Natural calcareous clays from off the coasts of Great Britain and Africa were subjected to strength, compression, and soil classification .tests to examine the effect of carbonate content on engineering behavior. Carbonate contents ranged from 10 to 90 percent. In general, specimens with carbonate contents greater than 40 percent exhibit granular behavior and less than 40 percent exhibit cohesive behavior. The data show that carbonate content is an index property and should be determined routinely along with other index properties.
Many marine sediments contain up to 90 percent calcium carbonated by dry weight; with sediments from vast areas of the deep ocean having carbonate contents greater than 30 percent. Consequently, the portion of calcareous materials will have a significant effect upon engineering behavior. The purpose of this paper is to discuss some features of calcareous clays which influence behavior and show the value of bulk carbonate content as an index property.
An index property is a parameter which describes or distinguishes the behavior of one soil from another. They are typically based upon field identification and inexpensive laboratory tests. Index properties may be divided into two categories:
soil grain properties, and
soil aggregate properties. Soil grain properties deal with the individual grain's size, geometry, mineralogy, and crystal structure while aggregate properties are concerned with the assemblage of particles or microstructure. For cohesive sediments, it is primarily water holding capacity and consistency which influence behavior.
Ewing-type core samples were taken by the U. S. Navy at two deepwater locations in the North Atlantic. Four gravity cores were from 13,000 to 15,000 foot water depths in the northeast Atlantic and four piston-rigged cores were from the Canary Basin in 15,000 to 18,000 feet of water. The core samples varied in length from about 10 to 23 feet. Radiographs and visual observation suggest that portions of the cores are slightly to highly disturbed with gravity cores showing the least disturbance. Radiographs were used to select test specimens with the least disturbance.
Keller2 reported that North Atlantic sediments are generally silts and clays plus the skeletal tests of planktonic plants and animals at any given size the relative proportions of clay minerals4 are expected to be constant with subbottom depth. However, the quantity of carbonates, as a percentage of dry weight, is highly variables5 and depends upon skeletal type, surface water temperature, presence of nutrients, and other factors. Also, the solubility of carbonate tests increases with increasing pressure, decreasing temperature and increasing chlorinity and may remove or alter carbonate at a carbonate rich time or location. The carbonate compensation depth (depth below which carbonates are mostly dissolved) for the North Atlantic is about 16,000 feet:
