Use of Finite Difference Numerical Technique to Evaluate Deep Patch Embankment Repair with Geosynthetics

Low-volume roads constructed in steep hillside terrain by cut and cast techniques may experience instability in the form of excessive subsidence that leads to large cracks and differential movement along the roadway bench. The technique of deep patch embankment repair with geosynthetics (DPERG) has been employed in the western United States; DPERG generally involves a 1- to 2-m-deep excavation that is backfilled with compacted granular soils and one or more layers of geosynthetic reinforcement. The design goal of a DPERG is not necessarily to eliminate future slope movement but to confine potential failure surfaces to a region of the slope well below the roadway bench and extending out to the slope face such that a failure surface does not extend up onto the roadway bench. This design results in movement along the roadway bench that is more uniform and less disruptive to traffic. This paper describes the results of a study to evaluate the DPERG technique by analytical methods supported by field observations for the purpose of determining the required depth of the DPERG and the optimum layer spacing of the reinforcement. The study showed that for a given slope geometry and set of soil properties that had led to failure in an unreinforced slope, there were several combinations of DPERG depth and number of reinforcement layers that satisfied the design goal. The study showed that more tightly spaced reinforcement layers were beneficial and that for widely spaced layers, the design goal of a DPERG could not be met, even for a thick DPERG depth.