Experimental Study of Dynamic Characteristics on Composite Foundation with CFG Long Pile and Rammed Cement-Soil Short Pile

Based on the idea of optimization design of pile type, the two kinds of the typical pile type are selected, which containing flexibility pile (e.g. rammed cement-soil pile is for short RCSP), and rigid pile (e.g. cement-flyash-gravel pile is for short CFGP). The three kinds of the composite foundation are designed, which are CFGP, CFG long pile and CFG short pile (for short CFGLP-CFGSP), CFG long-short pile and rammed cement-soil short pile (for short CFGLP-RCSSP). Natural earthquake is simulated by using the engineering blasting;the dynamic characteristics and dynamic response of the composite foundation are studied through field test. CFGLP-RCSSP is closed to linear relation. The bearing capacity of the four composite foundation of the CFGP, CFGLP-CFGSP, and CFGLP-RCSSP in the site are 225 kPa, 179 kPa, and 197 kPa, separately increases 150%, 98.8% and 119% compared to the natural foundation. The vibration main frequency is mainly depended on properties of foundation soil and piles between vibration source and measuring point, pilling load value. Horizontal vibration main frequency greater than the vertical vibration main frequency and the vertical vibration main frequency close to the first-order natural frequency of composite foundation. With the pilling load increasing, the CFGLP-RCSSP pile composite foundation combined frequency decreased. Under the same blast energy, the acceleration peak on the CFG pile composite foundation is less than CFGLP-CFGSP the corresponding values, as the load increases, the peak acceleration gently. CFG pile composite foundation is favorable on seismic. The distribution of peak acceleration is consistent within 4 m from pile top in the CFGLP_RCSSP composite foundation. The maximum of the horizontal acceleration peak along the pile body occurs at a distance of pile top 4 m or the pile top, and that of vertical acceleration peak occurred at a pile top.

Field testing of stiffened deep cement mixing piles under lateral cyclic loading

Construction of seaside and underground wall bracing often uses stiffened deep cement mixed columns （SDCM）. This research investigates methods used to improve the level of bearing capacity of these SDCM when subjected to cyclic lateral loading via various types of stiffer cores. Eight piles, two deep cement mixed piles and six stiffened deep cement mixing piles with three different types of cores, H shape cross section prestressed concrete, steel pipe, and H-beam steel, were embedded though soft clay into medium-hard clay on site in Thailand. Cyclic horizontal loading was gradually applied until pile failure and the hysteresis loops of lateral load vs. lateral deformation were recorded. The lateral carrying capacities of the SDCM piles with an H-beam steel core increased by 3-4 times that of the DCM piles. This field research clearly shows that using H-beam steel as a stiffer core for SDCM piles is the best method to improve its lateral carrying capacity, ductility and energy dissipation capacity.

Soil-cement mixture properties and design considerations for reinforced excavation

soil-cement is a mixture produced by grouting or mixing cement with soils. This paper reviews and discusses the general classifications of grouting techniques and the suitability of their applications.The mechanical properties of soil-cement mixture and the influence of sodium silicate added are discussed. Design considerations for deep soil mixed wall(DSMW) for excavation support and vault arch for tunnelling stabilisation are presented. Parameters for the numerical analysis of soil-cement mixture are evaluated and recommended.

Bearing Behaviors of Stiffened Deep Cement Mixed Pile

A series of investigations were conducted to study the bearing capacity and load transfer mechanism of stiffened deep cement mixed （SDCM） pile. Laboratory tests including six specimens were conducted to investigate the frictional resistance between the concrete core and the cementsoil. Two model piles and twenty-four full-scale piles were tested to examine the bearing behavior of single pile. Laboratory and model tests results indicate that the cohesive strength is large enough to ensure the interaction between core pile and the outer cement-soil. The full-scale test results show that the SDCM piles exhibit similar bearing behavior to bored and cast-in-place concrete piles. In general, with the rational composite structure the SDCM piles can transmit the applied load effectively, and due to the addition of the stiffer core, the SDCM piles possess high bearing capacity. Based on the findings of these experimental investigations and theoretical analysi , a practical design method is developed to predict the vertical bearing capacity of SDCM pile.

