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20 not develop strong foliation but instead develop a granular texture with larger crystals. Figure 11 - Outcrop of Metamorphic Gneiss (Pronounced “Nice”) Gneiss is identified by its alternating light and dark layers and usually wavy appearance. Quartzite usually resembles sandstone but the sand grains are fused together. Figure 12 shows an example of gneiss and quartzite. Sample of Gneiss Sample of Quartzite Figure 12 - Examples of Metamorphic Rocks Weathering Weathering is a process that decomposes all geologic materials into a soil over time. Weathering can be a physical process or a chemical process. Physical weathering occurs when wind, water, waves, ice, and other environmental conditions break down a rock and transform it into soil. This break-down process can occur in-place or it can result in erosion of the rock into particles that are transported and deposited as a soil at some other location. Generally, the properties of soils formed by in-place physical weathering retain some of the characteristics of the parent material. Chemical weathering occurs when chemical reactions between minerals and other compounds cause disintegration of rock. Exothermic chemical reactions result in an increase in volume that tends to break rocks apart. The most common chemical reactions are hydration, hydrolysis, solutioning and oxidation. The type of material resulting from chemical weathering is based on the chemical composition of the parent rock and the type of reaction. For example, hydrolysis is the reaction of a mineral with water to produce a new mineral. An example of this is when feldspar reacts with water to form kaolinite – rich clay soil. The weathering process is very complex and entire books have been devoted to characterizing the properties of weathered rocks and how they behave. Weathered rocks have a wide range of properties and are probably the most difficult material to characterize. As a result, they have been the focus of many financial claims during construction projects. The effects of weathering degrade all rocks that are exposed to these physical and chemical weathering processes, so it is highly likely that most excavations will encounter weathered rock. When dealing with weathered rock, it is important to recognize that engineering properties may change over very short distances and depths. Table 1 presents descriptions for weathered rock and some basic field recognition tests. Table 1 - Weathering Descriptions and Field Identification Weathering Description Field Recognition Fresh No discoloration; hammer rings when rock is hit Slightly Weathered Surface discoloration only; rock strength unaltered Moderately Weathered Discoloration penetrates rock slightly; iron minerals have rusty appearance; rock is slightly weakened Highly Weathered Discoloration penetrates throughout rock; iron minerals altered to clay; rock is weak and can be broken by hand or with light hammer blows Decomposed Completely discolored, feldspar and iron altered to clay; quartz may be unaltered;, partial rock structure may remain, but mostly resembles soil