# Geosynthetic Reinforced Steep Slopes: Current Technology in the United States
## 1 Introduction  
Reinforced slopes are a form of mechanically stabilized earth that incorporate planar reinforcing elements for the construction of sloped structures with inclinations less than 70\({}^{\circ}\). Structures with inclinations over 70\({}^{\circ}\) are classified as walls [1]. For uniform fill soil, there is a limiting slope angle under which an unreinforced slope may be built safely. The limiting angle of the slope is equal to the friction angle of the soil in the case of a cohesionless and dry material. Therefore, a slope with an angle greater than the limiting slope angle is referred to as a steep slope, which requires additional forces to maintain equilibrium [2]. The purpose of steep slope construction is to solve problems in locations of restricted right-of-way and at marginal sites with difficult subsurface conditions and other environmental constraints. Since soil has limited tensile strength, similar to concrete, reinforcement must be used to overcome this weakness [3]. To improve strength and make a soil structure self-supporting, tensile reinforcing elements are placed in the soil. The reinforcing elements can also withstand bending from shear stresses, providing increased stability to steep slopes [4]. Soil reinforcement concepts and technologies originated in prehistoric times. Straw, sticks, and branches were traditionally used to improve the quality of adobe bricks, reinforce mud dwellings, and even reinforce soil for erosion control. However, modern techniques for mechanically stabilizingsoil were introduced in the 1960s. First used in France, a method known as \"reinforced earth\" used embedded narrow metal straps to reinforce soil [5]. In 1972, this technique was adopted in the United States by the California Division of Highways for construction of retaining walls. Many other soil reinforcement methods were researched and implemented following the first applications in the United States [6]. Geosynthetics are modern materials used to improve soil conditions by providing tensile resistance and stability. In the case of soil reinforcement, a primary application of geosynthetics involves reinforcing steep slopes. With the use of geosynthetics, the construction of reinforced steep slopes is often more affordable and technically feasible when compared to traditional construction techniques [7]. Furthermore, geosynthetic reinforcement is a cost-effective solution for stabilizing recurring slope failures and constructing new permanent embankments [8; 9]. Geosynthetic reinforcement of steep slopes was initially utilized to repair failed slopes. Instead of importing soil to reconstruct the slopes, the slide debris was salvaged and reused with the addition of geosynthetic reinforcement, resulting in reduced costs. To provide stability, multiple layers of geosynthetic materials were placed in a fill slope during reconstruction. As per Figure 1, in addition to repairing failed slopes, steep slope reinforcement has also been introduced for the construction of new embankments, the widening of existing embankments, and as an alternative to retaining walls [10]. As reinforced slopes became constructed to perform as permanent structures, the structural analysis method used for near-vertical reinforced walls was adopted, which relies on analytical predictions, factors of safety, and reduction factors. Consequently, design methods for reinforced steep slopes are reasonably conservative [8]. Research efforts regarding the design and application of geosynthetics are rapidly increasing on the international level as well, including investigations in Canada, Taiwan, and the United Kingdom [11; 12; 13]. This study provides a thorough review of the current technology on geosynthetic reinforced steep slopes. Findings from state, national, and international sources were summarized and compared to establish best practices.