The Shaking Table Test on the Performance of Cement-mixed Piles in Liquefiable Railway Foundations

Cement-mixed piles,as countermeasure against liquefaction of silt and sand ground,can improve the shear strength and bearing capacity of foundation soil,meaning cement-mixed piles are capable of resisting displacement when an earthquake happens. However,investigations of cement-mixed piles by experimental methods such as the shaking table test is few and far between. It is especially true for the seismic performance of cement-mixed piles in liquefiable railway foundations in high seismic intensity regions. To this end,a cross-section of the Yuxi-Mengzi railway was selected as the prototype and studied by the shaking table test in this study. The results showed that composite foundation of cementmixed piles was not liquefied when the seismic acceleration was lower than 0. 30g. In the process of acceleration increasing from 0. 30g at 2Hz to 0. 60g at 3Hz,the upper middle silt outside slope toe was partly liquefied. The foundation soil under the shoulders and center of subgrade was far from the initial liquefaction criterion during the test. Cementmixed piles can effectively reduce the embankment settlement and differential settlement. It can be concluded that, the design of cement-mixed piles can ensure the seismic performance of the subgrade,and satisfy the seismic design requirements of the YuxiMengzi railway in areas of VIII degrees seismic fortification intensity.

Numerical computation of anti-liquefaction effect of lattice-type cement-mixed soil countermeasure

Continuous soil-cement wall confinement method to resist liquefaction is a new kind of process. However, whether it also has a good effect on anti-liquefaction or not needs to be urgently answered for earthquake engineering. Quiet boundary is adopted on the lateral face while free field boundary is employed at the bottom. Byrne model on dynamic pore water pressure generation is accepted and natural seismic wave EI Centro whose peak acceleration is adjusted to 0.2 g in proportion is used for input. A double-layer foundation with sandy soil in the upper portion while clay soil in the lower part is chosen as the calculation model, which is 30 m in length and 20 m in width. The groundwater level is on the ground surface. Excess pore water pressure rate is considered as a liquefaction index in the three-dimensional non-linear earthquake response computation. The anti-liquefaction effectiveness and its influencing factors, such as confinement element area are studied. For the natural double-layer foundation, it is liquefied when the excess pore water pressure rate reaches 1.0 under the seismic load. Under the same earthquake load, the peak excess pore water pressure reduces to 0.56 after adopting reinforcement of the continuous soil-cement wall, which is 46% lower than before. It indicates that continuous soil-cement wall confinement method can attain the purpose of anti-liquefaction. Accordingly, it can be a sort of engineering measure to carry on the anti-liquefaction foundation treatment.

Deformation and stability of the seawall,considering the strength uncertainty of cement mixing piles

The cement mixing (CM) pile is a common method of improving soft offshore ground. The strength growth of CM piles under complex conditions is affected by many factors, especially the cement and moisture contents, and shows significant uncertainty. To investigate the stochasticity of the early strength of CM piles and its impact on the displacement and stability of a seawall, a series of laboratory tests and numerical analyses were carried out in this study. Vane shear tests were conducted on the cement-solidified soil to determine the relationships between the undrained shear strength s_(u) of the cement soil curing in the seawater and the cement content a_(c), as well as the in situ soil moisture content w. It can be inferred that the 24 h undrained shear strength follows a normal distribution. A numerical model considering the random CM pile strength was established to investigate the deformation of the seawall. Due to the uncertainty of CM pile strength, the displacement of the seawall demonstrates a certain discreteness. The decrease of the mean undrained shear strength of CM piles causes a corresponding increase in the average displacement of the seawall. When the mean strength of CM piles is lower than a certain threshold, there is a risk of instability. Furthermore, the heterogeneity of the strength within an individual CM pile also has an impact on seawall displacement. Attention should be paid to the uncertainty of CM pile strength to control displacement and stability.

