LEAP-2017: Comparison of the Type-B Numerical Simulations with Centrifuge Test Results

This paper presents comparisons of 11 sets of Type-B numerical simulations with the results of a selected set of centrifuge tests conducted in the LEAP-2017 project. Time histories of accelerations, excess pore water pressures, and lateral displacement of the ground surface are compared to the results of nine centrifuge tests. A number of numerical simulations showed trends similar to those observed in the experiments. While achieving a close match to all measured responses (accelerations, pore pressures, and displacements) is quite challenging, the numerical simulations show promising capabilities that can be further improved with the availability of additional high-quality experimental results

Mechanisms of shear strain accumulation in laboratory experiments on sands exhibiting cyclic mobility behavior

The factors and mechanisms controlling the accumulation of shear strains of clean uniform sands exhibiting cyclic mobility behavior under level-ground conditions are examined. This phenomenon is investigated through a series of constantvolume cyclic direct simple shear (DSS) tests subjected to uniform and irregular loading conditions, and undrained cyclic element tests collected from the literature. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and effective overburden stress. Mechanisms of shear strain accumulation are investigated by decoupling the shear strain developed in each loading cycle in two components: g0, developed at nearzero effective stress, and gd, developed during dilation. Results show that g0 mostly depends on the shear strain history, while gd depends on the cyclic stress amplitude and relative density. These dependencies of gd and g0 are used to provide an explanation for the gradual decrease of the rate of shear strain accumulation that is observed while increasing the number of posttriggering loading cycles in tests performed on dense specimens

A constitutive model for clays and plastic silts in plane-strain earthquake engineering applications

A plasticity model for representing clays and plastic silts, as opposed to purely nonplastic silts or sand, in geotechnical earthquake engineering applications is presented. The PM4Silt model builds on the framework of the stress-ratio controlled, critical state based, bounding surface plasticity PM4Sand model, and is coded as a user defined material for use with the program FLAC. The model was developed to provide reasonable approximations of monotonic undrained shear strength, cyclic undrained shear strength, and shear modulus reduction and hysteretic damping responses. The model does not include a cap, and therefore is not suited for simulating consolidation or reconsolidation settlements (i.e., volumetric strains) or strength evolution with consolidation stress or seismic loading history. The primary input parameters are the undrained shear strength ratio (or undrained shear strength), the shear modulus coefficient, and the contraction rate parameter. All secondary input parameters are assigned default values based on a default calibration, but may be adjusted when calibrating against advanced laboratory test data or performing sensitivity studies. The calibration process is described and illustrated by calibrations for three different normally consolidated, fine-grained soils with plasticity indices ranging from 4 to 20. The model is shown to provide reasonable approximations of behaviors important to many earthquake-engineering applications and to be relatively easy to calibrate

A constitutive model for clays and plastic silts in plane-strain earthquake engineering applications

A plasticity model for representing clays and plastic silts, as opposed to purely nonplastic silts or sand, in geotechnical earthquake engineering applications is presented. The PM4Silt model builds on the framework of the stress-ratio controlled, critical state based, bounding surface plasticity PM4Sand model, and is coded as a user defined material for use with the program FLAC. The model was developed to provide reasonable approximations of monotonic undrained shear strength, cyclic undrained shear strength, and shear modulus reduction and hysteretic damping responses. The model does not include a cap, and therefore is not suited for simulating consolidation or reconsolidation settlements (i.e., volumetric strains) or strength evolution with consolidation stress or seismic loading history. The primary input parameters are the undrained shear strength ratio (or undrained shear strength), the shear modulus coefficient, and the contraction rate parameter. All secondary input parameters are assigned default values based on a default calibration, but may be adjusted when calibrating against advanced laboratory test data or performing sensitivity studies. The calibration process is described and illustrated by calibrations for three different normally consolidated, fine-grained soils with plasticity indices ranging from 4 to 20. The model is shown to provide reasonable approximations of behaviors important to many earthquake-engineering applications and to be relatively easy to calibrate

Lateral Thrust Distribution of Column-Supported Embankments for Limiting Cases of Lateral Spreading

Lateral spreading analysis of column-supported embankments (CSEs) requires an understanding of lateral thrust distribution. This includes quantifying the portion of thrust that is resisted by tension in geosynthetic reinforcements installed in the load transfer platform. Results from a three-dimensional (3D) numerical parametric study using a half-embankment domain and totaling 140 scenarios are presented in terms of lateral thrust distribution. Forces examined include the lateral thrusts in the embankment and foundation soil, the geosynthetic tension, and the base shear at depth, and results are presented for the limiting cases of lateral spreading (i.e., undrained end-of-construction and long-term dissipated). Results show that lateral thrusts induced by embankment loading are significant in the embankment, foundation soil, and base shear beneath the columns. However, the portion of lateral thrust carried by the geosynthetic is limited, though it increases with the geosynthetic stiffness. Results also indicate that lateral spreading in CSEs is more critical at the undrained end-of-construction condition than in the long-term condition after excess pore water pressures have dissipated. Correlations for the thrust distribution at these limiting conditions and different embankment locations (i.e., centerline, shoulder, and toe) are provided

