Rotational Restraint of Pile Caps during Lateral Loading

A pure fixed-head (zero-rotation) condition at the top of a group of laterally loaded piles is seldom achievable in the field, even when piles are installed in a group that is “rigidly” constrained by a stiff concrete pile cap. Assuming complete fixity during design (zero rotation at the pile head) can result in underestimated values of pile-head deflection, and incorrect estimates of the magnitude and the location of maximum bending moments. A simple and practical approach is presented for estimating the moment restraint that is provided by the pile cap at the top of a pile group. The moment restraint, represented by the rotational restraint coefficient (KMθ), serves as a boundary condition for analyzing groups of laterally loaded piles. Full-scale field tests performed on two pile groups with concrete pile caps show that the proposed method for estimating rotational restraint provides results that are in good agreement with measured field performance.     

Adapting Lift-Slab Technology to Construct Submerged Pile Caps

Many techniques are used in practice to construct underwater foundations, including caissons and piled foundations. The main challenge of underwater construction is cutting off water seepage so as to always provide dry working conditions. Water seepage can be eliminated by continually pressurizing inside caissons and dewatering the intercepted space of sheet-pile cofferdams. Although these methods alleviate water seepage, they have negative aspects, including unhygienic working conditions and elevated costs. This paper presents a method to partly construct the submerged pile caps above water, sink to place, and complete work in totally dry working conditions. This method adapts the lift-slab technology to lower the pile cap to the underwater permanent position. Only activities necessary to make the pile cap monolithic with the pile group are achieved after the cap is lowered in secured dry working conditions. This method was used successfully in many bridges on the Nile River in Egypt during the last decade. The method is illustrated in this paper along with the problems encountered during construction. Finally, the method is evaluated and compared with other methods.     

朝阳大桥主墩承台大体积混凝土温控过程

大体积混凝土有害裂缝产生的主因是混凝土中胶凝材料水化热快速升高引起的温差应力与混凝土本身强度增长慢之间矛盾发展的直接结果。提前采取措施控制大体积混凝土的内、外温度及内外温差能有效预控大体积混凝土裂缝的产生,是混凝土质量控制的重点。     

Determination of Pile Base Resistance in Sands

Advances in the design of axially loaded piles are desirable because significant cost savings may result. Well-designed piles settle by amounts that are well tolerated by the superstructure and induce strains around the pile base that are far removed from failure. To investigate the development of base resistance for a given soil condition and increasing settlements, piles embedded in sand are modeled using the finite-element method with a nonlinear elastic-plastic model. Based on the load-settlement response obtained from the finite-element analysis and cone penetration resistance obtained from cavity expansion and stress rotation analyses, values of normalized base resistance, defined as base resistance divided by cone penetration resistance, are obtained. The relationship between base resistance and cone resistance is useful in the design of deep foundation using cone penetration test results. The effect of the initial coefficient of earth pressure at rest K0 on normalized base resistance values is also investigated. Several case histories, including both nondisplacement and displacement piles, are used for comparison with the theoretical results.     

Ultimate Lateral Resistance of Pile Groups in Sand

Experimental investigations on model pile groups of configuration 1 × 1, 2 × 1, 3 × 1, 2 × 2, and 3 × 2 for embedment length-to-diameter ratios L∕d = 12 and 38, spacing from 3 to 6 pile diameter, and pile friction angles δ = 20° and 31°, subjected to lateral loads, were conducted in dry Ennore sand obtained from Chennai, India. The load-displacement response, ultimate resistance, and group efficiency with spacing and number of piles in a group have been qualitatively and quantitatively investigated. Analytical methods have been proposed to predict the ultimate lateral capacity of single pile and pile groups. The proposed methods account for pile friction angle, embedment length-to-diameter ratio, the spacing of piles in a group, pile group configuration, and soil properties. These methods are capable of predicting the lateral capacity of piles and pile groups reasonably well as noted and substantiated by the comparison with the experimental results of the writers and other researchers.     

