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1.
S. Moshfeghi 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2019,13(1):1-19
Due to variety of current pile bearing capacity methods based on cone penetration test (CPT) measurements, there is always a need for evaluating performance of existing methods to make proper choices of methods as well as safety factors for optimum design. In this regard, geotechnical databases are known as useful tools which facilitate evaluation of existing methods. This paper deals with axial bearing capacity of driven piles in sand using CPT-based methods. A database of seventy-six records is employed to analyze different criteria of interpreting static pile load test results to select the most consistent approach with the CPT-based methods. Then, performance of nine commonly used direct CPT-based methods was evaluated. Finally, via a failure probability and cost optimisation approach, optimum safety factors are presented and discussed. Analysis of different failure criteria shows that the Hansen 80% criterion leads to more consistent results with the CPT-based methods. In addition, almost all of the investigated methods showed promising performance. The attained safety factors range from 1.6 to 3.1 for all records, 1.4 to 3.1 for piles in compression, and 1.4 to 2.2 for the piles in tension. Then, efficiency of methods was evaluated and the methods with higher efficiency are introduced. 相似文献
2.
In this paper, a framework based on Bayesian theory and proof pile load test results was used to update resistance factors of axially loaded driven piles. Prior to implementation of the framework, resistance factors were calibrated based on the distribution of the measured-to-predicted pile ultimate bearing capacity using the results of static pile load tests conducted to failure. These resistance factors and the distribution were considered to be “prior.” The prior distribution of the measured-to-predicted ultimate bearing capacity was updated based on Bayesian theory to incorporate additional proof pile load test results. Using the measured-to-predicted load distributions and the updated (or posterior) measured-to-predicted bearing capacity distributions, resistance factors were calibrated (or updated) from the first-order reliability method (FORM) for two different target reliability indices, 2.33 and 3.0. This research attempted to use the results of proof pile load tests, which are generally conducted to verify pile designs, to update resistance factors. The updated resistance factors varied substantially depending on the proof pile load test results. Therefore, the Bayesian implementation can contribute to economical pile designs. 相似文献
3.
考虑参数和模型不确定性的基桩正常使用极限状态可靠度分析 总被引：1，自引：0，他引：1
考虑承载力计算模型和荷载不确定性,利用可靠度分析方法和概率统计理论,推导出承载能力极限状态（ULS）和正常使用极限状态（SLS）下可靠度指标的计算公式,给出了两种极限状态下可靠度指标间的线性关系式,研究了桩顶容许沉降 随机性对正常使用极限状态可靠度分析结果的影响。研究结果表明,土体类别和桩型对正常使用极限状态模型因子影响很小;正常使用极限状态下基桩可靠度指标随承载力计算模型和荷载不确定性的增大而减小,但减小幅度逐渐降低,且可靠度指标总变化量不大,工程应用中可忽略承载力计算模型和荷载不确定性在可靠度分析中的影响; 随机性对正常使用极限状态可靠性分析结果的影响很大,随 的增加,正常使用极限状态模型因子和可靠度指标逐渐增大,而模型因子变异性逐渐减小,但桩本身性质并没有任何改变,只是所允许的沉降条件不同。研究结果可为规范修订和工程应用提供参考。 相似文献
4.
高应变动测法目前得到了越来越广泛的应用,研究其测试结果的可靠性对实际工程具有重要意义。对PHC桩应用高应变法、竖向静载方法进行试验,以确定桩基的单桩极限承载力。首先对2组不同直径的6根试桩进行高应变测试,采用国际上推荐的GOBLE-CAPWAPC-CASE方法,得到试桩的单桩极限承载力; 然后按慢速维持荷载法进行竖向抗压静载试验,将所得极限承载力与高应变法所得结果进行对比,高应变法所得结果高于静载试验的结果, 2种方法结果不同是由于2种方法的测试原理、计算方法、影响因素等不同所致。高应变法的理想化桩-土体系以及主要参数的设定对结果都有很大的影响,实际试验中应充分考虑如何取值。 相似文献
5.
