Sinkholes
are a common, naturally occurring geologic feature and one of the
most obvious landforms throughout Northampton and Lehigh County,
Pennsylvania. Sinkhole collapse poses hazards to property and the
environment. It is often important to be able to evaluate the risk
of sinkhole collapse for a specific site, for insurance purposes,
for pre-purchase evaluation, for land planning purpose, and for
development of the site. Even after a road or a building is in
place, such a risk analysis can help determine whether the ground
may collapse.
Subsidence
is the sinking or collapse of a portion of the land surface. The
mechanisms of collapse, and sometimes the conditions existing before
the collapse, result from natural physical processes. Groundwater
facilitates ground collapse through dissolution of limestone and
other carbonate rocks. These rocks are nearly insoluble in pure
water but are readily dissolved by carbonic acid, a common
constituent of rainwater. The weathering attack occurs mainly along
fractures and other partings and openings in the carbonate bedrock.
The resulting features include caves, sinkholes, and
karst topography. In many cases the conditions leading up to
a subsidence event are exacerbated or even created by human actions.
A sinkhole
is a large dissolution cavity that is open to the sky. The
formation of sinkholes can be due either to the sudden wholesale
collapse of the roof of a cave or by a more gradual downward
movement of unconsolidated material into an open, chimney-like
passageway. The downward movement of material eventually leaves the
roof materials unsupported; surface fractures begin to develop and
the roof eventually collapses.
Sinkholes are enlarged whenever groundwater levels are high,
but the pressure of the water in the caverns and passageways helps
support the weight of the overlying rocks. Collapse results from
the lowering of the water table due to drought and/or excessive
pumping of water wells which leaves underground spaces and
passageways unsupported, facilitating the collapse of the overlying
rocks.
The terrain that is characterized by the presence of caverns
and sinkholes is called karst
topography. The most common
type of karst terrain is sinkhole karst, a landscape dotted
with closely spaced circular collapse basins of various sizes and
shapes. Several factors control the development of karst landscapes:
New sinkholes are
widely viewed as random, unpredictable geologic phenomena. However,
BlackRock's experience suggests that the general probability of
having one sinkhole develop is dependent on the area of the site,
new sinkhole distribution and the sinkhole size. BlackRock’s staff
of licensed Professional Geologists performs sinkhole risk
assessments in karst using qualitative analysis based on
physiographic, geomorphic, geologic and hydrogeologic information.
