Laboratory experiments on the removal of soil plugs during soil piping and internal erosion

Authors: WANGER, MIKAYLA - North Carolina State University; FOX, GAREY - North Carolina State University; Wilson, Glenn; NIEBER, JOHN - University Of Minnesota

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2018
Publication Date: 2/13/2019
Citation: Wanger, M., Fox, G., Wilson, G.V., Nieber, J. 2019. Laboratory experiments on the removal of soil plugs during soil piping and internal erosion. Transactions of the ASABE. 62(1):83-93. https://doi.org/10.13031/trans.13092.
DOI: https://doi.org/10.13031/trans.13092

Interpretive Summary: Plugging of soil pipes can result in erosion of hillslopes. A soil pipe becomes plugged when erosion inside the pipe is greater than the ability of the pipe to transport the sediment such as when the side-wall of a soil pipe collapses. When a pipe becomes plugged, a water pressure buildup occurs above the plug. This pressure may be enough to remove the plug, or the pressure may continue to build in the soil which can lead to hillslope failures. Almost no data exists to understand how or when plugs get removed or pressure buildup occurs or how to model these processes. This study involved controlled laboratory experiments to determine pressure buildup above soil plugs and conditions for which a plug is removed. Laboratory experiments were conducted with smooth and rough walled PVC pipes that were 100 cm long and at 0% slope. A pipe plug was established by packing soil inside the pipe. Experiments were conducted in triplicate with two pipe diameters, two soil types (sand and sandy loam), two plug lengths (3 and 6 cm), three pipe roughness, various packing densities, and with both changing and constant water pressures. Pressure gauges were installed along the pipe to monitor pressures both before and after the plug. The upslope pressure and the length of time that the plug withstood the pressure before removal were recorded. Regardless of time under pressure, all plugs were removed as intact plugs. Some sandy loam plugs withstood pressures of 100 cm of water; more cohesive plugs could withstand much higher pressures. Also, pressurized times exceeded 1000 s for some plug conditions even with short (3 to 6 cm) plugs. Therefore, hillslopes upslope of a plugged soil pipe may experience considerable pressure buildup for extended periods of time. Physical characteristics of plugs are important; for example, the plug’s bulk density had a positive relationship to the duration of pressure on the plug before removal. Soil water content changes inside of the plug will need to be considered as well as the type of condition in which flow was provided into the soil pipe. This experimental data uniquely provides insight into the changes of plugged soil pipes with the eventual goal of developing models to predict erosion within a pipe and hillslope failure.

Technical Abstract: Plugging of soil pipes can be detrimental to hillslope stability. A soil pipe becomes plugged through internal erosion when the sediment supply exceeds the transport capacity such as when the soil pipe wall collapses. When a pipe becomes plugged, a pressure buildup occurs upslope from the plug. This pressure may be enough to remove the plug, or the pressure may continue to build in the soil matrix which may lead to hillslope failures. Field observations have indicated occurrences of both processes, but limited to no data exists to understand if and when plug removal or pressure buildup occurs or how to model these processes. This study involved well-controlled laboratory experiments to determine instantaneous pressure buildup behind soil plugs and conditions for which an idealized plug in a soil pipe will be removed. Laboratory experiments were conducted with a smooth and roughened 100-cm long clear polyvinyl chloride (PVC) pipe with a 0% slope. A pipe plug (3 or 6 cm) was established 90 cm along the pipe length. Triplicate experiments were conducted with two pipe diameters, two soil types (sand and sandy loam), two plug lengths, three pipe roughness, various packing densities, and with both dynamic and constant heads. Digital pressure gauges were installed along the second half of the pipe to monitor pressures both before and after the plug. The upslope pressure and the length of time that the plug withstood the pressure before removal were recorded. Regardless of pressurized time, all plugs were removed as intact plugs. Some sandy loam plugs withstood pressures of 100 cm of water; more cohesive plugs could withstand much higher pressures. Also, pressurized times exceeded 1000 s for some plug conditions even with short (3 to 6 cm) plugs. Therefore, hillslopes upslope of a plugged soil pipe may experience considerable pressure buildup for extended periods of time. Plug physical characteristics are important; for example, the plug’s bulk density had a positive exponential relationship to the pressurized time. Soil water dynamics inside of the plug will need to be considered as well as the type of hydraulic boundary providing inflow to the soil pipe. This experimental data uniquely provides insight into the dynamics of plugged soil pipes with the eventual goal of assisting in being able to better predict internal erosion and hillslope failure. Future experiments are needed across a wider range of soil types.