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<br /> <br /> <br /> <br />17 <br />Geology 101 – Dam Good Foundations <br />Introduction <br />The behavior and performance of manmade structures <br />(buildings, dams, roads, tunnels, etc.) depend on the <br />foundation that supports them. A foundation is formed <br />by existing geology at the site. Being familiar with <br />geology and being able to identify the foundation’s <br />rock or soil type assists in developing an understanding <br />of what types of issues should be considered when <br />designing, constructing, modifying, or repairing a <br />structure. <br />Understanding how to deal with issues related to <br />geology can prevent costly mistakes. (What makes the <br />situation better, what <br />makes it worse?) What <br />are the limitations of <br />different soil and rock <br />types? <br />Recognizing basic <br />geologic conditions <br />can help evaluate how <br />dams will behave with <br />respect to seepage, <br />settlement, slope <br />stability, piping, and <br />other problems that <br />affect dams. <br />Civil engineers like <br />Terzaghi, Peck, and <br />Leggett, recognized <br />that major engineered structures were constrained by <br />the foundations they were placed upon or the <br />materials available for their construction. It could be <br />argued that these men coined the term “Engineering <br />Geologist”, which is a way to differentiate the study of <br />geology as it is applied to the use of earth’s natural <br />materials to house (tunnels, pipelines, underground <br />structures), found (structures with shallow to deep <br />foundations), and build (dams, dikes, levees, concrete) <br />engineered structures. <br />This article is intended as a geologic primer for the <br />basics of geology and geologic principles, different rock <br />and soil types and simple ways to distinguish between <br />them, and potential issues affecting dams as related to <br />different types of geology. <br />Rock <br />The three classes of rock are igneous, sedimentary, <br />and metamorphic. Igneous rocks are formed by cooling <br />and crystallization of liquid rock materials and as a <br />result have distinctive texture and composition. <br />Sedimentary rocks form at the earth’s surface through <br />the activity of the hydrologic system. Two main types <br />of sedimentary rocks are clastic rocks, consisting of <br />rock and mineral fragments, and chemical or organic <br />rocks consisting of chemical precipitates or organic <br />material. The conglomerate shown in Figure 2 is an <br />example of a clastic sedimentary rock. <br /> <br />Figure 2 - Outcrop of Conglomerate <br />Metamorphic rocks result from changes in <br />temperature and pressure and the chemistry of pore <br />fluids. Igneous and sedimentary rock can each be <br />subjected to these forces to become metamorphosed. <br />Minerals are formed by compounds of elements. The <br />most common compounds are silicates, carbonates, <br />oxides, and sulfates. These compounds form hundreds <br />of minerals, but there are a few common rock-forming <br />minerals that can be easily identified in the field to <br />help classify rocks. Silica and feldspar are the most <br />abundant minerals in the upper part of the earth’s <br />crust. Using a quartz, alkali, plagioclase (QAP) diagram, <br />the percent of quartz, plagioclase feldspar, and alkali <br />feldspar is estimated and gives the rock type. Figure 3 <br />shows a QAP diagram for intrusive igneous rocks and <br />Figure 4 shows a QAP diagram for extrusive rock. <br />Figure 1 - Leaning Tower of Pisa