Glossary of Soils and Rocks
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
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.
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.
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