Levees: Problems and Solutions
Argus began developing cutting-edge methodologies for assessing factors of levee integrity in 2004, geared toward the unique conditions in the Sacramento-San Joaquin Delta (1). Geophysical methods are recommended for levee investigations to non-invasively record lateral changes in embankment and foundation materials (2, 3), and to "interpolate between borings, guide additional borings in anomalous areas, and as a basis to avoid unneeded borings" (4). In addition, over five years of research and correlation of data with the results of excavation has propelled development of the Buried Hazards Toolbox-What's in your levee?TM The Buried Hazards Toolbox targets sources of uncertainty for internal erosion such as rodent dens, relict and/or forgotten pipes, cables, concrete blocks and pads, storage tanks, origins of seepage pathways, and zones of unconsolidated, coarse-grained materials in levees.
With sufficient contrast between material properties of any of these hidden hazards and their surroundings a change in the data described as an anomaly may be identified on a qualitative basis. The actual cause of an anomaly is determined by conducting some level of invasive exploration such as probing, drilling, or excavation, which also allows for the geophysical data to be calibrated with known ground conditions and geology. There are two levels of effort that produce complementary data sets, Phase 1 and Phase 2.
Phase 1 is a rapid scoping or reconnaissance survey of an entire levee, which includes three lines on the levee crest. The Phase 1 geophysical data provides information suggesting starting points for investigations aimed to explain the agents, factors, and mechanisms that lead to deterioration and potential failure of the levee, as well as the course of action aimed to reinforce and/or repair it. This information will be used, in consultation with the Reclamation District, the District Engineer, and Geotechnical Engineer to select/prioritize areas that will be further investigated based on the data, current issues, and past performance. The majority of Phase 1 anomalies, being high contrast with a narrow spatial distribution, are relict objects such as pipes that are easily exhumed. As relict pipes and other unknown subsurface objects may create conduits that lead to piping and levee failure removing objects such as these prior to levee fortification has obvious benefits.
Phase 2 consists of quantitative analysis (inversion) of Phase 1 data and/or intensive surveys with appropriate instrumentation in the vicinity of anomalous areas and areas of concern, to obtain higher resolution geophysical data and develop correlations with geotechnical data. Phase 2 include can be a comprehensive approach to obtain data from deeper into the foundation across the entire levee, and can include data collected on the waterside and landside slopes and toes, or from the water. It can also be a targeted diagnosis approach using 2 or more methods to address specific problem areas, current and historic with quantitative methods such as inversion.
Inversion is a numerical method in which quadrature (apparent conductivity) and in-phase (magnetic susceptibility) data, collected at 1-, 2-, and 3.6-meter coil separations are transformed into a locally layered layered earth model. The horizontal layers vary in conductivity and thickness. EM3 data (in combination with EM-34 data for greater depths) are inverted using the Workbench (Hydrogeophysics Group, 2008). The software, originally designed for airborne electromagnetic data, was customized for the EM3.
The images in the Photo Gallery constitute representative levee problems and findings from anomaly excavations.
- Johnson, M.J., K. Tremaine, J. A. Lopez, Y. Liao. (2005). Geophysical Investigations at Webb and Holland Tracts-Toward Developing Levee Integrity Assessment Methods, Sacramento-San Joaquin Delta, California. Report to Department of Water Resources Bay Delta Office, Sacramento, CA
- Llopis J. L. & Simms J. E., Geophysical Surveys for Assessing Levee Foundation Conditions, Feather River Levees, Marysville/Yuba City, California, US Army Corp of Engineers, September 2007.
- Rutlege, F.A., M. Mauldon, and C.J. Smith, (2005) Geophysical Primer for Geotechnical Engineers. Center for Geotechnical Practice and Research at Virginia Tech, 123pp.
- U.S. Army Corps of Engineers (2005). ETL 1110-2-569 Design Guidance for Levee Underseepage. Washington D.C.