Soil–Geosynthetic Interaction Under Triaxial Conditions: Shear Strength Increase and Influence of the Specimen Dimensions

Abstract Geosynthetics are extensible reinforcements used to enhance the engineering performance of a soil. The transfer of stresses from the soil to the reinforcement is achieved through soil–geosynthetic interaction. The proposals from the literature to assess the shear strength of the reinforced soil under triaxial conditions use three different approaches. These involve analysing the reinforced soil: (i) as a homogeneous composite material (Approach A), (ii) as two different materials (Approach B), or (iii) as soil having the same fundamental shear strength, with the effect of the reinforcement represented as an additional lateral or confining stress (Approach C). In this paper, triaxial tests data of a soil reinforced with a geosynthetic, and specimens with different dimensions (diameters 70 and 150 mm) were used to assess changes in shear strength and to carry out a statistical analysis. The increases in shear strength of the reinforced soil and of the soil–geosynthetic interface were analysed using equations from the literature. The difference between the triaxial results obtained from specimens of different sizes was assessed using an Analysis of Covariance (ANCOVA). When the joint term of the regression model was not statistically significant, the characterisation from different specimen sizes was used to generate soil failure envelopes. Thus, a new methodology to obtain a failure envelope with different specimen sizes is presented.