On the interface shear resistance of a novel geogrid with in-plane drainage capability

Abstract

The enhancement in the short-term interface shear strength (critical case for embankment construction) achieved by a novel geogrid that combines both reinforcement and in-plane drainage functions was studied. Marginal fill (wet gravely clay) was standard Proctor compacted to 92% of its maximum dry density and tested under consolidated-undrained conditions in the large shearbox apparatus. The interface shear resistance (τ_s−g) values mobilized for the novel geogrid were similar to the undrained shear strength of the surrounding soil. In contrast, the τ_s−g values mobilized for a conventional geogrid, which had similar physical and tensile strength properties, were only between 82% and 85% of the undrained shear strength of the soil. Overall, the undrained shear resistance mobilized along the soil–geogrid interface was between 20% and 30% greater for the novel geogrid than for the conventional geogrid.

Introduction

Geosynthetics are widely used in reinforced-earth and landfill construction and soft ground improvement (Bergado et al., 2006; Long et al., 2007; Tatsuoka et al., 2007; Rowe and Taechakumthorn, 2008), and have many advantages including speed of construction, flexibility and durability, use of local readily-available soils rather than imported quarry product, and cost effectiveness. More recently, innovative dual-function geosynthetics that combine both reinforcement and drainage functions have been developed (Kempton et al., 2000; Bergado et al., 2002; Lorenzo et al., 2004; Zornberg and Kang, 2005), which can facilitate the use of marginal fill in the construction of reinforced-earth structures. Marginal fill is defined as predominantly granular material that includes high silt and/or clay fractions (more than 15% by dry weight passing the 0.063-mm sieve) and often has high water content, with typically only 90–95% of its maximum dry density achieved under standard Proctor compaction. Dual function geosynthetics included in the earth structures provide reinforcement and preferential drainage channels, thereby increasing the factor of safety against slope instability.

The use of geosynthetics requires a proper understanding of the soil–geosynthetic interaction mechanisms. Utilizing marginal fill would involve considerable savings (Lorenzo et al., 2004) on condition that the intended engineering purpose can be achieved. However, most of the previous research has studied the interaction parameters (pull-out resistance and shear stress–strain characteristics) between geosynthetics and granular soils. Few studies have been carried out in relation to the interaction parameters between cohesive soils and geosynthetics (Bergado et al., 1991; Nagahara et al., 2004; Almohd et al., 2006; Long et al., 2007). For example, the angle of interface friction value for conventional geogrids is lower than the angle of shearing resistance of the surrounding soil and this must be considered in analysing the factor of safety on slope instability where potential slip surfaces can align with the soil–geosynthetic interface. This paper presents a laboratory study of the short-term interface shear resistance (τ_s−g generally critical case) achieved for using a novel dual-function geogrid in the construction of earth embankments using wet cohesive fill.