Field Studies of Well-Instrumented Barrette in Hong Kong

A large excavated rectangular pile (barrette) with lateral earth pressure and pore-water pressure cells was successfully constructed and tested in a sequence of marine, alluvial, and weathered granite soils. A “soft” base formed beneath the bottom of the barrette permitted over 100 mm of vertical settlement, completely mobilizing the shaft friction at the barrette-soil interface. During the vertical load tests, an unusual and complex response of pore-water pressures and earth pressures at the barrette-soil interface was measured. During each vertical loading cycle (except the last one) and before interface slippage of the barrette occurred, excess positive pore-water pressures were recorded in all soil layers. Upon the initiation of slip at the barrette-soil interface, a sudden drop in the measured pore pressures as well as a substantial drop in lateral earth pressures generally resulted. Subsequent loading or unloading slippage events did not show the same dramatic behavior unless a period of consolidation∕recovery was allowed first. This implies that caution must be used in design of barrettes relying heavily on skin friction when shearing induces contractive soil behavior. The current test results indicated that the empirical uncorrected SPT-N value approach and the effective stress β-method were inconsistent.     

Effect of Soil Permeability on Centrifuge Modeling of Pile Response to Lateral Spreading

This paper presents experimental results and analysis of six model centrifuge experiments conducted on the 150?g-ton Rensselaer Polytechnic Institute centrifuge to investigate the effect of soil permeability on the response of end-bearing single piles and pile groups subjected to lateral spreading. The models were tested in a laminar box and simulate a mild infinite slope with a liquefiable sand layer on top of a nonliquefiable layer. Three fine sand models consisting of a single pile, a 3×1 pile group, and a 2×2 pile group were tested, first using water as pore fluid, and then repeated using a viscous pore fluid, hence simulating two sands of different permeability in the field. The results were dramatically different, with the three tests simulating a low permeability soil developing 3–6 times larger pile head displacements and bending moments at the end of shaking. Deformation observations of colored sand strips, as well as measurements of sustained negative excess pore pressures near the foundations in the “viscous fluid” experiments, indicated that an approximately inverted conical zone of nonliquefied soil had formed in these tests at shallow depths around the foundation, which forced the liquefied soil in the free field to apply its lateral pressure against a much larger effective foundation area. Additional p-y and limit equilibrium back-analyses support the hypothesis that the greatly increased foundation bending response observed when the soil is less pervious is due to the formation of such inverted conical volume of nonliquefied sand. This study provides evidence of the importance of soil permeability on pile foundations response during lateral spreading for cases when the liquefied deposit reaches the ground surface, and suggests that bending response may be greater in silty sands than in clean sands in the field. Moreover, the observations in this study may serve as basis for realistic practical engineering methods to evaluate pile foundations subjected to lateral spreading and pressure of liquefied soil.     

Design Charts for Piles Supporting Embankments on Soft Clay

This paper describes the development and application of design charts for piled embankment designs. It outlines the computational approach adopted, the geotechnical profiles used, and the application of the design procedure using the charts. The soil profile used for the charts is representative of a Malaysian soft clay profile, involving a more or less normally consolidated soil, with a strength and stiffness that varies linearly with depth. Such a profile is typical of the ground conditions in a variety of countries in the Southeast Asian region. The design charts address the issues of pile capacity, settlement due to embankment load, settlement due to a temporary piling construction platform, and lateral response of piles near the edge of the embankment. The charts consider variations in ground conditions, embankment height, pile length, and pile spacing. An illustrative example is given to demonstrate the use of the charts.     

