Numerical investigation of pile installation effects in sand using material point method

The installation of displacement piles in sand leads to severe changes in the stress state, density and soil properties around the pile tip and shaft, and therefore has a significant influence on the pile bearing capacity. Most current numerical methods predicting pile capacity do not take installation effects into account, as large deformations can lead to mesh distortion and non-converging solutions. In this study, the material point method (MPM) is applied to simulate the pile installation process and subsequent static pile loading tests. MPM is an extension of the finite element method (FEM), which is capable of modelling large deformations and soil-structure interactions. This study utilizes the moving mesh algorithm where a redefined computational mesh is applied in the convective phase. This allows a fine mesh to be maintained around the pile tip during the installation process and improves the accuracy of the numerical scheme, especially for contact formulation. For the analyses a hypoplastic constitutive model for sand is used, which takes into account density and stress dependent behaviour. The model performs well in situations with significant stress level changes because it accounts for very high stresses at the pile tip. Numerical results agree with centrifuge experiments at a gravitational level of 40 g. This analysis confirms the importance of pile installation effects in numerical simulations, as well as the proposed numerical approach’s ability to simulate installation and static load tests of jacked displacement piles.     

Pile driving formulas based on pile wave equation analyses

Pile driving formulas, which directly relate the pile set resulting from a hammer blow to the static load capacity of the pile, are often used to decide whether a pile will have the required design capacity. However, existing formulas do not consider soil or pile type, and do not, in general, reliably predict pile capacity. In this paper, an advanced model for dynamic pile driving analysis was used to develop accurate pile driving formulas. The proposed driving formulas are validated through well-documented case histories. Comparisons of predictions from the proposed formulas with results from static and dynamic load tests show that they produce reasonably accurate predictions of pile capacity based on pile set observations.     

Application of a continuum numerical model for pile driving analysis and comparison with a real case

This paper presents the results of a so-called continuum numerical model for wave propagation analysis and soil-pile dynamic response during pile driving. An axisymmetric finite difference numerical model is developed having solid elements for both pile structure and the soil media surrounding and below the pile. Interface elements are used between the pile shaft and the soil to facilitate the sliding between the two media. The performance of the developed model is verified in two stages. First, a simple rod is subjected to a half sine-wave force function at the rod head and the corresponding reflections of force and velocity (multiplied by impedance) are presented for different boundary conditions at the rod tip. The model is then used for signal matching analysis of a real driven pile for which complete information of soil layering, dynamic test signals, and static load test results are available. The signal matching analysis was performed successfully and comparison between several other predicted and measured parameters proved the reasonably good performance of the developed continuum model.     

Modelling of vibratory pile driving in sand

A model for vibro-driving of rigid piles in sand is proposed incorporating the interaction of the vibrator–pile–soil system. The vibro-driver force and the non-linear soil resistance (dynamic f–w and q–w relationships) have been quantified in terms of in situ stress, relative density and particle size. The influence of relative density (0·65 and 0·90), particle size (effective grain size of 0·2 mm and 1·2 mm) and in situ stress (up to 20 psi) on the vibro-driving of a closed-ended pipe pile was investigated experimentally using a large-scale laboratory testing system. The vibro-driving model predicted the observed driving history reasonably well. A computer program (UHVIBRO) has been developed to analyse vibratory pile driving using the dynamic soil resistance relationships developed along with correlation factors from the systematic laboratory study.     

沉桩引起的三维超静孔隙水压力计算及其应用

引入时间、深度参数分析在饱和软粘土中沉桩时引起的超静孔隙水压力,给出了考虑固结效应的超静孔隙水压力的三维解析解;分析了超静孔隙水压力的消散过程中桩周土发生曼德尔效应的时间和区域,提出了在群桩施工过程中土体中的超静孔隙水压力是消散与累加的综合过程,施工完毕后则变为单一的消散的计算模型,并给出计算公式。通过算例,分析了桩群不同桩距、不同入桩顺序对超静孔隙水压力的影响。     

抗液化排水刚性桩沉桩过程的土压力响应

抗液化排水刚性桩是一种将刚性桩与竖向排水体相结合的新桩型。基于某建筑桩基工程,开展了抗液化排水刚性桩和不含排水体的普通刚性桩的沉桩对比现场试验,采用了动态土压力传感器实时监测沉桩过程中桩周土体内产生的土压力响应,对比了排水桩与普通桩沉桩对桩周土体水平方向应力及有效应力影响的差异。试验结果表明：抗液化排水刚性桩能够有效减小沉桩过程对桩周深部可液化土体的扰动,在桩身近侧（距桩心0.6 m）深部埋深（-15 m）位置,排水桩的水平土压力响应峰值仅为普通桩的1/4;排水桩能够有效降低沉桩对可液化土层有效应力的影响,使桩周土体更加稳定;在单次沉桩过程中,对于浅部埋深（-5 m）,排水桩对桩周土压力峰值的影响作用较小,对于存在可液化土层的深部埋深（-10、-15 m）,排水桩对土压力峰值的有效影响半径可达4倍桩径。现场试验数据为抗液化排水刚性桩的桩间距选择提供了有力的设计参考依据。     

