Since the reintroduction of reinforced soil in the early 1960s, many innovative reinforced soil wall systems have been developed to deal with earthwork construction where an abrupt change in grade is desired or needed. Reinforced soil wall systems deploy horizontal layers of tensile inclusion in the fill material to improve or achieve stability. These wall systems have demonstrated many distinct advantages over their conventional counterparts such as cantilever reinforced concrete earth retaining walls, gravity concrete walls, crib walls, etc. In addition to high load‐carrying capacity, reinforced soil walls are typically more ductile (hence less susceptible to sudden collapse), more flexible (hence more tolerant to differential settlement), faster and easier to construct, more adaptable to low quality backfill, require less over‐excavation, more economical to construct, and have lower life‐cycle maintenance costs. To date, reinforced soil walls are being constructed at a rate of over 100,000 m² (in terms of total face area) annually in the U.S. alone.

Modern technologies of reinforced soil walls incorporate metallic strips/mats or synthetic polymeric sheets (termed geosynthetics) as tensile inclusion in the backfill during fill placement. Reinforced soil walls have commonly been designed by considering tensile inclusion as quasi‐tieback elements to stabilize the fill material through soil‐reinforcement interface bonding, and are collectively referred to as mechanically stabilized earth (MSE) walls. MSE walls with geosynthetics as reinforcement have been referred to as geosynthetic mechanically stabilized earth, or simply GMSE. To date, over 60,000 GMSE walls have been built along highways in the U.S.

Reinforcement spacing used in GMSE walls has been relatively large. This stems from a fundamental design concept that spacing of quasi‐tieback elements hardly matters to performance, and that larger spacing would result in shorter construction time. The beneficial effect of deploying geosynthetic reinforcement on tight spacing, however, is gaining increased attention. The significant benefits of close reinforcement spacing were first realized through actual wall construction, and later verified by field‐scale loading experiments. It has been shown that close reinforcement spacing will increase considerably the load‐carrying capacity and, more importantly, improves stability of the reinforced soil mass. Studies have suggested that the behavior of reinforced soil mass with closely spaced reinforcement can be accurately characterized as soil–geosynthetic composites.

Geosynthetic reinforced soil (GRS) emerged as a viable alternative to GMSE in the early 2000s. GRS takes advantage of soil–geosynthetic interaction by which the soil mass is reinforced internally. To activate a significant beneficial effect of soil–geosynthetic interaction, reinforcement spacing in GRS is much smaller than in GMSE. Note that in the literature the term “GRS” has sometimes been used for all soil structures reinforced by geosynthetic inclusion without any regard to reinforcement spacing or the design concept.

GRS bears strong resemblance to GMSE, in that both systems are composed of three major components: facing, compacted backfill, and horizontal geosynthetic inclusion. The main difference between the two systems lies in the design concept. GRS considers closely spaced geosynthetic inclusion as a reinforcing element of a soil–geosynthetic composite (hence the term “reinforced” in GRS). GMSE, on the other hand, considers the geosynthetic inclusion as frictional tieback tension members to stabilize potential failure wedges (hence the term “stabilized” in GMSE). Because of this difference, the role of facing for the two systems is also very different. In GRS, the soil mass is internally reinforced to form a stable mass. The wall facing serves primarily as an aesthetic façade. It also serves to prevent soil sloughing and as a construction aid. In GMSE, however, facing is a major load‐carrying component; if facing fails, failure of the GMSE wall will usually be imminent.

In today’s practice, GMSE is enjoying a much wider popularity than GRS. This is in part because there is a lack of understanding of GRS, and in part because GMSE is similar to conventional earth retaining walls in design concept. Most designers are not entirely comfortable with a soil wall that achieves stability through internal reinforcing of the soil behind the wall rather than through the resistance offered externally by the facing.

Lately, a number of renowned designers and wall builders have estimated about 5–10% failure rate for GMSE, with a majority being associated with serviceability (i.e., excessive deformation). The National Concrete Masonry Association has also estimated a 2–8% failure rate of various types for GMSE walls. Whether it is structural failure or serviceability failure, the failure rate is much too high compared to other types of earth structures. Studies into the causes of failure have not lead to conclusive solutions to the problem. By employing tight reinforcement spacing to form soil–geosynthetic composites of higher stiffness and ductility, GRS has slowly but gradually affirmed itself as a viable alternative wall system to GMSE. GRS has promised some advantages as a sound wall system of the future, including (i) closely spaced reinforcement of GRS improves fill compaction efficiency and relaxes requirement of stiffness/strength of geosynthetics, (ii) GRS tends to be much less susceptible to long‐term creep when well‐compacted granular fill is employed, (iii) GRS provides much better seismic stability, and (iv) GRS mass exerts less earth pressure against facing and improves facing stability. Failure of GRS is practically nonexistent as long as well compacted granular fill is used. This is likely because GRS does not rely on the stability of any single structural component (e.g., facing or tension anchors) to maintain overall stability.