Liquefaction-Induced Settlement of Pile Groups in Liquefiable and Laterally Spreading Soils

The results of a series of dynamic centrifuge tests on model pile groups in (level) liquefied and laterally spreading soil profiles are presented. The piles are axially loaded at typical working loads, which has enabled liquefaction-induced settlements of the foundations to be studied. The development of excess pore pressures within the bearing layer (dense sand) was found to lead to a reduction in pile capacity and potentially damagingly large coseismic settlements. As the excess pore pressure increased, these settlements were observed to exceed postshaking downdrag-induced settlements, which occur due to the reconsolidation of liquefied sand around the pile shaft. In resisting settlement, the pile cap was found to play an important role by compensating for the capacity lost by the piles. This was shown to be achieved by the development of dilative excess pore pressures beneath the pile cap within the underlying loose liquefied sand which provide increasing bearing capacity with settlement. The centrifuge test data show good qualitative and quantitative agreement with the limited amount of model and full-scale data currently available in the literature. The implications of settlement for the design of piled foundations to serviceability conditions in both level and sloping ground are discussed, with settlement becoming an increasingly important consideration for laterally stiffer piles. Finally, empirical relationships have been derived from the test data to relate suitable static safety factors to given increases in excess pore pressure in the bearing layer within a performance-based design framework (i.e., based on limiting displacements).     

Use of Deep Cement Mixing to Reduce Settlements at Bridge Approaches

Differential settlements between a bridge and the backfill behind the abutment have been a major problem in the construction of expressway embankments over a soft clay foundation. Deep cement mixing (DCM) columns with varying lengths were used to reduce such differential settlements along the Fu-Xia Expressway, Fujian Province, China. The performance and the feasibility of the DCM method were investigated in a trial embankment constructed prior to the actual construction. This paper presents the results of the instrumentation including total settlements, multipoint settlements, soil pressures on both the DCM columns and soil surface, pore-water pressures, and lateral movements obtained from in situ monitoring. The strength of the soil-cement from laboratory mix tests and from in situ quality control tests on DCM columns is presented in the paper. Study results indicate that DCM columns with varying lengths were a simple and effective method to reduce the total and differential settlements from a soft clay foundation at a bridge approach.     

Deformation and Pore-Water Pressure Responses of Elastic Viscoplastic Soil

The deformation and pore-water pressure responses of clayey soils are of great interest to civil engineers. In this paper, displacements and pore-water pressures of a clay subjected to the loading of a strip footing are simulated using a fully coupled finite-element (FE) consolidation method incorporated with a newly developed 3D elastic viscoplastic (EVP) model for the clay. A brief introduction to the 3D EVP model and its implementation in the FE analysis is presented. The 3D EVP model can describe the time-dependent stress-strain behavior of clayey soils, including volumetric creep. The main objective of this paper is to examine how the viscosity (or creep parameter ψ/V) of the clay affects the deformation and pore-water pressure responses of the clay. For this, the value of the creep parameter ψ/V is varied in the FE analysis. When viscous nature is taken into account, the pore-water pressure in the soil is higher than that without consideration of the viscous nature. The phenomenon of pore-water pressure increase due to creep is studied in this paper. It is found that larger creep parameter ψ/V results in higher pore-water pressure and larger deformation in the soil. The difference of the pore-water pressure due to the Mandel-Cryer effects and the creep is investigated using the FE model and discussed in this paper. In addition, a few other parameters (Poisson's ratio ν, permeability k, clay layer thickness h, and thickness h to a half footing width a ratio h∕a) are also varied to investigate their influence on deformation and pore-water pressure of the soil with creep. It is found that, the lower the permeability of soil, the higher is the pore-water pressure and the larger is the local deformation. The thickness of the soil layer also has a great influence on the pore-water pressure induced by the viscous effect. All these increased pore-water pressures result from a balance of the pore-water pressures induced by creep (and the Mandel-Cryer effects or both) and dissipated because of drainage.     

Behavior of an Atypical Embankment on Soft Soil: Field Observations and Numerical Simulation

This paper compares the behavior of an embankment with nonsymmetric geometry built on soft soil with that predicted numerically using four elastoplastic soil models. Two of these models are based on isotropic conditions (Modified Cam-Clay on its own or in association with Von Mises) and two other are derived from anisotropic conditions (Melanie on its own or conjugated with Mohr Coulomb). The performance of the models, whose parameters are derived from experimental data, is checked against triaxial tests results. For the embankment, the measured and computed displacements and excess pore pressure are compared, with the isotropic models performing best. The maximum horizontal displacements versus settlements, the change in excess pore pressure versus vertical stress, the extent of the yield domain and the contours of the effective vertical and horizontal stress increments are also examined. The numerical results are explained based on the characteristics of the numerical models, namely the size and shape of the yield surface. The embankment, despite its nonsymmetric geometry, exhibits some similarities with typical behavior.     

