New design equations for estimation of ultimate bearing capacity of shallow foundations resting on rock masses

Rock masses are commonly used as the underlying layer of important structures such as bridges, dams and transportation constructions. The success of a foundation design for such structures mainly depends on the accuracy of estimating the bearing capacity of rock beneath them. Several traditional numerical approaches are proposed for the estimation of the bearing capacity of foundations resting on rock masses to avoid performing elaborate and expensive experimental studies. Despite this fact, there still exists a serious need to develop more robust predictive models. This paper proposes new nonlinear prediction models for the ultimate bearing capacity of shallow foundations resting on non-fractured rock masses using a novel evolutionary computational approach, called linear genetic programming. A comprehensive set of rock socket, centrifuge rock socket, plate load and large-scaled footing load test results is used to develop the models. In order to verify the validity of the models, the sensitivity analysis is conducted and discussed. The results indicate that the proposed models accurately characterize the bearing capacity of shallow foundations. The correlation coefficients between the experimental and predicted bearing capacity values are equal to 0.95 and 0.96 for the best LGP models. Moreover, the derived models reach a notably better prediction performance than the traditional equations.

Evaluation of ultimate bearing capacity of Y-shaped vibro-pile

Based on Mindlin stress solution, a numerical computational method was proposed to calculate the stresses in the ground induced by side friction and the resistance of Y-shaped vibro-pile. The improved Terzaghi's and ЪерезанцевВГ's methods for ultimate bearing capacity evaluation were proposed by considering the stress strength induced by friction resistance at pile head level of Y-pile. A new method to calculate the ultimate bearing capacity of Y-pile was also proposed based on the assumptions of soil failure mode at the tip of Y-pile and the use of Mohr-Coulomb soil yield criterion and Vesic compressive correction coefficient with the induced stresses in the ground. Based on the comparisons with the field static load test results, it is found that the improved Terzaghi's method gives higher ultimate capacity, while the other two methods shows good agreement with the field results.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Methods for Estimating the Ultimate Bearing Capacity of Layered Foundations

The Meyerhof and Hanna′s(M-H) method to estimate the ultimate bearing capacity of layered foundations was improved. The experimental results of the load tests in Tianjin New Harbor were compared with predictions with the method recommended by the code for the foundations of harbor engineering, i.e. Hansen′s method and the improved M-H method. The results of the comparisons implied that the code and the improved M-H method could give a better prediction.

Undrained Capacity of a Bucket Foundation with a Limited-Tension Interface for Offshore Wind Turbines

In this paper,the influence of the limited-tension interface between lid and soil on the undrained bearing capacity of the wide-shallow bucket foundation is examined by finite element(FE)analysis.The interface between the lid and the soil is modeled using a simplified approach called the surface-based cohesive behavior,with the aim of simulating the limited-tension interface.Initially,the interaction between the lid and the soil is explored under the zero-and unlimited-tension conditions by small-scale experiments.Afterward,the effects of the embedment ratio,soil strength heterogeneity,and lid-soil interface on the bearing capacity are outlined,and the failure mechanisms are explained by FE analysis.A modified closed-form formula is given to compute the moment bearing capacity with the limited-tension interface between the lid and the soil for different embedment ratios and soil strength heterogeneities.The numerical results reveal that the existing approximating solutions,which assume fully bonded interaction,accurately exhibit the shape of the normalized failure envelopes in hm and vh load space for the limited-tension interface.However,the shape of the vm envelopes differs,requiring a novel solution to estimate the combined bearing capacity of the bucket foundation based on the embedment ratio and soil strength heterogeneity with a zero-tension interface between the lid and the soil.

Experimental Study on Horizontal Bearing Characteristics of Screw Pile

The horizontal bearing capacity of the screw pile and monopile was analyzed by model tests.Results showed that the horizontal bearing capacity of the screw pile was significantly greater than that of the monopile under the same loading conditions.With the increase in horizontal loading speed,the ultimate horizontal bearing capacity of the two piles also increases,and the difference decreases gradually.Moreover,the influence of vertical loading on the horizontal bearing capacity of screw pile and monopile is studied at the horizontal loading speed of 2 mm s-1.The findings indicate that vertical load evidently affects the horizontal bearing capacity of common piles,but slightly influences the horizontal bearing capacity of screw piles.

