短芯劲芯水泥土桩承载路堤失稳破坏模型试验

在劲芯水泥土桩承载路堤的工程应用中,芯桩比外桩短的情况比较普遍。但是,人们对短芯劲芯水泥土桩承载路堤的桩体破坏模式和失稳破坏机理的认识较为欠缺,无法合理评价路堤的稳定性。鉴于此,本文通过室内模型试验,开展短芯劲芯水泥土桩承载路堤失稳破坏模式研究。通过监测芯桩导电通路变化、芯桩桩身应变和桩土竖向应力,并结合PIV技术,综合分析桩体渐进式破坏模式和路堤整体失稳规律。研究结果表明,在路堤顶面下,荷载主要由芯桩承担,并随着超载增加,芯桩外桩荷载分担作用减小,但路堤失稳后芯桩仍具有一定荷载分担作用。在路堤失稳破坏过程中,路堤下桩体表现出受压破坏且芯桩底部局部鼓胀破坏,坡面下桩体表现出压弯和拉弯破坏模式,地基滑动面并不完全穿过桩体破坏位置。     

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。     

Modeling of Stone Column-Supported Embankment Under Axi-Symmetric Condition

In the present work, a simplified model has been developed to study the behavior of stone column-supported embankment under axi-symmetric loading condition. The rate of consolidation of stone column-reinforced soft ground under axi-symmetric condition has also been presented in the paper. Mechanical model elements such as Pasternak shear layer, spring–dashpot system are used to model the different components such as granular layer, soft soil, stone columns etc. The governing differential equations are solved by finite difference technique. Parametric study has also been carried out to show the effect of different model variables on the settlement, stress concentration ratio of the foundation system. It is observed that for lower diameter ratio, at a particular time, the degree of consolidation predicted by the present method for axi-symmetric loading condition is almost same or lower than the degree of consolidation obtained by unit cell approach, but as the diameter ratio increases present analysis predicts higher degree of consolidation as compared to the unit cell approach. The maximum settlement decreases as the modular ratio increases and beyond the modular ratio value 30, the reduction rate of settlement decreases.     

Geosynthetic-reinforced and jet grout column-supported embankments on soft soils: Numerical analysis and parametric study

Using a computer code based on the finite element method, a study is conducted to analyse the time-dependent behaviour of a geosynthetic-reinforced and jet grout column-supported embankment on soft soils, as well as the influence of three factors: the embankment height, the elastic modulus of column and the column spacing. The cylindrical unit cell formulation is used. The numerical model incorporates the Biot consolidation theory with soil constitutive relations simulated by the p–q–θ critical state model. Special emphasis is given to the analysis of several parameters: settlement, excess pore pressure, effective stress, stress level, tension in the geosynthetic, soil arching effect and overall efficiency coefficient.     

现浇X形桩桩承式加筋路堤三维有限元分析

桩承式加筋路堤在高速公路软基处理中得到了广泛应用,但目前对其承载变形机制还缺乏深刻认识。结合南京长江第四大桥北接线段软基加固工程,基于数值方法对现浇X形桩桩承式加筋路堤中桩土沉降、桩土应力、桩身轴力及超静孔隙水压力的发展变化规律进行了分析。研究结果表明,受土拱效应影响,路堤填筑荷载主要由桩体承担,桩及桩间土土压力在路堤填筑过程中均逐渐增大,在软基固结过程中,桩顶土压力继续增大而桩间土土压力逐渐减小,最终趋于各自的稳定值;路堤填筑过程中桩身轴力增长较快,现浇X形桩桩身上部较大部分区段存在负摩阻力,桩身中性点位置经历了先逐渐上移、而后向下移动,最终趋于稳定的过程。     

Design method for calculating settlement of stiffened deep mixed column-supported embankment over soft clay

Acta Geotechnica - As a composite column, stiffened deep mixed (SDM) column is formed by inserting a precast concrete core pile into the center of a deep mixed (DM) column. The SDM columns have...     

