Evaluation of Deep Patch Landslide Mitigation Design Methodology

As described in the final report, the research team evaluated the deep patch slope repair methodology by analytical methods and field observations for the purpose of developing a simple design method suitable for use by Federal Lands Highway and Forest Service personnel. Literature was reviewed, current design methodologies were documented and site visits were conducted to better understand how the deep patch methodology has been used in the past, to evaluate the performance of in-service deep patch sites and to help authenticate the newly proposed design method. An analytical study was conducted to model the effects of various slope configurations, failure mechanisms, deep patch design geometries, and type of geosynthetics using 2D and 3D computer modeling software. The effects of depth and vertical spacing of the reinforcement on the performance of the deep patch was analyzed in addition to the geometry of the reinforcement at the face of the slope. From these efforts, a new design procedure was developed.

The purpose of this project is to evaluate and improve current FHWA Office of Federal Lands Highway (FLH) and US Forest Service (USFS) deep patch design and construction methods.

FLH and its partners are responsible for thousands of miles of low-volume roads through steep mountainous terrain. Roads in mountainous terrain were often constructed by cutting into the uphill side of the road and casting the material onto the downhill side with little or no compaction, known as “cut and cast” construction. Over time, the soil materials settle and begin to creep down the slope, becoming an active, but relatively small scale and slow moving landslide under the roadway pavement. Stabilization of the pavement can require annual or even semiannual maintenance, and the traditional reconstruction method is an expensive process. A technique that has occasionally been utilized by the USFS and FLH to mitigate these small scale and slow moving landslides on low volume roads is the “deep patch”. A deep patch consists of subexcavating 1.5 to 6 feet below the surface and reconstructing a geosynthetic reinforced fill section over the landslide. Although the deep patch does not usually fully stabilize the landslide it has proven to be a very cost effective means of slowing the development and propagation of surface distress and settlement. The USFS has developed a simplified design method for the deep patch process, but actual field performance of deep patch sections has not been well documented. Through this project, WTI researchers will visit and collect data from deep patch construction sites, evaluate the performance of the deep patch process, and recommend improvements to current FLH and FS deep patch design and construction methods.