Vertical bearing capacity of pile based on load transfer model

The load transfer analytical method is applied to study the bearing mechanism of piles with vertical load in this paper. According to the different hardening rules of soil or rock around the pile shaft, such as work-softening, ideal elasto-plastic and work-hardening, a universal tri-linear load transfer model is suggested for the development of side and tip resistance by various types of soil (rock) with the consideration of sediment at the bottom of the pile. Based on the model, a formula is derived for the relationship between the settlement and load on the pile top to determine the vertical bearing capacity, taking into account such factors as the characteristics of the stratum, the side resistance along the shaft, and tip resistance under the pile tip. A close agreement of the calculated results with the measured data from a field test pile lends confidence to the future application of the present approach in engineering practice.

Construction of improved rigid blocks failure mechanism for ultimate bearing capacity calculation based on slip-line field theory

Based on the slip-line field theory, a two-dimensional slip failure mechanism with mesh-like rigid block system was constructed to analyze the ultimate bearing capacity problems of rough foundation within the framework of the upper bound limit analysis theorem. In the velocity discontinuities in transition area, the velocity changes in radial and tangent directions are allowed. The objective functions of the stability problems of geotechnical structures are obtained by equating the work rate of external force to internal dissipation along the velocity discontinuities, and then the objective functions are transformed as an upper-bound mathematic optimization model. The upper bound solutions for the objective functions are obtained by use of the nonlinear sequential quadratic programming and interior point method. From the numerical results and comparative analysis, it can be seen that the method presented in this work gives better calculation results than existing upper bound methods and can be used to establish the more accurate plastic collapse load for the ultimate bearing capacity of rough foundation.

Model tests on uplift capacity of double-belled pile influenced by distance between bells

To optimize the distance between the bells in pile design,this paper reports a series of small scale tests on the uplift capacity of double belled piles embedded in dry dense sand considering different bell space ratios.Finite element modelling is also performed to evaluate the range of soil failure around the piles during pile uplift displacement.Test results show that when bell space ratio is 6 or 8,the uplift capacity reaches the peak value.The upper bell bears more load than the lower one for the piles with bell space ratio less than 6,while the lower bell bears more load than the upper one for the piles with bell space ratio larger than 8.

Research on pile performance and state-of-the-art practice in cold regions

A pile foundation is commonly adopted in geotechnical engineering to support structures, and its application has been extended to cold-regions engineering. In past decades, a host of scholars investigated pile behaviors and proposed design guidelines for seasonally frozen ground or permafrost. This paper reviews the research with respect to pile performance and engineering practice in cold regions, organized as follows:(1) creep tests and bearing capacity,(2) frost-jacking hazards,(3) laterally loaded piles,(4) dynamic responses,(5) refreezing due to concrete-hydration heat, and(6) improved countermeasures and design methods. We first summarize previous research and recent progress; then, predict the development trend of pile foundations in cold regions and recommend further research.

Stability Analysis of Piles Subjected to Lateral Load in Static and Seismic Conditions

It is disclosed a method for the stability analysis of foundation piles and piers subjected to lateral loading, both static and seismic conditions. The stability analysis for stratified soil is based upon the models of foundation soil-structure interaction and the Rankine＇s theory of earth passive pressure. In addition, its application is simpler and it can be solved using a spreadsheet. The procedure described in this work can be used in homogeneous soils as in stratified soils, considers the horizontal drag forces exerted by the soil mass against the foundation during an earthquake, can be used easily in the four pile and piers boundary cases, and considers the pore pressure generated in a fine saturated soil during an earthquake or during a rapid application of the horizontal load. The solution of two examples are shown, one in static condition and one in seismic condition, detailing the procedure step by step.

基于Meyerhof机构的吸力式沉箱基础抗拔承载力极限分析上限解

为了研究吸力式沉箱基础上抗拔荷载作用下的极限承载力上限解,构建出反向Meyerhof破坏模式.根据极限分析上限定理,引入反向地基承载力,基于Meyerhof破坏模式,将原基础下的主动区变为被动区,对数螺旋线方向相反且延伸至沉箱侧壁,建立了相应的机动许可速度场,推导出理论上更为合理的吸力式沉箱基础极限抗拔承载力上限解.分析了上限解与黏聚力、内摩擦角、摩擦系数以及基础下地基反向承载力之间的关系.采用Matlab软件编程计算出上限解,并与文献中的试验数据进行对比分析.结果表明,上限解与试验值的误差最大值为28%,最小值为9%,从而证明了破坏模式的合理性及所提方法的适用性.

