Soil–geosynthetic inclined plane shear behavior: influence of soil moisture content and geosynthetic type Maria-Lurdes Lopes*, Fernanda Ferreira, Jose ´ Ricardo Carneiro and Castorina Silva Vieira This paper deals with the inclined plane shear on three different geosynthetics (a geocomposite (GC), a non-woven geotextile (GTX), and an extruded geogrid (GGR)) with a residual soil from granite. Soil and geosynthetic properties, test equipment, and procedures are described. The influence of soil moisture content and geosynthetic type on soil–geosynthetic interaction behavior is discussed by analyzing the results of the inclined plane shear tests. The main conclusions that can be outlined from the present study are the following: (1) the influence of soil moisture content was relevant for the soil–GTX and soil–GC interfaces. Indeed, the resistance of those interfaces decreased with the increase of soil moisture content. No significant differences were observed between the behavior of those geosynthetics; (2) the influence of soil moisture content on the behavior of the soil–GGR interface was less evident. A slight decrease on the interface friction angle was only observed for the highest soil moisture content; (3) the dry soil–GGR interface resistance was lower than that observed for the other two geosynthetics due to the relevance of soil–soil friction at the GGR apertures, to the high percentage of fines of the soil used in the research (D 50 51?00 mm), and to the smoother solid lateral surface of the extruded GGR when compared with the surface of the GTX or GC. Keywords: Geosynthetics, Soil–geosynthetic interaction, Inclined plane shear behavior, Soil moisture content Notation Basic SI units are given in parentheses. b 5slipping angle of upper box (u) t 5shear stress (N m 22 ) c max d 5maximum dry unit weight of soil (N m 23 ) c min d 5minimum dry unit weight of soil (N m 23 ) s n 5normal stress (N m 22 ) w sg 5friction angle of soil–geosynthetic interface (u) A 5soil–geosynthetic contact area (m 2 ) C u 5uniformity coefficient of soil (dimensionless) C c 5curvature coefficient of soil (dimensionless) D 10 5diameter corresponding to 10% passing of soil (m) D 30 5diameter corresponding to 30% passing of soil (m) D 50 5diameter corresponding to 50% passing of soil (m) D ma ´x 5maximum diameter of soil (m) D min 5minimum diameter of soil (m) f (b) 5force required to restrain the empty upper box at inclination of b (N) F v 5vertical force acting on soil–geosynthetic interface (N) I D 5relative density of soil (%) w opt 5Optimum soil moisture content (%) Introduction Inclined plane shear tests are used to characterize the interaction mechanism at soil–geosynthetic or geosyn- thetic–geosynthetic interfaces when the relative movement that occurs is of shearing and the geosynthetics are placed over an inclined surface. Inclined plane shear behavior at soil–geosynthetic and geosynthetic–geosynthetic interfaces have been studied by several authors (among others: Izgin and Wasti, 1998; Faculty of Engineering, University of Porto, Porto, Portugal *Corresponding author, email lcosta@fe.up.pt ß 2014 W. S. Maney & Son Ltd Received 25 October 2013; accepted 17 January 2014 DOI 10.1179/1939787914Y.0000000047 International Journal of Geotechnical Engineering 2014 VOL 000 NO 000 1