Estimating Saturated Conductivity and Modeling Infiltration Rates on Highway Embankments for Stormwater Management

Focus Area

Water Quality/Wetlands


Natural Resources






Over 3 years

Research Idea Scope

Background: Natural and engineered dispersion are best management practices (BMPs) designed to receive highway runoff from sheet flow for the purposes of water quality treatment and flow control. These BMPs are sloped vegetated areas, typically on or immediately adjacent to the highway embankment. The design criteria for these BMPs in the Washington State Department of Transportation’s (WSDOT) Highway Runoff Manual (HRM) (WSDOT 2014) require knowledge of the saturated hydraulic conductivity (Ksat) of underlying soils to estimate the dispersion area needed. Currently, Ksat can be determined by in-situ testing using the Guelph Permeameter, large or small scale Pilot Infiltration Test (PIT) methods, or by a numerical model based on soil gradation. However, all these methods have limitations. The Guelph Permeameter is only approved for use in eastern Washington and along existing highway embankments that are retrofitted to meet stormwater requirements. Since the Guelph Permeameter is recommended for homogeneous, fine grain soils, it is not appropriate for all soil conditions in the state. PIT methods are expensive, time consuming, involve digging a large hole, and require lots of water to perform the tests. PIT methods are more appropriate for large facilities, such as infiltration ponds. Finally, the numerical model was developed using native and laboratory created soils subjected to minimal compaction. These methods are inappropriate for estimating Ksat along highway embankments, since most embankments are constructed of coarse grained soils and compacted to between 90 and 95 percent of maximum density. A new method for estimating Ksat of compacted soils along highway embankments is needed. This new method should enable hydraulic conductivity values to be calculated using grain size to extrapolate to the compacted state. This method should also allow designers to determine the hydraulic conductivity for proposed slopes using the grain size distribution of specified fill materials. Research Objective Phase I: Estimate saturated hydraulic conductivity (Ksat) along highway embankments using a numerical model for the purpose of designing stormwater sheet flow best management practices (BMPs). The objective of this research is to measure infiltration of highway runoff into embankment side slopes to support future revisions to WSDOT’s continuous hydrologic model for stormwater facility design (MGS Flood Version 4) (MGS Flood 2015). The scope of this work is focused on the geotechnical characterization and analysis of several test sites where inflow and outflow are monitored so that the data can be properly interpreted. Specific Tasks Task 1: Literature Review, Compilation of Data and Survey of State Practices and Experience Task 1 continues a literature search to determine accepted methods for estimating Ksat using numerical methods, and includes testing both in-situ and in the laboratory. Based on the findings of the literature search, a research numerical model and testing methods will be defined. This task will consist of two parts; summarizing the findings from a similar study performed at WSDOT, and performing a literature search for similar research studies that have been performed outside WSDOT. An initial literature review did not find specific information on determining soil infiltration rates for stormwater facilities. Task 2: Field Data Task 2 will focus on the selection of two new study sites as well as documenting and collecting field data (i.e., soil samples) for a numerical model. The task also includes testing Ksat in-situ. This work involves drilling borings to complete the geotechnical investigation of five existing stormwater monitoring sites along Interstate 5 (I-5) north of Everett, Washington, and two new sites to be selected and monitored as part of this study. Each boring will have a piezometer with a data logger to monitor ground water levels during the study period. Samples from each boring will be tested to obtain gradation parameters and soil permeability. In addition, permeability values will be obtained in the field during test hole drilling using the Packer Permeability Test. Water quality and surface water flow data will be collected from all study sites following guidance from the Washington State Department of Ecology’s Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies: Technology Assessment Protocol – Ecology (TAPE) (Ecology 2011). Geotechnical and surface flow data will be used to model the water losses due to infiltration. Lastly, the model will be used to make revisions to MGS Flood Version 4. Task 3: WSDOT Materials Lab Perform Testing Task 3 will be performed by the WSDOT materials lab and involves all laboratory testing including collecting any field samples necessary to perform laboratory tests and performing in-situ PIT tests. In addition, the materials lab will perform any sieve analyses necessary from the soil samples collected. Task 4: Summarize Field and Lab Testing Task 4 will be performed by a researcher with guidance and review from the project manager and materials lab. This task will use the field and lab testing data to develop a correlation to the numerical models and summarize the study and recommendations in a draft report. Task 5: Peer Review Task 5 will be performed by three recognized experts from outside WSDOT. The review panel will perform an independent evaluation of the research report. Their comments will be incorporated into a final report. Task 6: Ecology Review Task 6 involves presenting report findings to the Washington State Department of Ecology (Ecology) in support of revising stormwater BMP guidance defined in the WSDOT Highway Runoff Manual (HRM) (WSDOT 2014). This will include time for the researcher to prepare and present the research findings to Ecology as well as revise the HRM based on the Ecology-approved final design guidance. In addition, time is included in this task for the project manager and materials lab to meet with Ecology and review both the presentation and HRM revisions. Research Objective Phase II: Develop a representative method for estimating the volume of stormwater that will infiltrate into the highway embankment and through the subsoil considering the underlying soil characteristics. This phase of the study will address Ecology’s concern with modeling infiltration through embankments that have underlying impermeable soil layers. Impermeable layers may prevent soils from infiltrating water through the embankment vertically. Instead, water may reemerge out the side of the embankment as interflow. Task 1: Literature Review Conduct a literature search for studies that evaluated methods for testing how compact soils affect rates of hydraulic conductivity. Task 2: Field Data Collection and Groundwater Modeling Task 2 will perform infiltration testing along highway embankments and use the field data along with a groundwater modeling program to estimate the stormwater detention time of various embankment soils. Task 3: Final Report A final report will recommend a numerical model(s) (based on groundwater modeling) that can be used during stormwater BMP design to estimate the final volume of stormwater present as runoff.

Urgency and Payoff

If saturated hydraulic conductivity (Ksat) is underestimated, dispersion and infiltration best management practices (BMPs) are overdesigned, directly impacting the cost of projects. This often requires an expansion of the roadway footprint to allow for additional area. This can result in unnecessary right of way (ROW) acquisition and construction costs, as well as additional ecological impacts to established native vegetation or environmentally sensitive areas. To keep highway construction costs down and ecological impacts to a minimum, an accurate and cost effective method for estimating Ksat rates and water losses on highway embankments is needed.

Suggested By

Fred Bergdolt Washington State Department of Transportation 360.570.6648

[email protected]