Conference Agenda

A DEM-FEM coupled numerical model was used to investigate the behaviour of a plat-form reinforced by geotextile (GTX) over soft soil subjected to cyclic loading. The ability of the numerical model to consider the settlement of the granular and the subgrade soil surface and the stress transmitted to the top of subgrade layer was established by compari-sons with the experimental data for the first loading cycle. A parametric study was carried out using two GTX tensile stiffnesses, one with a medium tensile stiffness and the other with a low tensile stiffness. The comparison between the stress acting on the upper and the lower face of each GTX showed the effect of the tensile stiffness on the tensioned mem-brane mechanism and confirm the role of the GSY sheet in the numerical model.

<p>Geosynthetics have been used to increase the bearing capacity of poor foundation soils and/or to reduce excessive settlements. In the literature, particular attention has been given to the case of shallow foundations on reinforced soil with a horizontal surface. Several authors have studied this problem using experimental, numerical and/or analytical approaches. The experiments reported in the literature included a wide range of soils (sandy soils, clayey soils, aggregates and pond ash). However, the case of shallow foundations close to the crest of a slope has been less studied. In the literature there are some studies based mostly on small scale physical models that include recommendations for the reinforcement layout. Usually the reinforcement layout is normalised to the width of the footing, <em>B</em>, and the recommendations can vary significantly. For example: depth of the upper reinforcement layer, <em>u</em>, can vary between 0.5<em>B</em> and 0.75<em>B</em>; vertical spacing between consecutive layers, <em>h</em>, from 0.3<em>B</em> to 0.<em>5B</em>; distance of the foundation pad to the crest of the slope, l, from 1.25<em>B</em> to 5<em>B</em>; number of reinforcement layers, <em>n</em>, from 1 to 4. The studies based on small-scale physical models have significant limitations, particularly regarding the unrealistically low stress levels applied. To address such limitations, this paper will present numerical analysis of full-scale structures using the finite element method. The problem analysed refers to a footing close to the crest of a slope reinforced with geosynthetics. The parametric analysis will focus on the effect of selected parameters on the response of the soil slope to loading. Such parameters may include: type of soil, drainage conditions, properties of the reinforcement, number of reinforcement layers, reinforcement layout. The main conclusions of the parametric analysis will be compared to results from physical models available in the literature.</p>

A pile support system is a common solution for embankments constructed over soft, normally consolidated fine-grained, or organic soils. Piles are installed to reach the stronger layers of the foundation soil, while the transfer of the embankment load to the piles relies on the development of the stress distribution often referred to as arching. In addition, geosynthetic reinforcement is used in a load transfer platform to enhance the transfer of the load to the piles. A series of numerical simulations (FEM) were carried out to study the formation of arching in embankment fill placed over a square-grid pile support system. The development of the load transfer process was also studied, quantified by the system efficacy. The efficacy of the support system without geosynthetic can easily exceed 80%, but it can be larger than 90% if a geosynthetic-reinforced load transfer platform is used. Numerical simulations provided some insight into the development of the load transfer mechanism, but distinct arching as assumed in many design methods was not detected.

Pile-supported earth platforms provide an economical and effective solution for embankments constructed on soft soils, particularly during their rapid construction or when strict control over their deformation is necessary. Based on a three-dimensional assumption of soil arching, an analytical approach is developed to explore the soil arching of the pile-supported earth platform with geosynthetics under the embankment load and uniform pressure. Further, the expression of stress concentration ratio in the pile-supported earth platform with geosynthetics is improved through the analysis of the two regions on top of the pile caps. The results from the in-situ and centrifuge tests are compared with those from the analytical method.

<p>The cooperative national research project ASIRI+ (2019 - 2023) gathers about forty organizations and aims at proposing dimensioning rules in the field of soil reinforcement by rigid inclusions. Its first research axis aims to improve the understanding of mechanisms in the Load Transfer Platform (LTP).</p>

<p>Full-scale tests were carried out in the experimental bench of the CEREMA in Rouen on a plot containing 16 inclusions. These experiments should make it possible to study the load transfer mechanisms in LTP by controlling the settlement of the soft soil. Characterization tests of the soft soil have been carried out on a scale 1 prior to the GEOMAS laboratory. The soft soil is composed of 50 cm of rubber granulates which rest on 10 cm of biodegradable honeycomb cardboard. The objective was to be able to model a settlement in two steps, firstly with the compressibility of rubber granulates and secondly with the biodegradation of biodegradable honeycomb cardboard by water.</p>

