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Western Dam Engineering Technical Note 2 Soil Characterization – Here’s the Dirt (Part 1) Introduction Soil characterization can be a dirty job. So dirty that it makes jobs on Mike Rowe’s Dirty Jobs TV show look clean by comparison. There’s the obvious dirt involved with drilling and soil sample collection in the field, then it gets even dirtier in the laboratory where soil gets scooped, mixed, wetted, baked, squeezed, and shaken in an effort to discover its properties. Perhaps the “dirtiest” (i.e., not clean or straightforward) aspect of soils characterization happens in the engineer’s office where the usually challenging task of characterizing soil properties and utilizing the results begins. But the trail of dirt actually begins long before drill rigs mobilize and samples get shuttled to the laboratory. The need for site-specific soil characterization is usually justified with a trigger event such as:  Dam hazard reclassification  Owner’s decision to significantly modify the dam  Flooding or a seismic event that puts an unprecedented or extreme loading on the dam  Periodic dam inspection that identifies a dam safety deficiency. The trigger event can lead to a dam safety evaluation or other study, which in turn may trigger geotechnical investigation, analyses, and dam modification. Soil characterization is a critical step in the dam modification process – it is the culmination and product of data review, use of empirical correlations, and/or geotechnical investigation, and it is the basis of analyses and design. For a Dam Owner or small engineering firm with limited dam design experience, trying to understand soil characterization can seem daunting. This article helps the reader through the soil characterization labyrinth by presenting the fundamentals of soil characterization pertinent to dam design and providing some key resources that can be useful. So let’s dig in and get dirty! The Challenge of Soil Characterization Failure to develop meaningful, representative soil parameters can result in faulty analyses and design, unacceptable dam safety risks, and wasted money. The acronym GIGO (Garbage In - Garbage Out) captures it well: the outcome of geotechnical analyses, design, and construction will only be as good as the input soil parameters upon which they are based. Soil characterization is usually an inexact process, requires considerable experience and judgment, and sometimes resembles more of an art than a science. To the unenlightened, dirt is…well, just plain ol’ dirt. But to the geotechnical engineer and enlightened Dam Owner, dirt is anything but, considering the following:  There are many types of soils used as engineering materials, each with different properties and behavior  There are a plethora of soil characterization tools to choose from – investigation methods, laboratory tests, equations, correlations, “index” properties, classification systems, and soil behavior models  Unlike other engineering materials with uniform or isotropic composition / behavior such as steel or concrete, soil is a 3-phase material (solid particles, water, and air) with potential for complex physical interactions among the three constituents and under various loadings