SOIL THERMODYNAMICS APPLIED TO THE MOVEMENT OF MOISTURE IN ROAD FOUNDATIONS

RESEARCH IS SUMMARIZED ON THE PROBLEM OF STABILIZING OR CONTROLLING THE MOISTURE CONDITIONS IN ROAD FOUNDATIONS WHICH ARE CONSIDERED IN RELATION TO ALL THE FACTORS WHICH GOVERN THE MOVEMENT OF WATER IN SOIL. THE WATER IN SOIL CAN BE DIVIDED BROADLY INTO TWO CATEGORIES: (1) FREE WATER CONSTITUTED BY THE GROUND WATER BENEATH THE WATER TABLE, AND GRAVITATIONAL WATER FLOWING TOWARDS THE WATER TABLE UNDER THE ACTION OF GRAVITY, AND (2) HELD WATER RETAINED IN THE SOIL BY SURFACE TENSION AND ABSORPTIVE FORCES. THE MOVEMENT AND CONTROL OF FREE WATER AND HELD WATER ARE DISCUSSED. MATHEMATICAL ANALYSES ARE PRESENTED ON THE RELATIONSHIPS BETWEEN SOIL MOISTURE SUCTION, RELATIVE HUMIDITY OF SOIL VERSUS MOISTURE CONTENT. VARIATIONS OF SOIL VAPOR PRESSURE, PF VALUE OF SUCTION, WITH TEMPERATURE ARE ANALYZED. THE CHIEF FACTORS TO BE CONSIDERED IN RELATION TO ROAD FOUNDATIONS ARE: (1) THE DISTRIBUTION OF HELD WATER ABOVE THE WATER TABLE AND THE EFFECT OF VARIATIONS OF THE WATER TABLE LEVEL ON THIS DISTRIBUTION, (2) SEASONAL MOVEMENTS OF MOISTURE BETWEEN THE ROAD FOUNDATION AND THE VERGES, (3) MOISTURE MOVEMENTS DUE TO TREES AND OTHER VEGETATION NEAR THE ROAD, AND (4) MOISTURE MOVEMENTS ASSOCIATED WITH THE DAILY AND YEARLY TEMPERATURE CYCLES. DIFFERENCES IN TEMPERATURE IN SOILS OF LOW MOISTURE CONTENT ARE SHOWN TO FAVOR VAPOR MOVEMENTS. THESE MOVEMENTS, BY VIRTUE OF THE LATENT HEAT EXCHANGE, TEND TO SET UP A SECONDARY TEMPERATURE GRADIENT IN OPPOSITION TO THE MAIN GRADIENT. IT IS CONCLUDED THAT IN ALL THE CASES OF MOISTURE TRANSFER WHICH HAVE BEEN CONSIDERED, THE THERMAL EFFECTS WILL PRODUCE THE GREATEST TEMPERATURE EFFECTS WHEN THE UNSATURATED PERMEABILITY OF THE SOIL IS HIGH AND WHEN THE SPECIFIC HEAT AND THERMAL CONDUCTIVITY OF THE SOIL ARE LOW.