Hardening Soil Constitutive Model Research Articles

Stochastic numerical analyses to investigate the effects of the spatial nonuniformity of offshore ground on the serviceability of monopile foundations

ABSTRACT The evidence of soil nonuniformity has been reported in several site investigation reports and is well-confirmed for any soil in general. However, studies related to the spatial soil variability of offshore ground are somewhat limited, and current design practices for monopile foundations also overlook the uncertainty of the offshore ground. This paper presents the probabilistic evaluation of the serviceability limit state of monopile foundations considering the spatial variability of offshore ground. The nonuniformity of the ground is modeled utilizing the site-specific undrained shear strength (su) as a stochastic variable, using a spatially correlated Gaussian random field. A hardening soil constitutive model is utilized in 3D finite element analysis of offshore ground, with a focus on parameter calibration. The reliability of the serviceability limit state is assessed based on the allowable load on the pile head. The stochastic behavior of the maximum bending moment and maximum shear force along the depth of the monopile is also investigated. Besides, the paper also highlights the importance of incorporating ground nonuniformity during the design process of a monopile foundation.

Finite Element Analysis on Load-Settlement Behavior of Axially Loaded Pile on Sand

The analysis of geoengineering problems using various constitutive models necessitates substantial field and laboratory research, which entails a significant investment of time and money. As a result, the application of Finite Element Analysis in geoengineering projects is limited. This research focuses on numerically examining the behavior of a pile in a sandy soil layer. The study employs different relationships to estimate soil parameters from standard penetration tests and simple lab tests. These derived parameters, required for the Mohr-Coulomb & Hardening Soil constitutive model and the linear elastic model of the pile, are implemented in the PLAXIS 3D software. This software is used to simulate the settlement of the pile in sandy soil incorporating factors like mesh density and soil-pile interaction. The results of the study reveal that employing a finer mesh size leads to more precise results. Additionally, an interface strength of 67% proves to be more accurate for this study. Furthermore, the correlation introduced by Papadopoulos (1982) exhibits a close relation with field settlement values. In contrast, other correlations either overestimate or underestimate the results. Moreover, the study conclude that Mohr-Coulomb constitutive model satisfactorily predicts the behavior of the pile for the study area.

Overconsolidated flysch-type clays. Engineering considerations for the Strait of Gibraltar tunnel project

The stress-strain behaviour of 85 overconsolidated clay samples from Campo de Gibraltar Flysch Through Domain (Algeciras Unit, South Spain) is presented and discussed. The samples were identified and classified following ASTM standards while their chemical and mineralogical composition were determined by chemical and X-ray techniques. Several samples were tested under triaxial as well as oedometric conditions. Given the results, a detailed comparison was made between different theoretical constitutive models and real testing data, using the finite-elements method. The comparison indicated a good fit between experimental data and those found with finite-elements modelling when the Hardening Soil constitutive model was used. This model showed a better fit than did the Modified Cam- Clay model (historically used for modelling clayey soils), although the latter fit proved better for lower strain values (<5%) than higher ones. These results clarify this intermediate material (hard soils – weak rocks) behaviour and will help in Strait of Gibraltar tunnel project design, as these materials are widely involved in this tunnel design.

Numerical simulations of displacement piles in a tropical soil

The behavior of pile foundations under axial loading is directly influenced by the effects that its installation process induces in the surrounding soil. Consequently, the consideration of these effects is essential for the correct numerical modeling of these geotechnical structures. In the present study, numerical simulations of driven cast-in-situ piles under axial loading have been carried out using finite element analysis. Three 3.5 m long piles with diameters ranging from 114.3 to 219.1 mm were analyzed. The pile installation effects have been considered indirectly by employing two distinct approaches, both based on the concepts of cylindrical cavity expansion. The behavior of the tropical soil profile is described with the Hardening Soil constitutive model. The load-displacement response and load distribution along the pile obtained with the numerical simulations have been analyzed and compared with in-situ load tests results. In the failure conditions, both approaches accurately predicted the bearing capacity of the piles, with an average error of only 2% compared to the measured values. The results in terms of load distribution over depth were also satisfactory. The difference between measured and numerical ultimate base resistance values ranged from 0% to 30%. The good agreement between the numerical and experimental results indicates that the proposed numerical approaches have been effective in simulating the piles installation process and reinforces the importance of considering the installation effects in the numerical modeling of these geotechnical structures. Both approaches can also be used to predict the bearing capacity of displacement piles.

