Rocks
Most foundations consist of rock of various sorts.
Geologists tend to call every formation a rock, even if it is soft clay shale,
unless it is a surface deposit that has been weathered into something that can
grow vegetation.
Origins.
Some formations consist of every hard rock that are normally
quite good support for foundation and other earth-related structures. However,
they can be very difficult and costly to excavate. Rock can include igneous
deposits such as granite or sedimentary deposits such as limestone or shale.
Igneous rocks are those that are formed from molten rock (lava or magma).
Sedimentary rocks are formed when mineral precipitates or shells of marine
organisms are deposited in water and are hardened by pressure or cementation
over long time periods.
Metamorphic rock is another major rock type. These rocks are
formed from any of the igneous or sedimentary rocks by transformation from heat
and pressure. Of course, within each of these types of rocks there are quite
large variations in strength and excavation difficulty. Some of the more
difficult rock formations to evaluate for foundation construction purpose can
be those with layers of milestone or sandstone intermingled with layers of clay
or clay-shale.
Identifying rock.
A useful engineering definition of rock is a material that
will not slake when submerged in water. This indicates a virtually irreversible
cementation, which will allow the rock to retain its structure throughout its
use. Another way to determining rock is if it has a brittle fracture or if a
small flat piece can be broken with a detectible “snap”. The definition of
shale is especially confusing at times. Shale is generally considered to be a
material meeting the rock tests named above but also is thinly layered or
“fissile”. Shale can weather into clay, and frequently there is a change in the
profile with depth, with weathered clay like materials near the surface and the
more rock0like shale at deeper depths.
Rock as a foundation support.
Rock is an excellent material for supporting foundations
because of the high strength and consequent high bearing values that can be
assigned to this material. Naturally, the higher the bearing value of a
materials, the smaller and lighter a foundation can be to distribute the loads
of the structure to the soil. Rock strength comes in many grades, ranging from
rock that is only somewhat stronger than a hard clay up to material that may be
stronger than concrete.
The below table showing presumptive bearing values for rock
and soil, is taken from the 2015 IBC. To apply the presumptive bearing values
to any materials, the materials must be classified carefully.
The presumptive bearing values given I the above table are
maximum but are generally conservative numbers, provided the rock or soil
classification is reasonably accurate. To determine more precise and possibly
higher bearing values for rock formation, a geotechnical investigation is
necessary in which cores of the rock are obtained, lab testing is utilized, and
some theoretical calculations are done to determine the allowable bearing
capacity of the rock.
While rock formations are generally thought to be strong and
uniform, they can be variable. For example, some formations have hard rock
layers intermixed with clays and silts. While the foundation may be in contact
with a hard rock layer, the softer underlaying clay and silt layers may control
the bearing capacity. Even where no clay and silt layers exists, rock
formations may be fractured or jointed, resulting in lower strengths. Because
of the uncertain effects of fractures and joints in rock, a higher factor of
safety is commonly used for rock than soils.
Where the rock formations are variable, the presumptive
bearing values included in the above table may not be conservative. Section 1803.5.6
of the IBC 2015 indicates that where variable rock conditions exist, a suitable
number of borings should be drilled to assess the competency of the rock and
its load bearing capacity.