桩的平均侧阻力和桩身沉降 ,而且将显著提高桩侧岩层的局部应力。文中结论可为桩基工程的科研、设计和施工提供参考     

单桩承载力对刚性桩复合地基路堤稳定性的影响

现行规范中刚性桩复合地基路堤稳定分析方法不能反映单桩承载力、桩帽等对路堤稳定性的影响,计算的路堤稳定安全系数严重偏大,导致部分刚性桩复合地基路堤滑塌。为克服刚性桩复合地基路堤现有稳定分析方法的缺陷,在研究刚性桩复合地基路堤滑塌原因的基础上,分析了路堤滑塌时桩土相互作用,及刚性桩提高路堤稳定性的机理。然后,在分析刚性桩复合地基路堤现有稳定分析方法的缺陷的基础上,将修正密度法完善为修正重度法。最后,为研究单桩承载力对刚性桩复合地基路堤稳定性的影响,利用修正重度法分析了刚性桩长度、间距、扩底、桩帽等对路堤稳定性的影响,对比了不同稳定分析方法计算结果,并对刚性桩复合地基路堤设计提出了建议。研究表明：单桩承载力对密桩复合地基路堤稳定性影响很小,对疏桩复合地基路堤稳定性影响很大;增大单桩承载力比减小桩间距更合理;刚性桩在持力层中扩底比加大桩长更有效;利用桩帽、土拱等措施将大部分路堤荷载转移到桩顶方可发挥单桩承载力对路堤稳定性的作用;软土强度随深度增大不明显时应慎用悬浮桩复合地基。刚性桩复合地基路堤宜采用“强桩大帽”的疏桩复合地基方案。     

Axial bearing capacity of socketed single cast-in-place piles

In this paper, a comparison is made of the axial bearing capacities estimated with pile load tests and empirical methods for seven rock-socketed single cast-in-place piles constructed in Turkey. The unconfined compressive strength of rock, obtained from pressuremeter tests, is used in the empirical correlations. It is commonly assumed that axial loads applied at the top of a socketed pile are transferred to the sides of the socket until a certain displacement is reached and that the end bearing capacity contributes only after this threshold displacement is exceeded. In practice, however, due to typically small displacements occurring in rock sockets, most, if not all, of the axial capacity is estimated to derive from the side shear. The limit displacement up to which the side frictional capacity of a socketed pile governs and the end bearing capacity starts mobilizing is examined, and no such threshold value is observed based on the findings of this study. Nevertheless, the bearing capacities obtained from the empirical correlations agree reasonably well with those calculated from pile load tests, when a systematic approach for estimating the threshold value from pile load tests is utilized and the unconfined compressive strength of socketed rocks can be estimated within reasonable accuracy applying actual field conditions.     

基于CPTU测试的K_0固结黏土中静压桩时变承载力研究

基于空间滑动面(SMP)准则改进的K_0固结各向异性修正剑桥模型,考虑K_0固结饱和黏土初始应力各向异性、应力历史及应力诱发各向异性对土体三维力学特性的影响,推导了静压沉桩柱孔扩张问题的弹塑性解析解。在此基础上,根据桩侧土体应力状态与单剪试验中试样应力状态的相似性,结合桩周土轴对称固结理论提出采用孔压静力触探仪(CPTU)锥尖阻力、锥肩孔隙水压力及相应孔压消散数据预测静压桩时变承载力的理论方法。通过离心机模型试验实测结果和理论预测值的对比,验证了理论方法的有效性,研究了静压桩承载力随时间的变化规律。研究结果表明,本文理论预测方法避免了土体基本参数测定等繁琐过程,且可以较为合理地预测静压桩时变承载力;静压桩沉桩结束后其承载力在短时间内迅速增加,之后承载力增加幅度变缓且逐渐趋于稳定值;静压桩桩径越大,沉桩结束后承载力增加的速度越慢,承载力达到稳定值的时间越长。     

组合桩的竖向承载力特性研究

组合桩是钢筋混凝土桩插入水泥土搅拌桩复合而成的新型桩.为掌握组合桩的荷载传递机理、破坏模式和竖向承载力特性,通过对3根14m长桩的载荷试验,分析了桩身轴力和桩周的侧摩阻力的分布及影响组合桩承载力因素.试验得到了组合桩的Q-S曲线、S-logt曲线、桩的极限承载力.为分析该桩竖向力的传递路线,应用有限元方法分析了混凝土桩、水泥土和桩周土的传力特点.试验和有限元分析结果表明混凝土桩的插入改变竖向荷载的传递规律,形成了从混凝土桩到水泥土再到土的传递模式,更有效地发挥了桩周的侧摩阻力;水泥土的固化效应、混凝土桩的挤土效应和混凝土桩的荷载传递是组合桩高承载力的主要来源.组合桩具有较高的单桩竖向承载力且造价低,在软土地基工程中具有广泛的应用价值.     