Nonlinear Efficiency of Bored Pile Group under Lateral Loading

A 3×3 bored pile group consisting of nine cast-in-drilled-hole reinforced concrete shafts and a comparable single-shaft were subjected to reversed cyclic, lateral head loading to investigate group interaction effects across a wide range of lateral displacements. The piles had the same diameter of d = 0.61?m and similar soil conditions; however, various equipment constraints led to two differences: (1) a fixed head (zero rotation) boundary condition for the single pile versus minor pile cap rotation in the vertical plane for the group and (2) shaft longitudinal reinforcement ratios of 1.8% for the single pile and 1% for the group piles. To enable comparisons between the test results, a calibrated model of the single pile (1.8% reinforcement) was developed and used to simulate the response of a single shaft with 1% reinforcement. Additional simulations of the pile group were performed to evaluate the effects of cap rotation on group response. By comparing the simulated responses for common conditions, i.e., 1% reinforcing ratio and zero head rotation, group efficiencies were found to range from unity at lateral displacements <0.004×d to 0.8 at small displacements ～ 0.01–0.02×d and up to 0.9 at failure (displacements >0.04×d). Hence, we find that group efficiency depends on the level of nonlinearity in the foundation system. The general group efficiency, although not its displacement-dependence, is captured by p-multipliers in the literature for reinforced concrete, fixed-head piles.     

Undrained Lateral Pile Response in Sloping Ground

Three-dimensional finite element analyses were performed to study the behavior of piles in sloping ground under undrained lateral loading conditions. Piles of different diameter and length in sloping cohesive soils of different undrained shear strength and several ground slopes were considered. Based on the results of the finite element analyses, analytical formulations are derived for the ultimate load per unit length and the initial stiffness of hyperbolic p-y curves. New p-y criteria for static loading of piles in clay are proposed, which take into account the inclination of the slope and the adhesion of the pile-slope interface. These curves are used through a commercial subgrade reaction computer code to parametrically analyze the effect of slope inclination and pile adhesion on lateral displacements and bending moments. To validate the proposed p-y curves, a number of well documented lateral load tests are analyzed. Remarkable agreement is obtained between predicted and measured responses for a wide range of soil undrained shear strength and pile diameter, length, and stiffness.     

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance (qt) and penetration pore-water pressure at the shoulder (u2). Parametric studies for normalized cone tip resistance (Q = qcnet/σv0′) and normalized excess pressures (Δu2/σv0′) as a function of overconsolidation ratio (OCR = σvy′/σv0′) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter (Bq = Δu2/qcnet). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δu2/σv0′ space is superior to Q–Bq space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and Δu2/σv0′ be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling.     

黏土中静压沉桩离心模型

采用西澳大学室内鼓轮式离心机,在预先固结的高岭黏土中开展不同离心力场(50g,125g及250g,g为重力加速度)条件下的模型压桩试验、T-bar试验和静力触探试验,分析了模型桩在贯入过程、静置稳定过程中桩身径向应力(σ_r)的变化规律,并对后期桩体拉伸载荷阶段的径向应力变化值(Δσ_r)及桩侧摩阻力变化情况行了探讨,揭示了在不同超固结比(OCRs)黏土中静压桩侧摩阻力的演变特性.在此基础上,通过两种经验公式方法对桩侧摩承载力进行了预测计算和对比分析.研究结果表明:沉桩过程中桩端相对高度(h/B)对桩身径向应力的发展变化有很大的影响,桩身不同位置(h/B)的总径向应力对同一贯入深度而言,存在桩侧径向应力退化现象;基于静力触探试验提出的经验方法,能有效考虑静力触探锥端阻力(q_t)和桩端相对高度(h/B)因素的影响,将其应用于黏土沉桩时桩侧摩阻力的预测,可取得与试验实测结果较吻合的结果.研究成果对软土地区静压桩施工与承载力设计具有一定的工程指导意义.      

