Discussion of “Engineering Behavior and Structure of Compacted Clay”

Abstract

CABRERA AND SMALLEY. THE AUTHORS CONCLUDED, FROM THEIR INVESTIGATIONS AT HIGH MAGNIFICATIONS, THAT THE STRUCTURE OF A COMPACTED KAOLINITE SOIL CONSISTS OF PACKETS OR DOMAINS, I.E., THE TURBOSTRATIC STRUCTURE OF AYLMORE AND QUIRK. OUR INVESTIGATIONS HAVE SUGGESTED THAT THE HIGH-MAGNIFICATION SCANNING ELECTRON MICROSCOPE STUDY OF COMPACTED KAOLINITE CAN REVEAL MORE THAN THIS ABOUT THE IMPORTANT PROBLEMS OF PARTICLE ASSOCIATION AND SOIL STRUCTURE. IT APPEARS THAT BETWEEN THE INITIAL FORMATION OF THE KAOLINITE PARTICLES AND THE EVENTUAL FORMATION OF A COMPACTED KAOLINITE SOIL SEVERAL IMPORTANT EVENTS OCCUR AND THESE ALL HAVE STRUCTURAL SIGNIFICANCE: (1) THE BOOKHOUSE STRUCTURE IS DISTURBED AND THE LARGE KAOLINITE PARTICLES ARE SUCCESSIVELY BROKEN UNTIL THEY REACH THEIR SOIL SIZE OF ABOUT 1 MICRON IN DIAMETER. THE PARTICLES EFFECTIVELY GET STRONGER AS THEY GET SMALLER AND 1 MICRON REPRESENTS A BREAKAGE LIMIT. (2) THEY REFORM DISCRETE DOMAINS, BUT THE INITIAL REGULARITY IS NOT RESTORED, AND STEPPED STRUCTURES TEND TO BE FORMED. (3) THE STEPPED STRUCTURE IS EMPHASIZED BY THE COMPACTION PROCESS AND BECOMES THE PREDOMINANT FORM OF PARTICLE ASSOCIATION. ALCOTT AND SCHMIDT. THE EFFECTS OF SEVERAL SOIL PROPERTIES ON THE COMPACTION CHARACTERISTICS OF CLAY RESIDUAL SOILS DEVELOPED ON THE SALEM PLATEAU OF MISSOURI WERE STUDIED BY ALCOTT WITH REFERENCE IN REGARD TO THEIR POOR PERFORMANCE IN EARTH DAM CONSTRUCTION. AS PART OF THE INVESTIGATION INTO THE NATURE OF THE COMPACTED CLAYS, MICROGRAPHS WERE MADE BY SCANNING ELECTRON MICROSCOPY FROM SOIL SAMPLES COMPACTED 3% DRY OF OPTIMUM MOISTURE CONTENT (W = 34%) AND 5% WET OF OPTIMUM (W = 42%). AT RELATIVELY LOW MAGNIFICATION (100X) THE SAMPLE COMPACTED DRY OF OPTIMUM APPEARS MORE ORIENTED AND HAS A MORE UNIFORM TEXTURE. THE VOLUME OF INTERPEDULAR VOIDS, OR MACROSPACES, FOR THE DRY SAMPLE GREATLY EXCEEDS THAT OF THE WET SAMPLE. THESE DIFFERENCES ARE ENHANCED IN THE MICROGRAPHS TAKEN AT 300X. THE MICROGRAPHS TAKEN AT HIGH MAGNIFICATION (3,000X) PRESENT A DIFFERENT PATTERN WITH NO OBVIOUS DIFFERENCES IN CLAY PARTICLE-TO-PARTICLE STRUCTURE BETWEEN THE SAMPLES COMPACTED WET AND DRY OF OPTIMUM. THE COMPACTION MOISTURE CONTENT APPEARS TO HAVE HAD LITTLE EFFECT ON THE CLAY PARTICLE STRUCTURE. IT IS SUGGESTED THAT, AT COMPACTION MOISTURE CONTENTS WET OF OPTIMUM, DENSITY DECREASES AS WATER IS ATTRACTED TO MANY OF THE CLAY PARTICLES AND SMALL PEDS. THESE MICROGRAPHS, ALONG WITH OTHERS STUDIED DURING THE INVESTIGATION, APPEAR TO SUBSTANTIATE AND EXTEND THE WORK OF THE AUTHORS. THE STRUCTURAL SIMILARITY OF AN ILLITIC GLACIAL BOULDER CLAY AND A KAOLINITIC RESIDUUM OF LIMESTONE WOULD SUGGEST THAT THE GENERALIZATIONS MADE REGARDING MICROSTRUCTURE OF CLAYS ARE INDEPENDENT OF THE ORIGIN AND MORPHOLOGY OF THE CLAY. /AUTHOR/