Synthesis of Geosynthetic Reinforced Soil (GRS) Design Topics

CHAPTER 1. INTRODUCTION

This report provides technical synopses on six topics related to geosynthetic reinforced soil (GRS) systems: embedment length, pullout check, eccentricity, lateral pressures, the W equation, and reduction factors. The synopsis for each topic includes a summary of the relevant research and a review of pertinent issues. GRS refers to closely spaced layers of geosynthetic reinforcement and compacted granular fill material. The GRS systems examined included, but were not limited to, walls, abutments, and the integrated bridge system (IBS).

Embedment length refers to the length of reinforcement for GRS walls or load-bearing walls. The synopsis on this subject addresses the issues of structures with short reinforcement lengths-lengths less than 0.7 of a wall height at its face. The synopsis includes a review of research and case histories on short reinforcement. GRS walls involving a "constrained fill" zone (that is, where rock, heavily overconsolidated soil, or a nail wall is present behind a wall) are also discussed.

The pullout check refers to the implications of performing, or not performing, the check for geosynthetic pullout of a GRS wall or abutment. The pullout check is performed to determine required reinforcement length, but there is some uncertainty about the check's efficacy. The synopsis examines the research and performance data, discusses the history of this particular design component in traditional geosynthetic mechanically stabilized earth (GMSE) theory, and considers the appropriateness of extending the pullout check to GRS technology.

Eccentricity is another name for overturning. As part of the design of conventional cantilever and gravity retaining walls with relatively rigid footings, overturning failure is commonly checked to determine whether the design meets the required margin of safety. This check typically consists of taking moments of all the stabilizing forces acting on the free body of the wall system about the toe of the wall and comparing them with the moments for the destabilizing forces about the same point. An alternative approach involves basing overturning criteria on a minimum base area in compression. This synopsis examines the purpose and theoretical justification of the eccentricity design component.

Lateral pressures at a GRS facing differ from those in the soil portion. The internal lateral stress in the GRS soil is governed by compaction-induced stresses and by supplementary confining effects that the reinforcement adds to the soil. The pressure at the facing is governed by those two factors as well as the movement of the facing, especially in facings with little or no connection strength. The synopsis discusses the effects of compaction-induced stresses on the internal lateral stress of both the unreinforced and reinforced soil. Measured thrusts against facing elements with and without purely frictional connection are presented. Methods for estimating this thrust are compared with the measured values.

The W equation is used to estimate the vertical capacity and required reinforcement strength of GRS walls and abutments. The W refers to a term or factor that attempts to explain why an increase in the reinforcement strength does not have the same effect as a proportional decrease in the reinforcement spacing. Proportional effects between strength and spacing have been assumed in simplified GMSE design. The synopsis discusses the various terms in the W equation and previous research and theory leading to its development. Based on load test results from the literature, the reliability of the equation is considered.

Lastly, the topic of reduction factors is examined. The synopsis includes a summary of the research and associated theory behind the reduction factors of creep, degradation-or durability-and installation damage. Deficiencies in current practices are also discussed. The reviewed literature includes reports on the impact of the use of various polymer types.