Assessment of the Landfill Barrier System through Numerical Analysis: Rehabilitation and Expansion of Belgrade Landfill Case Study

There are still many unmanaged landfills around the world that pose significant potential environmental problems. One of the largest unmanaged landfills in Europe, which has been used for more than 40 years to deposit waste from Serbia’s capital, Belgrade, is the Vinča landfill. A revitalization and extension of this landfill was proposed that would allow its sustainable operation in the future. The revitalization project considered building a capping layer on the surface of the current landfill, which will close it and which will serve as a liner on the bottom of the new landfill. The use of a composite system including a HDPE geomembrane is considered in the project. New landfill settlements were predicted using the FEM method utilizing a Hardening-soil constitutive model for subgrade. Both immediate settlements of subgrade caused by waste deposition and primary consolidation settlement were calculated. The results show that a substantial increase in the settlement of the geomembrane subgrade can be expected during the primary consolidation period, due to the high rate of filling compared to the permeability of the subgrade. The total settlement of the new landfill in its crown is expected to be between 2.73 and 4.52 m. The axial force in the geomembrane will not exceed the tensile strength of the membrane at any time during or after the new landfill operation.

Evaluation of the Installation Effect on the Performance of a Granular Column

The procedure of granular column installation impacts soil properties such as stress state, stiffness, and permeability in the near-field of the column. For an accurate and efficient design of relatively costly geostructures on improved subsoil with granular columns, a reliable estimation of the column installation effects on the properties of natural subsoil deposits is necessary. To achieve this goal, two phases are adopted in numerical simulations: (1) the installation phase based on the cavity expansion method using a 2D model, and (2) the construction phase in conjunction with the improved soil properties after column installation using a 3D model. The latter phase includes the construction of an embankment and the column is considered as an independent unit. The soil profile, i.e., stress and stiffness, is spatially updated from the first simulation (i.e., installation phase). In this frame, the stiffness was calculated according to a procedure suggested by the authors to determine the final stiffness based on the formulation of the Hardening Soil constitutive model. The numerical models were validated through a comparison with the recorded data of a field test obtained from the Klagenfurt site. Results of numerical analyses for the case study indicated that application of proposed methodology led to a more accurate estimate of the settlement, demonstrating that the installation effects have be taken into consideration to assure reliable evaluation of granular-column performance.

The influence of face and shield annulus pressure on tunnel liner load development

Three dimensional (3D) finite-difference analysis simulations together with rare field data from strain gauges installed in segmental lining rings at the Seattle Sound Transit Northgate Link tunnel project are examined to provide a better understanding of segmental liner loading developed during earth pressure balance shield tunneling. A simplified 3D numerical model is presented for pressure balanced mechanized tunneling, where the annulus between the shield and excavated ground is full of pressurized material, simplifying the shield shape, and the modeling of the annular gap. This simplified model is based on the assumption that the shield annulus pressure controls the tunnel convergence prior to lining installation. While the presented model simplifies the modeling of the TBM shield, it captures the important components in shield tunneling, including the jointed segmental lining, grout injection pressure, time-dependent grout hardening, and the non-liner soil behavior using the hardening soil constitutive model. The results of this study show that the final lining loads are controlled by the chamber pressure for pressure balance TBMs with pressurized material in the shield annulus. In the case study presented here, pressure drops of the chamber pressure (common in EPB tunneling due to excavation standstill), result in a significant influence on tunnel lining loads.