Effect of Installation Method on External Shaft Friction of Caissons in Soft Clay

The influence of the installation method on the soil flow pattern, resulting external radial total stress changes, and final external shaft friction after consolidation has been investigated for caissons in soft clay by means of centrifuge model tests, large deformation finite-element (FE) analysis, and a simple cavity expansion approach. Both the centrifuge measurements and the FE results show that more soil is forced into the caisson under suction than under jacking. However, the difference in the resulting external radial total stress changes or penetration-induced excess pore-water pressure is much less significant, since the expansion-induced excess pore pressure is smaller for thin-walled caissons than for driven piles. After subsequent consolidation, the influence of the installation method reduces further, and the final shaft friction ratios are close for the two installation methods. Based on the magnitude of heave ratios derived from the centrifuge measurements and the FE analysis, a simple form of cavity expansion approach can reasonably estimate external radial stress changes during installation and after consolidation, and final shaft friction ratios for the caissons. An approach for estimating the external shaft friction ratios for vertical pullout of sealed caissons is proposed.     

Behavior of Slender Piles Subject to Free-Field Lateral Soil Movement

Soil movements associated with slope instability induce shear forces and bending moments in stabilizing piles that vary with the buildup of passive pile resistance. For such free-field lateral soil movements, stress development along the pile element is a function of the relative displacement between the soil and the pile. To investigate the effects of relative soil-pile displacement on pile response, large-scale load tests were performed on relatively slender, drilled, composite pile elements (cementitious grout with centered steel reinforcing bar). The piles were installed through a shear box into stable soil and then loaded by lateral translation of the shear box. The load tests included two pile diameters (nominal 115 and 178?mm) and three cohesive soil types (loess, glacial till, and weathered shale). Instrumentation indicated the relative soil-pile displacements and the pile response to the loads that developed along the piles. Using the experimental results, an analysis approach was evaluated using soil p-y curves derived from laboratory undrained shear strength tests. The test piles and analyses helped characterize behavioral stages of the composite pile elements at loads up to pile section failure and also provided a unique dataset to evaluate the lateral response analysis method for its applicability to slender piles.     

Observed Performance of Long Steel H-Piles Jacked into Sandy Soils

Full-scale field tests were performed to study the behavior of two steel H-piles jacked into dense sandy soils. The maximum embedded length of the test piles was over 40?m and the maximum jacking force used was in excess of 7,000?kN. The test piles were heavily instrumented with strain gauges along their shafts to measure the load transfer mechanisms during jacking and the subsequent period of static load tests. Piezometers were installed in the vicinity of the piles to monitor the pore pressure responses at different depths. The time effect and the effect of installation of adjacent piles were also investigated in this study. The test results indicated that, although both piles were founded on stiff sandy strata, most of the pile capacity was carried by shaft resistance rather than base resistance. This observation implies that the design concept that piles in dense sandy soils have very large base capacity and small shaft resistance is likely to be inappropriate for jacked piles. It was also found that the variation in pore pressures induced by pile jacking was closely associated with the progress of pile penetration; the pore pressure measured by each piezometer reached a maximum when the pile tip arrived at the piezometer level. A nearby pile jacking was able to produce large tensile stresses dominating in the major portion of an installed pile; both the magnitude and distribution of the induced stresses were related to the penetration depth of the installing pile.     