Numerical study of the 3D failure envelope of a single pile in sand

The paper presents a comprehensive study of the failure envelope (or capacity diagram) of a single elastic pile in sand. The behavior of a pile subjected to different load combinations is simulated using a large number of finite element numerical calculations. The sand is modeled using a constitutive law based on hypoplasticity. In order to find the failure envelope in the three-dimensional space (i.e. horizontal force H, bending moment M and vertical force V), the radial displacement method and swipe tests are numerically performed. It is found that with increasing vertical load the horizontal bearing capacity of the pile decreases. Furthermore, the presence of bending moment on the pile head significantly influences the horizontal bearing capacity and the capacity diagram in the H–M plane manifests an inclined elliptical shape. An analytical equation providing good agreement with the 3D numerical results is finally proposed. The formula is useful for design purposes and the development of simplified modeling numerical strategies such as macro-element.     

Vertical dynamic response of an inhomogeneous viscoelastic pile

The vertical dynamic response of an inhomogeneous viscoelastic pile embedded in layered soil subjected to axial loading has been investigated. The interaction between pile and soil is simulated by a general Voigt model, one that has been demonstrated by earlier investigators to be capable of representing the plane strain case of soil adequately. The analytical solutions of pile responses in the frequency domain are obtained by using the (two-sided) Laplace transform. The corresponding semi-analytical solutions in the time domain for the case of a pile subjected to an instantaneous half-sine exciting force applied at the pile top are obtained via Fourier transform inversion. Using these solutions, a parametric study of the influence of the pile and soil properties on the vertical dynamic responses has been undertaken. It is shown that an abrupt variation of the soil properties with depth cannot yield evident reflection signal that may lead geotechnical engineers to assess the pile integrity wrongly from the velocity curve of the pile top, and the influence of viscosity of the pile material on the response is different from that of the damping of the soil surrounding the pile. The theoretical model developed in the present paper has also been validated in field studies, where it is shown by means of three examples that the solution developed in this study has been adequately verified by comparison of the theoretical pile model and field measurements of the dynamic responses.     

Analytical layer-element solution to axisymmetric consolidation of multilayered soils

After the application of a Laplace–Hankel transform, the governing equations of Biot’s consolidation are solved analytically by using the eigenvalue approach. Then the analytical layer-element of a single soil layer can be obtained in the transformed domain by synthesizing the generalized displacements and stresses, which are both expressed by six arbitrary constants. The elements of the analytical layer-element only contain negative exponential functions, which leads to a considerable improvement in computation efficiency and stability. The global stiffness matrix equation of multilayered soils is further obtained by assembling the interrelated layer-elements, and the actual solution is achieved by numerical inversion of the Laplace–Hankel transform after the solution in the transformed domain is obtained. Numerical examples are given to demonstrate the accuracy of this method and to study the influence of the layered soil properties and time history on the consolidation behavior.     

应用圆孔柱扩张理论对预制管桩的挤土效应分析

假定预制管桩在沉桩时的挤土过程是一个有初始孔径的圆柱形孔的扩张过程,初始孔径等于管桩的内径,最终孔径等于管桩外径,应用前人提出的圆孔扩张理论对管桩进行弹塑性分析,得到塑性区半径、土体位移等解析表达式,也对实际工程中常遇到的土塞效应对该理论应用的影响进行了讨论,并对某电厂扩建工程中预制管桩施工引起的土体位移进行计算,通过现场监测,验证了上述解析表达式的合理性,得出可以参考该组解析解来预测预制管桩施工的挤土效应的结论。     

Torsional dynamic response of a pile embedded in layered soil based on the fictitious soil pile model

The torsional dynamic response of a pile embedded in layered soil is investigated while considering the influence of the pile end soil. The finite soil layers under the end of the pile are modeled as a fictitious soil pile that has the same cross-sectional area as the pile and is in perfect contact with the pile end. To allow for variations of the modulus or cross-sectional area of the pile and soil, the soil surrounding and below the pile is vertically decomposed into finite layers. Using the Laplace transform and impedance function transfer method, the analytical solution for the dynamic response of the pile head in the frequency domain is then obtained, and the relevant semi-analytical solution in the time domain is derived using the inverse Fourier transform and convolution theorem. The rationality and accuracy of the solution is verified by comparing the torsional dynamic behavior of the pile calculated with the fictitious soil pile with those based on a rigid support model and a viscoelastic support model. Finally, a parametric study is conducted to investigate the influence of the properties and thickness of the pile end soil on the torsional dynamic response of the pile.     

