Accumulation of moisture in soil under an impervious surface

THE OBJECTIVE OF THIS STUDY WAS TO COMPARE THE LONG TIME ACCUMULATION OF MOISTURE IN A SOIL SUBGRADE BENEATH AN IMPERVIOUS SURFACE WITH THE ESTIMATED EQUILIBRIUM MOISTURE CONTENT BASED ON MEASUREMENTS OF THE MOISTURE RETENTION CHARACTERISTICS OF THE SOIL AND THE ELEVATION OF THE GROUND WATER. MOISTURE ACCUMULATION DUE TO THE FORMATION OF ICE WAS NOT CONSIDERED. A THEORETICAL APPROACH, BASED ON THERMODYNAMICS, WAS USED TO EVALUATE THE FREE ENERGY PER UNIT MASS OF WATER IN A SOIL WATER SYSTEM IN TERMS OF COMPONENT FREE ENERGIES. THUS THE EFFECTS OF ADSORPTIVE AND GRAVITATIONAL FORCE FIELDS, SURFACE TENSION, PRESSURE AND DISSOLVED MATERIALS WERE CONSIDERED. BY ASSUMING THAT THE OSMOTIC AND ADSORPTIVE FORCE FIELD EFFECTS WERE SMALL, THE THERMODYNAMIC ARGUMENT REDUCED TO THE CAPILLARY POTENTIAL CONCEPT. THE EXPERIMENTAL INVESTIGATION WAS CONDUCTED IN TWO PHASES. THE FIRST PHASE INVOLVED THE ROUTINE TASKS OF PERIODICALLY DETERMINING SOIL MOISTURE CONTENTS, SOIL TEMPERATURES AND WATER TABLE ELEVATIONS UNDER AN IMPERVIOUS SURFACE CONSTRUCTED OF ALTERNATE LAYERS OF ASPHALT ROOFING PAPER AND ASPHALT CEMENT. THE SECOND WAS CONDUCTED TO DETERMINE THE SOIL-MOISTURE RETENTION CHARACTERISTICS, IN THE FORM OF DESORPTION CURVES, AND OTHER PROPERTIES OF A SERIES OF UNDISTURBED SOIL SAMPLES TAKEN FROM UNDER, AND ADJACENT TO, THE IMPERVIOUS SURFACE NEAR THE CLOSE OF THE FIELD INVESTIGATION, OR FIRST PHASE. IT WAS FOUND THAT THE TERMINAL EQUILIBRIUM MOISTURE CONTENTS IN A SOIL COLUMN UNDER AN IMPERVIOUS SURFACE CAN BE PREDICTED FROM DESORPTION CURVES RUN ON UNDISTURBED SAMPLES OF THE SOILS PROVIDING WATER TABLE AND TEMPERATURE DATA ARE AVAILABLE. SOIL STRATIFICATION DOES NOT AFFECT THE PREDICTED VALUES. TEMPERATURE HAS ONLY A MINOR EFFECT ON THE TERMINAL MOISTURE CONTENTS THUS PREDICTED EXCEPT WHERE EXTREME TEMPERATURES ARE ENCOUNTERED. ACTUAL CHANGES IN MOISTURE CONTENT RESULTING FROM TEMPERATURE CHANGES CAN BE ESTIMATED WITHIN CLOSE LIMITS USING A PROPOSED APPROXIMATE METHOD. TWO SAMPLES OF A GIVEN SOIL AT DIFFERENT DENSITIES WILL HAVE DIFFERENT EQUILIBRIUM MOISTURE CONTENTS AT A GIVEN MOISTURE TENSION, THE LESS DENSE SAMPLE HAVING A HIGHER MOISTURE CONTENT. THEREFORE, UNDER NORMAL FIELD CONDITIONS WHERE INCREASING SOIL DENSITY IS NOTED WITH INCREASING DEPTH, IT IS POSSIBLE TO NOTE INCREASING MOISTURE CONTENTS WITH INCREASING HEIGHT ABOVE THE WATER TABLE, THEREBY GIVING THE FALSE IMPRESSION THAT SOME MECHANISM IS AT WORK WHICH CAUSES SATURATION OF THE SOIL BENEATH AN IMPERVIOUS SURFACE. /AUTHOR/