Numerical Investigation of Soil Arching in Dense Sand

Soil arching is relevant in various engineering applications particularly in cases where there is relative movement between soil and structure. When a portion of the soil mass yields, redistribution of stresses take place between the yielding mass and its surrounding. Several trapdoor experiments have been performed with different improvements and extensions over the past few decades to study soil arching. Numerical modeling of a trapdoor mechanism is performed in this study and the suitability of Hardening Soil constitutive model over the Mohr-Coulomb model has been studied. In the present study, extensive parametric studies were conducted to examine the extent of the zone of influence and zone of yielding. The zone of influence, beyond which there is no influence of soil arching, is bounded by straight lines to form a trapezoidal shape. The dimensions of the influence zone depend on the fill height (H) and trapdoor width (B). The two families of failure surfaces, internal and external, are described and their categorization with respect to the H/B ratio is well explained. Furthermore, the zone of yielding and the region enclosed within limits of influence and failure planes are classified as active and passive zones, considering the dilation and compression in the direction of trapdoor displacement. The equilibrium of stress transfer in soil arching that remained as an unjustified assumption until now is systematically brought out in the present study. The present study also highlights salient characteristics of the plane of equivalent settlement and critical height. The present study throws some light on the need for further understanding of arching in various field applications, such as piled embankments, tunnels, culverts, buried pipelines, excavations supported by contiguous pile walls, and slopes stabilized by a row of piles.

Static earth pressures on a pile supported excavation in Santiago gravel

Concrete soldier piles anchored in multiple levels are the most common method for supporting deep excavations in gravel. Their static design is based on limit equilibrium principles, although these procedures were originally developed for sheet piles or anchored walls on shallow excavations supporting medium-dense sand. The present study draws on the applicability of current design tools to model the static response of deep excavations in stiff gravels and their implications for design. For this purpose, the static response of an anchored pile system was evaluated with a detailed finite element model of a case study, and the results are compared with simplified hand-calculation procedures. The study unit was the 28 m deep excavation of the Beauchef Poniente building located in the fluvial deposits of the Mapocho River in Santiago. A plain strain model was developed in PLAXIS using the Hardening Soil constitutive model, with parameters determined from large-scale triaxial tests of local gravels, and the computed pile displacement were compared to actual displacement profiles measured with inclinometers. The analysis shows that the simplified procedures provide a reasonably good approximation to the computed earth pressures, internal forces on the piles, and stress in the anchors.

Dynamic behavior of NATM tunnel under harmonic loading

In modern societies, underground structures like subways, water transport and urban tunnels are using widely due to the rapid population growth and limitations of above ground spaces. Although these structures especially urban tunnels have been reported to experience less damages in comparison with above-ground structures during earthquakes, their damages have direct effects on human lives. In this study, 2D plane strain simulation of a NATM tunnel subjected to harmonic loading is employed to investigate the dynamic effects of the presence of tunnel on the ground responses and on the drift values of a tunnel section using advanced hardening soil constitutive model. The surface settlement, acceleration and drift in both tunnel section and free field are presented as time histories for different PGA and results have been discussed.

Reliability and 3D Modeling of Flexible Walls with EPS Inclusion

AbstractLengthy retaining walls are typically required in several geotechnical applications. Compacted soil is the classic backfill behind such walls, yet soil has a high unit weight that imposes large lateral loads and may lead to outsized wall dimensions. Geofoam or foam made of expanded polystyrene (EPS) can substitute the soil backfill due to its light weight and high compressibility and can reduce the lateral static loads imposed on walls. In this paper, EPS mechanical properties were measured in addition to the interface friction between this geomaterial and other materials, such as sand and concrete. A reliability analysis was conducted to develop the partial resistance factors required for the main EPS properties. Using a three-dimensional (3D) numerical analysis, the reliable EPS properties were used in the hardening soil constitutive model to simulate flexible walls with EPS inclusions of various thicknesses. Analysis outcomes were verified against measurements from a physical prototype and were...

Reduction of lateral pressures on retaining walls using geofoam inclusion

Expanded polystyrene geofoam has long been used as a compressible lightweight alternative for soil backfills to reduce lateral pressure behind retaining walls. Uncertainty in material properties and lack of design parameters are the main constraints to using local geofoam products. In this study, the main goal was to characterise geofoam properties and to calibrate a standard numerical model to capture the behaviour of rigid and flexible walls with geofoam inclusions. A laboratory-testing programme was conducted to measure the shear strength and interface properties between geofoam–geofoam, geofoam–soil, and geofoam–concrete. These measurements were then used to calibrate the numerical model in which geofoam was represented using a hardening soil constitutive model. For both wall types, outcomes were verified against results from the literature. From the main outcomes, a geofoam inclusion thickness-to-wall-height ratio (t/h) of 0.085 was sufficient to change the behaviour of the backfill soil behind rigid walls from at-rest to active conditions. In addition, an inclusion with t/h ratio of 0.065 reduced the lateral pressure behind flexible walls by 28%, and represents a borderline criterion between reduced pressure and zero pressure results.

