International Journal of Geomechanics

A multiarch culvert embankment is a new type of filling structure for which several precast arch culverts are installed continuously in the direction of the road extension. The key points in the design are to estimate the practical, optimal spacing between installed arch culverts and to clarify the interactive seismic behavior of the filling material and the culvert structure. In the current study, first, dynamic centrifuge model tests and a numerical analysis were carried out to clarify the basal earthquake behavior of the structure and verify the numerical approach. Then, the full-scale numerical analysis was performed to investigate the influence of spacing between multiarch culverts and the mechanical behavior under seismic conditions. From the results, it is confirmed that when the unit spacing is narrow, the whole rigidity of the ground and the arch culvert increases relatively. This is because the volume in the fill part, where the rigidity is small, decreases comparatively. Hence, the section force and the deformation are controlled.

This paper mainly focuses on numerical implementations in slope instability computations through three simplified numerical procedures, i.e., FEM, mesh-free method (MFM), and spectral-element method (SEM), in one sample problem that was originally solved by the FEM. This work validates all three numerical procedures through comparison of the results with the sample problem, and also briefly describes all three numerical procedures and their scopes and limitations. Among the three procedures, the SEM-based procedure is found to be more effective in handling simple to complex problems of a small- to large-scale size because of its effective computational capacity as well as higher degree of work accuracy. For this, a newly released open-source program has been used along with two other programming codes developed in FEM and MFM platforms. This work also presents a sample simulation of large-scale slope stability using the $h$-refinement (i.e., meshing), $p$-refinement (i.e., mapping), and $hp$-refinement (i.e., meshing and mapping) technique of the SEM approach in different instability conditions, such as soil saturation and pseudostatic seismic loading. Furthermore, the SEM procedure can be effectively applied to slope stability modeling for numerical stability and accuracy in small- to large-scale mountain failure.

In this paper, liquefaction-triggering potential of soil is evaluated in terms of probability of liquefaction ($P_L$) using a first-order reliability method (FORM). First, an empirical model for determining the cyclic resistance ratio (CRR) of the soil was developed using multigene genetic programming (MGGP), an evolutionary artificial intelligence technique, based on the postliquefaction cone penetration test (CPT) data. This developed resistance model, along with the existing cyclic stress ratio (CSR) model, formed a limit-state function for a reliability-based approach to liquefaction-triggering analysis. The model uncertainty of the developed limit-state function was determined through an extensive reliability analysis following a trial-and-error approach, using Bayesian mapping functions that were calibrated with actual liquefaction field-performance observations of a high-quality, postliquefaction case-history database. A deterministic model with a mapping function relating $P_L$ and factor of safety against liquefaction ($F_s$) also was developed for use in the absence of parameter uncertainties. An example is presented to compare the present MGGP-based reliability method with the available ANN-based reliability method.

Based on the unit-cell concept, analytical solutions are presented to predict deformation behaviors of geotextile-encased stone columns at any depth below the top plane of the columns. Under vertical loads at the tops of the stone columns, an axial compression deformation occurs that is often accompanied by a lateral expansion near the top. This deformation characteristic of stone columns was incorporated directly into the proposed analytical method. The shear stress between the encased stone column and the surrounding soil in the vertical direction also was taken into account. In this method, the confining pressure provided by the soil was analyzed based on an analogy with passive earth pressure. The method was verified via comparison with two other analytical solutions. Parametric studies were conducted to investigate the effects of geotextile encasement, vertical applied stress, and column spacing and diameter on the deformation behaviors of columns. The results indicated that geotextile encasement with higher stiffness has a beneficial effect on reducing column bulging and settlement. Moreover, selection of geotextile stiffness for encased stone columns should be done in relation to column diameter and spacing because increased column diameter and decreased spacing have a great effect on settlement reduction.

