Pile Foundations, Geosynthetics, Finite Element Method

Evaluation of Slope stability is one of the day-today practices of geotechnical engineers. Nowadays, different methods are available to evaluate the stability of a particular slope. Despite the advances that have been made in site exploration, evaluating the stability of slopes remains a challenge. Recently, Ethiopia has been trying to construct a newly planned railway routes to connect the country's development centers and link with ports of neighboring countries. However, this newly planned railway routes will pass in the heart of highly fragile mountainous terrains and earthquake prone regions. Therefore, the prime objective of this paper is to investigate the stability of the railway embankment by using three different stochastic approaches (First Order Reliability Method, Point Estimate Method and Monte Carlo Simulation) with commercially available finite element programs. Moreover, the seismic response of the railway embankment was studied by using a nonlinear analysis (FLAC2D v 7.0) program. The first order reliability method (FORM), Monte Carlo Simulation (MCS) and Point-estimate method (PEM) gave 3.2%, 4.14% and 1.5% of probability of failure respectively. In the mean time, there was no any indication of liquefaction observed due to stiff foundation clay soils and deep groundwater table.

Geotechnical projects typically achieve load transfer to the ground using shallow or deep foundations. The conventional design approach does not provide for the combination of these two types of foundation. The piled raft philosophy allows the association of the soil elements, raft, and piles to obtain technical and economic advantages over conventional design. The city of João Pessoa, in northeastern Brazil, has developed foundation practices with hollow auger piles in piled raft design. The coastal area of the city has topsoil layers with favorable conditions for using such a technique. This paper addresses the results of a research project with instrumented load tests on foundation systems of hollow auger piles and a piled raft. The analysis is based on the load–settlement curve through extrapolation criteria. The Poulos–Davis–Randolph (PDR) method is applied according to a trilinear and hyperbolic approach to simulate the load–settlement curve of piled rafts. The results indicate that the raft absorbs most of the load, and the raft–soil contact significantly increases the load capacity of the foundation. The PDR hyperbolic method could apply to practical use in the foundations of the region, as it allows a more detailed assessment of the behavior of the foundation and can forecast the behavior of the (locally nontraditional) piled raft foundation system. Résumé : Dans les projets géotechniques, le transfert de charge vers le sol est habituellement réalisé a ` l'aide de fondations superficielles ou profondes. La méthode traditionnelle de conception ne prévoit pas l'association de ces deux types de fonda-tions. L'approche des radiers sur pieux permet de combiner les éléments de sol, le radier et les piles de manière, ce qui comporte des avantages techniques et économiques par rapport a ` la méthode de conception traditionnelle. La ville de João Pessoa, dans le Nord-Est du Brésil, a mis en place des pratiques de construction des fondations prévoyant l'utilisation de pieux a ` tarière creuse lors de la conception de radiers sur pieux. La couche superficielle du sol dans la zone côtière de la ville est favorable a ` l'utilisation d'une telle technique. Le présent article présente les résultats d'un projet de recherche comportant des essais instrumentés effectués sur des systèmes de fondations constitués de pieux a ` tarière creuse et d'un radier sur pieux. L'analyse est basée sur la courbe charge–tassement et fait appel a ` des critères d'extrapolation. La méthode de Poulos–Davis–Randolph (PDR) est appliquée en suivant une approche trilinéaire et hyperbolique afin de simuler la courbe charge–tassement de radiers sur pieux. Les résultats montrent que le radier absorbe l'essentiel de la charge et que le contact entre le radier et le sol a pour effet d'augmenter de manière significative la capacité de charge de la fondation. La méthode hyperbolique PDR pourrait s'appliquer dans la pratique aux fondations construites dans la région de João Pessoa étant donné qu'elle permet d'évaluer plus en détail le comportement de la fondation et de prédire celui du système de fondation (localement non traditionnel) comportant un radier sur pieux. [Traduit par la Rédaction] Mots-clés : radier sur pieux, essai de charge, instrumentation, pieu a ` tarière creuse.

Assessment of liquefaction susceptibility is essential for seismically active area when pile
foundations are designed or existing under seismic loading. During the liquefaction soil loss
its strength and stiffness, thus remove the support from pile that can affect the overall
stability of the pile foundations. The main objective of this study is to investigate the extent
of the liquefaction zone that will make unsupported to pile and to assess the effect of the
liquefaction on the pile considering structural and geotechnical aspects. The simplified
empirical procedure of Seed & Idriss based on standard penetration test, liquefaction of soil is
predicted. Besides, a simple approach of Euler load to predict the buckling capacity of an
axially loaded partially embedded pile in sand is formulated using the conventional Davisson
and Robinson method combined with the ACI & area transform theory’s flexural stiffness
equation of the slender concrete column. Example solved as a case study based on above
analytical procedures that an axially loaded existing concrete pile under foundation can be
laterally unsupported in liquefied soils and susceptible to buckling failure and pointer towards
the inclusion of slenderness ratio limit of pile in the respective building code.

Laboratory and field data showed that deep mixed (DM) columns accelerated the rate of consolidation of the soft foundations. Most analyses of consolidation of DM column-improved foundations so far have been based on the elastic theory. In reality, the DM columns may yield due to the stress concentration from the soft soil and its limited strength. The influence of column yielding on the degree of consolidation of the soft foundation improved by DM columns has not been well investigated. A three-dimensional mechanically and hydraulically-coupled numerical method was adopted in this study to investigate the degree of consolidation of the DM column foundation considering column yielding. A unit cell model was used, in which the soil was modeled as a linearly elastic material. For a comparison purpose, the DM column was modeled as an elastic or elastic-plastic material. This study examined the aspects of stress transfer, settlement, and degree of consolidation of the foundations without or with the consideration of the yielding of the DM column. A parametric study was conducted to investigate the influence of the column yielding on the stress concentration ratio, settlement, and average degree of consolidation of the DM column foundation. The stress concentration ratio increased and then decreased to reach a constant value with the increase of the column modulus and time. A simplified method was proposed to calculate the maximum stress concentration ratios under undrained and drained conditions considering the column yielding. The simplified method based on a composite foundation concept could conservatively estimate the consolidation settlement. An increase of the column modulus, area replacement ratio, and/or column permeability increased the rate of consolidation.