Lateral Spreading Forces on Bridge Piers and Pile Caps in Laterally Spreading Soil: Effect of Angle of Incidence

In this paper, the kinematic forces which may be applied to bridge piers or pile caps from laterally spreading surficial cohesive soil layers (nonliquefied crusts) through which they pass are considered. Such forces often represent the largest load component acting on a structure and/or foundation during liquefaction-induced lateral spreading. Both circular and square structural inclusions are considered, and particular attention is paid to the orientation of the inclusion to the direction of spreading, here defined as the angle of incidence (θ). Experimental modeling was conducted using a modified direct shearbox to simulate the spreading of kaolin past structural inclusions at various θ. Load-displacement data and particle image velocimetry analysis revealed that the ultimate load for both square and circular cases may be determined using a wedge-based upper-bound plasticity analysis. For circular sections, this ultimate load is independent of θ due to radial symmetry. The ultimate load on square sections was found to depend more significantly on θ and a simple analytical method is presented to account for this. The method suggests that the ultimate loads acting on square bridge piers or pile caps will be a maximum when the spreading soil impinges on the corners of the inclusion, at which time the ultimate load will be 19–26% larger (depending on the soil-structure interface roughness) than for spreading impinging on the edge of the inclusion. Experimental tests suggested a value of 22%. Finally, the tests support previous results suggesting that when the underlying soil is unable to carry redistributed shear stress (i.e., when it is liquefied) load-displacement curves in the crustal layers are less stiff than for typical retaining structures under static conditions. The displacement at soil yield was found to be between 20–30% of the height of the inclusion in the layer, and also depends on θ in the case of square inclusions.     

Lateral Resistance of Full-Scale Pile Cap with Gravel Backfill

A static lateral load test was performed on a full-scale 3×3 pile group driven in saturated low-plasticity silts and clays. The steel pipe piles were attached to a concrete pile cap which created a “fixed-head” end constraint. A gravel backfill was compacted in place on the backside of the cap. Lateral resistance was therefore provided by pile–soil–pile interaction, as well as base friction and passive pressure on the cap. In this case, passive resistance contributed about 40% of the total resistance. The log–spiral method provided the best agreement with measured resistance. Estimates of passive pressure computed using the Rankine method significantly underestimated the resistance while the Coulomb method overestimated resistance. The cap movement required to fully mobilize passive resistance in the gravel backfill was about 6% of the cap height. This is somewhat larger than reported in other studies likely due to the underlying clay layer. The p-multipliers developed for the free-head pile group provided reasonable estimates of the pile–soil–pile resistance for the fixed-head pile group once gaps adjacent to the pile were considered.     

Evaluation of Reliability of Platform Pile Foundations

A primary consideration and concern in development and implementation of risk assessment and management based hurricane and earthquake criteria for design and requalification of platforms in the Bay of Campeche criteria is the reliability characteristics of the pile foundations that support the platforms. Analyses of the ultimate limit state (ULS) performance characteristics of the platform pile foundations are summarized. These analyses indicated that in many cases when the capacities of the piles were based on American Petroleum Institute “static” pile capacity analysis methods, the piles were the most likely to fail elements in the platforms. Results from these analyses were in dramatic contrast with the performance of pile foundations supporting more than 250 platforms during the 100-year hurricane Roxanne (1995). Detailed analytical and field studies were performed to determine the ULS performance characteristics of pile foundations subjected to hurricane induced lateral and axial loadings. This study addressed all of the phases in the life cycle of the piles. The results of this work indicate that, in general, there can be large biases in the predicted capacities of piles that support the platforms.     

Lateral Resistance of Single Pile Located Near Geosynthetic Reinforced Slope

An extensive program of laboratory tests was carried out to study the effect of reinforcing an earth slope on the lateral behavior of a single vertical pile located near the slope. Layers of geogrid were used to reinforce a sandy slope of 1 (V):1.5 (H) made with sands of three different unit weights representing dense, medium dense, and loose relative densities. Several configurations of geogrid reinforcement with different numbers of layers, vertical spacing, and length were investigated. The experimental program also included studies of the location of pile relative to the slope crest, relative density of sand, and embedment length of pile. The results indicate that stabilizing a soil slope has a significant benefit of improving the lateral load resistance of a vertical pile. The improvement in pile lateral load was found to be strongly dependent on the number of geogrid layers, layer size, and relative density of the sand. It was also found that soil reinforcement is more effective for piles located closer to the slope crest. Based on test results, critical values are discussed and recommended.     

