CS25C - Concurrent Session 25C: Influence of Wet-Dry Weathering on Hydraulic Conductivity and Infiltration Rate for Fine-Grained Soils and Weathered Shales
Graduate Student South Dakota School of Mines and Technology Whitewood, SD
Abstract Description: Spillways and spillway gates are crucial components of our nation’s hydro-geo-structures. Providing overflow relief during times of extreme precipitation events to help prevent ultimate failure of dams and levees systems. These crucial components are a key barrier between the saturated earth behind the wall and the spillway itself. During seasonal moisture fluctuations, such as wet-dry weathering, the soil behind the spillway walls can experience drying and form tensile cracks in the bulk soil mass. These tensile cracks have been shown to increase the overall infiltration rate of moisture in a soil mass and result in the build up of excess pore water pressure. With the average age of dams being 60+ years in the United States the associated spillways are already at an aged and degraded state. The build up in excess pore water pressure increases the stresses behind the wall, promoting failure of the already weakened structure. This study was dedicated to investigating how seasonal wet-dry cycles impact the hydraulic conductivity values of a fine-grained soil and a weathered residual shale. While these geo-materials do not represent optimum soil types, they do represent commonly encountered geo-materials in aging infrastructure that was constructed with based on availability of nearby geo-materials. This study was dedicated to utilizing real-world geomaterials and not laboratory derived soils. Each soil in this study was molded at its optimum moisture content and then exposed to 0, 1, 2, 10, and 20 wet-dry cycles. It was found that both soils experienced an increasing trend in their hydraulic conductivity values. It was also found that soils with higher plasticity experienced increased rates of degradation due to the high amount of shrink/swell associated with each wet-dry cycle. This resulted in a larger impact to the plastic soils hydraulic conductivity with increasing cycles.
Learning Objectives:
To demonstrate that design/maintenance of spillways should take into consideration seasonal effects of weathering.
To show readers that seasonal wet-dry weathering cycles can significantly alter the hydraulic conductivity value of in-situ soil masses.
To demonstrate to new engineers that sometimes geo-structures are built with sub-par geo-materials and their long-term values need to be considered in design.
