Two-Dimensional Physical and Numerical Modeling of a Pile-Supported Earth Platform over Soft Soil

This paper focuses on the mechanisms occurring in a granular earth platform over soft ground improved by rigid piles. Two-dimensional physical model experiments were performed using the Schneebeli’s analogical soil to investigate the load transfer mechanisms by arching and the settlement reduction and homogenization. Experimental outputs are compared to results obtained on a numerical model using a plane strain continuum approach. The impact of the constitutive model complexity to simulate the platform material behavior was first assessed, but no large difference was recorded. As far as the proposed model, which takes the main features of the observed behavior satisfactorily into account, the numerical procedure could be validated and the parametric studies extended numerically. Both approaches of this study underlined the main geometrical and geotechnical parameters which should inevitably be taken into account in a simplified design method, namely the capping ratio, the platform height, and the platform material shear strength.     

Structural Behavior of a Pile-Supported Embankment

The stress field in a pile-supported 3.9-m-high embankment was interpreted through three-dimensional finite-element modeling, and evaluated by field measurements involving strain gauges on the piles and earth pressure cells at the top and bottom of a 0.9-m-thick geogrid-reinforced platform. Analyses of the numerical results and the experimental data suggest that a vaultlike arch developed within the embankment, such that the vertical stress at the top of the platform was concentrated above the piles and virtually no vertical stress was measured between the piles. A similar situation existed within the platform, where an almost stress-free region between the piles was experimentally detected and numerically verified. From a structural point of view, a supporting skeleton was formed from a pile extension through the platform, a type of stress diffusion problem, and an arching effect appeared mainly in the embankment due to the very large stiffness of the piles in comparison to the surrounding media.     

Numerical Modeling of Seismic Response of Rigid Foundation on Soft Soil

A numerical model was developed to simulate the response of two instrumented, centrifuge model tests on soft clay and to investigate the factors that affect the seismic ground response. The centrifuge tests simulated the behavior of a rectangular building on 30?m uniform and layered soft soils. Each test model was subjected to several earthquakelike shaking events at a centrifugal acceleration level of 80g. The applied loading involved scaled versions of an artificial western Canada earthquake and the Port Island ground motion recorded during the 1995 Kobe Earthquake. The centrifuge model was simulated with the three-dimensional finite-difference-based fast Lagrangian analysis of continua program. The results predicted with the use of nonlinear elastic–plastic model for the soil are shown to be in good agreement with measured acceleration, soil response, and structural behavior. The validated model was used to study the effect of soil layering, depth, soil–structure interaction, and embedment effects on foundation motion.     

Numerical Solution for Laterally Loaded Piles in a Two-Layer Soil Profile

Piles are often embedded in a layered soil profile, such as sand or clay layer underlain by rock. Several existing solutions are available for laterally loaded piles in a layered soil system. However, these solutions are only applicable to constant soil stiffness for each layer. In this paper, a variational approach is employed to numerically solve the problem of laterally loaded piles in layered soils using beam on an elastic foundation model. The soil stiffness can be either constant with depth or linearly varying with depth. The numerical solution is validated against an existing solution for linearly varying soil stiffness in a single soil layer system and an existing solution for a two-layer soil system with constant soil stiffness. Case studies using the proposed solution for field lateral load tests on full size drilled shafts embedded in weak rock with an overlying sand layer are presented. The simplicity and the relative ease of using the solution make it a good alternative approach for estimating the deflection and moment responses of a laterally loaded pile in a two-layer soil profile.     

Monitoring and Modeling Grout Efficiency of Lifting Structure in Soft Clay

An inclined eight-story reinforced concrete building on a thick soft clay deposit was leveled by compensation grouting with short gel time grout injected through sleeved pipes. The monitoring system is used to record the injected grout volume, the mat foundation’s heaved volume after grouting, and the mat foundation’s settled volume during pore pressure dissipation. The grouting efficiencies improved from negative value to less than one, and the stress histories of clay soils changed from normally consolidated to overconsolidated states. A final compensation efficiency of 9.78% was achieved and the building was successfully leveled. A series of numerical simulations were conducted to assess the capability of compensation grouting modeling. The numerical simulation results indicate that the consolidation behavior and the stress history of clayey foundation soils can be modeled reasonably well. However, the computed final grout efficiency is larger than that from the monitoring data because the simulation of fracture grouting by volumetric strain input is more like compaction grouting mode rather than fracture grouting mode, and this induces lesser excess pore pressure, lesser settlement and higher grout efficiency. These findings are also confirmed from other researchers’ laboratory and field testing results.     