Bearing Capacity of Mixed Pile with Stiffness Core

To study load transfer mechanism and bearing capacity of a mixed pile with stiffness core （MPSC）, which is formed by inserting a precast reinforced concrete pile （PRCP）, in-situ tests involving MPSCs with different lengths, diameters, water cement ratios and PRCPs, cement mixed piles, and drilling hole piles, were carried out. Limit bearing capacities, load-settlement curves and stress distribution of MPSCs and mixed piles were obtained. The load transfer between cement soil and PRCP was analyzed by finite element method （FEM）. Test results and FEM analysis show that an MPSC has fully utilized the big friction from a cement mixed pile and the high compressive strength from a PRCP which transfers outer top load into the inner cement soil, and that inserting a PRCP into a mixed pile changes the stress distribution of a mixed pile and improves frictional resistance between a mixed pile and soil. The length and the section area on PRCP of an MPSC both have an optimum value. Adopting MPSC is effective in improving the bearing capacity of soft soil ground.

Reliability-based settlement analysis of embankments over soft soils reinforced with T-shaped deep cement mixing piles

This paper presents a reliability-based settlement analysis of T-shaped deep cement mixing(TDM)pile-supported embankments over soft soils.The uncertainties of the mechanical properties of the in-situ soil,pile,and embankment,and the effect of the pile shape are considered simultaneously.The analyses are performed using Monte Carlo Simulations in combination with an adaptive Kriging(using adaptive sampling algorithm).Individual and system failure probabilities,in terms of the differential and maximum settlements(serviceability limit state(SLS)requirements),are considered.The reliability results for the embankments supported by TDM piles,with various shapes,are compared and discussed together with the results for conventional deep cement mixing pile-supported embankments with equivalent pile volumes.The influences of the inherent variabilities in the material properties(mean and coefficient of variation values)on the reliability of the piled embankments,are also investigated.This study shows that large TDM piles,particularly those with a shape factor of greater than 3,can enhance the reliability of the embankment in terms of SLS requirements,and even avoid unacceptable reliability levels caused by variability in the material properties.

The Dynamic Characteristic Experimental Method on the Composite Foundation with Rigid-Flexible Compound Piles

Based on the idea of optimization design of pile type, the composite foundations, which include cememt-flyash-gaavel (for short CFG) long piles and cement-soil (for short CS) short piles, and CS piles with CFG core as well, are formed. The method of the site dynamic characteristic tests of the composite foundations is discussed. The test results show that fireworks bomb may replace demolitions as the vibration resource. Vibration time is about 0.1 sec. Horizontal vibration major frequency is at 22.476 - 56.436 Hz, and vertical vibration major frequency is at 15.538 - 55.884 Hz. The pile arrangements of the composite foundation in the same site have more effect on the acceleration peak value. From the point of vibration, the anti-seismic effect of the CS piles with CFG core is better than others.

新近吹填土下卧软土层地基基础方案比较研究——以孟加拉国Dasherkandi污水厂为例

鉴于孟加拉地质条件特殊,工程中普遍存在新近吹填土下卧软土层,加之当地粗骨料需要进口且价格昂贵、供应量难以保证。针对上述问题,以Dasherkandi污水厂为例,提出了穿越软土层(桩基础)和加固软土层(地基处理)两种处理方法。又因孟加拉国桩基规范(BNBC-2012)中国规范(JGJ94-2008)存在差异,根据分析结果,提出了采用具有挤土效应的沉管灌注桩。同时,提出了因吹填土和软黏土的存在造成的场地变形大的地基处理方案,并对上述两种方案在场地沉降、混凝土用量、工期和业主接受度多方面进行了比较分析。

