Evaluation of soil-reinforcement composite interaction in geosynthetic-reinforced soil structures

Soil reinforcement has become a well-established technology, providing alternatives to an increasingly large number of critical geotechnical structures. While significant advances have been made to characterize the soil-reinforcement interaction of individual reinforcement layers, field evidence has been collected that suggests unaccounted benefits in structures where the vertical spacing between reinforcements is comparatively small. The nature of the complex interactions that may develop between contiguous reinforcement layers, possibly leading to a “composite” behavior of the reinforced soil mass, requires full characterization. The degree of interaction between adjacent reinforcement layers is expected to impact, perhaps significantly, the mechanical response of the reinforced soil mass. The added benefits from interaction among reinforcement layers would be particularly relevant for critical structures, such as reinforced soil bridge abutments and piers, reinforced soil pile platforms, and reinforced soil foundations. Accordingly, this study aims at assessing the effect of geosynthetic reinforcements on the behavior of the surrounding soil, contiguous reinforcements, and the geosynthetic-reinforced soil mass at large. The findings of this study on the behavior of soil-reinforcement interaction is expected to lead to practical implications such as the selection of the reinforcement vertical spacing in geosynthetic-reinforced soil (GRS) structures. The specific objectives of this research are to (1) evaluate mechanisms involved in soil-reinforcement interaction, (2) identify and characterize the shear zone of influence surrounding a reinforcement layer under tension, (3) evaluate the interaction that develops between a reinforcement and its neighboring reinforcement layers, and (4) understand and quantify the potential benefits of closely-spaced reinforcements in GRS structures. Significant information was initially gained by reevaluating data collected from other research studies in order to assess soil-reinforcement interaction with focus on the impact of reinforcement vertical spacing on GRS structures. Specifically, a detailed evaluation of data sources was conducted, including actual experimental and field monitoring data. The data sources reevaluated in this dissertation include (1) evaluation of the performance of large-scale experimental GRS structures, (2) analysis of soil arching in GRS structures, (3) evaluation of the performance of GRS structures using geotechnical centrifuge, and (4) assessment of the performance of the Founders/Meadows GRS bridge abutments. A state-of-the-art device was developed as part of this research to comprehensively assess the soil-reinforcement composite interaction under both working stress and failure conditions. The new equipment was able to assess the mechanical behavior of a geosynthetic-reinforced soil mass considering varying reinforcement vertical spacings. In addition, it allowed investigating the interface shear stress transfer mechanisms. The device provided suitable measurements of the strains developed in both actively tensioned and the adjacent reinforcement layers. It allowed direct visualization of the kinematic response of soil particles adjacent to the geosynthetic reinforcement layers, which facilitated evaluation of the soil displacement field via digital image analysis. Evaluation of the soil displacement field allowed quantification of the extent of the zone of shear influence around a tensioned reinforcement layer. Finally, the device allowed monitoring of dilatancy within the reinforced soil mass, providing additional insight into the effect of reinforcement vertical spacing on the reinforced soil mass. A comprehensive testing program was conducted using the newly developed experimental device. The testing program was tailored to evaluate the following aspects: (1) test repeatability; (2) effect of reinforced soil confinement on the soil-reinforcement composite interaction behavior; (3) effect of reinforcement vertical spacing on the soil-reinforcement composite interaction behavior; (4) effect of reinforcement properties on the soil-reinforcement composite interaction behavior; (5) effect of boundary type on the soil-reinforcement composite interaction behavior; and (6) effect of backfill properties on the soil-reinforcement composite interaction behavior. Analysis of the experimental results revealed that the existence of the zone of shear influence and its extent can be directly related to the interaction between contiguous reinforcement layers. In particular, for the uniform gravel evaluated in this study, the zone of shear measured from the soil-reinforcement interface ranged from 0.10 to 0.30 m for the normal stress range involved in this study. It was concluded that reducing the vertical spacing between reinforcement layers in a GRS mass increases the strain compatibility between the reinforcement layers and the soil mass in between.