Geosynthetic Drains for Slope Stability and Rehabilitation

The Arkansas State Highway and Transportation Department spent approximately $600,000 per year for road repairs damaged by landslides between the years of 1986 and 1990, according to a memorandum that surveyed landslide correction costs on state highway systems, prepared at the Federal Highway Administration. On January 8, 1999, the Arkansas State Highway and Transportation Department (AHTD) opened the final section of Interstate 540 (I-540) which connects Ft. Smith to Fayetteville. The new 43 mile span traverses the rugged terrain of the Ozark Plateau in Northwest Arkansas and has 13 major bridges as well as a 430 m twin bored Tunnel. The overall cost for this project was $460 million (approximately $11 million per mile), which is an expensive project for this area of the country. During the construction of this new highway, repair of slope failures and modifications to slide prone areas cost approximately $65,000 per mile per year, according to information sourced from the "Hazard Mitigation Plan in Clay County, Arkansas, prepared for the Clay County Disaster Preparedness Council in Piggot, Arkansas. Additionally, AHTD spent over $33 million on slope repairs for this section of road during the first four years of service. The AHTD has documented 21 slope failures between May of 2001 and May of 2002. These expenditures have made this section of I-540 the most expensive interstate ever built in Arkansas. A southern extension of this highway system through similar terrain is anticipated. As a result, there is a need to develop cost effective repair techniques to existing slope failures or accident-prone areas in the state of Arkansas. Existing slopes can fail as a result of erosion, tension or shrinkage crack development, loading at the crest of the slope, and the removal of earth at the toe of the slope. More commonly, however, fluctuations in the water table can increase pore water pressures, decrease the shear strength of the soil, and cause slope instabilities, especially if the drainage capacity is limited. Currently, the principal remediation technique involves the removal of failed material in conjunction with rock buttressing. The AHTD has expressed an interest in investigating cost effective, preventative measures to install drainage systems and remotely monitor performance for areas that are prone to slope instability issues. This proposal will discuss the possibility of incorporating the use of geosynthetic drains to address the problem in the state of Arkansas. There are a variety of geosynthetic drainage products available to suit this application. Additionally, geosynthetic manufacturers are willing to accommodate a new application with customized products. The principal investigators not only have an expertise in geotechnical engineering and slope stability, but they have contacts in the industry to make these kinds of opportunities available for research. Geosynthetic drains typically consist of a polypropylene core that provides the necessary flow capacity, and a geotextile filter jacket that prevents soil adjacent to the core from impeding the channels. Various geosynthetic products designed for drainage are commonly installed in the vertical direction to promote preconsolidation in geotechnical engineering applications, or adjacent to roadways in pavement design applications. In order to extend the drainage function, the author suggests installing geosynthetic drains horizontally to provide additional drainage and reinforcement for slope stability. Full-scale field testing is needed to further evaluate the performance aspect and cost benefit of this concept for the purpose of providing a design and construction procedure for slope rehabilitation that will ultimately improve the safety of our highway structures.