上海软土地基超长打入PHC桩工程性状研究

通过对上海软土地基中64～77m的超长打入PHC桩打桩过程中锤击数、土塞高度随深度变化的跟踪监测及土塞的标准贯入试验和竖向荷载下的静载试验,探讨了超长PHC桩在上海软土地基中的贯入度与土层的关系,分析了打入承压水层中对土塞闭塞效应的影响以及桩的竖向承载力性状。这对软土地基中的超长PHC桩的设计、施工和深入研究具有重要指导作用。     

Centrifuge studies on the effects of existing piles on the end resistance and shaft friction of a newpile

The effects of existing piles on the vertical bearing capacity of piles of a new building were examined using vertical static loading centrifuge tests on a new pile located among existing piles. The results suggest the following conclusions: (1) Existing piles increased the total shaft resistance of the new pile with a rough surface because the existing piles restrained the soil around the new pile and the positive dilatancy of the sand increased the confining pressure of the soil. (2) The shaft resistance of the lower part of the new pile with a rough surface fell rapidly during the loading tests, regardless of whether there were existing piles or not. The diminution of shaft resistance, known as ‘friction fatigue’, was probably caused by sand particle crushing in the vicinity of the pile end. (3) For a new pile with a rough surface, existing piles did not affect the end resistance when the new pile head settlement normalized by the pile diameter, s/D_p, was less than 0.2. (4) Existing piles did not affect the shaft resistance or the end resistance of the new pile with a smooth surface. Dominance of the sliding displacement along the pile–sand contact surface engenders the extremely small variation of the confining pressure of the soil around the new pileshaft.     

打入承压水层中的管桩土塞的稳定分析

在开口管桩的打入过程中,土体进入桩内形成土塞。土塞的形成及其承载力受诸多因素的影响。综合评述不同条件下土塞的稳定理论,并以一个特殊工程为例,根据土塞的一维平衡理论和地下水渗流原理,探讨承压水条件下打桩过程中土塞的形成特点及打桩过后承压水对土塞稳定的影响。理论分析和工程实践均表明承压水能促使土塞涌动,减弱土塞的闭塞作用。     

Effects of inner sleeves on the inner frictional resistance of open-ended piles driven into sand

In open-ended piles, inner friction is developed between inner pile shaft and the inner soil. Inner frictional resistance depends largely on the degree of soil plugging, which is influenced by many factors including pile diameter, relative density and end conditions of piles. In this paper, effects of inner sleeves on inner frictional resistance are discussed. The experiments were conducted on a medium-dense sandy ground using laboratory-scale piles. It was observed that the piles penetrated under partially-plugged or unplugged state. The results suggest that inner frictional resistance, Q_in increases with sleeve height, l linearly and requires 2D (D is pile outer diameter) of l to produce a large as 50% of Q_t by Q_in (Q_t is total resistance). The results also indicate that bearing capacity increases with wall thickness at the pile tip, which can be attributed to the increase in annular area. The results also indicate that soil plug height is independent of sleeve height. The results also reveal that the penetration of straight piles is closer to unplugged state than the sleeved piles. The results of incremental filling ratio and plug length ratio also indicate that the degree of soil plugging is affected by the sleeve height.     

Seismic Bearing Capacity of Piles in Liquefiable Soils

An investigation into the base capacity of piles in passing through loose, liquefiable sand and founded in underlying dense sand is presented based on the results of a series of dynamic centrifuge tests on instrumented model pile groups. Excess pore pressures equal in magnitude to the initial effective vertical stress were observed to be generated in the bearing layer of dense sand at both shallow (15 m) and deep (26 m) depths. This induced a dramatic reduction in base capacity and consequently, large settlements of the piles by as much as ~5D₀. A spherical cavity expansion solution for base capacity was validated against measured values showing good agreement, provided that excess pore pressure and dynamic shear stiffness in the bearing layer are known. A simple closed-form relationship, applicable to end-bearing piles, between the degree of liquefaction and the initial pile static safety factor was then developed against plunging failure at the pile base which can be used in design.     

