Glossary

Glossary - Glossary of Soils and Rocks A) CLASSIFICATIONS:...

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Glossary of Soils and Rocks A) CLASSIFICATIONS: Bedrock Bedrock is the common name for the parent rock, but generally implies rock at a depth in the ground on which a structure may be founded. All other rocks and soils are derived from the original bedrock formed from cooling of molten magma and subsequent weathering. Bedrock extends substantially downward to igneous rock formed by cooling of the molten magma. This may, or may not, be overlain by one or more layers of more recently formed sedimentary rocks such as sandstone, limestone, shale, etc., formed from indurated soil deposits. The interface layers between igneous and sedimentary rocks may be metamorphic rocks formed from intense heat and pressure acting on the sedimentary rocks. In some cases a bedding rock layer, usually sedimentary in origin, may overlie a soil deposit. In earthquake areas the parent rock may much fractured. Past uplifts may produce zones of highly fragmented parent rock at the bedrock level. Considering these factors, one might say that generally, bedrock makes a satisfactory foundation, but good engineering practice requires that one check the geological history of the site. In this context it is fairly common to refer to the bedrock as the geological age of estimated formation as Cambrian, pre-Cambrian, etc. Boulders and Cobbles Boulders are large pieces of rock fractured from the parent material or blown out of volcanoes (called bombs in this case). They may have volume ranging from about 1 to 10 m ; and weight from about one to several hundred tonnes. They may create disposal or excavation problems on or near the ground surface, and pose problems in soil exploration or pile driving at greater depths when suspended in the soil matrices such as glacial tills. Large boulders may be suitable to found a pile or caissons; however, the size determination may be difficult and placing a large load on a small suspended boulder may be disastrous. Gravels Rock fragments smaller than boulders are: cobbles, pebbles, gravel, sand, slit, and colloids in decreasing order of size. Crushed stone is gravel manufactured by crushing rock fragments from boulders or are obtained from suitable rock formations by mining. Bank run gravel is a common term for naturally occurring gravel lenses deposited along rivers or from glaciers. Pea gravel is a type of gravel screened (via sieves) to contain only sizes in a certain range (usually about 6 mm to 3 mm) and is, of course, poorly graded.
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Sands, Silts and Clays. Gravels, sands, and silts are granular materials and exist in deposits ranging from a loose to dense and coarse to fine state. Most deposits, however, are in a medium to fairly dense state. These materials can have cohesion from clay materials in the fine sand and silt filler that may be present. Silts and clays are particular interest in foundation engineering because they tend to be most troublesome in terms of strength and settlements. Usual size range for general soil classification
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Glossary - Glossary of Soils and Rocks A) CLASSIFICATIONS:...

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