MOVEMENT AND DISTRIBUTION OF WATER IN SOIL IN RELATION TO HIGHWAY DESIGN AND PERFORMANCE

RESEARCH IS REPORTED ON THE APPLICATION OF THERMODYNAMICS TO THE DESIGN OF ROAD FOUNDATIONS. STUDY OF THE STRENGTH CHARACTERISTICS OF UNSATURATED SOILS FOCUSED ATTENTION ON THE INFLUENCE OF SOIL SUCTION BOTH IN RELATION TO SOIL MOISTURE MIGRATION AND SHEAR STRENGTH. MOISTURE CONTENT ON A WEIGHT BASIS PROVIDED THE SIMPLES METHOD OF EVALUATING THE WATER IN SOIL. SINCE THE VALUE OF SUCTION FOR A SOIL INCREASES AS THE SOIL BECOME DRIER ANY LOCAL CHANGE IN MOISTURE CONTENT WILL CAUSE A REDISTRIBUTION OF WATER IN THE SOIL. THE MIGRATION OF WATER IN SOIL IS ACCOMPANIED BY SOME CHANGE OF VOLUME. VOLUME CHANGES RESULTING FROM APPLIED PRESSURE CAN BE MEASURED UNIDIRECTIONALLY BY THE OEDOMETER APPARATUS OR THREE-DIMENSIONALLY BY A TRIAXIAL CELL. THE FORCES BY WHICH WATER IS RETAINED IN SOIL LOWER THE FREEZING POINT. THE THERMODYNAMIC RELATIONSHIP BETWEEN SOIL TEMPERATURES BELOW THE FREEZING POINT OF FREE WATER AND THE SUCTION OF THE UNFROZEN SOIL MOISTURE ENABLES THE SUCTION DISTRIBUTION ASSOCIATED WITH ANY GIVEN TEMPERATURE DISTRIBUTION WITHIN THE FROZEN ZONE TO BE CALCULATED. THE EXTENT TO WHICH WATER WILL MOVE INTO THE FROZEN ZONE TO CAUSE THE PHENOMENA KNOWN AS FROST-HEAVE, DEPENDS ON SUCTION OF THE SURROUNDING UNFROZEN SOIL, AS WELL AS ON THAT OF FROZEN SOIL, AND ON THE PERMABILITY OF THE LIQUID FLOW OF THE SOIL AT THE HIGH SUCTIONS PRESENT DURING FREEZING. THE THERMODYNAMIC APPROACH TO SOIL MOISTURE MOVEMENT CAN BE USED TO EVALUATE MOISTURE MIGRATION IN TERMS OF MOISTURE CONTENT ON A WEIGHT BASIS IF THE APPROPRIATE RELATIONSHIP BETWEEN SUCTION AND MOISTURE CONTENT ARE KNOWN. THE SUCTION PLATE APPARATUS IS DESCRIBED. THE NATURE OF THE RELATIONSHIP BETWEEN SUCTION AND MOISTURE CONTENT DEPENDS ON THE CHARACTERISTICS OF THE SOIL. THIS RELATIONSHIP IS CONSIDERED FOR THE FOLLOWING CATEGORIES: (1) INCOMPRESSIBLE SOILS AND MATERIALS OF RIGID STRUCTURE, (2) COMPRESSIBLE SOILS WHICH MAY REMAIN SATURATED AT HIGH SUCTIONS, ANY CHANGE OF MOISTURE CONTENT BEING ACCOPANIED BY A CHANGE IN VOLUME AS FOR HEAVY CLAYS, AND (3) INTERMEDIATE SOILS SUCH AS SANDY CLAYS. IN NATURAL SOIL THE PORE-WATER PRESSURE IS DETERMINED BY THE RAINFALL, EVAPORATION, TRANSPIRATION, FROM VEGETATION AND THE SOIL PERMEABILITY. THESE FACTORS GIVE RISE TO A WATER TABLE WHOSE DEPTHS CAN BE DETERMINED FROM A BORE HOLE. FROM ANY KNOWN DISTRIBUTION OF PORE-WATER PRESSURE WITH DEPTH THE CORRESPONDING VARIATION OF SUCTION WITH DEPTH CAN BE REDUCED USING A GIVEN EQUATION. IN CONJUNCTION WITH THE APPROPRIATE RELATIONSHIPS BETWEEN CONTENT AND SUCTION, THE SUCTION DISTRIBUTION CAN BE CONVERTED TO A DISTRIBUTION OF MOISTURE CONTENT.