Environmental Engineering Reference
In-Depth Information
Rock Purity and Cavity Growth
Purer limestone
, normally found as thick beds of dense, well-indurated rock, is the most
susceptible to cavity growth. At least 60% of the rock must be made up of carbonate mate-
rials for karst development, and a purity of 90% or more is required for full development
(Corbel, 1959).
Impure limestone
is characteristically thinly bedded and interbedded with shale and is
resistant to solution.
Jointing
Groundwater moves in the rock along the joints, which are usually the result of strain
energy release (residual from early compression) that occurs during uplift and rebound
subsequent to unloading by erosion. This dominant origin causes most joints to be
normal to the bedding planes. Major joints, cutting several beds, usually occur in paral-
lel sets and frequently two sets intersect, commonly at about 60°, forming a conjugate
joint system.
Cavity Growth, Subsidence, and Collapse
Solution
Groundwater moving through the joint system at depth and rainfall entering the joint sys-
tem from above result in the solution of the rock. As rainwater passes through the surface
organic layer, it becomes a weak acid that readily attacks the limestone. Solution activity
is much greater, therefore, in humid climates with heavy vegetation than in dry climates
with thin vegetation.
Geologic Conditions and Cavity Growth Form
Horizontal beds
develop cavities vertically and horizontally along the joints, which grow
into caverns as the solution progresses. Cavern growth is usually upward; surface subsi-
dence occurs when the roof begins to deflect, or when broken rock in the cavern provides
partial support, preventing a total collapse. When a cavern roof lacks adequate arch to
support overburden pressures, collapse occurs (see
Figure 6.24)
and a sinkhole is formed.
Horizontal beds
overlain by thick granular overburden are also subject to sudden ravel-
ing into cavities developing along joints. An example of a large sink developing under
these conditions is given in
Figure 10.15.
Sowers (1975) states, “Raveling failures are the
most widespread and probably the most dangerous of all the subsidence phenomena that
are associated with limestone.” The author considers this statement to apply to conditions
like those in central Florida, i.e., relatively thick deposits of granular alluvium overlying
limestone undergoing cavity development from its surface.
Dipping beds
develop cavities along joint dips, as shown in
Figure 10.17,
creating a very
irregular rock surface, characterized by pinnacles. As the cavity grows, the overburden
moves into the void, forming a soil arch. With further growth the arch collapses and a
sinkhole results. In granular soils, the soil may suddenly enter a cavity by raveling,
wherein the arch migrates rapidly to the surface, finally collapsing. A pinnacled rock sur-
face is typical of eastern Pennsylvania, as illustrated in
Figure 10.18.
In addition to sink-
holes resulting from soil raveling into cavities, the very irregular rock surface poses
difficult foundation conditions.
Natural Rate of Cavity Growth
It has been estimated that the rainfall in the sinkhole region of Kentucky will dissolve
a layer of limestone 1 cm thick in 66 years (Flint et al., 1969). Terzaghi (1913), reporting on