Centrifuge Model Tests of Liquefaction-Induced Downdrag on Piles in Uniform Liquefiable Deposits

Earthquake-induced soil liquefaction can cause settlement around piles, which can translate to negative skin friction and the development of drag load and settlement of the piles. A series of centrifuge model tests were performed to assess liquefaction-induced downdrag and understand the interplay and effects of (1) pile embedment and pile-head load, (2) excess pore pressure generation and dissipation, and (3) reconsolidation and ground settlement on pile response during and postshaking. The model included a layered soil profile (clay, liquefiable sand, and dense sand) with two 635-mm-diameter instrumented piles. One pile was placed with its tip at the bottom of the liquefiable deposit; the other pile was embedded five diameters into the dense sand layer. The model was shaken with multiple earthquake motions with their peak horizontal accelerations ranging from 0.025 to 0.4 g. For each shaking event, the drag load on the piles first decreased during shaking and then increased during reconsolidation, exceeding its preshaking value. With multiple shaking events, the net drag load on the piles increased. The maximum observed drag load was found equal to the drained interface shear strength calculated from the interface friction angle of δ=30° and a lateral stress coefficient of K=1. Larger drag loads and smaller settlements were observed for the pile embedded deep in the dense sand layer. Most of the pile settlements occurred during shaking; postshaking pile settlement was less than 2% of the pile's diameter. The mechanisms behind the development of liquefaction-induced drag load on piles and settlements are described. Select ramifications concerning the design of piles in liquefiable soils are also described

Numerical Modeling of Liquefaction-Induced Downdrag: Validation against Centrifuge Model Tests

Earthquake-induced soil liquefaction can cause soil settlement around piles, resulting in drag load and pile settlement after shaking stops. Estimating the axial load distribution and pile settlement is important for designing and evaluating the performance of axially loaded piles in liquefiable soils. Commonly used neutral plane solution methods model the liquefiable layer as an equivalent consolidating clay layer without considering the sequencing and pattern of excess pore pressure dissipation and soil settlement. Moreover, changes in the pile shaft and the tip resistance due to excess pore pressures are ignored. A TzQzLiq numerical model was developed using the existing TzLiq material and the new QzLiq material for modeling liquefaction-induced downdrag on piles. The model accounts for the change in the pile's shaft and tip capacity as free-field excess pore pressures develop or dissipate in soil. The developed numerical model was validated against data from a series of large centrifuge model tests, and the procedure for obtaining the necessary information and data from those is described. Additionally, a sensitivity study on TzLiq and QzLiq material properties was performed to study their effect on the developed drag load and pile settlement. Analysis results show that the proposed numerical model can reasonably predict the time histories of axial load distribution and settlement of axially loaded piles in liquefiable soils both during and postshaking

3D Numerical Analyses of Column-Supported Embankments: Failure Heights, Failure Modes, and Deformations

Design of column-supported embankments (CSE) requires the evaluation of global stability using the conventional limit equilibrium method (LEM). Yet, for CSEs using unreinforced concrete columns and load transferring geogrids, the failure mechanisms and corresponding soil-structure interactions are not well understood. There is increasing evidence pointing to large bending moments in columns and failure of columns in flexure, as opposed to a failure by shear as assumed in limit equilibrium analyses. In response to these design uncertainties, the failure height, failure mode, and deformations of eight column-supported embankment scenarios were investigated using three-dimensional (3D) numerical analyses. For the same embankment scenarios at failure height, factors of safety (FS) were then calculated using the two-dimensional (2D) LEM for investigating its applicability in evaluating global stability of CSEs. The 3D numerical analyses examined CSE stability for the limiting conditions at undrained end-of-construction and after long-term dissipation of excess pore water pressures. The numerical model included representations of flexural tensile failure in the concrete columns and tensile failure in the geosynthetic reinforcement. Scenarios consisted of a base case with typical concrete column design, five single-parameter variations using base case conditions, and two multiparameter variations using base case conditions. The undrained condition was the most critical, and two failure modes were found: (1) multisurface shearing in the embankment coupled with bending failure of columns and near-circular shear failure in the clay, and (2) multisurface shearing in the embankment coupled with bending failure of columns and shearing in the upper portion of the soft foundation clay. Both failure modes were accompanied by a rupture of the geosynthetic when included in the load transfer platform. Soil-column interactions were complex, and many columns failed in bending at lower embankment heights than those that produced collapse. The factors of safety calculated using the LEM were overstated. This is because the LEM assumes failure by shear, which has limited applicability for examining the complex mechanisms by which CSEs fail. The practical implication is that the LEM should not be used for evaluating global stability of this system type and, by extension, other system types in which soil-structure interactions result in failures controlled by mechanisms other than shear