This paper reports results from centrifuge model tests that provide insight into the behavior of rectangular foundations (representing earthmoving equipment) during penetration, with freedom in rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). A parametric study was used to explore the relevant range of the setback ratio, slope height ratio, normalized clay strength and its nonhomogeneity, and foundation base geometry. The effect of these nondimensional parameters on the penetration resistance profile is discussed in the context of the likelihood and severity of failure. The failure of a flat-based rectangular foundation was less severe than that associated with a strip foundation, and the corresponding measures reduced with the increasing setback ratio ($\lambda$) and slope height ratio ($\eta$). No rotational failure occurred for $\lambda /{\left(s_{u\text{,}\mathit{av}}/{\gamma }_{c}B\right)}_{\mathit{cr}}>8$ (where $s_{u\text{,}\mathit{av}}/{\gamma }_{c}B$ is the average normalized strength of the lower clay layer) or $\eta \ge 0.604$, regardless of strength nonhomogeneity and the normalized strength of the bottom clay layer. The addition of small skirts to the base of the rectangular foundation assisted in reducing the likelihood and severity of failure significantly, with the exception of $\lambda =0.19$ and $\eta =0.264$. Expressions were proposed for calculating the modified bearing capacity factors.

This paper reports the three-dimensional (3D) nonlinear dynamic analysis of floating pile groups subjected to seismic loading. For the nonlinear dynamic analysis of foundations in general, and pile foundations in particular, a perfect accuracy in modeling the frequency dependence of the unbounded soil support, as well as the local temporal nonlinearities, was ensured. In the current work, the finite elements (FEs) and boundary FEs were coupled in a hybrid domain of alternating frequency-time domains, to analyze the nonlinear dynamics of pile foundations in a 3D space. The frequency dependence of the stiffness and the damping of the unbounded far-field soil were completely accounted for by the consistent infinitesimal finite-element cell method (CIFECM) in the frequency domain, whereas the nonlinearities and transient conditions of the near-field were accounted for by the FE in the time domain. The hybrid frequency time domain algorithm is used for coupling the two domains. For the evaluation of nonlinear effects, an advanced, plasticity-based, hierarchical single surface (HISS) soil model was used. The applicability of this rigorous approach to pile foundations was validated using well-established results in the literature. The transient dynamic response of a $2\times 2$ pile group embedded in clayey soils was performed. Using the results, the frequency response of the pile foundations is presented under elastic and inelastic soil conditions in terms of dynamic impedances and kinematic interaction factors for different pile spacings. The results showed significant reduction, caused by soil plasticity, in both the lateral stiffness and the damping of the pile groups.

In this paper, predictions of pile settlement determined by appropriate superposition of two-pile interaction factors are compared with those computed from continuum analysis. A finite-element package is used in conjunction with the nonlinear Hardening Soil model and the pile group sizes studied are larger than in previous research. The study is presented in two phases: (1) examination of the modulus variation within the group allowing the elimination of some variables, and (2) comparison of pile settlement predictions in the context of the significant variables. The research shows that predictions using the former approach match the continuum analyses very closely for friction piles and reasonably well for end-bearing piles, and show potential for reducing computational time and effort in such problems. The paper also distinguishes between alternative definitions of interaction factor and shows that most accurate predictions arise from interaction factors calculated when only one of the two piles is loaded.

Mobilization of negative skin friction (NSF) along a pile shaft increases the axial load (dragload) and pile head settlement (downdrag) of the pile, which may exceed the structural capacity of the pile and violate its serviceability performance, respectively. Therefore, the dragload estimation is essential for a pile embedded into a settling ground, where NSF is very likely to occur. This paper presents the results of a series of fully coupled consolidation numerical parametric studies of a pile embedded into a consolidating ground. The investigation focuses on the influences of pile geometry, ground compressibility, and loading condition on pile responses. The soil compressibility and time to apply the pile head load are found to have little influence on the degree of skin-friction mobilization in the long term. The long-term neutral plane (NP) lies within the 60–70% range of the pile embedded length when the pile head load is absent. The presence of a head load shifts NP upward, and the amount of upshift increases with decreasing pile diameter and length. A simple design chart is proposed to estimate the skin-friction distribution in the long term in cases where the pile head load is absent. Finally, an illustrative example is provided to demonstrate how the chart is used and performs. The study offers practicing engineers a simple and quick approach to estimating the dragload of a pile subject to NSF.