This book addresses both GRS and GMSE, with a much stronger emphasis on the former. Details of GMSE have been given by several design guides, such as the AASHTO bridge design specifications, the Federal Highway Administration NHI MSE walls and steepened slopes manual, and the National Concrete Masonry Association design manual. For completeness, this book begins with a review of shear strength of soils (Chapter 1) and classical earth pressure theories (Chapter 2). Chapter 3 addresses the observed behavior of soil–geosynthetic composites, reinforcing mechanisms of GRS, and GRS walls of different types of facing. Chapter 4 addresses geosynthetics as reinforcement, with emphasis on mechanical properties of geosynthetics, including load–deformation properties, creep properties, stress–relaxation properties, and soil–geosynthetic interface properties. Chapter 5 discusses design concepts of GRS walls and describes a number of prevalent design methods for GRS walls. In addition, recent advances on design of GRS and a new design method incorporating the recent advances is delineated. Design examples for each of the design method are given to help illustrate the design methods. Chapter 6 addresses construction of GRS walls, including construction procedure of GRS walls and general construction guidelines. It is my hope that the civil engineering community will become more familiar with GRS through this book, and makes better use of this novel technology in earthwork construction.

This book would not have been possible without the contribution of many of my colleagues and friends. Foremost is Professor Gerald A. Leonards, who was an inspiration for my lifelong interest in the theories and practice of geotechnical engineering. I must acknowledge Bob Barrett, a true innovator of reinforced soil technology, with whom I have had the privilege to work on many fact‐exploring projects over the past three decades. From Bob I leaned many key issues of GRS. I also wish to thank Mike Adams and Jennifer Nicks, two relentless FHWA researchers whose field‐scale experiments allowed me to learn the behavior of GRS. I also wish to acknowledge an outstanding wall builder, Calvin VanBuskirk, who had the vision to suggest separation of GRS from GMSE. I am especially in debt to Fumio Tatsuoka, who kindly shared many valuable experimental techniques and his unique experiences during my two sabbatical leaves at the University of Tokyo. Fumio was extremely instrumental for many field‐scale experiments of GRS that I was involved in.

I was fortunate to have worked with many outstanding research associates on GRS and related subjects, including (in alphabetical order) Noom Aksharadananda, Daniel Alzamora, Vasken Arabian, John Ballegeer, Bill Barreire, Michael Batuna, Melissa Beauregard, Richard Beck, John Billiard, David Bixler, Harold Blair, Eric Y. Chen, Nick S.‐K. Cheng, Nelson N. Chou, Alan Claybourn, Phil Crouse, David Curran, Mark Davis, Gary Dieward, Gene Dodd, Robert Duncanson, Nicolas El‐Hahad, Chris Ellis, Egbal Elmagre, Zeynep Erdogen, Barbara Evans, Tony Z‐Y. Feng, Seth Flutcher, Brian Francis, Chris Gemperline, Dave Gilbert, Justin Hall, Khamis Haramy, Mark Hauschild, Matthew Hayes, Sam Helwany, Dennis Henneman, Jason Hilgers, Zhenshun Hong, Kanop Ketchart, Elaheh Kheirkhahi, Cassie Klump, Ilyess Ksouri, W.T. Hsu, Kevin Lee, S.L. Lee, K.H. Lee, J.C. Lin, C.W. Ma, Paul Macklin, David Manka, Mike May, Rick McCain, Araya Messa, John Meyers, Greg Monley, Larry Moore, Mike Nelson, James Olson, Jean‐Baptiste Payeur, Breden Peters, Thang Pham, John Pierce, Michele Pollman, Xiaopei Qi, Mark Reiner, Zac Robinson, Jerzy Salamon, Brett Schneider, Pauline Serre, Rennie Seymour, Daming Shi, Barry Siel, Saeed Sobhi, C.K. Su, Omar Takriti, Damon Thomas, Sheldon C.‐Y.Tung, Alan Tygesen, Mark Vessely, Diane TeAn Wang, Roy Wittenburg, Derek Wittwer, Temel Yetimoglu, and Sam S.‐H. Yu.

Finally, I wish to express my gratitude to my mother Umeko to whom I owe everything.