<p>Eight test sections were set up to analyze the mechanisms developed inside the granular platform with or without geosynthetic reinforcement, inside load transfer platform made with treated soil, or to find the equal settlement plane inside the embankment. For that, many sensors were installed in the central grid to measure the stress inside the LTP, the settlement in the soft soil, the LPT and the embankment, the geosynthetic strain, the load at the toe of the pile.</p>

<p>From these experimental results, the understanding of complex mechanisms developed inside the load transfer platform were investigated. In particular, these results highlighted the efficiency of geosynthetics and the different behavior between a geogrid and a geotextile.</p>

<p>The geosynthetic reinforcement in a pile-supported (GRPS) embankment can be designed using the CUR226 (2016) design guideline. This design guideline adopted the Concentric Arches model (Van Eekelen et al, 2013, 2015). The validated application of this model is limited to GRPS embankments whose geometry and materials meet the conditions used for the validation of the Concentric Arches model. This means for example that: the whole embankment should be located above the groundwater table and that at least one geogrid layer should be used as reinforcement. These two requirements were studied using two years of field measurements that were conducted in a partially submerged GRPS embankment. It was concluded that the Concentric Arches model matches the measurements very well. This means that it is feasible to apply the Concentric Arches model and therefore the CUR226 (2016) guideline for GRPS embankments where the reinforcement consists of two layers of woven geotextile without geogrid. This conclusion is valid for woven geotextile like the geotextile applied in this case. Furthermore, a varying groundwater level made it possible to study how ground water in the geosynthetic-reinforced embankment affects the geotextile strain. It was shown that a high groundwater level resulted in less strain in the geotextile. This decrease in strain is clearly visible but remains limited. The paper analyses possible explanations for these measurements, including the reduction in soil weight due to the higher water level, and swell of the underlying peat layers. A reduction in the arching because of a lower effective stress in the embankment fill seems not important in this case.</p>

<p>The Hong Kong International Airport has expanded its existing two-runway system to a three-runway system based on a reclamation project over a seabed of soft marine clay. In the reclamation area, a layer of load transfer platform (LTP) was designed with geosynthetic reinforcement to bridge the overlaid surcharge loadings of reclamation fills and underlaid marine clay improved by deep cement mixing method. In this study, firstly, a small-scale physical model test of geotextile-reinforced sand layer over soft marine clay improved by cement-treated soil columns was performed to investigate the load transfer mechanism among columns, soils, and geotextile under different surcharge loadings. The results from the scaled model test were then adopted to verify the parameters used in a finite element model established using PLAXIS. Furthermore, the finite element model was used to reveal the development of soil arching.</p>

<p>Geosynthetics have been widely used on improving the stability of embankments. However, limited studies focus on the stability and failure modes of geosynthetic-reinforced embankments considering different layers or stiffness of geosynthetic. In this paper, a finite difference approach is adopted to analyze the stability of embankment reinforced by geosynthetic with various layers and stiffnesses. The stability and failure mechanism of reinforced- embankment and unreinforced-embankment are compared. The stiffness and layers of geosynthetic both have positive influences on the embankment stability. A deep-seated failure occurs when the geosynthetic stiffness is low. When the stiffness and layers increase to a certain value, the failure mechanism changes to lateral spreading failure. Besides, the progressive failure of is preliminarily investigated.</p>

<p>Several methods for the design of a geosynthetic-reinforced fill spanning a void appear in the literature and in national standards and codes in the UK, Germany and in the Eurocode. In this paper, the reinforced void problem is reexamined with special focus on the role of geosynthetic stiffness taking into consideration the rate-dependent behaviour of polymeric materials (i.e. load-strain-time–dependent behaviour). In current analytical model approaches, a single-value (constant) estimate of reinforcement stiffness is used for the design of reinforced fills over voids. Hence, the choice of a constant and representative (elastic) stiffness value requires careful consideration. A simple hyperbolic stiffness model proposed by the writers is shown to be a useful approximation to constant-load isochronous creep-strain behaviour of these materials, at least at low load levels, which are applicable to operational (serviceability) conditions of geosynthetic-reinforced soil structures. The paper uses the UK method for the solution of the reinforced fill over a void problem to compute tensile load and strain for a prescribed surface deformation (or maximum reinforcement settlement). However, the general approach applies to other analytical solutions for the same problem. A useful empirical approximation that links the isochronous stiffness of the reinforcement to the reinforcement ultimate strength is used to improve the utility of the general approach. Margins of safety for reinforcement failure and stiffness limit states are expressed deterministically using a conventional factor of safety approach. A design chart approach is presented that includes the void deflection limit state plus strain, strength and reinforcement stiffness limit states for the reinforcement.</p>