Bounding surface model for soil resistance to cyclic lateral pile displacements with arbitrary direction

The development of a two-surface elastic–plastic bounding surface P–Y model for cyclic lateral pile motions is described. The kinematic-hardening model is applicable to the analysis of pile foundations subjected to loading with arbitrary azimuths relative to the pile axis. The model realistically captures the hysteretic energy damping associated with dynamic loading of subsea foundations through physically correct plastic mechanisms and provides results consistent with those observed in physical tests including cyclic loading. Its performance is demonstrated in element states of stress and in pile foundation analyses. The development based on the incremental theory of plasticity results in more robust solutions than may be obtained using alternative elastic, variable moduli and deformation plasticity formulations.     

Analysis of laterally loaded pile groups in multilayered elastic soil

The paper presents a semi-analytical method of calculating the response of a pile group. The approach is based on tying the displacement at any point of the soil mass around a pile or group of piles to the displacements experienced by the piles themselves. This is done by multiplying the pile displacements by decay functions. Application of the principle of minimum potential energy and calculus of variations to the resulting displacement field formulation leads to the differential equations for the soil and piles. Solution of these differential equations using finite differences and the method of eigenvectors leads to the desired displacement field in the soil and deflection profiles of the piles. The method produces displacement fields that are very close to those produced by the finite element method at a fraction of the cost. To illustrate the ease of application of the method, it is then used to prepare pile group efficiency charts for some typical soil modulus profiles.     

软黏土中PHC管桩打入过程中土塞效应研究

当开口管桩打入土层中,土体进入桩内形成土塞,土塞效应对桩的打入特性和承载能力具有重要影响。基于上海典型软土地基中长PHC桩的现场试验,统计分析3个不同场地共44根桩打桩过程中的土塞数据,探讨软土地基中PHC桩打桩过程中土塞长度与内径之比、土塞增量与桩打入深度增量之比(IFR)随打入桩长与内径之比变化的规律,并线性拟合出土塞增量与桩打入深度增量之比与土塞高度和桩打入深度之比(PLR)的经验公式。研究表明,大部分PHC桩在打桩过程中,土塞部分闭塞,桩从浅部较硬土层打入较软土层,IFR值减小,土塞闭塞作用大;桩从较软土层打入深部较硬土层,IFR值逐渐增大,土塞闭塞作用小,且土塞长度增量与桩打入深度增量之比与土塞长度与桩打入深度之比基本呈线性关系。     

沉桩超孔隙水压力对码头边坡稳定的影响

沉桩会对码头边坡稳定产生不利影响,一是引起桩周土体超孔隙水压力的急剧上升,导致土体有效应力降低;二是沉桩的振动加速度会产生对边坡稳定不利的瞬时惯性力。对于灵敏度低的土质岸坡来说,前者是影响其稳定性的主要因素。考虑沉桩时初始超孔隙水压力的分布,根据Biot固结方程超孔隙水压力消散解的一般表达式,建立了沉桩引起的超孔隙水压力随时间消散的解析式,在条分法的基础上考虑沉桩产生的超孔隙水压力的不利影响,建立了沉桩时边坡稳定安全系数的计算公式。根据沉桩顺序对某码头进行边坡稳定分析,结果表明：考虑打桩作用的岸坡稳定安全系数明显降低,沉桩产生的超孔隙水压力逐渐消散,边坡稳定安全系数随沉桩工序历时变化,施工中期由于超孔隙水压力叠加,岸坡最危险,沉桩结束3个月以后,超孔隙水压力基本消散,边坡稳定安全系数接近不考虑沉桩时的值。工程中要根据打桩计划进行边坡稳定计算。     

基于二次抛物线型强度包络线的管桩挤土效应分析

假定管桩在沉桩时的挤土过程是一个圆柱形孔扩张过程。基于具有抛物线型强度包络线和圆孔扩张理论对管桩的挤土效应进行分析研究。分析了桩周土体周围土体的弹塑性力学行为,得到应力场和位移场等的表达式,并求得其塑性区半径、孔内最终压力。其结果可以为管桩工程问题提供理论依据。     

