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