Centrifugal and Numerical Modeling of a Reinforced Lime-Stabilized Soil Embankment on Soft Clay with Wick Drains

A centrifuge test was performed on a reinforced embankment backfilled with lime-stabilized soil on soft clay installed with wick drains. The finite-element program PLAXIS was employed to simulate the centrifuge test on the basis of the test dimensions. Verifications of the numerical model were taken by comparing the numerical results with the measurements obtained from the centrifuge test, and it was found that both were in good agreement. Three cases of unreinforced, one-layer reinforced, and two-layer reinforced embankments were simulated and compared on the basis of the numerical model. A centrifuge similarity drainage relationship was also proposed in this paper on the basis of the equal degree of consolidation between the model and prototype drains.     

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.     

Behavior of Laterally Loaded Large-Section Barrettes

A barrette is a large cross section rectangular pile. Due to dependence of the flexural stiffness of the rectangular section on its orientation and the nonlinear behavior of barrette materials, loading direction affects the lateral resistance of the barrette. Recently, full-scale lateral load tests were conducted on two barrettes in Hong Kong, one (DB1) with a cross section of 2.8 m by 0.86 m and a length of 51 m and the other (DB2) with a cross section of 2.7 m by 1.2 m and a length of approximately 30 m. This paper aims to investigate the response of laterally loaded large-section barrettes based on the load tests, to simulate the response of the two test barrettes, and to study the influence of loading direction on the lateral response of barrettes. Nonlinear p–y curves for soils and nonlinear stress–strain relations for barrette concrete and reinforcement are used to simulate the lateral response of the test barrettes considering five loading directions. The simulations were able to capture the apparently different behaviors before and after cracking of the barrette section. Sudden increases of displacement and rotation under a small lateral load increment and reduced depths of load transfer in the ground are predicted when the barrette section cracked. Based on this study, the direction of the resultant horizontal displacement is different from the loading direction if the barrette is not loaded along the major or minor axis of the cross section.     

Mechanically Stabilized Earth Wall Failure at Two Soft and Sensitive Soil Sites

Two highway bridge approaches, about 10 and 12?m in height, near Kolkata (Calcutta), India constructed with mechanically stabilized earth (MSE) failed recently. These structures were founded on sensitive, soft and compressible, fine-grained soils of the intertidal flats and backswamps of the Ganges delta. One of these MSE walls, which failed in the final stages of its construction, was constructed after foundation soils were strengthened with prefabricated vertical drain installation and preloading. The second MSE wall that failed within a month of its opening for traffic was constructed on unimproved ground. Fortunately, immediate collateral damage from these incidents was small. Using pre and postconsolidation shear strengths the MSE walls were redesigned. Reconstruction involved prefabricated vertical drain installation at the second site and construction of stabilizing berms at both locations. The facilities are now operational and appear to be performing satisfactorily. Details of the failures, postfailure investigations, and monitoring, redesign, and reconstruction are presented in this paper.     

Investigation of Load-Transfer Mechanisms in Geotechnical Earth Structures with Thin Fill Platforms Reinforced by Rigid Inclusions

The reinforcement of soft soils by rigid inclusions is a practical and economical technique for wide-span buildings and the foundations of embankments. This method consists of placing a granular layer at the top of the network of piles to reduce vertical load on the supporting soil and vertical settlement of the upper structure. The study focuses on the modeling of load-transfer mechanisms occurring in the reinforced structure located over the network of piles with a coupling between the finite-element method (geosynthetic sheets) and discrete element method (granular layer; concrete slab in some cases). The importance of granular layer thickness to increase load-transfer intensity and to reduce vertical settlement was observed. However, without a basal geosynthetic sheet, the compressibility of soft soil has a great influence on the mechanisms. A method predicting the intensity of load transfers was proposed, based on Carlsson’s solution. The main parameters concerned are the geometry of the work and the peak and residual friction angles of the granular layer.     

