Conference Agenda

<p>The use of geosynthetic reinforcements in the base of embankments on piles/columns or in earth structures bridging voids belongs to their more sophisticated applications.</p>

<p>In the first case the goal is to decrease the number of piles and to avoid inclined piles at the edge of embankment. The geosynthetic reinforcement bridges the increased space between piles and takes over the lateral trust below the embankment slope.</p>

<p>In the second case the goal is to bridge an open sinkhole if the arching capacity of the soil body above is not sufficient, which is often the case.</p>

<p>Common in both cases is the bridging of zones below the base of embankments without sufficient supporting capacity: under piled embankments the subsoil between the piles is usually too soft, and in the case of sinkholes there is simply no support at all.</p>

<p>Also common is the need to control by the reinforcement both the ultimate (ULS) and serviceability (SLS) limit states. It should be noted that in both cases second order theory has to be used for design calculations, say deformations and forces are coupled even when analyzing the ULS.</p>

<p>In Germany applications and intensive parallel research started beginning of the 90ies.</p>

<p>Global knowledge and design level were at that time modest.</p>

<p>It has been by some extent learning by doing trying to stay on the safe side.</p>

<p>The German and global knowledge and experience were then synthesized in recommended design procedures in two chapters of EBGEO 2010 (Recommendations for the design of geosynthetic reinforcement).</p>

<p>This is still one of the very limited number of codes handling these issues.</p>

<p>Design basics in EBGEO, philosophy and background, pros and contras will be reported.</p>

<p>Both excessive subsidence and slippage are concerns in the construction of embankments or retaining walls on the soft soil foundation. Various techniques can be used to overcome these problems. The geosynthetic basal reinforcement is known as a powerful solution. This paper reports the results of a case study of it in Japan.</p>

<p>The geosynthetic basal reinforcement technique can be classified into independent types with laying geosynthetics on the ground surface and hybrid types with combining geosynthetics and other techniques such as vertical drains, piles, and deep mixing. The current technique needs to be updated to make it easier for engineers to consider the influence of complex site conditions and required performances level for the structures in the design and construction. The case study of the actual project is important to achieve the update. The authors have collected numerous Japanese case histories and examined the improvement points.</p>

<p>In this paper, the authors briefly introduce the Japanese design method for the geosynthetic basal reinforcement techniques. And they report the Japanese case histories. These include cross-sections of the reinforced foundations, geotechnical site conditions, soil types, geosynthetics properties, and actual performance with observation results. Finally, the authors clarify the challenges in migration to the performance-based design with their numerical and physical model test results.</p>

<p>BS8006, Code of practice for strengthened/reinforced soils and other fills was first published in 1995, after 10 years of development. At the time it was the first comprehensive design code on geosynthetics and was used around the world for the design of geosynthetic structures and systems. Section 8 of the code covers the design of embankments over soft soil, over piles and over areas prone to subsidence. The code uses limit state principles in design and is, in part, Eurocode 7 compliant.</p>

<p>This paper will chart the historic development of the design methodologies, including the ultimate and serviceability limit states, for basal reinforcement in BS 8006. The principal changes made to the basal reinforcement section of the code over the past 25 years will be documented and their significance discussed. These changes are primarily centred on the design of piled embankments, where two approaches are now given for estimating the arching in the fill and guidance was added on incorporating subsoil support into design and estimating the lateral loads transmitted to the piles. The importance of these changes will be discussed with reference to the significant research in the last 10-15 years and the development of Dutch and German design codes in this area.</p>

<p>The design output for embankments over soft soil, over piles and over areas prone to subsidence for typical cases, soil conditions and geometry encountered in practice will be benchmarked against the Dutch, German and French design standards. A discussion on the difference between the design outputs will be presented and discussed in terms of their underlying assumptions. The limitations in the BS 8006 approach to design will also be highlighted.</p>

<p>Finally, recommendations on updating BS 8006 based on recent research and in the context of Eurocode 7 will be made.</p>

<p>The stability of embankments on soft foundation soil is most critical during the construction period, because of the relatively low permeability of the soft foundation resulting in excess pore water pressures that decrease the effective shear strength of the soil. In this scenario, Geosynthetics reinforcements placed across at foundation level enhance the short-term stability of an embankment over soft soil by preventing lateral sliding of the fill, extrusion of the foundation soil, and rotational failure. The reinforcement has the double function of carrying the outward shear stresses transmitted by the embankment fill and of providing inward shear stresses to restrain the foundation soil from lateral squeezing-out. For each of these limit states the associated reinforcement strength and bond length should be checked to ensure that the required tensile load can be generated in the reinforcement. Moreover, while the consolidation of the soft soil develops, the base of the embankment is subject to non-uniform settlements, usually higher at the centre of embankment and progressively decreasing towards the toes of the lateral slopes. Hence Geosynthetics for basal reinforcement are progressively subject to out-of-plane deformations, which trigger the tensioned membrane mechanism. When consolidation ends, the related out-of-plane deformations and the tensioned membrane force become constant over time. Hence the long-term design of Geosynthetics for basal reinforcement of embankments on soft soil requires the calculation of the final and constant tensioned membrane strength, which has to be compared with the long-term tensile strength of the Geosynthetics for getting the associated long-term Factor of Safety. This paper presents a new design method to evaluate the reinforcement strength and bond length required at short-term for the limit states of lateral sliding and foundation extrusion, and then presents a new design method to evaluate the strength required at long-term when the tensioned membrane mechanism is fully developed.</p>