Influences of soil consolidation and pile load on the development of negative skin friction of a pile

Negative skin friction (NSF) along a pile caused by soil consolidation is of great concern to engineers. The development of NSF is time-dependent because soil consolidation is also time-dependent. In this paper, a numerical solution is provided for the development of negative skin friction of a pile in nonlinear consolidated soil under different loads on a pile top. A hyperbolic interface model is also developed. This model considers the development of shear strength during soil consolidation and loading–unloading scenarios at the pile–soil interface. One-dimensional nonlinear consolidation theory is invoked to estimate the soil settlement and shear strength. The distributions of NSF and the axial force along the pile are obtained using the differential quadrature method (DQM). The influences of soil consolidation and different pile loads on the negative skin friction of a pile are discussed.     

水资源需求驱动因素分析

根据我国1980-2000年国民经济实际用水状况,简要分析了水资源需求的驱动因素,建立了水资源需求驱动因素综合分析模型,并根据模型进行了实例计算和分析.结果表明:(1)1980-2000年全国总用水增加了1 220亿m3,其中工业和生活用水增长是国民经济用水增长的直接原因;(2)全国而言,排在前四位的正向驱动因素分别是工业增加值、农田灌溉面积、人口以及林牧渔需(补)水面积.负向驱动因素分别是农田灌溉定额的下降,其次是工业用水定额的下降;(3)对生活用水而言,北方省份人口增长对生活需水增长的贡献较突出,南方省份则是生活用水定额的提高对生活需水增长的贡献较突出;(4)对工业用水而言,工业规模的扩大是驱动工业需水增长的主要因素;北方省份工业用水定额下降对抑制工业需水增长的贡献率明显;(5)农田灌溉面积变化对农田灌溉需水变化影响较大.分析结果为把握我国未来水资源需求变化态势,合理配置水资源提供依据.     

砂土中刚性短桩的p-y模型案例研究

基于Winkler地基模型的p-y曲线法在水平受荷桩的分析与设计中应用非常广泛。该方法最初主要针对海洋石油气平台,基于试桩桩径主要不超过1.2 m、长径比大于20的现场水平荷载试验结果,推导了半经验半理论方法。在过去的十年间,快速发展的风能行业（尤其海洋风机）所采用的桩基础尺寸已经远远超出了当初提出现有p-y模型时的试桩尺寸。目前普遍认为,针对大直径（如桩径 6 m）水平受荷桩的设计,现有p-y模型的可靠性值得商榷和进一步研究。通过两组水平受荷桩基试验实测结果,对当前API规范建议的砂土中p-y模型及其他研究者提出的修正方法进行了案例研究。研究结果表明：不同的p-y模型计算得到的桩身弯矩差异较小,可忽略不计;桩头变形主要受p-y曲线初始刚度值及曲线表达式影响;确定地基刚度常量时,除依据砂土地基的密实度与内摩擦角外,还应考虑地基形成历史。最后,提出了进一步研究方向。     

Pile responses to side-by-side twin tunnelling in stiff clay: Effects of different tunnel depths relative to pile

In densely built areas, the development of underground transportation systems often involves twin tunnels, which are sometimes unavoidably constructed adjacent to existing piled foundations. Because soil stiffness degrades with induced stress release and shear strain during tunnelling, it is vital to investigate the pile responses to subsequent tunnels after the first tunnel in a twin-tunnel transportation system. To gain new insights into single pile responses to side-by-side twin tunnelling in saturated stiff clay, a three-dimensional coupled-consolidation numerical parametric study is carried out. An advanced hypoplasticity (clay) constitutive model with small-strain stiffness is adopted. The effects of each tunnel depth relative to pile are investigated by simulating the twin tunnels either near the mid-depth of the pile shaft or adjacent to or below the pile toe. The model parameters are calibrated against centrifuge test results in stiff clay reported in literature. It is found the second tunnelling in each case resulted in larger settlement than that due to the first tunnelling with a maximum percentage difference of 175% in the case of twin tunnelling near the mid-depth of the shaft. This is because of the degradation of clay stiffness around the pile during the first tunnelling. Conversely, the first tunnelling-induced bending moment was reduced substantially during the second tunnelling. The most critical location of twin tunnels relative to the pile was found to be the tunnels below the pile toe. This is because the entire pile was located within the major influence zone of the twin tunnelling. Two distinct load transfer mechanisms can be identified in the pile, namely downward load transfer in case of tunnels near mid-depth of the pile shaft and next to the pile toe and upward load transfer in case of twin-tunnelling below the pile toe. These two transfer mechanisms can be useful for practitioner to assess the pile performance due to twin tunnelling.