Conference Agenda

<p>Geosynthetic reinforcement materials such as geogrids, geotextiles and polymer straps are rate-dependent materials which means that their mechanical tensile properties are time-, load- and strain-dependent. Nevertheless, analytical and numerical analyses of geosynthetic reinforced soil walls, slopes, thin fills and embankments are most often carried out using a single-value estimate of the reinforcement stiffness consistent with the notion of a single equivalent elastic modulus for the reinforcement material. The designer or modeller is left with the question of what value of stiffness is best used for simple analyses of the structures identified above if the reinforcement stiffness will change with time (i.e., creep). This paper offers practical guidance to answer this question for structures under operational (serviceability) conditions. The authors first fitted a hyperbolic model to isochronous load-strain time curves constructed from more than 600 creep tests for 89 different reinforcement products falling within seven different product categories. The data are restricted to polyolefin and PET geogrids and straps because these are the most common materials used to perform the reinforcement function in geotechnical earthworks construction. The fitted data were then used to find parameters for a hyperbolic secant stiffness model that can be used in both analytical and numerical models to account for creep. The formulation can be differentiated to provide a simple tangent stiffness expression that is easily implemented in numerical codes. Hyperbolic parameters for typical reinforcement products and representing a range of elapsed time after construction and strain are provided. The model has important applications to simple analytical and more advanced numerical modelling of reinforced soil walls, embankments, slopes, footings and for reinforcement of thin soil layers over voids. The details of the proposed model and the methodology to find the hyperbolic model parameters are thus of practical interest to designers and researchers.</p>

<p>Geogrids are used to reinforce ballasted railway embankments thanks to their high tensile strength and strong mechanical interlock with the granular material encountered in railway substructures. Typically, large aperture biaxial geogrids are placed in the ballast layer due to their ability to let the granular material strike through them and hence develop a strong mechanical bond with the unbound aggregate. Geogrids reduce the magnitude of vertical stresses transferred to the natural formation as well as the lateral spreading of the ballast material and increase the service life of the ballast layer, thereby solving some of the most important issues associated with problematic railway substructures. Railway tracks built in seasonally cold regions must also be designed to withstand harsh environmental conditions such as freezing and thawing cycles and the corresponding frost heave and thaw soften-ing. In that context, it is crucial to understand how extreme temperature variations affect the performance of geogrids and their ability to fulfil their functions within a railway em-bankment. To do so, an experimental campaign is devised to assess how temperature changes in the range of -30°C to 20°C affect the tensile strength of a polymeric geogrid and a geogrid composite designed for use in ballasted railway tracks.  </p>

<p>Geocells have been used as basal reinforcement in improvement of foundation soils, embankments and highway subgrades to increase bearing capacity and reduce total and differential settlements. Attempts have been made in engineering practice to somehow reduce the extensibility of the geocells whenever they are subjected to considerable tensile forces. This paper presents pullout test results on conventional (ordinary) and diagonally enhanced geocells under different surcharge pressures to evaluate feasibility of their applications when subjected to significant planar forces. Extensive pullout tests on scaled geocells embedded in silica sand are performed to investigate the effects of improvements on load-deformation response, strength and stiffness. Conventional web-shaped geocells are having a small stiffness when subjected to planar tension attributed to deformability of webs. Therefore, conventional geocells may not function properly when subjected to tensile forces along the main plane in service. A special geocell is fabricated in this study, similar to tendoned geocells, through adding diagonal members along the induced tensile load to overcome the shortcomings of conventional geocells. The test results have shown that both the stiffness and ultimate resistance of the diagonally enhanced geocells have significantly improved with respect to the conventional ones.</p>

The assessment of soil-geosynthetic interface shear strength properties is essential for the safe design of geosynthetic-reinforced soil systems. In this study, a series of inclined plane and direct shear tests was carried out to evaluate the shear strength parameters of the interfaces between a residual soil from granite and two different geosynthetics: an extruded geogrid and a geocomposite reinforcement. The influence of soil moisture content was analysed under inclined plane and direct shear modes by compacting the soil at the optimum moisture content (wopt) and 2% wet of the wopt. The direct shear test results show that the increase in soil moisture content may lead to a considerable reduction in the apparent cohesion of the soil-geosynthetic interface. In general, higher shear stresses were reached at the interface involving the geocomposite reinforcement. Moreover, the interface shear strength parameters established from direct shear test results generally exceeded those obtained by inclined plane tests, particularly in terms of the apparent cohesion value.

