01 July 1997
PILE DRIVING IN CORAL SANDS (CASE HISTORY)
5-1. BACKGROUND. A very specific yet a very important application of pile driving equipment and driven
piles is discussed, namely the application of driven piles to coral sand and calcareous soils. To illustrate
the potential problems of such installations, a case history of the installation of piles for a drydock at the
Kwajalein Atoll will be discussed. Additional details concerning this project, along with a bibliography, can
be found in the U.S. Army Engineer Waterways Experiment Station' Miscellaneous Paper GL-92-23,
" wajalein Drydock Pile Foundation Analysis."
a. Overview of Calcareous Soils. Calcareous soils are some of the most challenging types of soils for
the design and installation of piling. Because they frequently appear in areas where offshore oil is found
(i.e., southeast Asia, the Persian Gulf, Australia, etc.), a great deal of research has been done on these
soils. Because of the complex nature of these soils and the variable way in which they are formulated, their
properties are complex and not as well quantified as other types of soils.
(1) Definition and Origin. Calcareous soils are those which are composed of primarily sand size
particles of calcium carbonate, which may be indurated to varying degrees. They can originate from
biological processes such as sedimentation of skeletal debris and coral reef formation. They can also
occur because of chemical precipitation of particles such as oolites. Because of their association with coral
reefs, these soils appear mostly between the latitudes of 30EN and 30ES.
(2) Important Properties of Calcareous Soils. The brittle, crushable nature of calcareous sands
complicates the site investigation. This makes both the site investigation itself and a meaningful correlation
of test data to actual soil properties difficult. However, there are some important soil properties to watch for.
(a) Carbonate Content. By definition, these soils have higher than average carbonate content.
The calcareous soils most prone to difficulties have a carbonate content by weight above 50 percent.
Problems are especially pronounced above 80 percent, where many pile driven into these soils have
abnormally low capacities.
(b) Degree of Cementation and Grain Structure. The grain structure of these soils is highly
variable due to the diverse nature of the soils. This variability is one of the most important factors in the
unpredictability of these soils. This variability can manifest itself in the angularity, size, or void structure of
the grains or other factors. Light cementation can lead to both low shaft friction and toe capacity.
(c) Bulk Density. Void ratios for calcareous sands can vary from 0.8 to 1.4 as opposed to
0.4 to 0.9 for noncarbonated sands. The tendency to voids of all sizes is one of the most difficult problems
encountered with calcareous sands.
(d) Specific Gravity. This normally varies from 2.75 to 2.85 with these soils.
(e) Friction Angle. This is generally greater than 35 degrees and can be greater than
50 degrees. This may decline with increased confining pressure, and the surface friction angle may
decrease with surface roughness.
(3) Loading Response. Calcareous soils are highly compressible under pressure loading and are
subject to softening under cyclic loading.