Page 52 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 47

Caverns and Sinks
Whereas clay pockets penetrate from the ground surface downward, caverns
occur at the level of a groundwater table and remain as open voids until the top
caves in. Caverns are difficult to locate as they can readily be missed by borings,
and geophysical seismic and resistivity measurements focus on what is there and
not what is gone. Ground-penetrating radar would be most useful but the
penetration depth is limited, particularly in clay.

Caves are of obvious concern in foundation engineering, as a roof collapse can
drop part or all of a building or other structure into the ground without advance
warming. The consequences can be devastating. A cavern collapse under a
railroad track in South Africa left a train suspended in midair by welded rails.

A cavern that already has collapsed is a sink, and cavernous ground often can be
recognized from the occurrence of sinks. Sinks may be obvious, as in Fig. 3.4, or
also can be detected from streams that disappear and have no visible outlet.

Sinks also are a major concern for groundwater supplies, as they are direct conduits
leading into aquifers. Whatever falls or is thrown into a sink therefore may be only
a step away from the well, and the once-common practice of dumping everything
into a sink from bald tires to dead dogs and old automobile batteries must stop.

Sinks also are a challenge for investigators because a floor may only conceal part
of a deeper cavern. A hint as to consequences of a brash action may be found in
animal bones lying in the bottom of a sink.

Cavernous ground impacts the feasibility of a dam because of the potential for
leakage. Sealing of leaky dam foundations and abutments can be attempted by

Figure 3.4
Collapsed caverns,
or sinks, commonly
dot the landscape
in limestone areas.
It is the caverns
that are not known
and have not yet
collapsed that are
more likely to
create serious
foundation
problems.

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