Experimental study of the bearing capacity of a drainage pipe pile under vacuum consolidation

In this study,we propose a drainage pipe pile under vacuum consolidation to integrate foundation treatment and pile foundation engineering in soft soil areas.To study its bearing capacity characteristics and foundation treatment performance,single pile static load tests,vane shear tests,and water content tests were carried out for ordinary piles,perforated piles,and drainage pipe piles under conditions of static and vacuum consolidation.Based on the results,the concept of strong and weak reinforcement areas was proposed and used for bearing capacity prediction.The results showed that the drainage pipe pile did not become silted under vacuum consolidation.The single pile bearing capacity was much higher than that of an ordinary pile,and the pile side friction was exerted mainly in the middle and lower parts.Good results were achieved using the shear strength at the junction of the strong and weak reinforcement areas to estimate the ultimate bearing capacity of a single pile.This study provided important insights into the design and construction of drainage pipe piles in a soft soil foundation.

砂土中冲刷条件下三筒基础水平承载特性研究

通过开展小比尺三筒基础承载力模型试验,研究冲刷条件下三筒基础水平承载特性,建立数值模型对模型试验结果进行验证与扩展,分析水平和弯矩荷载下,冲刷率e和相对筒间距S/D对极限承载力的影响,提出三筒基础极限承载力的冲刷修正系数dhe、dme的计算方法;采用固定荷载比方法,通过数值模型计算M-H加载条件下三筒基础的破坏包络线,分析冲刷率e和相对筒间距S/D对复合加载特性的影响,提出冲刷条件下三筒基础M-H破坏包络线公式。研究结果表明:冲刷条件下的三筒基础受水平和弯矩作用直至破坏的过程,经历弹性变形—塑性变形—失稳破坏3个阶段;三筒基础的水平极限承载力随冲刷率e的增大而下降,且下降速率逐渐增大,相同长径比的三筒基础的水平极限承载力随相对筒间距的减小而降低;随着冲刷率e的提高,三筒基础的M-H破坏包络线向内移动,随着相对筒间距S/D增大,归一化破坏包络线呈上凸的趋势。

后顶扩臂压浆桩竖向承载机理及其桩盘力学性能研究

为了有效提高桩基础承载力,创新性提出后顶扩臂压浆技术与后顶扩臂压浆桩基础形式。其桩臂顶扩压浆后形成的桩盘力学以及承载特性直接影响基础承载力。为此,在砂土中开展后顶扩臂压浆桩模型试验以及桩盘力学性能试验,通过分析荷载位移变化规律、荷载分担比、桩盘破坏模式等,探究后顶扩臂压浆桩竖向承载特性以及桩盘的力学性能,揭示桩盘承载机理以及后顶扩臂压浆桩承载力提升机制。试验结果表明:通过后顶扩臂压浆技术,不仅提高桩土接触面积同时提升桩周土体强度,压浆效果呈圆柱型,桩臂包裹效果较好。以压浆量250mL为例,单盘后顶扩臂压浆桩承载力是普通桩的5.0倍左右,是其他桩型的2.0~3.0倍,承载力提升效果显著。单桩盘分担荷载占总荷载的80%~90%,且盘径比越大、桩盘个数越多,桩盘分担荷载越高。桩盘承载力与压浆量成线性关系,压浆量越高桩盘承载力越大。根据试验结果,建立了桩盘力学模型以及后顶扩臂压浆桩竖向承载力简化计算模型,提出桩盘承载力计算方法以及后顶扩臂压浆桩承载力计算方法。并与试验结果进行对比验证,以期可为后顶扩臂压浆桩承载力计算提供参考。

上砂下黏地层中桩-筒复合基础V-H承载特性

海上风机基础不仅受自重等竖向力V作用,也因水流、波浪和风等影响而承受水平荷载H。为探讨上砂下黏地层中一种由单桩和吸力筒组成的新型海上风机桩-筒复合基础受V-H组合作用时的承载特性,自主设计完成了一系列室内桩-筒复合基础V-H组合加载模型试验,获得不同组合参数下桩-筒复合基础的荷载-位移曲线和桩身弯矩分布曲线,并绘制出V-H承载力包络线。在此基础上,采用ABAQUS建立了上砂下黏地层中桩-筒复合基础三维数值分析模型,经模型验证与参数分析,进一步讨论了砂土厚度、筒径、筒高以及加载高度等参数对桩-筒复合基础承载特性的影响曲线,并拟合出桩-筒复合基础承载力简化计算公式,分析结果表明:桩-筒复合基础能显著提高桩身水平承载力,增幅达30%~90%,且增加筒径比增加筒高更有利于提高基础水平承载力;上部砂土层较厚时,桩-筒复合基础存在一个使复合基础水平承载力达到最大的预加竖向荷载最佳值,其值随不同载荷工况在(0.4~0.7)Vult范围内变化。