Experimental Investigation of Clear-Water Local Scour at Pile Groups

Experiments of local scour around pile groups are carried out under steady clear-water scour conditions. A variety of conditions including different pile group arrangements, spacing, flow rates, and sediment grain sizes are considered. In total, 112 experiments are carried out. It is observed that the scour-hole depth for some cases of pile groups increases as much as two times more than its magnitude for the case of single piles. The data from this study and some laboratory experiment data from previous works are used to derive a correction factor to predict the maximum local scour depth for the pile groups. Two well-known equations, i.e., Federal Highway Administration, Hydraulic Engineering Circular No. 18, HEC-18 (reported by Richardson and Davis in 2001) and the New Zealand pier scour equation (reported by Melville and Coleman in 2000) are considered. The prediction of scour hole based on the present correction agrees well with the observations.     

Lateral Resistance of Short Single Piles and Pile Groups Located Near Slopes

Many transmission towers, high-rise buildings, and bridges are constructed near steep slopes and are supported by large-diameter piles. These structures may be subjected to large lateral loads, such as violent winds and earthquakes. Widely used types of foundations for these structures are pier foundations, which have large diameter with high stiffness. The behavior of a pier foundation subjected to lateral loads is similar to that of a short rigid pile, because both elements seem to fail by rotation developing passive resistance on opposite faces above and below the rotation point, unlike the behavior of a long flexible pile. This paper describes the results of several numerical studies performed with a three-dimensional finite-element method (FEM) of model tests and a prototype test of a laterally loaded short pile and pier foundation located near slopes, respectively. Initially, in this paper, the results of model tests of single piles and pile groups subjected to lateral loading, in homogeneous sand with 30° slopes and horizontal ground were analyzed by the three- dimensional (3D) finite-element (FE) analyses. Furthermore, field tests of a prototype pier foundation subjected to lateral loading on a 30° slope was reported. The FE analyses were conducted to simulate these results. The main purpose of this paper is the validation of the 3D elasto–plastic FEM by comparisons with the experimental data.     

Load Resistance Factor Design of Axially Loaded Pile Based on Load Test Results

The present study proposes a procedure to determine partial factors in reliability based design format for pile foundations, considering bias as well as uncertainty in the parameters that represent soil-pile interaction. These issues are addressed using pile load-settlement test data from case studies obtained from the literature. The pile ultimate capacities are evaluated considering three different failure criteria. The uncertainties in the pile-soil interface parameters as well as pile ultimate capacity are quantified in Monte Carlo framework from the measured data by utilizing the closed form “t-z” method. Considering dead load to live load ratios as calibration points, the target reliability index is calculated based on existing code safety-checking format. The optimal partial factors are determined such that the difference between reliability index based on limit state equations expressed in terms of partial factors and target reliability index is minimum. Finally, it is observed that optimal partial factors enable rational choice of allowable load on pile foundation.     

陶瓷蓄热体阻力特性的实验研究

依据所选通道截面之间实际流体的伯努力方程提供的沿程阻力、局部阻力与截面压力降之间的关系 ,分别研究具有较小高度的陶瓷球及陶瓷蜂窝体内流体压降的影响因素及特性关系     

Dynamic Thermal Buckling of Functionally Graded Spherical Caps

In the present work, dynamic buckling behavior of clamped functionally graded spherical caps suddenly exposed to a thermal field is studied using the finite-element procedure. The material properties are graded in the thickness direction. The temperature load corresponding to a sudden jump in the maximum average displacement in the time history of the shell structure is taken as the dynamic buckling temperature. Numerical study is carried out to highlight the influences of shell geometries and material gradient index on the critical buckling temperature.     

Axisymmetric Vibrations of Reinforced Orthotropic Shallow Spherical Caps

Axisymmetric vibrations of reinforced shallow. spherical caps manufactured from orthotropic materials are considered. The closed form solution is obtained for the natural frequency of the cap with a clamped and immovable circular edge by assuming that the motion component parallel to the cap boundary plane (in plane) is negligible. Parametric studies are performed to assess the effect of various geometric and structural parameters on the natural frequency of the cap and, most importantly, to identify the most influencing parameters of the problem. From the generated data, it is concluded that the national frequency increases with increasing extensional stiffness and eccentricity of reinforcements and to a lesser extent with increasing bending stiffness of reinforcements. Other important parameters include the base circle radius and the initial rise of the cap.     