<p>To handle the sustainable construction required by the modern world, designers of geo-synthetic reinforced soil structures should search for alternative backfill materials, such as recycled materials. Since installation damage is responsible for significant changes in the geosynthetic stress-strain behaviour, the ones caused by the recycled ones must be carefully assessed and quantified. This study aims to assess the damage to geosynthetics caused by the backfill material-dropping process. Four geosynthetics (two geogrids and two non-woven geotextiles) and five types of recycled aggregates (with different grain–size distributions) were tested. The experimental program consists of laying the geosynthetic on the area of the recycling plant and the individual launching of the tested backfill materials (using a backhoe loader) from two drop heights: 1.0 m and 2.0 m. The geosynthetic samples were then exhumed to obtain specimens, perform wide-width tensile tests, and assess their ultimate tensile strength. The geotextiles suffered a reduction in the property of interest for all scenarios investigated. These reductions were higher than the ones suffered by the geogrids. Within some limitations, the results show that the damage increase as the maximum grain size of the backfill increases. Further investigations are required using sophisticated statistical analysis using a broader database.</p>

<p>The long-term analysis of the geosynthetics is mandatory in the design phase. Once the performance of creep-rupture tests is too time-consuming, adopting temperature to accel-erate the geosynthetics creep behaviour is an attractive solution. The ASTM D 6992 pro-vide the guidelines to perform accelerated creep rupture tests on geosynthetics and rec-ommends the adoption of the ramp and hold tests to help assess the initial creep rupture behaviour. In this way, this paper aims to assess the initial creep behaviour of two ge-ogrids and two non-woven geotextiles. A universal testing machine was used to perform ramp and hold tests. The specimens were loaded (at a similar load condition to the ones adopted in the accelerated creep rupture tests) to load levels ranging between 10% and 90 % of the geosytheticss ultimate tensile strength. Each load level was maintained during 3,600s and 10,800s for geogrids and non-woven geotextiles, in this order. Three speci-mens were tested for each load level, and a best-fit equation was used to obtain the initial creeps train rate and the initial axial strain of the mean curve obtained. The results re-vealed the higher development of creep strains for the non-woven geotextile than the ge-ogrids. The effects of the non-woven geotextile mass per unit area, and the polymer type of the geogrids were pointed out. The results help to identify and compare the initial axial strain of these geosynthetics after the performance of the accelerated creep rupture tests.</p>

The present study aims at the development of a model for the shear strength prediction of any type of soil reinforced with synthetic fibers of circular cross-section. A database was created from the available literature comprising experimental results of direct shear tests conducted in soils ranging from sands to clays, reinforced with polypropylene and nylon fibers. The shear stress at failure of fiber-reinforced soils was correlated to independent variables pertinent to the soil, fiber, and laboratory test by performing multivariable ordinary linear regression analyses of the experimental results with suitable statistical software. The model with the best performance exhibits a coefficient of multiple determination, R2, equal to 0.96 and estimates successfully the experimental results used for model testing at a rate equal to 77%. The predictions of the shear strength parameters of fiber-reinforced soils, based on the proposed model, are in reasonable agreement with the measured values after applying appropriate reduction factors.

In landfill cover systems, a geogrid or reinforced geomat is often used, placed just above the upper drainage geocomposite, to ensure the stability of the topsoil. In these cases, a specific interface occurs, which could be defined as a mixed interface, as there is a simultaneous contact between two geosynthetics and between the geosynthetics and the soil. The paper presents some preliminary results of a research on the interface shear strength of two interfaces of this type, corresponding to the contact between a drainage geocomposite and a geogrid or a reinforced geomat. A series of tests were carried out with a not standard direct shear device, operating at increased shear stress, to compare the response of the soil-less interface with that measured in the presence of the soil. The results showed how the presence of soil leads to an increase in the shear strength compared to the condition without soil and they also highlighted how the influence of soil on the behavior of the interface depends on the type of reinforcement used.

<p>Experimental and theoretical investigations on the stress-strain responses of geocell-reinforced clay are very limited. In this paper, an analytical method was formulated for the prediction of stress-strain responses of the geocell-reinforced normally consolidated clay. In addition, a series of conventional triaxial compression tests were conducted on the geocell-reinforced normally consolidated clay to investigate the reinforcement effects and validate the effectiveness of the proposed method. The study results show that the geocell-reinforced normally consolidated clay exhibits strain-hardening. The reinforcement effect on the clay increases with increase of the axial strain and the reduction of the confining pressure. The predicted stress-strain responses are in good agreement with those measured in the tests, showing the effectiveness of the proposed method.</p>

<p>Geofoam is a popular lightweight material block, used for the structural filling for the con-struction of vertical approach embankments of road bridges over soft subsoils. The design of such embankments is driven by the differential settlement, as the overall settlement of the geofoam blocks is lower than the fascia wall. Thus, the critical interface is the geofoam blocks with the concrete fascia wall. In the present study, an attempt has been made to determine the geofoam-concrete interface characteristics using a series of inter-face tests in a direct shear box. Geofoam with varying densities and concrete with varying roughness were utilised for the study. Based on the observed test results, a non-linear model for predicting interface friction angle with the roughness index was derived. This study can be utilised for the design and site quality assurance based on the roughness in-dex of the concrete retaining wall cast at the site.</p>

The vertical permeability coefficient is an important hydraulic characteristic index of needle-punched nonwoven geotextiles. A theoretical model has been proposed to predict the vertical permeability coefficient of nonwoven geotextiles, considering the fiber orientation distribution characteristics. The permeability coefficient and parameters of four needle-punched nonwoven geotextiles were obtained from the literature and image analysis. By comparing the predicted results of the permeability model with the experimental results, it is confirmed that the permea-bility model proposed in this paper could accurately predict the vertical permeability coefficient of nonwoven geotextiles with low thicknesses.