A Case Study: Post construction performance of rockfill columns installed in clay shale – Theodore Dam Right Abutment

ABSTRACT: Rockfill columns replace weaker native soils with a granular material of high frictional strength to develop a new shear strength comprised of both the native soil and the replacement material. This new strength from the combined materials is typically analyzed as an equivalent shear key. Application of rockfill columns for slope stabilization provides the ability to extend through zones of weak soil with minimal disturbance to the surrounding ground, resulting in a general improvement in factor of safety throughout construction. However, before this strength is realized, deformations are required to mobilize the friction of the rockfill columns with deformations typically ceasing within two years. In November 2020, 72 rockfill columns were installed into the natural valley wall in the right abutment to Theodore dam. The site consists of a clay till underlain by a pre-sheared, weathered clay shale and then by intact clay shale. Displacements have been shown to take place along an existing shear zone within the disturbed clay shale. Perched groundwater discharges into the clay till and disturbed clay shale from upslope. This perched groundwater is heavily influenced by precipitation. To provide a full understanding of movements within this slope, four shape accelerometer arrays (SAA) were installed before and after the construction in order to observe deformations. Two SAAs were installed three months before construction located above and below the rockfill column placement to observe overall site deformations. After construction, two additional SAAs were installed, each in a separate rockfill column to observe deformations within the rock column shear key. The deformation monitoring post-construction indicates continued deformations are driven by seasonal fluctuations. This paper provides a summary of the observations made from instrumentation and provides comments on the performance of the remediation method with correlation to laboratory data conducted on select clay shale specimens from the site.