FEM Analysis of the Unsteady Seepage during Excavation

The seepage is the main reason of many excavation accidents. Currently, most of the excavation designs ignore the seepage function or pay little attention on it. This paper focuses on the excavation’s stress and deformation influenced by the unsteady seepage, leading into the Van Genuchten seepage model and the Hardening-Soil constitutive model. And compare and analyze the different influences of flow field, stress field, displacement field and force caused by the steady and unsteady seepage

Estimation of Fill properties behind a deep excavation by back analysis

Accurate prediction of soil parameters is a challenge to braced excavation systems design, especially when the wall retains “Fill” containing artificial materials. This paper presents a case study of a braced diaphragm in situ wall supporting 14 m of Fill in Egypt with the objective of estimating representative parameters for the Fill. The wall, designed based on conservative strength and deformation parameters, was monitored using inclinometers. Back analysis is conducted to determine representative Fill parameters using the beams on elastic foundation (BEF) and finite element (FE) methods. The FE analysis is conducted using the hardening soil constitutive model, and a comparison is made with Mohr–Coulomb (MC) model. The results show that the hardening soil model can reproduce the wall movement better than MC model and the BEF method. Hence, representative parameters are estimated for the retained Fill. It is illustrated that adopting the deduced values could result in considerable savings.

The extended Kalman Filter and the unscented Kalman Filter for Material Parameter Identification with Application in Tunneling

AbstractWe present in this work the use of the extended Kalman filter (EKF) and unscented Kalman filter (UKF) for identification of constitutive material parameters with application in mechanized tunneling. Although both filters are based on the principle of recursive least squares estimation, one differs from another in terms of where approximation is made. Whereas in the EKF first‐order Taylor series expansion is used to approximate the nonlinear modeling equation, in the UKF approximation of the probability density of the state is made using a small number of well defined points. To validate the methods, we performed parameter identification of the Hardening Soil constitutive model used for describing the soil behavior in an tunnel excavation model. Both methods showed fast and stable convergence of the considered soil parameters ‐ the four parameters of the Hardening Soil model. Although the EKF requires less number of forward calculations of the numerical model, the UKF is favored since it does not require calculation of the derivatives of the observables with respect to the identifying parameters. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses

This paper is the second of a two-part series describing an investigation of the mechanical behavior of rammed aggregate piers in supporting rigid square footings. In this paper, the performances of two pier-supported footings and three isolated piers during compressive load tests were simulated using an axisymmetric finite element model and compared to experimental data. A hardening-soil constitutive model with parameters estimated from in situ and laboratory tests was used to characterize the constitutive behaviors of the pier material and the matrix soil. Pier groups were modeled as unit cells with the tributary area determined from the center-to-center spacing. Cavity expansion modeling was used to simulate the pier installation process. Verifications of the numerical model were carried out by comparing the numerical results with the data obtained from full-scale, instrumented load tests. Interpretation of the numerical results focused on the load-deformation behavior, group effect, stress concentration ratio, and the development of stresses in the matrix soil. The distributions of vertical stress underneath the pier-supported footings are also characterized.

Modeling the mechanical behavior of railway ballast using artificial neural networks

Ballast is one of the most commonly used construction materials in railway tracks. Under heavy train loads, ballast is subjected to a high stress level that is always associated with significant track deformation. Consequently, an accurate prediction of the mechanical behavior of ballast under static and dynamic loading conditions is important for the stability of railway tracks. In this paper, the feasibility of using artificial neural networks (ANNs) for modeling the mechanical behavior of railway ballast under static loading is investigated. The database used for the development of the ANN model is obtained from selected literature and comprises a series of 29 large-scale drained triaxial compression tests conducted on three types of commonly used ballast (i.e., basalt, dolomite, and granite). Predictions from the ANN model are compared with the results of experimental tests and with those obtained from the hardening-soil constitutive model in PLAXIS finite-element code. The results indicate that the ANN model is able to accurately predict the stress–strain and volume change behavior of ballast. The plastic dilation and contraction of ballast at various confining pressures and the strain-hardening and postpeak strain-softening behavior of ballast are also well simulated.Key words: ballast, modeling, neural networks, prediction, railway, triaxial tests.