波浪荷载下海上风力机桩基础滞回效应及水平承载力变化特性研究

该研究以海上风电桩基础为研究对象,采用自主设计的水平加载装置模拟波浪荷载,结合粒子图像测速(PIV)技术开展缩尺模型试验。着重探究不同幅值的波浪荷载加载过程中,桩周土体的循环变形特征及加载前后,海上风力机桩基础水平承载能力变化。试验结果表明:桩顶累积转角随加载次数呈线性增长、缓慢增长和稳定发展的三阶段变化特性;桩周土体呈现典型的刚性桩两区域破坏形式,桩右被动区土体位移矢量场和位移云图影响范围受加载幅值变化影响显著;桩顶循环加载荷载位移曲线表现出明显的滞回性,滞回圈随加载次数右移并产生累积位移。伴随着滞回圈面积减小和割线刚度增大,桩周土体由以塑性变形向弹性变形为主导变化;循环加载后的滞回圈有明显捏缩,耗能能力减弱。桩基础水平承载能力及弹性变形能力较加载前均明显提高。

静压桩残余应力对极限承载力影响

为研究静压桩回弹特性,明确静压桩残余应力性状,使静压桩承载力能够得到有效发挥,通过静压桩沉桩及静载荷试验,结合有限元模拟方法,对不同沉桩速率和卸荷速率作用下静压桩的回弹特性进行分析,总结桩端残余应力对静压桩极限承载力的影响规律。结果表明:卸荷过程中桩体回弹量占桩体总沉降量的80%,卸荷完成桩体回弹位移逐渐稳定;沉桩结束后,桩周土体应力最大达到被动土压力的4.5倍,卸荷后浅部土体桩周侧压应力消散程度大于深部土体;桩端土体应力消耗越多,桩端残余应力越小,卸荷后桩端土体回弹量越多;沉桩速率和卸荷速率增加都会导致桩端残余应力减小,从而桩端土体能够提供的承载力降低,致使桩体承载力降低。研究认为静压桩残余应力是影响承载力的主要因素,卸荷速率对静压桩残余应力的影响最为明显。

卷积神经网络与随机场分析桩梁基础承载力

岩土体的参数在空间上随机分布,为能更好地反应实际工程地质条件,在桩基础承载力研究中考虑土体的不确定性,并建立具有重要工程价值的承载力预测模型,将基于随机场理论的岩土参数空间变异性引入桩梁基础的研究中,采用数值方法建立桩梁基础与群桩基础的二维随机有限元模型分析承载能力,并与模型试验结果验证。随后通过卷积神经网络建立土体参数随机场图像与基础极限承载力之间的模型进行承载力预测,并基于预测模型研究不同参数的影响。结果表明:考虑土体空间变异性的基础承载力与试验结果基本吻合,随机结果均高于确定性分析;随机场下桩梁基础与群桩基础的承载力均为正态分布;采用卷积神经网络建立的基础承载力预测模型精度较高,且可以用于参数分析,基础承载力随着土体参数的增加而增加,随变异系数的增加而下降。随机条件下,桩梁基础的承载力高于群桩基础,可以充分发挥土体强度并抵御参数不确定性带来的承载力损失。

基于浸水环境的湿陷性黄土灌注桩承载力计算

研究目的:当铁路桥梁桩基穿越大厚度湿陷性黄土场地时,目前规范按照最不利状态下的极限值进行负摩阻力计算,导致计算桩身长度过大,造成大量工程投资浪费。本文提出场地浸水环境的概念与分类,考虑地形地貌,重点分析场地的浸水条件与程度,将场地浸水环境分为完全浸水环境、长期浸水环境、短期浸水环境、综合措施环境四种状态。针对不同浸水环境,在负摩阻力计算时,湿陷性土层下限深度取值考虑折减。研究结论:(1)现行规范在大厚度湿陷性黄土评价方面,适合湿陷性土层较薄的工业民用建筑工程,对于湿陷性土层厚度较大的铁路工程不适用;(2)对于完全浸水环境,按照目前规范推荐值进行计算;对于长期浸水环境,桩周湿陷性土层下限深度计算值可考虑10~15m;对于短期浸水环境,桩周湿陷性土层下限深度计算值可考虑5~10m;(3)对大厚度湿陷性黄土场地采用综合措施处理后,可不考虑负摩阻力作用,能够大幅缩短桩长,降低施工难度,节约投资;(4)本研究结果可为湿陷黄土地区的桩基工程设计提供参考。