结合上海软土地区12个场地57根单桩水平静载荷试验,研究建筑桩基单桩在水平荷载作用下的受力特点。根据试验实测数据,探讨不同桩径、不同试验加载方法及灌芯对预应力空心桩受力性状影响等因素,认为建筑工程中,可选择桩径较大或者采用在桩顶一定范围内扩大桩径的做法进行单桩水平承载力优化设计,试验加载方式建议可采用单向单循环恒速水平加载法,预应力空心桩顶部灌芯可作为水平承载力的安全储备,为软土地区类似工程的单桩水平承载力的设计和理论研究提供参考。 相似文献
6.
确定大直径扩底桩承载力的最直接的方法是静力载荷试验。但由于加载的困难和试验费用高等原因,静力载荷试验往往不可行。目前进行大直径扩底桩设计时通常借用建筑桩基技术规范的经验参数法,对大直径灌注桩端阻力值和侧阻力值折减确定,容易造成大直径扩底灌注桩的承载能力得不到充分发挥。结合大直径扩底桩的规范编写,对收集的完整试桩资料进行统计分析,并参考《建筑桩基技术规范》及北京、天津、浙江、福建、广东等地方给出的经验值,最终按不同扩底直径给出了端阻承载力特征值和侧阻承载力特征值的经验值表,并与各试桩的承载力实测值以及按桩基规范确定的承载力特征值进行比较,结果表明,本文建议的大直径扩底桩端阻力特征值和侧阻力特征值具有一定的可靠性和较大的适用性。 相似文献
7.
基于天津于家堡南地下车库工程扩底桩抗拔极限承载力试验,结合数值模拟手段对扩底桩抗拔承载特性、破坏模式和受力机制开展了分析研究。计算分析表明,扩底桩（有效桩长19 m）相比等截面桩抗拔极限承载力提高约50%,材料增加仅8.5%,扩底桩扩大头周边土体提供的抗力显著提高了抗拔承载力;荷载较小时抗拔力主要由等截面段侧摩阻力提供,扩头段抗拔力占桩顶加载的比值随加载近似呈线性增加;扩底桩等截面段沿桩土界面先发生剪切破坏,扩头段周边土体后发生受压破坏,抗拔承载力达到极限;扩头段位于同一土层时,不同桩长扩头段提供的极限抗拔力相差不大,桩长越长,扩头段抗拔力贡献率越低;扩头段抗拔力主要由自重、扩头段法向力竖向分力和侧摩阻力组成,其中法向力竖向分量提供了扩头段的主要抗拔力,占扩头段总抗拔力约70%。 相似文献
8.
为深入研究静压管桩桩土的作用机制及其竖向承载力的确定方法,以长春地区建筑工程使用的静压管桩为例,采用数值模拟方法和桩的载荷试验,分析了桩对桩周土的挤压作用以及桩端阻力变化规律。结果表明：桩被压入土体过程中,主要受到桩侧摩阻力和桩端阻力的作用,并造成桩周及桩端附近土体受到挤压而变形;随着桩入土深度的增加,桩顶荷载逐渐增大,而且桩径越大,相应深度的桩顶荷载就越大;同时,随着桩入土深度的增加,桩端阻力在单桩竖向承载力中的比例有规律地下降。根据桩端阻力在单桩承载力中所占比例与入土深度的关系,提出了静压管桩单桩承载力特征值的计算方法;对比建议公式和经验公式计算结果,其比值为0.57~1.26,两者结果接近。因此,文中所提出的单桩承载力特征值的确定方法是可行的。 相似文献
9.
10.
打入桩承载力确定方法对安全系数的影响 总被引：1，自引：0，他引：1
在传统的容许应力设计方法中桩基承载力安全系数取决于桩基承载力的确定方法,然而,基于经验选择的承载力安全系数缺少理论依据,为此,基于贝叶斯理论和线性回归方法提出了评价桩基承载力确定方法对承载力安全系数影响的方法,给出了现行规范对于不同承载力确定方法采用不同安全系数的理论依据,阐明了桩基动力测试降低安全系数的理论依据,并以7种桩基设计方法为例,分析了承载力确定方法对安全系数的影响。结果表明,采用所提出的方法计算的安全系数基本上与现行的经验值相一致,表明精确的承载力确定方法可以有效地降低承载力安全系数,设计方法、承载力确定方法和水平都对安全系数有影响,但当承载力试验桩的数目足够多时,安全系数对于设计方法的精确性不敏感。 相似文献
11.
