Removal of water and rearrangement of particles during the compaction of clayey sediments - review

Abstract

Among the factors that influence the amount of water held in clayey sediments under overburden pressures between 0 and about 50 kg per cm are particle size, clay minerals, exchangeable cations, and interstitial-electrolyte concentration. The amount of water held in clayey sediments varies inversely with particle size through a wide range of pressures and chemical conditions. The influence of different clay minerals on water content is related mainly to their particle size. The influences of exchangeable cations and electrolyte concentration also vary with particle size: in fine-grained clays (montmorillonite, for example), water content varies inversely with the valence of exchangeable cations and the concentration of monovalent electrolyte; in coarse-grained clays (kaolinite, for example), water content varies directly with the cation valence and the electrolyte concentration. Under overburden pressures greater than about 50 kg per em ll' the major influences on the water content of clayey sediments seem to be particle size, clay minerals, and temperature. As temperature increases, less pressure is needed to compact the clay. Variations in exchangeable cations and in electrolyte concentration do not seem to influence the water content under higher pressures, but they do affect the clay-particle fabric. Although preferred and turbostratic orientations of claymineral particles can be produced by compressing clays in the laboratory, there is little evidence to show that they are formed readily during natural compaction. Experimental evidence suggests that preferred orientation of particles, normal to overburden pressures, would be most likely to form in sediments that have large clay-mineral particles, low concentration and low acidity of interstitial electrolyte, and low valence of exchangeable cations. The formation of a preferred orientation also seems to be favored by the presence of organic material and the existence of a partly oriented fabric at the onset of compaction. The formation of a turbostratic fabric (particles oriented in domains which are oriented at random with respect to other domains) instead of generally preferred orientation might be favored by high electrolyte concentration and high