Soil–Pile Response to Blast-Induced Lateral Spreading. I: Field Test

Two full-scale experiments using controlled blasting were conducted in the Port of Tokachi on Hokkaido Island, Japan, to assess the behavior of a single pile, a four-pile group, and a nine-pile group subjected to lateral spreading. The test piles were extensively instrumented with strain gauges to measure the distribution of bending moment during lateral spreading which allowed the backcalculation of the loading conditions, as well as the assessment of damage and performance of the piles. Based on the test results, it was concluded that using controlled blasting successfully liquefied the soil, and subsequently induced lateral spreading in the 4–6% surface slope test beds. The free-field soil displacements in the vicinity of the test piles were over 40 cm for both tests. When compared with the results from the single pile case, the effect of pile head restraint from the pile cap improved overall pile performance by decreasing the displacement of the pile groups and lowering the maximum moments in individual piles within each group. Finally, backcalculated soil reactions indicated that the liquefied soil layer imparted insignificant force to the piles. In the companion to this paper (Part II), an assessment of the potential of using the p–y analysis method for single piles and pile groups subjected to lateral spreading is presented.     

Anisotropic Strength Evaluation of Clay Reinforced with Grout Piles

Grout piles are often used to reinforce the base soil against base heave when carrying out deep excavations in soft clay. However, there is still a lack of an adequate criterion to describe the shear strength of clay reinforced with grout piles. In general, the anisotropic strength characteristic of clay reinforced with grout piles is more significant than that of clay. The objective of this work is to develop an anisotropic strength criterion for the reinforced soil mass. Only four parameters are needed in this anisotropic strength criterion: two are the strength properties of the in situ clay, namely, the axial compressive and axial extensive undrained shear strengths; another is the undrained shear strength of treated soil; and the final is the improvement ratio which is related to the spacing and layout pattern of the grout piles. To be used in two-dimensional undrained stability analysis, the suitability of this anisotropic strength criterion under plane strain conditions is verified by comparing the results with true triaxial test. The maximum difference between the calculated and laboratory measured shear strengths is less than 8%. The results of this study indicate that the anisotropic undrained shear strength of clay reinforced with grout piles under plane strain condition decreases with an increase in the angle between the vertical direction and the major principal stress and decreases with a decrease in the strength anisotropy ratio of clay reinforced with grout piles. However, there will be a greater improvement in the effect if the grout piles are installed in the active zone rather than in the passive zone. This is because the shear strength of a grout pile mobilized in the active zone is close to its maximum level.     

Analytical and Numerical Modeling of Soft Soil Stabilized by Prefabricated Vertical Drains Incorporating Vacuum Preloading

This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed (trapezoidal) vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data.     

Pile Responses Caused by Tunneling

In this paper, a two-stage approach is used to analyze the lateral and axial responses of piles caused by tunneling. First, free-field soil movements are estimated based on an analytical method, and, second, these estimated soil movements are imposed on the pile in simplified boundary element analyses to compute the pile responses. Through a parametric study, it is shown that the influence of tunneling on pile response depends on a number of factors, including tunnel geometry, ground loss ratio, soil strength, pile diameter, and ratio of pile length to tunnel cover depth. Simple design charts are presented for estimating maximum pile responses and may be used in practice to assess the behavior of existing piles adjacent to tunneling operations. A published case history has been studied in which the measured lateral pile deflections are compared with those computed using the present method and fair agreement is found.     

Group Interaction Effects on Laterally Loaded Piles in Clay

This paper presents the results of static lateral load tests carried out on 1×2, 2×2, 1×4, and 3×3 model pile groups embedded in soft clay. Tests were carried out on piles with length to diameter ratios of 15, 30, and 40 and three to nine pile diameter spacing. The effects of pile spacing, number of piles, embedment length, and configuration on pile-group interaction were investigated. Group efficiency, critical spacing, and p multipliers were evaluated from the experimental study. The experimental results have been compared with those obtained from the program GROUP. It has been found that the lateral capacity of piles in 3×3 group at three diameter spacing is about 40% less than that of the single pile. Group interaction causes 20% increase in the maximum bending moment in piles of the groups with three diameter spacing in comparison to the single pile. Results indicate substantial difference in p multipliers of the corresponding rows of the linear and square pile groups. The predicted field group behavior is in good agreement with the actual field test results reported in the literature.