Reliability-based design (RBD) can play a useful complementary role to overcome some limitations in the Eurocode 7 (EC7) design approach, for example in situations with parameters not covered in EC7, different parametric sensitivities across different problems, cross-correlated or spatially correlated parameters, design aiming at a target reliability or failure probability, or when uncertainty in unit weight of soil is modeled. The complementary role played by RBD under these circumstances is illustrated and discussed for a shallow foundation, a reinforced rock slope, a Norwegian clay slope with spatial variability, a laterally loaded pile requiring implicit numerical analysis, and an anchored sheet pile wall. A pragmatic RBD approach involving first-order reliability method (FORM) only and a more rigorous RBD approach involving both first-order and second-order reliability method (SORM) are offered. Both approaches are implementable using either spreadsheet-based FORM and SORM procedures, or using various commercially available FORM/SORM packages. 相似文献
12.
Many state departments of transportation and consulting firms continue to use dynamic formulas because they are simple and inexpensive as compared to the preferred dynamic loading test with signal matching. Efforts to reduce the error and uncertainty associated with dynamic formulas are therefore warranted until dynamic monitoring becomes standard for every driven pile. However, dynamic formulas calibrated to national test pile databases have indicated inaccuracy and high uncertainty in the capacity prediction using dynamic formulas. A region-specific dynamic load test database was used to assess the accuracy and uncertainty in the Janbu, Danish and FHWA Gates formulas, recalibrate the equations for local conditions (construction practice, geology) and to generate resistance factors for use with Load and Resistance Factor Design (LRFD). Following recalibration, the capacity predictions became more accurate, and an observed dependence of the accuracy on the magnitude of resistance was eliminated for most driving conditions (e.g. end-of-driving, restrike). Previously reported static load test data were used to incorporate the transformation error associated with using the dynamic capacity to predict an equivalent static capacity. Resistance factors for use in Load and Resistance Factor Design (LRFD) were developed in consideration of American Association of State Highway and Transportation Officials–recommended load statistics and target reliability indices. Efficiency factors were used to assess the economic performance of each dynamic formula. Comparison of the accuracy and uncertainty of the recalibrated equations to a nationwide calibration illustrated the advantage of using a geologic-specific database for the calibration of resistance factors. 相似文献
13.
The support vector machine (SVM) is a relatively new artificial intelligence technique which is increasingly being applied to geotechnical problems and is yielding encouraging results. In this paper SVM models are developed for predicting the ultimate axial load-carrying capacity of piles based on cone penetration test (CPT) data. A data set of 108 samples is used to develop the SVM models. These data were obtained from the literature containing pile load tests and each sample contains information regarding pile geometry, full-scale static pile load tests and CPT results. Moreover, a sensitivity analysis is carried out to examine the relative significance of each input variable with respect to ultimate strength prediction. Finally, a statistical analysis is conducted to make comparisons between predictions obtained from the SVM models and three traditional CPT-based methods for determining pile capacity. The comparison confirms that the SVM models developed in this paper outperform the traditional methods. 相似文献
14.
以中铁西安中心超高层建筑为工程背景,采用后压浆钻孔灌注桩技术对该工程进行地基处理。工程中测试钻孔灌注桩压浆前、后单桩竖向抗压静载试验及桩身内力。采用建筑桩基技术规范、侧阻、端阻分项增强系数法、基于极限承载力增强系数法和只考虑桩端改善承载力5种方法进行了后压浆桩的桩基极限承载力计算。结果表明,后压浆后桩基极限承载力提高了24%,侧阻、端阻分项增强系数法和基于极限承载力增强系数法与现场实测最接近;建筑桩基技术规范与只考虑桩端承载力改善与现场实测结果相差较大,其计算值偏于安全保守。 相似文献
15.
