Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2012
Publication Date: 4/29/2013
Citation: Wilson, G.V., Nieber, J., Sidle, R.C., Fox, G.A. 2013. Internal erosion during soil pipeflow: State of the science for experimental and numerical analysis. Transactions of the ASABE. 56(2):465-478. Interpretive Summary: Erosion caused by water flowing underground, often called piping, is a leading cause of dam and levee failures, landslides, and gully formation. In this paper we review the terms used in various fields of science and engineering to describe piping processes and highlight the experimental and numerical work done to date on the specific processes involved in flow through soil pipes and the internal erosion of soil pipes. Mathematical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes in order to calculate effective critical shear stress and erodibility properties of soils. The most common mathematical models for pipe flow have been based upon Richards' Equation with the soil pipe treated as a highly conductive porous material instead of a void. Incorporation of internal erosion into such models is difficult due to enlargement of the soil pipe with time, by flow through the pipe being turbulent instead of smooth, and by temporary clogging of soil pipes causing unsteady flow and erosion characteristics. The current state of science for modeling these processes is identified, gaps in our understanding of pipe flow and internal erosion are identified and recommendations for future research are offered.
Technical Abstract: Many field observations have lead to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved and inconsistent use of terms have exasperated the problem. One such piping process that has experienced a lot of field observations but very limited mechanistic experimental work involves flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes result in internal erosion, stabilize hillslopes by acting as drains, result in hillslope instability by causing pressure buildups, and result in ephemeral gully formation or reformation of filled-in gullies. The objective of this paper was to review discrepancies in terminology to better explain the piping processes and highlight the experimental work done to date on the specific processes of soil pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies reforming due to mass failures caused by flow into discontinuous soil pipes, the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion, and the process of embankment failure due to internal erosion of soil pipes. In some of these studies the soil pipes were simulated with perforated tubes placed in the soil, while in other studies the soil pipes were formed out of the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes in order to calculate critical shear stress and erodibility properties of soils. The most common numerical models for pipeflow have been based upon Richards' equation with the soil pipe treated as a highly conductive porous media instead of a void. Incorporation of internal erosion into such models has proven complicated due to enlargement of the pipe with time, turbulent flow, and temporary clogging of soil pipes. These studies and modeling approaches are described and gaps in our understanding of pipe flow and internal erosion processes and our ability to model these processes are identified, along with recommendations for future research.