Model tests comparing the behavior of pre-bored grouted planted piles and a wished-in-place concrete pile in dense sand

The compressive bearing capacity of wished-in-place (WIP) concrete piles and pre-bored grouted planted (PGP) piles in dense sand was investigated by means of model tests. In total, three model piles were tested. The load–displacement response, axial force and tip resistance of each model pile were measured in the static load test process. Several conclusions can be drawn from the model test results: the pre-bored grouted planted nodular (PGPN) pile and the pre-bored grouted planted pipe (PGPP) pile have ultimate skin friction 1.23–1.36 times and 1.34–1.46 times greater than the ultimate skin friction of the wished-in-place (WIP) pile, respectively. The tip bearing capacity of the PGP pile is similar to the tip bearing capacity of the WIP pile, and the hyperbolic model of normalized tip resistance (q_b/q_c) and normalized tip displacement (S_b/D) can represent the tip load–displacement response of the WIP and PGP piles well.     

Behavior of strip footing on fiber-reinforced cemented sand adjacent to sheet pile wall

In urban areas, shallow foundations are often placed along the ground surface above a sheet pile wall. In this research, the potential benefits of reinforcing the active zone behind a model sheet pile wall by using polypropylene fiber and cement kiln dust have been investigated experimentally and numerically. Tests were conducted by varying parameters including fiber ratio (R_F), cement kiln dust (CKD) ratio, thickness of reinforced layer, footing location relative to the sheet pile wall and curing time of reinforced layer. Finite element computer code PLAXIS 2D foundation was used for numerical modeling. Close agreement between the experimental and numerical results was observed (maximum difference 14%). Experimental and numerical results clearly show that fiber insertion into the cemented soil causes an increase in ultimate bearing capacity of footing and significant reduction in the lateral deflection of the sheet pile wall. At higher fiber ratios (R_F ≥ 0.75%), the bearing capacity ratio (BCR) increased by about 42% and the effect of CKD ratio on BCR is more pronounced. The addition of fibers changed the brittle behavior of cemented sand to a more ductile one. Critical values of reinforcing parameters for maximum reinforcing effects are established.     

钙质砂中群桩模型试验研究

 针对取自南沙群岛永暑礁的钙质砂,利用室内模型试验研究钙质砂中群桩的承载和变形性能及其影响因素,并进行石英砂中的对比试验。试验结果表明,钙质砂由于颗粒破碎作用,导致其群桩承载和变形性能与石英砂有着显著差异。相对密度对钙质砂中群桩承载力影响较大,闭口群桩的承载力比开口群桩高17%～20%,但与石英砂相比很小,仅为石英砂的56%～71%。钙质砂中桩身轴力衰减速率缓慢,桩侧阻力值仅为石英砂的21%～30%,但具有深度效应,而且钙质砂中桩侧阻力对相对密度的变化没有石英砂敏感,受相对密度影响较小。钙质砂的承载力群桩效应系数在Dr = 46%,75%时均小于1,且随着相对密度的增加而增加,与石英砂有着本质的区别。该研究成果可为钙质砂桩基工程设计提供有益的参考。     

软土中超长水泥搅拌桩复合地基承载力研究

对软土中18m长水泥搅拌桩复合地基,进行了现场单桩静载试验、承压板下有无褥垫层的单桩复合地基载荷试验、四桩复合地基载荷试验及桩中心插12m长微型加劲钢管的单桩复合地基载荷试验,研究了复合地基破坏模式、褥垫层、桩身强度等因素对复合地基承载力的影响,对软土中超长水泥搅拌桩复合地基承载力检验方法及承载力评价提出了建议。     

用于调整差异沉降的预留净空桩筏基础模型试验研究

预留净空桩筏基础是一种新的利用桩土相互作用的形式,可用于调整建筑物筏板–桩–土相互作用和减少差异沉降。进行了砂土中常规桩筏基础和桩顶预留净空桩筏基础的模型试验研究。分析表明:在相同荷载作用下,在试验条件下,相对于常规桩筏基础来说,预留净空桩筏基础的实质作用在于可使桩间土压缩提前发生,显著提高桩间土分担荷载的比例,在可压密土中可强化桩土相互作用,桩端刺入量显著下降,使得桩土更趋于整体沉降。预留净空桩筏基础可有效利用土的承载力,减小桩顶应力过分集中,且对桩端阻力有显著的强化作用。     