湿陷性黄土中成孔方式对桩基承载力影响试验研究 总被引：1，自引：0，他引：1
为了研究黄土地区成孔方式对桩基承载特性的影响,依托永寿至咸阳高速公路工程,对两个试验区4根试桩进行了静载试验,分析研究了不同成孔方式下桩基荷载传递规律。研究结果表明：成孔方式对桩基承载特性影响较大,黄土地区旋挖钻孔灌注桩承载力高于泥浆护壁循环钻孔灌注桩;循环钻孔灌注桩在成孔时需泥浆护壁,桩侧形成的泥皮会严重影响桩侧土体摩阻力的发挥,桩侧极限摩阻力值较小,桩端承担桩顶荷载比例较高;浸水后湿陷性黄土结构破坏,单桩承载力较原始状态有所下降;由于上覆黄土层湿陷量较小,浸水状态下桩侧土体并未产生负摩阻力。 相似文献
16.
螺旋群桩基础承载性状试验研究 总被引：3，自引：1，他引：2
根据螺旋群桩现场静载试验结果,分析了螺旋群桩基础承载性状的影响因素,讨论了极限荷载下群桩几何参数对螺旋群桩的桩端阻力和桩侧摩阻力的影响。结果表明,螺旋群桩承载性状受到相邻桩的叶片距离以及首层叶片到承台底面间的桩长度影响明显。在叶片桩距比（桩间距比叶片直径）小于2时,螺旋群桩静载试验的荷载-位移曲线呈缓变形,无明显的拐点,群桩破坏属于整体破坏。同时,螺旋桩的叶片有助于减小承台下桩间土变形活动区的高度,增强了桩-土-承台的相互作用,减小了群桩基础沉降量。 相似文献
17.
原位载荷试验是确定复合地基竖向承载力最可靠的方法。许多工程实例表明,当载荷试验加载不充分,无法通过极限荷载判断地基承载力特征值时,应根据《建筑地基处理技术规范》方法确定复合地基承载力特征值,但该值有时会偏小,这样的结果会导致设计人员采用的复合地基承载力特征值偏低,设计过分保守,由此造成技术投入加大,工程造价增加的不良结果。通过某工程同场地、同条件的一组素混凝土桩复合地基原位载荷试验数据与一组天然地基原位载荷试验数据对比,研究原位载荷试验测定复合地基承载力特征值方法的不足,讨论产生缺陷的原因。补充完善了《规范》通过载荷试验确定复合地基承载力确定方法,以期更好地促进复合地基技术及原位载荷试验的推广应用。 相似文献
18.
依据某工程中采用桩底注浆和没有注浆的2根试桩的单桩竖向抗压静荷载试验结果,通过对基桩的竖向极限承载力性状、桩身轴力传递特性以及桩侧阻力发挥特性和桩端阻力发挥特性的综合分析研究,揭示了桩底注浆对基桩承载性状的影响情况。 相似文献
19.
根据软土地基、非软土地基中长钻孔灌注桩静载荷试验和桩身轴力的测试结果,分析探讨了竖向荷载下长桩的受力性能及沉降特征的一些规律。桩侧土模量较高的非软土地区的长桩静力试桩所测得的结果表明,荷载传递和桩身压缩与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。极限侧阻力与端阻力不同步发挥,不同深度处的不同土层处基桩的侧阻也不能同步发挥;同时指出相关规范中极限承载力计算公式的不严密之处,并且探讨说明了竖向荷载下的长群桩基础变形性状及沉降计算有待进一步的研究。本项研究对今后超长桩的理论研究和工程设计应用具有一定的借鉴作用。 相似文献
20.
通过海洋环境条件下大直径管桩的垂直和水平荷载试验,分析了管桩在垂直和水平荷载作用下的受力特点,得到了管桩的垂直极限承载力、侧摩阻力及端承力、轴向反力系数等结果,以及水平荷载作用下桩顶位移和转角关系、弯矩分布、土抗力、水平地基反力系数的比例系数和最大弯矩点等参数。试验结果表明：垂直荷载作用下,极限承载力可达12000kN,在沉桩过程中部分桩有一定程度的闭塞;大直径管桩能够抵抗水平荷载的作用,弹性长桩的受力性质主要受上部土层的影响。根据试验结果计算的水平地基抗力比例系数m值,对本工程及同类地质条件的桩基设计具有参考价值。 相似文献