饱和地基水泥土复合桩近场主动隔振的BEM-FEM耦合分析

人工振动污染是当前城市发展过程中面临的一个难题,单排或多排桩是人工振动污染防治的一种重要方式。传统基桩施工容易造成泥浆污染、振动等各种负面环境问题,而水泥土复合桩是一种低振动、低噪声、无挤土、少排泥浆的新型基桩,特别适合在城区构建排桩屏障。针对饱和地基中单排水泥土复合桩的近场主动隔振问题,建立了动力机器基础环境振动影响的半解析BEM方程;在此基础上,分别采用半解析BEM对饱和地基、FEM对水泥土复合桩进行建模,根据饱和地基-水泥土桩接触面的平衡和相容条件,建立了水泥土复合桩近场主动隔振的半解析BEM-FEM耦合方程,给出了耦合方程稀疏矩阵的存储策略,并对单排水泥土复合桩的近场主动隔振效果进行了计算分析。研究结果表明:单排水泥土复合桩能够有效地对动力机器基础引起环境振动进行隔振;等长芯桩复合桩的隔振效果要优于短芯桩。单排水泥土复合桩的隔振效果随着桩数的增加而逐渐提高;较小桩间距并不一定取得更好的隔振效果,建议相邻桩桩间净距取2.0~2.5λ_(R)(Rayleigh波波长)。随着单排水泥土复合桩距振源距离的增加,屏障隔振效果逐渐降低,但降低幅度相对较小。此外,对水泥土复合桩而言,当内插预制桩的外轮廓尺寸相同时,预制芯桩型式对水泥土复合桩隔振效果影响相对较小,可根据实际工程条件选择合适的芯桩类型。

水泥搅拌桩施工过程中的智能化快速评估方法

水泥搅拌桩技术在地基处理和加固工程中广泛应用,其施工质量直接影响到整个工程的整体质量,而传统的质量评估方法既耗时又复杂。介绍了一种基于智能化技术的快速评估方法,其通过实时监测与人工智能算法快速评估桩体的质量,进而能够提升水泥搅拌桩的施工效率和质量控制水平。

水泥土加固桩水平循环承载性能足尺试验研究

选取宿迁典型饱和粉土场地,采用水泥土搅拌桩加固钻孔灌注桩桩周土,开展灌注桩水平循环加载足尺试验。探究水泥土加固试验桩(TPI)和未加固试验桩(TPU)在循环水平荷载作用下的滞回特性、弯矩、刚度退化、位移延性、水平承载力与残余变形能力。试验结果表明:TPI比TPU桩的弯矩减小47%,屈服位移减小41%,水平承载力提升150%,有效刚度提升240%,等效黏滞阻尼比提升233%,桩的开裂荷载提升150%,屈服荷载阶段桩体的残余变形能力减小17%。水泥土搅拌法不仅可以限制桩侧粉土液化,增加桩基础的耗能及位移延性,而且可以显著的降低桩头水平位移和弯矩的发展。从抗震性能的角度分析,水泥土搅拌桩加固既有桩基,有效抑制了加固范围内地基土刚度衰减,增强了桩-土结构的总耗能、等效黏滞阻尼系数与变形恢复能力,提高桩基础在大变形阶段的抗震性能。

玄武岩纤维提升水泥土抗压性能试验研究

水泥土搅拌桩常用于处治山区软弱土路基,但其抗压强度和变形性能较弱,易发生脆性破坏。为此,提出采用玄武岩纤维提升水泥土搅拌桩抗压性能的方法,并通过单轴抗压试验分析纤维掺量和长度对玄武岩纤维水泥土抗压性能的影响规律,最后通过电镜扫描试验(SEM)揭示玄武岩纤维对水泥土的抗压性能提升机理。结果表明:玄武岩纤维水泥土的应力-应变曲线先后经历孔隙压密、弹性变形、弹塑性变形及破坏4个典型阶段;玄武岩纤维的掺入有效提高了水泥土的抗压性能,抗压强度、峰值应变随着纤维掺量的增加先增大后减小,弹性模量随之先减小后小幅上下波动;抗压强度随着纤维长度的增加而减小,峰值应变随之先增大后减小,而弹性模量则先减小后增大,在纤维掺量为0.6%、长度为6 mm时抗压强度最大;玄武岩纤维通过与水泥土颗粒之间的摩擦力和机械锚固力对土体进行摩擦加筋,锚固水泥土内部的裂纹增强颗粒之间的连接作用力,但当纤维过多或者较长时,会出现“堆聚”和“交叉搭接”现象,减少有效加筋纤维数量,从而降低试样的抗压性能,因此在水泥土中掺入玄武岩纤维时,纤维掺量和长度要适宜。