土塞效应对管桩纵向动力特性的影响研究

基于附加质量法,研究考虑土塞效应时成层地基中管桩的纵向振动问题。首先,根据桩侧土、桩端土的成层性,将桩土系统沿纵向划分为有限个微元段,并采用虚土桩法和附加质量法建立了桩土系统的纵向振动控制方程。然后,运用Laplace变换技术和阻抗函数递推法,推导得到了考虑土塞效应时管桩纵向振动的桩顶速度频域响应解析解和半正弦脉冲激振力作用下相应的桩顶速度时域响应半解析解。最后,分析了附加质量法中Voigt模型参数的敏感性区间,讨论了土塞效应对管桩纵向动力特性的影响规律,并通过模型试验验证了所提理论解的合理性。结果表明：①当附加质量法中的Voigt模型参数逐渐增大时,土塞与管桩之间的相互影响将逐渐增强;②在桩顶速度时域响应曲线上,土塞顶部界面位置处会出现类似扩颈桩段的反向反射信号;③由于土塞的存在,填充土塞桩段的综合波速会小于管桩材料的一维弹性纵波波速,且土塞高度越高,填充土塞桩段的综合波速越小。     

Influence of relative density of soil on performance of fiber-reinforced soil foundations

An experimental study has been carried out for studying the influence of combinations of relative densities of two layered soil system. The model tests have been performed for the case of circular and ring footings resting on randomly distributed fiber reinforced sand (RDFS) layer overlying unreinforced sand bed. The influence of relative density on, different type of footings i.e. circular and ring (r_i/r_o = 0.3, 0.4, 0.5, 0.6) footings; percentages of fiber in RDFS layer i.e. 0.5%, 0.75%, 1.00%, and 1.25%; and thickness of RDFS layer i.e. 0.5B, 0.75B, and 1.00B have been studied. Results have indicated that relative density, of both the RDFS layer as well as the bottom unreinforced sand layer, significantly influences the ultimate bearing capacity as well as the settlement. Improvement in terms of bearing capacity ratio (BCR) is more when top RDFS layer is compacted at 70% relative density with bottom unreinforced sand having 30% relative density. Moreover, in terms of settlement reduction, maximum improvement is observed when both the layers were compacted at 70% relative density.     

Frequency- and intensity-dependent impedance functions of laterally loaded single piles in cohesionless soil

This paper investigates the frequency-dependent pile-head impedance characteristics of a model soil-pile foundation system under large amplitude loads, inducing soil yielding. Testing was conducted on a scaled single pile embedded in sand under a 1g condition. A laminar shear box mounted on a unidirectional shaking table was used to house the soil-pile foundation system. Quasi-static loads and dynamic loads were applied to obtain the force–displacement relationships and pile-head impedance functions, respectively, through the pile head connected to a loading actuator providing fixity to the pile head in all directions, except horizontal. In the quasi-static case, loads with three different velocities were applied to study the rate-dependent characteristics of the lateral bearing capacity of the pile. The Stereo-PIV system was employed to measure the surface soil displacement around the pile. The lateral bearing capacity changed with the loading velocity, but the soil near the pile showed a consistent failure pattern despite a significant change in velocity. Lateral pile-head dynamic impedance functions were obtained for low-to-high amplitude harmonic loading for a wide range of frequencies. The dynamic stiffness was seen to converge to that of the secant static stiffness with an increase in the amplitude of the dynamic loading for all the excitation frequencies.     

预应力混凝土开口管桩的极限承载力分析

预应力混凝土开口管桩在沉桩过程中存在土塞作用,因此其受力机理比闭口管桩更加复杂。根据现行规范中的经验公式以及修正公式计算单桩极限承载力时,开口管桩的计算值常常低于或高于载荷试验的结果。根据西安一高层建筑预应力开口管桩的设计和桩基检测结果,对单桩极限承载力计算公式的取值进行分析,阐述了"土塞效应"对单桩竖向极限承载力的影响,对计算公式中的侧阻力及端阻力进行修正,所得到的承载力计算值与试桩结果较为吻合。通过研究,建立了基于土塞效应下根据土的物理指标与承载力参数计算管桩承载力的修正公式。     