真空联合堆载预压下混凝土芯砂石桩复合地基固结特性理论分析

真空联合堆载预压与混凝土芯砂石桩复合地基相结合是处理深厚软土地基的一种新方法。该方法既能利用预制混凝土芯桩提高地基承载力,又能利用砂石外壳缩短排水路径、传递真空负压,加快固结。根据真空联合堆载预压下混凝土芯砂石桩复合地基的固结特点,将堆载预压和真空预压的固结效应分开考虑,然后再进行叠加,推导出了真空联合堆载预压下混凝土芯砂石桩复合地基的平均固结度解析解,并利用三维数值模拟对平均固结度解析解进行了验证。通过对解析解的参数分析,探讨了置换率、涂抹区大小及渗透系数、混凝土芯砂石桩长径比和芯桩率对混凝土芯砂石桩复合地基固结特性的影响。结果表明,混凝土芯砂石桩复合地基的固结速率随着置换率和芯桩率的增大而增大,随涂抹区和砂石外壳直径之比、未扰动区和涂抹区渗透系数之比、长径比的增大而减小。     

Numerical analysis of foundation columns to support widening of embankments

Increased traffic volume has made it necessary to increase highway capacities by widening embankments and pavements. Adding a new embankment to an existing embankment induces additional stresses and deformations beneath the widened and existing portions of the embankment. Differential settlement may develop between and within the new and existing portions of the embankment, especially over soft soils. This differential settlement often causes pavement distress, such as longitudinal cracks or the drop-off (or sinking) of pavement sections. Different techniques have been adopted to remedy these problems, including the use of foundation columns, such as deep mixed columns, vibro-concrete columns, stone columns, and aggregate piers. However, design procedures for foundation columns constructed for this purpose are not well developed. The analyses of eight cases of column-supported widened embankments and two untreated foundations are presented in this paper. The factors considered include the consolidation of foundation soils under existing embankments and the spacing, region, and modulus of foundation columns. Two-dimensional finite difference software was used after the calibration of the model against a field case study and numerical analyses were conducted to investigate stresses and deformations of the widened embankments over soft soil with or without the remediation of foundation columns. The results presented in this paper include the vertical and the horizontal displacements, the maximum settlements, the transverse gradient change, and the distribution of the additional stresses induced by the widening. Recommendations are made for the design of foundation columns to remedy roadway pavement failure due to widening of embankments.     

Two-dimensional parametric study of geosynthetic-reinforced column-supported embankments by coupled hydraulic and mechanical modeling

Geosynthetic-reinforced column-supported (GRCS) embankments have been increasingly constructed over soft soil. Current design methods were developed based on different assumptions and simplifications, which lead to great variations among them. Numerical studies have been conducted to improve the understanding of the GRCS embankment system. Most of the past numerical studies were limited to mechanical modeling only (i.e., not coupled with hydraulic modeling), which can be used to evaluate long-term or short-term behavior by using appropriate mechanical properties. However, the generation and dissipation of excess pore water pressure under GRCS embankments during and after construction make their behavior time-dependent. A two-dimensional (2D) parametric study based on coupled hydraulic and mechanical modeling was conducted to study the time-dependent behavior of GRCS embankments under various conditions. Special attention was paid to the influence of several key factors on the performance of the embankments indicated by maximum and differential settlements, tension in the geosynthetic, and degree of consolidation. The investigated key factors include modulus and permeability of the soft soil, modulus and spacing of columns, tensile stiffness of the geosynthetic, and average construction rate. The influence of these factors are compared and rated in terms of the degree of importance in this study.     

高速公路工程中粉喷桩复合地基最大侧向变形研究

分析了粉喷桩复合地基路堤填筑过程中最大侧向变形发展的特性,确定了坡脚处最大侧向变形与路中沉降呈双直线的关系,坡脚处最大侧向变形在大部分填筑过程中与路中沉降呈 0.115的关系。对比分析了粉喷桩复合地基和天然地基侧向变形发展规律的差异,并基于复合地基理论分析,认为导致它们差异变化的原因是粉喷桩复合地基桩-土应力分担比的变化。     

Numerical analysis of passive pile groups in offshore soft deposits

The performance of pile foundations in offshore soft deposits is frequently dominated by passive loading from approach embankments for reclaimed land in Korea. This is mainly because of a Korean offshore soil profile, involving an over 30 m normally consolidated soil, with a strength that varies linearly with depth. This paper presents the results of an investigation on short- and long-term passive loading on the piles using two dimensional elastoplastic-consolidation coupled FEM analyses with large strain mode. This numerical scheme is essentially based on an updated Lagrangian formulation, which is favorably validated in cases of both centrifuge tests and field measurements. Although a parametric study is limited by some of the application, it is shown that passive pile loading is primarily affected by soil profile, pile head boundary condition, magnitude of embankment load, and average degree of consolidation. Simultaneously, time-dependent effect of shear transfer at the pile head is explicitly identified and a useful starting point in design is recommended for passive pile loading in construction (short-term) and consolidation (long-term) phases.     