改良配方水泥搅拌桩现场试桩试验对比研究

水泥搅拌桩在深厚海相淤泥软土中成桩困难,改良配方的水泥搅拌桩研究具有重要的工程意义。本文针对珠海市横琴保税区一道路工程地质情况,研究了5种水泥搅拌桩(4种改良配方和1种未改良对比组)的试桩成桩效果,综合考虑桩身完整性及其强度,对各方案成桩效果进行了定量对比分析。分析结果表明:随着软土层深度的增加,各方案试桩的桩身完整性和强度减小;当粉煤灰掺量为40%或矿渣掺量为31%时,改良的水泥搅拌桩无侧限抗压强度较纯水泥搅拌桩分别提高了104.95%和43.56%;在本场区内,粉煤灰掺量为40%的改良水泥搅拌桩较其它方案更具有适用性;而对于矿渣—水泥改良体系,当矿渣掺入量过高时,水泥搅拌桩的成桩效果随着矿渣掺入量的增加反而降低。

混凝土芯水泥土复合桩竖向承载特性分析方法

基于Euler梁理论和状态空间法,建立了综合考虑非线性芯桩-水泥土-桩周土界面作用的双层叠合直梁分析模型及其状态方程,推导了成层土中不同组合形式的复合桩截面内力与变形的统一解析解,有效地考虑了复合桩分析中土与结构相互作用、结构局部参数发生变化等难点。通过与已有文献中现场试验和数值计算结果对比,验证了该方法的有效性,并讨论了桩径比、芯长比和水泥土弹性模量对桩承载特性的影响。研究结果表明:(1)桩径比增大,可提供的桩侧摩阻力和桩端阻力增大,复合桩竖向承载力基本呈线性增长;(2)芯长比增大,复合桩竖向承载力的增长幅度逐渐增大;(3)水泥土弹性模量对于桩竖向承载力影响较小。

基于扰动状态概念水泥土桩-土界面强度演化规律研究

通过设计并制作不同养护龄期(7、14、28、35及60 d)的水泥土桩-土单元体试样,开展水泥土桩-土界面的力学性能试验,测得了各龄期下水泥土桩-土界面的剪应力-剪切位移全过程曲线,并以此建立了水泥土桩-土界面峰值抗剪强度及残余抗剪强度随龄期变化的双曲线型演化模型.同时,开展了与单元体试样同龄期的水泥土桩体材料的单轴和三轴压缩试验以及土体的三轴压缩试验,得到了水泥土桩体的单轴抗压强度和抗剪强度指标(即黏聚力c和内摩擦角φ)随养护龄期的演化规律和演化模型,且试验结果表明水泥土桩-土界面抗剪强度与水泥土桩体单轴抗压强度之间呈线性关系.最后,引入扰动状态概念理论,基于试验结果建立了水泥土桩-土界面的剪应力-剪切位移关系随试样养护龄期的演化模型,该模型计算结果与实测值吻合良好,可为水泥土类桩的荷载传递规律的理论分析和数值模拟提供可靠的计算模型.

废弃泥浆制作水泥泡沫轻质土室内试验研究

为处理废弃泥浆,利用水泥协同泥浆制作泥浆水泥泡沫轻质土来修筑高速公路路基。制作了相同重度、不同水固比以及相同水固比、不同重度的泥浆水泥泡沫轻质土试块,测试在标准养护条件下28 d抗压强度,探究泥浆水泥泡沫轻质土水固比、水泥比例与重度对28 d抗压强度的影响。结果表明:泥浆泡沫轻质土的28 d抗压强度与水固比成负相关,与重度和水泥比例成正相关;配制合适的废弃泥浆水泥泡沫轻质土可用于高速公路路基修筑。此外,1 m3泡沫轻质土中,各掺量分别为水泥397 kg、干泥浆193 kg、水383.5 kg、发泡液27.1 kg时,流值为187.3 mm,试块28 d抗压强度可达2.62 MPa,且早期强度较高、水泥用量较少,可作为泥浆水泥泡沫轻质土的推荐配比。