Nonlinear Analysis of Torsionally Loaded Piles in a Two-Layer Soil Profile

This paper presents a method for predicting the nonlinear response of torsionally loaded piles in a two-layer soil profile, such as a clay or sand layer underlain by rock. The shear modulus of the upper soil is assumed to vary linearly with depth and the shear modulus of the lower soil is assumed to vary linearly with depth and then stay constant below the pile tip. The method uses the variational principle to derive the governing differential equations of a pile in a two-layer continuum and the elastic response of the pile is then determined by solving the derived differential equations. To consider the effect of soil yielding on the behavior of piles, the soil is assumed to behave linearly elastically at small strain levels and yield when the shear stress on the pile-soil interface exceeds the corresponding maximum shear resistance. To determine the maximum pile-soil interface shear resistance, methods that are available in the literature can be used. The proposed method is verified by comparing its results with existing elastic solutions and published small-scale model pile test results. Finally, the proposed method is used to analyze two full-scale field test piles and the predictions are in reasonable agreement with the measurements.     

General Axisymmetric Active Earth Pressure by Method of Characteristics—Theory and Numerical Formulation

Active pressure for axisymmetric problem under general conditions is formulated and solved by an iterative finite difference solution of the characteristic equations in the present paper. A general lateral stress coefficient is used instead of the Haar-von Karman hypothesis and numerical modeling has also confirmed that Harr-von Karman hypothesis is a reasonable assumption for the present problem if sufficient wall movement is allowed. The shape of the failure zone and variation of active pressure with depth and wall friction are investigated. Some interesting results on the active pressures arising from the arch action are found. Principle of superposition of the effect of soil weight, surcharge, and cohesive strength is discussed. Finally, the results in the present study are applied to bore pile construction and it is demonstrated that no casing for construction is required under some conditions.     

Load-Settlement Response of Rectangular and Circular Piles in Multilayered Soil

Traditionally, analyses developed for circular piles have also been used for rectangular piles by replacing in calculations the rectangular pile with a circular pile of equivalent area. In this paper, we present a settlement analysis that applies to piles with either rectangular or circular cross sections installed in multilayered soil deposits. The analysis follows from the solution of the differential equations governing the displacements of the pile-soil system obtained using variational principles. The input parameters needed for the analysis are the pile geometry and the elastic constants of the soil and pile. Pile displacements and vertical soil displacements calculated using this analysis match well those from finite-element analysis. A parametric study highlights some important insights for rectangular and circular piles in multilayered soil. A user-friendly spreadsheet program (ALPAXL) was developed to facilitate the use of the analysis. Examples illustrate the use of the analysis in design.     

Pile Bearing Capacity Factors and Soil Crushabiity

The existence of large magnitude stresses at the tip of a bearing pile is a well known phenomenon leading to crushing of soil grains and thus affecting pile behavior. Classical foundation design calculations which assume that the soil fails in shear and neglect volume change can be safely used where stress levels or particle strengths prevent crushing, however in the case of weak grains or high foundation stresses consideration should be given to the effects of grain crushing and the resulting volumetric compression. Model pile tests have been carried out in two skeletal carbonate sands and a standard silica sand with the aim of examining the variation of skin friction and end bearing capacities with degree of penetration. The mobilization of the strength of crushable soils requires a much higher strain level while at the same time the end bearing pressure on the model piles exceeded 10?MPa inducing considerable particle breakage. The peak skin friction for all sands occurred at a settlement normalized by pile diameter, S/D, of less than 0.1. At this point the carbonate sands generally had lower skin friction values than the silica sand. Further displacement caused a rapid decrease in skin friction for all three materials. At higher lateral stresses the less crushable Toyoura silica sand generated higher skin frictions. Samples of Chiibishi sand were sectioned and photographed. It was observed that a spherical plastic zone was formed at the base of the pile which expanded with increasing S/D and a degraded layer of broken particles developed around the pile as S/D increased. Large values of the Marsal particle breakage factor were restricted to a zone extending outwards to one pile radius. An end bearing capacity modification factor has been proposed to adapt the conventional bearing capacity equation for soil crushability. This modification factor is a function of soil compressibility and degree of penetration. The factor was shown to decrease with increasing soil compressibility and increase with normalized penetration S/D.     

Influence of Underlying Weak Soil on Passive Earth Pressure in Cohesionless Deposits

Finite-element simulations demonstrate the influence of underlying weak soil on mobilization of passive pressures in cohesionless deposits. Traditional passive earth pressure theories with typical angles of interface friction may overestimate passive forces in such cases. Simple analytical models that incorporate the underlying weak soil using traditional passive earth pressure concepts are shown to agree reasonably with the finite-element simulations. The studies presented herein are relevant for cases in which cohesionless soil deposits overlie soft clay, liquefiable sand, or other weak layers.     

