Consolidation of high replacement ratio stone column-reinforced ground:Analytical solutions incorporating clogging effect

The utilization of stone columns has emerged as a popular ground improvement strategy,whereas the drainage performance can be adversely hampered by clogging effect.Despite the ample progress of calculation methods for the consolidation of stone column-improved ground,theoretical investigations into the clogging effect have not been thoroughly explored.Furthermore,it is imperative to involve the column consolidation deformation to mitigate computational error on the consolidation of composite ground with high replacement ratios.In this context,an analytical model accounting for the initial clogging and coupled time and depth-dependent clogging of stone columns is established.Then,the resulting governing equations and analytical solutions are obtained under a new flow continuity relationship to incorporate column consolidation deformation.The accuracy and reliability of the proposed model are illustrated by degradation analysis and case studies with good agreements.Subsequently,the computed results of the current study are juxtaposed against the existing models,and an in-depth assessment of the impacts of several crucial parameters on the consolidation behavior is conducted.The results reveal that ignoring column consolidation deformation leads to an overestimate of the consolidation rate,with maximum error reaching up to 16%as the replacement ratio increases.Furthermore,the initial clogging also has a significant influence on the consolidation performance.Additionally,the increment of depth and time-clogging factors a and b will induce a noticeable retardation of the consolidation process,particularly in the later stage.

Liquefaction mitigation in silty soils using composite stone columns and dynamic compaction

The objective of this study is to develop an analytical methodology to evaluate the effectiveness of vibro stone column (S.C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquefaction in saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze soil densitication during S.C. installation and D.C. process, and (ii) identify parameters controlling post-improvement soil density in both cases, and (iii) develop design guidelines for densification of silty soils using the above techniques. An analytical procedure was developed and used to simulate soil response during S.C. and D.C. installations, and the results were compared with available case history data. Important construction design parameters and soil properties that affect the effectiveness of these techniques, and construction design choices suitable for sands and non-plastic silty soils were identified. The methodology is expected to advance the use of S.C. and D.C. in silty soils reducing the reliance on expensive field trials as a design tool. The ultimate outcome of this research will be design charts and design guidelines for using composite stone columns and composite dynamic compaction techniques in liquefaction mitigation of saturated silty soils.

Experimental study on soil improvement with stone columns and granular blankets

Stone column is one of the soil stabilizing methods that is used to increase bearing capacity and decrease the settlement of soft soils.Reinforced and unreinforced granular blankets are now being utilized to overcome the problems of soft soils.In this research,the bearing capacity of stone columns,granular blanket,and a combination of both methods in reinforced and unreinforced modes were studied using scaled physical models.Results show that using granular blanket,stone column,and combination of both improves bearing capacity of soft soils.Using geogrid as the reinforcement of granular blankets and geotextile as stone-column encasement increases the efficiency of granular blankets and stone columns significantly.Additionally,in the case of using geotextile around the stone column,the stress concentration ratio of the stone column will increase as well as its rigidity and bearing capacity.

Stone Columns and Tensioned Anchors to Completely Eliminate Tunnels Trough Settlements

The complex tunnelling constructive environment in urban area in similar green field situations is faced through analytical evaluations in order to control the design calculation process and subsequently manage the interventions techniques with the aim of totally reducing the typical settlements trough above the tunnel either during the construction stage or during the serviceability stage. Recently, the author has proposed an operative and mathematical method by an opportune choice of tensioned anchors to control the tunnel lining settlements. In order to completely eliminate the remainder typical soft soil trough which is normal to the line of the tunnel, it is here proposed to use and properly calculate the interventions of stone columns by the SAVE （silent, advanced, vibration-erasing） Compozer method, in combination with the anchorages.

Shear wave velocity-based evaluation and design of stone column improved ground for liquefaction mitigation

The evaluation and design of stone column improvement ground for liquefaction mitigation is a challenging issue for the state of practice. In this paper, a shear wave velocity-based approach is proposed based on the well-defined correlations of liquefaction resistance （CRR）-shear wave velocity （V）-void ratio （e） of sandy soils, and the values of parameters in this approach are recommended for preliminary design purpose when site specific values are not available. The detailed procedures of pre- and post-improvement liquefaction evaluations and stone column design are given. According to this approach, the required level of ground improvement will be met once the target V of soil is raised high enough （i.e., no less than the critical velocity） to resist the given earthquake loading according to the CRR-V relationship, and then this requirement is transferred to the control of target void ratio （i.e., the critical e） according to the V-e relationship. As this approach relies on the densification of the surrounding soil instead of the whole improved ground and is conservative by nature, specific considerations of the densification mechanism and effect are given, and the effects of drainage and reinforcement of stone columns are also discussed. A case study of a thermal power plant in Indonesia is introduced, where the effectiveness of stone column improved ground was evaluated by the proposed V-based method and compared with the SPT-based evaluation. This improved ground performed well and experienced no liquefaction during subsequent strong earthquakes.