基于不同贯入速率砂土地基中静压敞口混凝土管桩试验研究

贯入速率对静压敞口混凝土管桩沉贯及承载力特性具有重要影响。通过砂土地基中静压敞口混凝土管桩室内模型试验，分别对不同贯入速率下模型桩土塞特性、压桩力及极限承载力变化进行研究。结果表明，模型桩贯入速率由2.0mm/min增大至3.0、4.0mm/min时，土塞高度分别降低了12.8%、27.2%，土塞率降低幅值分别为12.9%、27.2%。不同贯入速率沉桩结束后，模型桩土塞增长率分别为35.4%、23.6%及19.8%；相比于4.0mm/min贯入速率，贯入速率2.0mm/min及3.0mm/min下模型桩最终压桩力增长幅度分别为92.6%及 25.1%，极限承载力增长幅值分别为77.8%及4.5%，这主要是由于贯入过程中土塞特性差异导致的内壁摩阻力不同所引起的。研究成果可为砂土地基中敞口混凝土管桩沉桩性状及极限承载力研究提供理论依据。     

Experimental study on face instability of shield tunnel in sand

Face stability is critical for ground settlement and construction safety control in shield tunneling. In this paper, a series of 3D large-scale model tests with a tunnel of 1 m diameter were conducted in dry sand for various cover-to-diameter ratios C/D = 0.5, 1, and 2 (i.e., relative depth; C is the cover depth and D is the diameter of tunnel). Each test provided a measurement of the support pressure and the ground settlement with the advance of face displacement. The evolution of soil arching during face failure was investigated by monitoring the redistribution of earth pressure in front of the face in the test case of C/D = 2. In the displacement-controlled face failure tests in the medium density sands, the support pressure dropped steeply to the minimum value, then increased to a steady state with the continuing increase in the face displacement. Relationships between the support pressure and face displacement for various cover depths were also verified by the numerical analysis using the finite difference program, FLAC^3D (Itasca, 2005). The limit support pressure increases with the increase of the relative depth C/D and then tends to be constant. A significant rotation of principal stress axes in the upward arches in the soil during face failure was found in the tests. A two-stage failure pattern is proposed based on the observation of earth pressure. The theoretical and empirical formulas for estimating limit support pressure were verified by the tests results.     

粉砂土中PHC管桩的工作性状试验研究

采用现场标准贯入试验 ,分析了粉砂土中PHC管桩的土塞特性、不同沉桩方式对桩周土力学性能的影响。进而通过系列载荷试验 ,研究了粉砂土中PHC管桩的单桩竖向极限承载力随休止期的变化规律及时间效应。研究结果显示 ,不同沉桩方式对粉砂土中PHC管桩产生显著不同的短期时间效应 ,休止期 2 5天后单桩竖向极限承载力趋于稳定     

钢管压入土体施工挤土机制与案例分析

 对钢管压入土体过程中钢管内土塞的受力状态、产生挤土效应条件及挤土机制进行分析,结果表明：产生挤土效应的条件为土塞底部应力超过钢管端部地基承载力;根据土塞底部应力与钢管端部地基承载力的关系,不同情况下可认为钢管压入土体为非排土桩、排土桩及不完全排土桩;当为不完全排土桩时,挤土率可作为钢管压入过程中的挤土程度评价指标。对钢管内土塞底部应力与钢管端部地基承载力理论计算方法进行分析与总结。最后,通过钻孔灌注桩护壁钢管旋压施工工程案例的实测结果与实施效果,对钢管压入土体施工挤土机制理论分析的合理性与实用性进行论证。     

Design and analyses of open-ended pipe piles in cohesionless soils

Large open-ended pipe pile has been found to be advantageous for use in transportation projects. The current design method, however, is not adequately developed. To close this practice gap, this paper first summarized different design methods for open-ended pipe piles in sandy soils. A major factor for all the design codes is to properly account for the formation and effects of soil plug. The comparison indicates that there is a large variation in the base capacity evaluation among different methods due to the complex behaviors of soil plug. To close the knowledge gap, discrete element method (DEM) was used to simulate the soil plugging process and provide insight on the plugging mechanism. The simulation results show that the arching effect significantly increases the internal unit shear resistance along pipe piles. The porosity distribution and particle contact force distribution from DEM model indicate a large stress concentration occurs at the bottom of the soil plug. Besides, nearly 90% of the plug resistance is provided by the bottom half portion of the soil column. The soil-pile friction coefficient has a significant effect on the magnitude of plug resistance, with the major transition occurred for friction coefficient between 0.3 and 0.4.