Soil-geocell reinforcement interaction by pullout and direct shear tests Nazri Mohidin & Marolo C. Alfaro Department of Civil Engineering- University of Manitoba,Winnipeg, Manitoba, Canada ABSTRACT This paper reports the results of a study to investigate the soil-geocell reinforcement using pullout and direct shear tests. The behaviour of soil-geocell interaction is characterized based on shear stress-displacement behaviour and shear strength properties. The results from pullout tests show strain hardening behaviour of pullout load-displacement curves. In direct shear tests, strain softening behaviour was observed on both reinforced and unreinforced soil where the provision of geocell increased the shear strength of the soil due mainly to higher apparent cohesion and internal friction angle. The degree of interlocking on dense soil and additional friction on the interface between soil and geocell reinforcement may have attributed to increase in shear strength. RESUMEN Este artículo presenta los resultados de un estudio para investigar el refuerzo del mecanismo suelo-geocelda ante pruebas de tensión y corte directo. El comportamiento de la interacción entre el suelo y la geocelda es caracterizado en función del comportamiento entre el desplazamiento y el esfuerzo cortante y las propiedades de resistencia del mecanismo suelo-geocelda. Los resultados de las pruebas de tensión demuestran un comportamiento de endurecimiento por deformación en las curvas de carga tensión vs desplazamiento. En pruebas de corte directo, se pudo apreciar un comportamiento de ablandamiento ante la deformación para las condiciones de suelo reforzado y sin refuerzo donde la geocelda incrementó la resistencia al corte del suelo debido principalmente a una mayor cohesión aparente y un incremento en el ángulo de fricción interno. El grado de entrelazado en suelos densos y la fricción adicional en el punto de contacto entre el suelo y el refuerzo de geocelda podría ser atribuido al incremento en la resistencia al corte. 1 INTRODUCTION Geocell is a geosynthetic product interconnected to form three-dimensional cellular networks. It provides working platform, reduces differential and total settlements, facilitates rapid construction, provide short term and long term slope stability, and increased bearing capacity of embankments (Bush et al. 1990, Cowland and Wong 1993, Krishnaswamy et al. 2000, Madhavi et al. 2006, Madhavi and Rajagopal 2007, Zhou and Wen 2008) The advantages of using geocell compared with planar types of reinforcements (e.g. geotextiles and geogrids) were reported by Dash et al. (2004). They showed that confinement by geocell allows redistribution of footing loads over a larger area makes a better composite material and reduces settlements. Planar reinforcements resist outward shear stresses induced by embankment fill and provide inward shear stresses to hold back foundation soil from lateral spreading (Jewell 1988). This is also the case for geocell reinforcements. Lateral confinement is due to the three dimensional structure of geocell. Geocell provides vertical confinement in two ways: (i) Friction between the filled soil and geocell walls (ii) geocell acts like mattress minimizing lateral spreading. Zhou and Wen (2008) pointed out that geocell reinforced- base can provide bending resistance, tensile strength and shear strength and intercept the failure plane. The use of geocell reinforcements is not as popular as planar reinforcements for embankments, shallow foundations and pavement applications. Although successful field applications and researches have been done, lack of understanding of operating mechanisms and influencing factors are the main reasons limiting the use of geocell reinforcements (Yuu et al. 2008). Additional research is needed for establishing design procedure to use geocell as basal reinforcement (Pokharel et al. 2010). The present study aims to investigate the pullout and direct shear interaction behaviour of soil and geocell reinforcements. The soil-geocell interaction is evaluated in terms of shear stress-displacement relationships and shear strengths. 2 TEST PROGRAM 2.1 Test Equipment and Instrumentations The test apparatus was composed of a box with dimensions of 1200 mm in length, 600mm in width and 410 mm in height. A schematic diagram of the testing equipment and photo used for both pullout and direct shear tests are shown in Figures 1 and 2, respectively. Detailed description of the test equipment is found in Alfaro et al. (2009). The soil thickness above and below the reinforcement were 160 and 250 mm, respectively. A rubber air bag was used to produce a uniformly distributed vertical pressure on top of the soil. The top cover of the equipment containing the rubber air bag was fitted with displacement rods. These rods measure the vertical displacement on top of the soil caused by soil dilation. Extreme care was taken to minimize the interference of tank wall in the pullout test results. Friction between the soil and the side walls of the box was minimized using two layers of thin plastic films lubricated buy silicon grease. In order to avoid front wall effects, a sleeve was fixed to the front wall as recommended by a number of researchers