Experimental investigations of bearing capacity of bored-cast-in-place tapered piles

Conclusions 1. The investigated types of bored-cast-in-place tapered piles with a taper angle of 1°20′, 2°, and 2°40′ have a specific bearing capacity which exceeds by 20–30% the specific bearing capacity of cylindrical piles having the same length. 2. The bearing capacity of tapered piles and the consumption of materials in them increase in direct proportion to the increase in the taper angle. The most rational type within the investigated limits is a bored-cast-in-place pile with a taper angle of 2.5°. 3. Introduction of tapered piles based on the above investigations has made it possible to obtain a significant economic effect. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 5–7, March–April, 1990.     

嵌岩扩底抗拔桩承载特性研究

近年来扩底桩用于建筑物抵抗上拔荷载作用的情况日趋增多,而对扩底抗拔桩的理论研究远跟不上工程实践的发展。分析了扩底抗拔桩的抗拔机理和常用的破坏模式,在合理假定的基础上给出了一种扩底嵌岩桩极限抗拔承载力的计算模式,该模式包含侧摩阻力、桩自重以及扩大头抗拔力对总抗拔承载力的贡献。结合南京绿地广场紫峰大厦工程中两根扩底嵌岩抗拔桩的自平衡法极限抗拔承载力试验结果,进行了该模式计算值与实测值的对比分析,吻合程度较好。     

碎石桩加固含粘土夹层的粉土地基

探讨了在地质条件相对复杂的徐州地区修建高速公路时，碎石桩处理含粘土夹层的粉土地基的施工工艺、质量检测。     

无黏性土斜坡地基承载力模型试验研究

采用缩尺模型试验对砂土斜坡地基的土压力分布、变形机制、破坏模式进行探索,并研究了斜坡坡角、基础尺寸、相对密度、基础形状对斜坡地基破坏形态及极限承载力的影响。结果表明：斜坡地基的破坏模式与Choudhury提出的破坏模式相近,破坏区域由不对称楔体、辐射向剪切区、被动楔体组成。斜坡地基的破坏区域长度随斜坡坡角、基础尺寸的增大而增大,但不随相对密度的变化而变化;而斜坡地基的极限承载力随斜坡坡角的增大而减小,随基础宽度、相对密度的增大而增大。对相同尺寸的基础而言,方形基础下的地基极限承载力和破坏区域长度均大于圆形基础。试验研究成果对斜坡地基变形特征、破坏形态和斜坡地基承载力影响因素的探究具有一定理论参考价值。     

桩基自平衡测试的可靠性分析

本文简要地介绍了桩基自平衡测试的Q-S曲线向传统静载试验Q-S曲线的等效转换原理。分析了测试数据的可靠性,考虑转换方法中各参数的随机变化,得出了各参数的分布。重点探讨了从两个随机因素影响的测试数据中获得每个因素的分布的简化方法以及如何利用多次试验数据(每次试验数据的影响因素不全相同)来补充一次试验得到的样本数据较少(本文为2个)的统计方法。最后分析了桩基自平衡测试方法的可靠性。     

稳定路基中刚性桩抗弯能力验算简易方法

稳定路基的刚性桩复合地基会产生一定的侧向位移,其刚性桩会承受一定的弯矩并可能开裂。为了保证刚性桩复合地基路基的长期稳定性,应验算路堤下复合地基刚性桩的抗弯能力。为减小路堤下复合地基刚性桩的抗弯能力验算难度,需要研究刚性桩弯矩简易计算方法。首先,基于大量公路路基软基试验和监测工程,对路基最大沉降与最大侧向位移的关系进行了统计研究,得到了两者之间的经验关系式;然后,根据路堤下复合地基和刚性桩的侧向位移曲线形状,提出刚性桩水平受力简化模型,并对复合地基刚性桩弯矩与最大侧向位移之间的关系进行了理论推导。在上述研究的基础上,提出了根据复合地基沉降得到桩身弯矩的简易方法。工程算例表明该方法简单、可行。研究表明,刚性桩位移与桩间土位移基本相等,可忽略不计桩土位移差对刚性桩弯矩的影响;当刚性桩进入硬土层的长度超过7倍桩径时,可认为刚性桩在软土层底面处固定。