Field Test Research on Embankment Supported by Plastic Tube Cast-in-place Concrete Piles

The plastic tube cast-in-place concrete pile (TC pile) with a small diameter consists of pre-driven plastic tube filled with concrete. Based on the case of TC pile-reinforced embankment on soft ground, and according to the monitoring data of the TC pile-reinforced embankment system, the treatment effect and reinforcement characteristics for this system were analyzed. The field monitoring results indicates that the critical height of embankment is about 1.1 times the pile net spacing, and the small-spacing arranged TC piles can be applied to low embankment engineering; the load share rate can reflect the degree of soil arching more better and steadily and exceeds 70 % at the end of monitoring period; the settlements of pile cap and soil between piles mainly occurs in the embankment construction period; the different settlement between pile cap and soil approaches the maximum and then reduces gradually when the embankment height is about 2.2 times the pile net spacing. The variation of layered settlement and pore water pressure illustrates that the embankment settlement is mainly caused by the compression of soils within pile length, which is about 90 % of the total settlement; the influence depth of pore water pressure is about 1/3 pile length.     

桩承式加筋路堤格栅分析

将单桩等效处理范围简化为圆柱体,假定桩间土上部的格栅受上覆土压力作用后变形为二次抛物面,考虑路堤中的土拱效应,得到了格栅受力分析的解析方法。当路堤中的土拱处于弹性状态时,格栅对提高桩体荷载分担比的作用不大;当路堤中的土拱处于塑性状态时,格栅对提高桩体荷载分担比有明显的作用;当格栅与地基的相对刚度较小时,格栅对减小桩间土沉降的作用很小。最后对一个模型试验结果和一个工程实例进行了分析。     

考虑砂石桩固结的混凝土芯砂石桩复合地基固结解析解

考虑了混凝土芯砂石桩复合地基中砂石桩的环形排水通道、砂石桩体内的径、竖向渗流和土体施工扰动,并采用桩土共同分担荷载的初始条件,得到了混凝土芯砂石桩复合地基固结问题的控制方程,给出了控制方程的解答;并分别给出了复合地基按应力和按变形定义的总平均固结度,分析了砂石桩桩体渗透系数、芯桩与砂石桩直径比对地基固结性状的影响。结果表明：对于混凝土芯砂石桩复合地基按应力定义的固结度与按变形定义的固结度表达式不同;地基的固结随着砂石桩桩体渗透系数增加而加快;砂石桩直径一定的情况下,固结速率随芯桩直径增大先增大后减小。最后对本文解和以往的两种解做了比较,与以往解相比本文解能够同时考虑环形通道和桩土荷载分担,给出的固结度介于以往的两种解之间。     

加芯水泥土桩复合地基桩土应力比计算方法研究

桩土应力比是研究复合地基的一项重要参数,目前的研究对象主要为常规复合地基,如搅拌桩、碎石桩等,而针对加芯水泥土桩的计算方法研究鲜有报道。从土体、水泥土及复合桩体单元变形模式出发,综合考虑桩体负摩阻力、桩顶和桩端刺入持力层的情况,分析了桩周土体、水泥土桩及复合桩体的压缩变形,导出了刚性基础下加芯水泥土桩复合地基芯桩、水泥土桩与土体3者之间的应力比的计算公式。并分析了加芯水泥土桩芯桩直径、芯桩桩长及水泥土桩桩长对桩土应力比的影响,其主要影响表现在随着加芯水泥土桩芯桩直径和桩长及水泥土桩桩长的增加,桩土应力比np亦逐渐提高。工程实例表明,计算结果与实测值具有较好的一致性,说明该方法具有一定的工程实用性,可为工程实践提供指导。     

路堤下现浇薄壁管桩复合地基工作特性分析

现浇薄壁管桩和土工格栅加筋碎石垫层所组成的复合地基加固软弱路基具有经济合理，施工时间短，承载力高，沉降减小明显等优点。采用非线性有限元对路堤荷载作用下的现浇薄壁管桩复合地基的工作特性进行了分析，结果表明：在路堤荷载作用下，路堤填土材料中的拱效应、土工格栅的拉膜效应或加筋垫层的刚性垫层效应、桩土间刚度差异引起的应力集中效应保证了大部分荷载由桩承担，有效地减小了总沉降和不均匀沉降。同时对荷载传递机理的影响因素做了分析.     

