uncertainty in their long-term performance under sustained loads. The most
significant limestone feature is its solubility. An extremely soluble one
can be riddled with solution caves, channels, or other open, water, or clay
filled features.
b. Identification. Presence of solution features may be checked by
geological reconnaissance, drilling, and other forms of bedrock
verification. Geophysical techniques, including shallow seismic refraction,
resistivity and gravimetry are often found to be valuable supplements.
c.
Coral and Coral Formation.
(1) Origin. Living coral and coralline debris are generally found
in tropical regions where the water temperature exceeds 20deg.C. Coral is
a term commonly used for the group of animals which secrete an outer
skeleton composed of calcium carbonate, and which generally grow in
colonies. The term "coral reef" is often applied to large concentrations
of such colonies which form extensive submerged tracts around tropical
coasts and islands. In general, coralline soils deposited after the
breakdown of the reef, typically by wave action, are thin (a few meters
thick) and form a veneer upon cemented materials (limestones, sandstones,
etc.).
(2) Geological Classification. Because the granular coralline and
algal materials are derived from organisms which vary in size from
microscopic shells to large coralheads several meters in diameter, the
fragments are broadly graded and range in size from boulders to fine-grained
muds. Similarly, the shape of these materials varies from sharp, irregular
fragments to well-rounded particles. Coralline deposits are generally
referred to as "biogenic materials" by geologists. When cemented, they may
be termed "reefrock," or "beachrock," or other names which imply an origin
through cementation of particles into a hard, coherent material.
(3) Characteristics. Coralline deposits are generally poor
foundation materials in their natural state because of their variability and
susceptibility to solution by percolating waters, and their generally
brittle nature. Coralline materials are often used for compacted fill for
roads and light structures. Under loads, compaction occurs as the brittle
carbonate grains fracture and consolidate. They can provide a firm support
for mats or spread footings bearing light loads, but it is necessary to
thoroughly compact the material before using it as a supporting surface.
Heavy structures in coral areas are generally supported on pile foundations
because of the erratic induration. Predrilling frequently is required.
Because of extreme variability in engineering properties of
natural coral formations, it is not prudent to make preliminary engineering
decisions on the basis of "typical properties." Unconfined compression
strengths of intact specimens may range from 50 tons/ft.2- to 300
tons/ft.2-, and porosity may range from less than 40% to over 50%.
For further guidance see Reference 24, Failure in Limestone in
Humid Subtropics, by Sowers, which discusses factors influencing
construction in limestone; and Reference 25, Terrain Analysis - A Guide to
Site Selection Using Aerial Photographic Interpretation, by Way.
7.1-43
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