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Western Dam Engineering Technical Note 3 The Soil Characterization Process The key steps in the soil characterization process leading to dam modification are outlined as follows: 1. Identify required soil properties to characterize an identified dam deficiency 2. Level One Soils Characterization – Paper Study (utilization of existing data, correlations, and graphical tools) 3. Level Two Soils Characterization - Site Investigation (subsurface investigation, field testing, laboratory testing) 4. Developing input parameters for analyses and design In the following paragraphs, we’ll take a more detailed look at each of these soil characterization steps. Identifying Required Soil Properties A trigger event (see above) usually establishes the need for further evaluation, investigation and/or analysis, which in turn governs which soil properties are required. For example, repair or modification became necessary for some dams that experienced excessive seepage and slope failure during the record- setting flooding along Colorado’s Front Range in the fall of 2013. Assuming flattening or buttressing the downstream slope and/or adding an internal filter zone is selected as the design remedy, the key required soil properties necessary for design would then be strength, gradation, and hydraulic properties (permeability). Or, erosive properties of a soil may be required for design and repair of an emergency spillway that experienced severe erosion during the flooding. Some of the key soil properties requiring characterization that are most common to dam modifications are:  Shear Strength – ability of soil to resist failure (rupture or sliding) under loading  Permeability – ability of water to seep or flow through void spaces in soil or through fractures and joints in rock  Compressibility – susceptibility to volume change under loading; can include immediate settlement, consolidation, shrink/swell, and collapse  Protective Filters* – used to direct flow and prevent migration of fines or piping between various zones and foundations of embankment dams *Although not a soil property, soil characterization is a key component of filter design  Erosion Resistance – ability of a soil to resist erosive seepage or water flow; includes internal erosion and surface erosion  Dispersibility – susceptibility of soil (typically clay) particles to break apart or disperse when wetted due to an unstable soil structure  Compaction Characteristics – the degree to which a soil can densify through mechanical compaction methods Table 1 at the end of this article provides a summary of key considerations, test methods, and required sample types for characterizing the soil properties listed above. There are of course numerous other soil parameters that may come into play for various dam rehabilitation projects (bearing capacity, lateral earth pressures, cyclic or seismic characteristics, etc.). However, the intent of this article is to discuss a few of the most common parameters. Level One Soils Characterization Depending on the availability and quality of existing soils data, Level One soils characterization may be all that is necessary (or affordable) for most low hazard, low cost structures. Level One typically involves review of existing data and use of correlations, and may also include use of graphical tools such as subsurface profiles or cross sections. Existing Soils Information Review Before spending money on a new geotechnical investigation, available documents should be researched and reviewed for pertinent soil It’s very important to check applicable state rules, regulations, and/or guidelines that provide specific requirements for dam modification / construction, analyses and design. These will guide the engineer’s determination of the required soil properties.