Stone column settlement performance in structured anisotropic clays:the influence of creep

The recently developed elasto-viscoplastic Creep-SCLAY1S model has been used in conjunction with PLAXIS 2D to investigate the effectiveness of vibro-replacement in a creep-prone clay. The Creep-SCLAY1S model accounts for anisotropy, bonding, and destructuration, and uses the concept of a constant rate of viscoplastic multiplier to calculate creep strain rate. A comparison of settlement improvement factors with and without creep indicates that ‘total’ settlement improvement factors （primary plus creep） are lower than their ‘primary’ counterparts （primary settlement only）. The lowest settlement improvement factors arise for analyses incorporating the effect of bonding and destructuration. Examination of the variations of vertical stress with time and depth has indicated that vertical stress is transferred from the soil to the column as the soil creeps. This results in additional column yielding. In addition, the radial and hoop stresses in the soil are lower for the ‘creep’ case. The reduced radial stresses lead to additional column bulging and hence more settlement, whereas the hoop stress reductions appear to be a secondary effect, caused by additional plastic deformation for the ‘creep’ case.

碎石桩复合地基非等应变非线性固结特性分析

现有碎石桩复合地基固结理论通常基于等应变假设,与工程实际情况有出入。基于分段线性差分法,建立了一种考虑非等应变刺入修正和任意桩土非线性本构关系的碎石桩复合地基非线性固结模型。该模型可以考虑桩土自重、涂抹区、井阻效应和变荷载,可以计算碎石桩向垫层或下卧层的刺入并对等应变假设进行修正,可以分析桩土附加应力沿深度方向的衰减,以及任意的非线性压缩性及渗透性关系。与工程实测数据对比发现,考虑非等应变修正的模型计算值与工程实测值基本吻合。结合算例分析,进一步研究了垫层及下卧层模量、桩土竖向应力衰减和应力路径对碎石桩复合地基土体固结特性的影响,结果表明:碎石桩向垫层或下卧层刺入使复合地基桩土应力比减小、沉降量增大、固结速率减缓;忽略桩土竖向应力衰减虽对固结速率影响不大,但会高估复合地基沉降变形;不同应力路径对碎石桩复合地基固结特性影响显著,简化的土层压缩性关系将对分析结果带来较大误差。

碎石桩-排水板组合型复合地基固结解析解

碎石桩-排水板组合型复合地基以成本低、固结速率快及加固效果好等优势被广泛应用于工程实际。虽然目前竖井地基或碎石桩复合地基固结理论已较为完善,但同时包含以上两种排水体的复合地基固结理论还鲜见报道。此外,以往固结模型中土体内孔隙水在径向仅发生单向渗流,这与实际情况存在差别。基于此,以碎石桩-排水板组合型复合地基为研究对象,考虑外荷载作用下土中孔隙水分别向碎石桩和排水板发生径向渗流,在复合地基顶面排水、底面不透水条件下建立碎石桩-排水板复合地基固结模型。利用桩体、桩周土体和竖井间流量相等的连续条件,获得该模型的解析解答。在一定条件下,该解答可退化为碎石桩复合地基固结解或竖井地基固结解,表明该解答的普适性及正确性。将该解答应用于工程实例及室内模型试验的沉降计算,计算结果与实测值吻合较好。开展大量计算研究此类复合地基不同工况下的固结性状,结果表明:在碎石桩桩间打设排水板,能有效提高复合地基固结速率,复合地基固结速率随排水板数量增加而加快;排水板渗透系数对复合地基固结速率及沉降速率的影响较大;与传统复合地基技术相比,碎石桩桩间增设排水板的工法具有更好的经济价值。