强夯碎石墩处治“山地型”软土路基的变形特征研究

为明确填方路堤荷载下强夯碎石墩的变形特征与其路堤稳定性的内在关联,针对“山地型”软土地区某高速铁路车站高填方路堤的失稳破坏现象,基于现场实测数据和三维数值分析模型,着重研究了不同位置强夯碎石墩墩体的鼓胀变形与侧向弯曲变形规律,并定义墩体鼓胀率和最大侧向弯曲值对其变形特征予以描述。研究结果表明:路堤荷载作用下不同位置的强夯碎石墩发生不同程度的鼓胀变形和侧向弯曲变形,其中鼓胀变形主要由上覆路堤荷载所产生的压缩鼓胀和路堤侧滑所产生的滑动剪切鼓胀所组成,且其最大鼓胀变形区域集中在距桩端1~2倍桩直径范围内;位于强夯碎石墩复合地基潜在滑裂面剪入口附近墩体的鼓胀率最大(0.75%),约为路堤中心处墩体鼓胀率的1.5倍,表明该处以滑动剪切鼓胀变形为主。沿路堤中心线向外不同位置的强夯碎石墩墩体的最大侧向弯曲变形的发展规律近似呈三段线模式,其中潜在滑裂面剪入口附近的17#墩体为其侧向弯曲变形开始陡增的拐点,表明17#与其附近墩体为控制路堤侧向变形与稳定的关键构件。路堤填筑过程中所出现数条裂缝的原因可能在于强夯碎石墩的剪切破坏而致使路堤出现整体滑移所引起的,建议在采用散体材料桩复合地基处治“山地型”软弱地基时,应配合其他加固措施以确保填方路堤的整体稳定性。     

新型桩板结构对高速铁路软基沉降控制作用离心试验

在CFG、PHC等刚性桩桩顶加设50 cm厚筏型钢筋混凝土形成板桩板结构控制深厚软土路基沉降,是高速铁路建设中提出的新型地基处理形式,以满足高标准铁路路基变形要求。通过对群桩处理的深厚软土路基在有无钢筋混凝土板时路基沉降的离心模拟,重点研究了钢筋混凝土板对路基沉降的控制作用以及桩、板与土体之间的相对变形。试验结果表明,在群桩处理地基上部铺设一定厚度的钢筋混凝土板,可以使路基整体沉降减小12 %,并使路基整体沉降更加均匀;桩端平面上部1/3桩长范围内土体由软塑状态转变为硬塑状态,密度和含水率变化显著;加载作用对路基变形影响深度大于40 m。     

考虑加载过程及桩体固结变形的碎石桩复合地基固结解析解

为了完善碎石桩复合地基固结理论,通过假设从桩体排出的水量等于流入桩体的水量与桩体体积变化之和以及地基扰动区土体水平渗透系数呈线性变化,并考虑上部荷载逐渐施加,推导了考虑桩体体积变化的碎石桩复合地基超静孔压及固结度解析解。当加载时间趋于零时,本文解可退化为瞬时加载情况下的解;当加载时间及桩径同时趋于零时,本文解可进一步退化为Terzaghi一维固结解,这证明了本文解的正确性。通过与已有解的比较,对地基固结性状进行了分析。结果表明,加载过程对地基固结度影响显著,加载历时越长,固结越慢;在各种条件下,不考虑桩体固结变形时地基固结始终比考虑桩体变形时快,并且其影响随着加载历时变小、桩径比变小、桩土模量比变小、桩土渗透系数变小而逐渐增大,这说明在实际工程固结计算中不考虑桩体固结变形是偏于不安全的。     

Centrifuge study on behavior of rigid pile composite foundation under embankment in soft soil

The rigid pile composite foundation is widely used in highway projects in soft soil area as it can effectively increase the bearing capacity and stability of the foundation. While the research on the behavior and failure mode of rigid pile composite foundation under embankment is not enough, instability failure of rigid pile composite foundation often occurs in practical projects. This paper presents a centrifuge model test to investigate the load transfer mechanism, settlement characteristic and failure mode of rigid pile composite foundation under embankment. The test results show that: the soil displacement of different region in rigid pile composite foundation was different, obvious vertical displacement occurred in the soil under the center of embankment and the horizontal displacement was very small in this region; both vertical and horizontal displacement occurred in the soil under the shoulder of embankment; and obvious horizontal displacement occurred in the soil under the slope toe of embankment; moreover, ground heave also occurred near the slope toe of embankment. The soil displacement in rigid pile composite foundation had a large influence on the stress characteristic and failure mode of rigid piles, the compressive failure and bending failure would probably occur for the piles under the center and shoulder of the embankment, respectively, and the tension-bending failure would probably occur for the piles under the slope toe of embankment. The different failure modes of piles at different regions should be considered in the design of rigid pile composite foundation under embankment. The test results can be used to improve the design method for rigid pile composite foundation under embankment in practical projects.