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Western Dam Engineering Technical Note May 2016 25 posts, and excavating trenches or pits downstream of a dam. See additional cautionary discussion below on drilling and excavating near the dam toe. Figure 14. Potential Defects to a Confining Layer Investigation and Evaluation In the cases where site observations have provided some indication of an adverse foundation seepage condition—what is the next step? First, take a step back and look at the big picture. Information obtained from the site observations should be combined with as much background information as available, such as past inspection reports, original design records, modifications that have been made, performance data, and geotechnical studies. Underseepage evaluation would be fairly simple if foundation soils were homogeneous and their properties were isotropic (the same in all directions). That is usually not the case. Dam foundations are typically in valleys with soils that can consist of alluvium (stream-deposited), colluvium (gravity- deposited at the base of slopes), till (deposited by glaciers), residual soils (created in-situ by weathering of bedrock), and past manmade fills. These soils can range from open-work gravels (very high hydraulic conductivity) to high plasticity clays (very low hydraulic conductivity) and can be layered, lensed, segregated, or more likely, can be combinations of all three. It is important to have a geologic understanding of the dam foundation soils to interpret the soil properties that influence foundation performance. Key properties that influence a soil’s seepage and internal erosion potential are the gradation, relative density, and plasticity, all of which also affect the hydraulic conductivity of the soil. Site-specific information on the variable foundation soils is often limited. For dams with little to no information on the foundation, publically available geologic data can provide some insight on the types of native soils that may have been deposited in the foundation. The Natural Resources Conservation Service (NRCS), previously called the Soils Conservation Service, offers an online tool called the web soil survey. This tool acts as a database containing soil maps available for more than 95 percent of the nation’s counties: NRCS Web Soil Survey. However, soil surveys only map shallow soils, and more information is usually required to understand the full foundation profile with depth. After all available data is gathered, the engineer should consider the following questions to help characterize the severity of the foundation seepage condition in terms of dam safety and establish a path forward: Are there signs of foundation saturation: Wet ground with ponding water, well-developed water loving vegetation)? Are there signs of material erosion: sand boils or sediment plumes at the toe? If not, could ponded water be obscuring such detection or flow be carrying it away? Is there geological evidence nearby of conditions that create a confined aquifer situation (fine-grained, cohesive surface soils; likely limited foundation preparation)? Low-permeable layers, if continuous, along with the embankment, could provide a roof- supporting layer as well as a confining layer. Does the ground near the downstream toe feel “spongy?” Based on the geology of the valley, is it likely that an erodible layer continuous from upstream to downstream could exist in the foundation? Are there piezometers that allow for a reliable estimation of foundation pore pressures? Measurement Instruments The condition of a deep pervious foundation, not overlain by a confining layer, is more easily characterized. If there is evidence that a pressurized foundation seepage condition exists, along with backward erosion potential, then understanding the foundation seepage pressures is critically important. If