基于割线模量的散体材料桩复合地基沉降计算

基于土体侧限条件下的应力-应变关系曲线均为双曲线的现象,且应用割线模量计算土体的竖向应变更有助于实现程序计算,首先利用土体割线模量与压缩指数和压缩模量的关系,得到桩周土体割线模量的确定方法.在此基础上,基于等应变假设条件得到考虑土体非线性压缩的桩土应力比,并提出改进的应力修正法和复合模量法,以实现散体材料桩复合地基非线性沉降计算.最后结合模型试验和实际工程算例,对2个改进的沉降计算方法开展验证分析.研究结果表明:室内模型试验中,改进的应力修正法和复合模量法的沉降计算值与试验值误差均小于1.00 mm,表明改进的沉降计算方法具有一定的可靠性;实际工程算例中,改进的复合模量法计算值与实测值间误差为8.70%,远小于JGJ 79—2012《建筑地基处理技术规范》中规定的复合模量法误差(55.06%),改进的应力修正法计算值与实测值误差为8.73%,远小于JGJ 79—2012中规定的应力修正法误差(55.11%).对比分析表明,改进的沉降计算方法优于JGJ 79—2012中的复合模量法和应力修正法,且改进方法计算值更接近实测值,说明改进方法具有更好的可靠性和适用性.

节段料石拼接构造对SCFST柱抗震性能影响有限元分析

为研究节段料石拼接构造对内填料石钢管混凝土(SCFST)柱抗震性能的影响,通过ABAQUS软件建立SCFST柱的数值模型。以轴压比、钢管厚度、料石尺寸、节段料石构造为变化参数,对12个试件进行低周反复荷载作用下的数值模拟。结果表明:内填料石可显著提高试件的初始刚度、承载力及耗能能力,但会导致试件延性系数降低;混凝土强度、轴压比对试件的初始刚度、峰值承载力及耗能能力影响不明显,但对延性系数影响较大;随着料石尺寸的增大,初始刚度基本不变,峰值承载力最多降低17.8%,试件延性系数略微降低,耗能能力显著提高;料石节段布置与料石整体布置试件的初始刚度、峰值承载力基本相当,但延性系数最多降低32.7%,且料石节段间填充混凝土拼接时可显著提高试件承载力。

Centrifuge modeling of a large-scale surcharge on adjacent foundation

This study investigates the ground and structural response of adjacent raft foundations induced by largescale surcharge by ore in soft soil areas through a 130g centrifuge modeling test with an innovative layered loading device.The prototype of the test is a coastal iron ore yard with a natural foundation of deep soft soil.Therefore,it is necessary to adopt some measures to reduce the influence of the large-scale surcharge on the adjacent raft foundation,such as installing stone columns for foundation treatment.Under an acceleration of 130 g,the model conducts similar simulations of iron ore,stone columns,and raft foundation structures.The tested soil mass has dimensions of 900 mm×700 mm×300 mm(lengthwidthdepth),which is remodeled from the soil extracted from the drilling holes.The test conditions are consistent with the actual engineering conditions and the effects of four-level loading conditions on the composite foundation of stone columns,unreinforced zone,and raft foundations are studied.An automatic layer-by-layer loading device was innovatively developed to simulate the loading process of actual engineering more realistically.The composite foundation of stone columns had a large settlement after the loading,forming an obvious settlement trough and causing the surface of the unreinforced zone to rise.The 12 m surcharge loading causes a horizontal displacement of 13.19 cm and a vertical settlement of 1.37 m in the raft foundation.The stone columns located on both sides of the unreinforced zone suffered significant shear damage at the sand-mud interface.Due to the reinforcement effect of stone columns,the sand layer below the top of the stone columns moves less.Meanwhile,the horizontal earth pressure in the raft foundation zone increases slowly.The stone columns will form new drainage channels and accelerate the dissipation of excess pore pressure.

智能化振杆密实法加固海陆交互相液化地基试验研究

长江三角洲地区广泛分布着海陆交互相液化地基,具有粉土、粉砂、淤泥质粉质黏土互层的工程地质特点。随着我国高速公路建设快速发展,对此类海陆交互相液化地基的处理已迫在眉睫。本文依托洋通高速公路工程项目,采用具有自主知识产权的智能化振杆密实法施工技术对海陆交互相液化地基进行了加固处理,通过室内试验、原位测试和填筑期沉降监测等方法,综合评价了该技术处理海陆相交互相液化地基的加固效果,并与碎石桩法的处理效果进行对比。研究结果表明:智能化振杆密实法处理后,试验场地土体密度、内摩擦角、压缩模量等均得到明显提高,含水量和孔隙比明显降低;处理范围内土层的锥尖阻力、侧摩阻力和标准贯入试验锤击数较处理前提高了80%以上,测试波速则提升了15%,预测工后沉降为10.36 mm,满足了高速公路建设要求。振杆密实法工程费用相比碎石桩法可节约38.0%,且施工过程自动化、可视化程度高,具有显著的经济效益与应用前景。

