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Geosynthetic Reinforced Steep Slopes: Current Technology in the United States

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Geosynthetic Reinforced Steep Slopes: Current Technology in the United States applied sciences Review Geosynthetic Reinforced Steep Slopes: Current Technology in the United States Yoo-Jae Kim 1,*, Ashley Russell Kotwal 1, Bum-Yean Cho 2, James Wilde 1 and Byung Hee You 1 1 Department of Engineering Technology, Materials Science, Engineering, and Commercialization Program, Texas State University, 601 University Drive, San Marcos, TX 78666, USA; [email protected] (A.R.K.); [email protected] (J.W.); [email protected] (B.H.Y.) 2 Department of Fire Safety Research, Korea Institute of Civil Engineering and Building Technology, 64 Ma-doro 182beon-gil, Mado-myeon, Hwaseong-si, Gyeonggi-do 18544, Korea; [email protected] * Correspondence: [email protected]; Tel.: +1-512-245-6309 Received: 5 April 2019; Accepted: 13 May 2019; Published: 16 May 2019 Abstract: Geosynthetics is a crucial mechanism in which the earth structures can be mechanically stabilized through strength enforcing tensile reinforcement. Moreover, geosynthetic reinforcement stabilizes steep slopes through incorporating the polymeric materials, becoming one of the most cost-effective methods in not only accommodating budgetary restrictions but also alleviating space constraints. In order to explicate on the applicability and widen the understanding of geosynthetic reinforcement technology, a synthesis study was conducted on geosynthetic reinforced steep slope. This study is very important because in not only highlighting the advantages and limitations of using geosynthetic reinforcement but also in investigating the current construction and design methods with a view to determining which best practices can be employed. Furthermore, this study also identified and assessed the optimal condition of the soil, performance measures, construction specifications, design criteria, and geometry of the slope. To further concretize the understanding of these parameters or factors, two case studies were reviewed and a summary of the best practices, existing methods, and recommendations were drawn in order to inform the employment of geosynthetics in reinforcing steep slopes. Keywords: reinforcement; geosynthetics; geogrid; soil; slope 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◦. Structures with inclinations over 70◦ 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 stabilizing Appl. Sci. 2019, 9, 2008; doi:10.3390/app9102008 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2008 2 of 24 soil 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 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 23 States [6]. Geosyntheticssoil were introduced are modern in the 1960s. materials First used used in toFrance, improve a method soil conditionsknown as “reinforced by providing earth” used tensile resistance and stability.embedded In narrow the casemetal ofstraps soil to reinforcement,reinforce soil [5]. In a1972, primary this technique application was adopted of geosyntheticsin the United involves States by the California Division of Highways for construction of retaining walls. Many other soil reinforcingreinforcement steep slopes. methods With were the researched use of geosynthetics,and implemented the following construction the first applications of reinforced in the steep slopes is often moreUnited aff ordableStates [6]. and technically feasible when compared to traditional construction techniques [7]. Furthermore,Geosynthetics geosynthetic are modern reinforcement materials used is a cost-eto improveffective soil solutionconditions forby stabilizingproviding tensile recurring slope failuresresistance and constructing and stability. new In the permanent case of soil embankments reinforcement, a [ 8primary,9]. application of geosynthetics involves reinforcing steep slopes. With the use of geosynthetics, the construction of reinforced steep Geosyntheticslopes is often reinforcement more affordable ofand steep technically slopes fe wasasible initially when compared utilized to totraditional repair failedconstruction slopes. Instead of importingtechniques soil to [7]. reconstruct Furthermore, the geos slopes,ynthetic the reinforcement slide debris is a was cost-e salvagedffective solution and reused for stabilizing with the addition of geosyntheticrecurring slope reinforcement, failures and constructi resultingng new in permanent reduced embankments costs. To provide [8,9]. stability, multiple layers of geosyntheticGeosynthetic materials reinforcement were placed of in steep a fill slopes slope was during initially reconstruction. utilized to repair failed As per slopes. Figure Instead1, in addition to of importing soil to reconstruct the slopes, the slide debris was salvaged and reused with the addition repairingof failedgeosynthetic slopes, reinforcement, steep slope resulting reinforcement in reduced has costs. also To been provide introduced stability, multiple for the layers construction of of new embankments,geosynthetic the materials widening were of placed existing in a fill embankments, slope during reconstruction. and as an As alternative per Figure 1, to in retainingaddition walls [10]. As reinforcedto repairing slopes failed became slopes, steep constructed slope reinforcement to perform has also as permanent been introduced structures, for the construction the structural of analysis methodnew used embankments, for near-vertical the widening reinforced of existing walls embank wasments, adopted, and as an which alternative relies to on retaining analytical walls predictions, [10]. As reinforced slopes became constructed to perform as permanent structures, the structural factors ofanalysis safety, method and reductionused for near-vertical factors. Consequently, reinforced walls designwas adopted, methods which for relies reinforced on analytical steep slopes are reasonablypredictions, conservative factors of [ 8safety,]. Research and reduction efforts fact regardingors. Consequently, the design design and methods application for reinforced of geosynthetics are rapidlysteep increasingslopes are reasonably on the internationalconservative [8]. levelResear asch efforts well, regarding including the investigations design and application in Canada, of Taiwan, and thegeosynthetics United Kingdom are rapidly [11 –increasing13]. on the international level as well, including investigations in Canada, Taiwan, and the United Kingdom [11–13]. FigureFigure 1. 1.Geosynthetic Geosynthetic Reinforced Reinforced Steep Steep Slope SlopeApplications Applications [10]. [10]. This study provides a thorough review of the current technology on geosynthetic reinforced Thissteep study slopes. provides Findings a thoroughfrom state, review national, of and the currentinternational technology sources were on geosynthetic summarized and reinforced steep slopes.compared Findings to fromestablish state, best practices. national, The and study international synthesized information sources weregathered summarized from a literature and compared to establishreview, best interviews, practices. and surveys The study of transportation synthesized agencies, information educational gathered institutions, from consulting a literature review, interviews, and surveys of transportation agencies, educational institutions, consulting engineers, manufacturers, material suppliers, and construction contractors. To obtain supplementary information and professional insight, experts were also identified from the survey. Appl. Sci. 2019, 9, 2008 3 of 24 2. Geosynthetic Materials Geosynthetics are planar products manufactured from a polymeric material used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system [14]. They are identified by the type of polymer, type of fiber or yarn, type of geosynthetic, mass per unit area or thickness, and any additional clarification needed to describe the geosynthetic. Geotextiles are defined as any permeable textile used with foundation
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