In many cases, slope instability can be attributed to the effects of groundwater and associated pore-water pressures. Typically, drainage considerations for embankments can be readily incorporated during the design and construction process. However, subsurface drainage design and installation for marginally stable existing or new cut slopes can be challenging and typically involves the installation of horizontal drains and interceptor trenches. Horizontal drains can function unpredictably if not designed or installed adequately, or sited in inappropriate materials. Subsurface drainage is generally understood to be an important concept; nevertheless, the underlying principles, design, construction and maintenance considerations are not fully realized nor is the state-of-the-practice adequately documented. As a result, subsurface drains are often installed in a makeshift manner with varying degrees of success. Although subsurface drainage and horizontal drains have been used for many years, the amount of applied research directed at improving subsurface drainage techniques for slope stabilization applications is limited. Research is needed to identify, collect and develop best practices and guidelines to raise the standards for subsurface drainage design, installation and maintenance. This research is especially important because subsurface drainage is typically the most cost-effective stabilization measure, often being an order of magnitude less than other commonly employed slope stabilization measures. In addition, the research should explore new applications of existing materials and technologies that can be advantageously applied to subsurface drainage systems for slope stabilization. Examples could include trenchless technologies as well as other innovative excavation equipment and techniques that could be adapted to provide more effective, efficient installation and function of subsurface drainage systems within marginally stable slopes. Properly designed, installed and maintained subsurface drainage systems can, in many instances, be the most cost effective solution to stabilize slopes by lowering and controlling groundwater flow. Unfortunately, concerns with the effectiveness and long-term performance of drainage methods have limited the application of such techniques as a primary means to mitigate slope instability. This uncertainty has limited their use even when subsurface information and stability analyses clearly indicate that stabilization could be achieved with proper modification and control of groundwater conditions. The research and development proposed herein will potentially advance more extensive use of subsurface drainage systems for slope stabilization, significantly reduce expenditures for slope stabilization, improve performance, and provide for more rapid installation.

The objectives of this research are as follows: - To provide best practices and guidance for subsurface drainage applications for slope stabilization, including subsurface investigation and testing, groundwater-flow characterization, analysis, drain configurations and design, installation methods, monitoring, and maintenance. - To evaluate new applications of existing materials and technologies, such as trenchless technologies (horizontal directional drilling, micro tunneling, guided boring, etc.) and other innovative technologies and materials, for stabilizing slopes using subsurface drainage.

The anticipated scope of the study would consist of the following tasks: - Perform a literature review of applications, design methodology, construction, and maintenance of subsurface drainage systems used for slope stabilization; - Survey an appropriate range of designers/owners regarding applications, types of systems, design methodologies, and function/performance; - Evaluate short- and long-term performance of a limited number of existing installations utilizing available or new instrumentation; - With currently available groundwater characterization and modeling techniques, develop methodology for determining appropriate applications and design of subsurface drainage for slope stabilization; - Produce a summary report that includes detailed recommendations/guidelines on site suitability; groundwater characterization; design methodology; detailing and generalized specifications; construction; and maintenance of subsurface drainage systems. A technical advisory group consisting of representatives from contributing agencies and invited participants will refine this scope of work and provide periodic technical input/guidance throughout the duration of the research.