装配式建筑柱脚钢纤维豆石灌浆料研究与应用

为了达到装配式建筑柱脚灌浆应用的特种高强、抗裂、无收缩水泥基灌浆料应用要求,研究C60型钢纤维豆石灌浆料。单因素试验研究了豆石、钢纤维对于灌浆料性能影响;单因素试验系统探究矿物掺合料对灌浆料性能影响规律;工程应用研究材料工程适应性。结果表明:(1)豆石在体系中凝结硬化后会使硬化体机构更加密实,抗冲切能力增强。最佳掺量为8.0%。(2)钢纤维掺加会明显提高体系黏度,施工性能变差,但会提高灌浆料后期强度,钢纤维最佳掺量4.0%。(3)矿粉对于后期强度提高明显,28 d抗压强度提高超过3.3%;粉煤灰使初始流动度增大。(4)工程应用得到钢纤维豆石灌浆料初始流动度达到300 mm,28 d抗压强度超过65 MPa,抗压强度富余值超过8.33%。

低净空条件下海上振冲碎石桩施工新技术及应用

本文依托港珠澳大桥香港口岸填海工程,在高度限制条件、环境保护等施工环境的限制条件下,首次开创了一种特殊的底部出料振冲碎石桩伸缩管施工技术,通过该项特殊的振冲碎石桩施工技术,不仅满足了本工程振冲碎石桩施工深度以及施工质量的要求,而且满足了低净空条件的限制以及各类环保要求,取得良好的施工效果。本文从高度限制条件、设计技术要求、振冲碎石桩伸缩管施工设备及工艺、施工及其效果分析几个方面综合阐述了该海上振冲碎石桩施工工艺工法,为广大填海工程等其它类海上地基改良工程提供了参考。

水下套管强夯碎石桩夯锤动力响应试验研究

设计了水下套管强夯碎石桩模型试验,研究了夯锤形状、夯击能、单次填料量、土体强度等因素对水下套管强夯碎石桩成桩过程和成桩质量的影响,分析了夯击的夯锤加速度峰值、夯沉量、桩体密度的变化规律。试验结果表明:新型锥形夯锤夯击时的反力加速度与传统平底夯锤相比峰值接近但时程减小,随夯击作用时间表现为先震荡增大再迅速减小的发展特征。建立了夯锤夯击时的归一化最大冲击力关于夯击次数、夯击能、土体强度、单次填料质量等参数的拟合公式,可用于预测不同地基强度中夯击成桩的最大冲击力。

筋箍碎石桩承载机制的三维离散-连续介质耦合数值模拟

筋箍碎石桩的承载机制由于涉及碎石-土工格栅-土体之间的相互作用而十分复杂。基于三维离散-连续介质耦合计算模式提出一种筋箍碎石桩的数值建模方法。该方法基于离散单元法建立考虑碎石颗粒真实几何形态的桩体模型,采用弹性膜单元模拟双向拉伸土工格栅。通过对比室内模型试验获得的筋箍碎石桩荷载-沉降曲线及桩体变形特征验证该方法的正确性。采用该方法模拟不同包裹长度下筋箍碎石桩单桩承载变形直至破坏失效的全过程,并结合数值模拟揭示的土体、筋材受力变形结果以及碎石桩颗粒力链网络结构,从细观尺度讨论碎石-土工格栅-土体的相互作用规律,从而揭示筋箍碎石桩的承载机理和破坏特征。建立的离散-连续介质耦合数值既克服了连续介质数值模型难以准确反映碎石集料与筋材土体细观相互作用的缺点,又较离散单元法数值模型在参数标定和计算效率上具有优势,能够为筋箍碎石桩复合地基的承载变形规律等相关研究提供一种数值模拟思路。