Evaluation of Pile Diameter Effect on Initial Modulus of Subgrade Reaction

This paper presents the results of a study on the effect of pile diameter on the initial modulus of subgrade reaction. A series of ambient and impact vibration tests were performed on four different diameters of cast-in-drilled-hole piles to determine the natural frequencies and damping of the soil-pile systems. The measured natural frequencies were then compared with those estimated from a numerical model. The soil springs in the numerical model were established by implementing two different concepts on initial modulus of subgrade reaction. One is based on Terzaghi’s concept in which the modulus of subgrade reaction is independent of pile diameter. The other was based on recent research suggesting that the initial modulus of subgrade reaction may be linearly proportional to pile diameter. It was found that the measured natural frequencies were in good agreement with the computed ones when the diameter-independent modulus of subgrade reaction was employed. In addition, the test results show that the damping ratio of the system varied with pile diameter from 3% for 0.4-m pile to 25% for 1.2-m pile.     

Evaluation of a Dike Damaged by Pile Driving in Soft Clay

This paper presents a case study detailing a riverbank dike damaged by pile driving in very soft clay in Shanghai, P.?R. China. Driven piles were designed to support the existing dike to be raised to a higher elevation. The subsoil mainly consisted of very soft clay with its natural moisture content greater than its liquid limit. This paper describes the phenomena of dike movement and crack development during pile driving based on field observations and instrumentation data. Cone penetration tests and vane shear tests were conducted after pile driving to investigate the slip surfaces. Degradation of soil strength was identified as the main cause for the failure of this dike. Slope stability analysis was conducted to back-calculate the degraded undrained shear strength of the clay. The results indicate that the soil strength in the disturbed area due to pile driving approached the level of its remolded strength.     

Shear Resistance of FRP RC Beams: Experimental Study

This study investigates the shear behavior of concrete beams reinforced with fiber-reinforced polymer (FRP) reinforcement. Six beams were subjected to two successive phases of testing. Half of the beams were reinforced in flexure with conventional steel reinforcement, while the other half were reinforced with glass fiber bars. Different shear span to depth ratios, ranging from 1.1 to 3.3, were analyzed in order to study the variation in the shear behavior of beams characterized by different types of shear failure. No shear reinforcement was provided in the first phase of testing, while in the second phase, just enough glass and carbon shear reinforcement was provided to enable failure due to shear. The results of these tests are presented and compared to predictions according to the design recommendations proposed by the ACI and the Institution of Structural Engineers, U.K. The results of this study show that these approaches, which are based on modifications of equations derived for steel reinforcement, underestimate the contribution of the concrete and the shear reinforcement to the total shear capacity of FRP RC beams. It is shown that both approaches can be modified to become less conservative.     

低合金高强度钢热变形阻力试验研究

低合金高强度钢是首钢当前开发板材品种的主流产品,本文借助GLEEBLE-2000热模拟实验机得到的实验数据为基础,研究了低合金高强度钢在不同温度下的热变形过程中,变形阻力与变形温度、变形程度和应变速率之间的关系,并分析了加热温度对晶粒度的影响,其结论对首钢当前板材主流产品的生产实践和进一步研发高强钢具有一定的参考价值。     

Improved Evaluation of Equivalent Top-Down Load-Displacement Curve from a Bottom-Up Pile Load Test

A modified procedure is presented in this study to evaluate the equivalent top-down load-displacement curve in a bottom-up pile load test considering elastic shortening. On the basis of the results of a parametric study on a bored pile in normally consolidated cohesive soils under undrained conditions, varying shear strength distribution and pile slenderness ratio, it was concluded that the pile shortening caused by the skin-friction component of the load in a top-down test can be related to the measured elastic shortening in a bottom-up test. A λ-factor is used to define this relationship, that is, the ratio of the top-down to bottom-up pile shortening. The factor λ = 1.0 is used for the case of a pile in soil with uniform shear strength profile, λ = 2.0 for linear profiles, 1.0<λ<2.0 for nonlinear profiles varying above linear, and λ>2.0 for nonlinear profiles varying below linear. In addition, the method suggests taking the corresponding readings of the skin-friction load component from the upward displacement curve of the top of the pile, which is a closer approximation to rigid pile displacement than the bottom when corrections for elastic pile shortening are to be applied. Assuming a fully mobilized skin-friction, a logarithmic relation for the factor λ to the normalized area under the shear strength profile was generally formulated and is limited to the assumptions on which they were derived. The suggested procedure in this study has produced the equivalent top-down load-displacement curves that are in close agreement with the measured top-down curve, as validated in the case studies.