Numerical Analysis of T-Bar Penetration in Soft Clay

The penetration resistance of a cylindrical T-bar penetrometer in soft clay is affected by features such as anisotropy, high strain rates, and gradual strain-softening during passage of the T-bar. In order to evaluate these effects, a detailed numerical study has been undertaken, comprising: (1) finite-element analysis; and (2) a strain path approach within the upper bound plasticity mechanism. These studies showed that the T-bar factor is relatively insensitive to the degree of strength anisotropy, provided the penetration resistance is normalized by the average shear strength. Strain rates were found to be six or seven orders of magnitude greater than typical laboratory testing rates, and about three orders of magnitude higher than in a standard vane test. However, the effect of high strain rates is partly compensated by remolding of the soil, where average strains of 400% are imposed on the soil. Charts are presented showing how the separate effects of high strain rates and partial softening may be combined to derive a T-bar factor for a given soil. The paper concludes with a discussion of the measurement of remolded shear strength using cyclic T-bar tests, and interpretation of the T-bar resistance in fully remolded soil.     

Free-Field Measurements to Disclose Lateral Reaction Mechanism of Piles Subjected to Soil Movements

Soil-pile interaction remains to be the most ambiguous yet one of the most crucial aspects in the design of laterally loaded soil-pile systems subjected to embankment-induced movements. This paper proposes a new method that is capable of producing soil stiffness degradation curves, which are the outcome of real field behavior through free-field measurements. Soil-pile interaction mechanism can be solved with the proposed method for any possible case either the piles are constructed before the embankment construction or during and after. For any time considered, the method enables the computation of resultant stress effects on the pile cross section and the accompanying deflections. To provide a basis of comparison, an example problem has been solved with the proposed method and with two well-known commercial finite-element softwares. Obtained results indicated the capability of the proposed method to disclose real field behavior, which can be attributed to its inherent property of being also an observational method.     

Large-Scale Passive Earth Pressure Load-Displacement Tests and Numerical Simulation

Passive earth pressure is recorded in two different tests, using a 6.7-m long, 2.9-m wide soil container. In these tests, sand with 7% silt content is densely compacted behind a moveable test wall to a supported height of 1.68 m (5.5 ft). Lateral load is applied to the vertical reinforced concrete wall section, which displaces freely along with the adjacent backfill in the horizontal and vertical directions. The recorded passive resistance is found to increase until a peak is reached at a horizontal displacement of 2.7–3% of the supported backfill height, decreasing thereafter to a residual level. In this test configuration, a triangular failure wedge shape is observed, due to the low mobilized wall-soil friction. Backfill strength parameters are estimated based on this observed failure mechanism. From these estimates, along with triaxial and direct shear test data, theoretical predictions are compared with the measured passive resistance. Using the test data, a calibrated finite-element model is employed to produce additional load-displacement curves for a wider range of practical applications (e.g., potential bridge deck displacement during a strong earthquake). Hyperbolic model approximations of the load-displacement curves are also provided.     

Assessment of the Axial Load Response of an H Pile Driven in Multilayered Soil

Most of the current design methods for driven piles were developed for closed-ended pipe piles driven in either pure clay or clean sand. These methods are sometimes used for H piles as well, even though the axial load response of H piles is different from that of pipe piles. Furthermore, in reality, soil profiles often consist of multiple layers of soils that may contain sand, clay, silt or a mixture of these three particle sizes. Therefore, accurate prediction of the ultimate bearing capacity of H piles driven in a mixed soil is very challenging. In addition, although results of well documented load tests on pipe piles are available, the literature contains limited information on the design of H piles. Most of the current design methods for driven piles do not provide specific recommendations for H piles. In order to evaluate the static load response of an H pile, fully instrumented axial load tests were performed on an H pile (HP?310×110) driven into a multilayered soil profile consisting of soils composed of various amounts of clay, silt and sand. The base of the H pile was embedded in a very dense nonplastic silt layer overlying a clay layer. This paper presents the results of the laboratory tests performed to characterize the soil profile and of the pile load tests. It also compares the measured pile resistances with those predicted with soil property- and in situ test-based methods.