碎石桩复合地基抗剪特性物理模型试验与数值模拟

碎石桩法在软基处理中应用广泛,目前对该型复合地基抗剪特性定量研究较少,不能很好支撑实际工程需求。以物理模型试验和数值试验为主要手段,研究了碎石桩复合地基抗剪机制,得到以下结论:①以实际的碎石桩置换率19.6%为参照,对复合地基分别开展了无包裹常规碎石桩和透水橡皮膜包裹碎石桩的不固结剪切,以及无包裹碎石桩固结剪切单剪物理模型试验。3种方案复合地基反馈的抗剪能力较天然软基均提升显著,但前2种方案抗剪强度参数差异不大,而固结剪切方案较前两者有更进一步提升,表明桩周软土对散体状碎石桩的柔性约束能使其呈现整体均匀式剪切特征,碎石桩对软基渗透性能的显著改善是复合地基抗剪强化的关键。②将碎石桩置换率由19.6%提升至24.5%开展一组不固结剪切对比试验,与置换率19.6%的固结剪切状态相比,两者强度差异不大,表明在较低置换率区间,从发挥该类型复合地基抗剪强度的角度来看,促进地基固结更为经济有效。③以不同排水固结状态桩间土抗剪强度参数为主控因素,开展复合地基剪切数值模型试验,分析认为物理模型试验中桩间土反馈的抗剪强度较相应排水固结路径均有所提高,验证了碎石桩能显著改善软基渗透性的判断,进一步分析揭示了复合地基剪切变形及剪应力传递规律。

堤坝软土碎石桩复合地基计算参数研究

目前确定碎石桩复合地基参数时,工程界常采用碎石与原状土参数按平面面积占比叠加的简化方法进行计算,对其适用条件较少关注。以卢旺达那巴龙格河二号水电站高土石坝深厚覆盖层软弱地基处理项目为背景,基于PLAXIS有限元平台,对不同面积置换率下含碎石芯软黏土复合试样进行三轴固结排水试验数值模拟,经室内三轴试验验证了数值模拟方案的合理性。对软土碎石桩复合地基的桩-土作用机制和土体硬化模型计算参数进行研究,将所获参数应用于坝基的变形分析,并与传统参数叠加法和碎石桩墙法进行对比。结果表明:采用数值复合试样法确定的土石坝软土碎石桩复合地基参数是合理的,在计算复合地基沉降时误差小;传统参数叠加法低估了软土碎石桩复合地基的沉降,仅适用于低应力水平、高面积置换率的情况,并且会高估复合地基的强度参数。采用数值复合试样法参数对坝基变形的二维有限元分析表明,根据坝体高度不同采取不同置换率的碎石桩分区加固地基的优化方案是可行的。

粉质黏土中包裹式散体材料桩复合地基承载特性

为探究包裹式散体材料桩在粉质黏土地基中的应用和包裹式散体材料桩复合地基的承载特性。根据实际路基工程的特点,采用室内模型静载试验方法,研究了粉质黏土中包裹式散体材料桩复合地基的承载特性、应力传递规律和孔隙水压力变化规律,试验结果表明:包裹式散体材料桩复合地基的承载能力和变形特征主要受包裹的长度和包裹材料的强度的影响。随着包裹材料的强度的提高,包裹式散体材料桩复合地基的承载能力也随之增加;包裹式散体材料桩复合地基的超静孔隙水压力随散体材料桩的包裹材料的强度和包裹长度的增大而减小,不同加固形式下复合地基的超静孔隙水压力随着应力的增加而非线性增长,加载初期,增长较快,随着加载地进行,后期的增长变缓,并会趋于稳定;最后根据现行的建筑地基设计规范和已有包裹式散体材料桩复合地基的设计方法的研究资料,对粉质土包裹式散体材料桩复合地基的设计计算方法进行了总结归纳,并与试验结果进行了对比,计算结果与试验结果较为一致。

碎石桩处理饱和砂土坝基地震响应离心模型试验

为了研究坝体与坝基相互作用下饱和砂土坝基的地震响应规律及碎石桩的抗液化效果,首先,设计和开展了2组超重力振动台试验,研究了碎石桩处理前后饱和砂土坝基不同位置的加速度、孔压和位移等地震响应规律;然后,评估了坝基液化的可能性及碎石桩的抗液化效果;最后,通过反演方法探究了坝体和碎石桩对坝基不同位置土体的动剪应力-剪应变响应的影响。研究结果表明:未处理坝基的加速度放大系数在坝体正下方随高程增加而逐渐减小,在坝趾下方和坝外自由场随高程增加而先减小后增大;碎石桩处理坝基具有更大的刚度和更小的阻尼,加速度放大系数整体比未处理坝基的大;坝体能够增强下方土体的抗液化能力;碎石桩处理能够降低超静孔压的累积速率和峰值,加速震后孔压消散,但在遭遇强震时处理区外土体仍会发生液化。坝基土体的动剪应力-剪应变响应受初始围压和超静孔压影响,坝体下方土体的剪切模量更大,碎石桩能够通过抑制超静孔压的发展来缓解处理区内土体剪切刚度的退化。