Traditionally, deflection data from a falling weight deflectometer (FWD) test and thickness data from pavement coring are used to backcalculate layer moduli. In this study, instead of coring, a neural network (NN) model is developed to determine layer thickness from FWD time-deflection histories. Using the NN predicted thicknesses, layer moduli are backcalculated using a commercially available backcalculation software. For validation, backcalculated moduli are compared with the laboratory determined moduli. Results show that backcalculated moduli are nearly equal to the laboratory moduli. Thus, the inclusion of NN-based thickness data has the potential to replace pavement coring, which is very expensive, and/or to enable a backcalculation method to run with a reasonable assumption of thickness whenever coring information is not available.

This research introduces a simple method for preliminary evaluation of the elastic settlement of piled foundations resting on rock. Despite a significant number of publications in this area, the present effort is distinguished by adopting a unique representation of rock nonhomogeneity. It also considers pile–subgrade stiffness ratios that have not been covered in the literature for piled foundations on nonhomogeneous media. The settlement behavior was investigated using an extensive three-dimensional finite-element (FE) analysis of different piled foundation models. Elastic settlement of single piles was also studied for comparison with the pile-group behavior. Charts were developed to estimate the elastic settlement of piled foundations and to help in choosing the optimum pile spacing and length that limit foundation settlement to tolerable levels. Settlement values yielded from the FE analysis were compared with those calculated from equations commonly used in estimating elastic settlement of foundations.

An approach to finding the optimal layout of prefabricated vertical drains (PVDs) in soft soil treatment, which has the minimum amount of PVDs and meets the requirements for the degree of consolidation, is proposed. Because explicit functions for settlement and consolidation in a multilayer ground for PVDs are not available, the current study uses the differential evolution method. The results show a stable relation between the optimal value with elapsed time, degree of consolidation, and embankment height.

The ultimate bearing capacity of a strip footing on soil reinforced by a trench has been studied in the framework of limit analysis. Existing contributions were reviewed and were compared with numerical results predicted by two methods. The main finding was how the numerical tools performed in predicting closed lower-bound (LB) and upper-bound (UB) solutions of the ultimate bearing capacity of a strip footing on soil reinforced by a trench, for four cases of reinforcement. As a result, by analytical methods, the gap between the LB and UB solutions becomes significant as the difference in shear strength characteristics of the reinforcing material and initial soil increases. Assessment of the suggested quasi-exact solutions of the ultimate bearing capacity of a strip footing on soil reinforced by a trench is strongly recommended.

Stability charts for soil slopes, first produced in the first half of the twentieth century, continue to be used extensively as design tools, and draw the attention of many investigators. This paper uses finite-element upper and lower bound limit analysis to assess the short-term stability of slopes in which the slope material and subgrade foundation material have two distinctly different undrained strengths. The stability charts are proposed, and the exact theoretical solutions are bracketed to within 4.2% or better. In addition, results from the limit-equilibrium method (LEM) have been used for comparison. Differences of up to 20% were found between the numerical limit analysis and LEM solutions. It also shown that the LEM sometimes leads to errors, although it is widely used in practice for slope stability assessments.

The analysis of seepage behavior through a rock mass is vital to the design of any underground oil storage project. A sufficient amount of water pressure is necessary for the hydraulic confinement of the stored product in unlined caverns in order to prevent any leakage to the outside environment. This water is supplied through a network of water curtain tunnels and boreholes located above the caverns some distance away. However, excessive leakage inside caverns is controlled by carrying out grouting of rock fissures such that seepage is brought under acceptable limits. Most flow studies of fractured rock mass are carried out using continuous models, taking into account the effect of the essential rock mass features in order to develop a firsthand idea of the prevailing seepage conditions around the excavated boundary. This paper presents the results of simplified numerical studies that were carried out to analyze seepage variations around the storage cavern under changing hydraulic conditions. A case study through a hydrogeologically sensitive zone was also analyzed during this study, where actual seepage measurements inside large caverns were compared with the model values. It was observed from the analyses that the hydraulic conductivity and thickness of the grouted rock mass zone, along with the excavation sequence, have a significant impact on seepage assessment inside caverns. It was shown from the analysis that the cavern invert contributes to most of the seepage measured inside caverns. The predictions of the simplified numerical model compares well with the field measurements.