FIELD STUDIES OF THE MOVEMENT OF SOIL MOISTURE

FIELD INVESTIGATIONS WERE CONDUCTED TO: (1) DETERMINE TO WHAT EXTENT THE FINAL DISTRIBUTION OF PORE-WATER PRESSURE WITH DEPTH IN THE SOIL BENEATH AN IMPERMEABLE SURFACING COMPARABLE IN DIMENSIONS WITH THE WIDTH OF THE ROAD, WAS IN PRACTICE GOVERNED BY THE POSITION OF THE WATER-TABLE, FOR SOILS COVERED AT DIFFERENT SEASONS OF THE YEAR, (2) DETERMINE WHETHER THE DISTRIBUTION OF MOISTURE CONTENT WITH DEPTH BENEATH THE SURFACING COULD BE ESTIMATED FROM PORE- WATER PRESSURE MEASUREMENTS, (3) STUDY THE SEASONAL CHANGES WHICH OCCUR IN THE DISTRIBUTIONS OF PORE-WATER PRESSURE AND MOISTURE CONTENT WITH DEPTH BENEATH GRASS COVER AND BARE SOIL, (4) ASCERTAIN TO WHAT EXTENT THE MOISTURE CONDITIONS IN THE SOIL ADJACENT TO A COVERED AREA AFFECT THE PORE-WATER PRESSURE AND MOISTURE-CONTENT DISTRIBUTIONS UNDER THE EDGES OF THE COVERED AREA, (5) RECORD THE SEASONAL CHANGES THAT OCCUR IN THE TEMPERATURE OF THE SOIL AT VARIOUS DEPTHS UNDER AN IMPERMEABLE SURFACING AND GRASS COVER, AND TO CONSIDER THE PROBABLE EFFECT OF THESE FLUCTUATIONS ON THE PORE-WATER PRESSURE AND MOISTURE-CONTENT DISTRIBUTIONS, AND (6) RELATE VERTICAL MOVEMENTS OF THE IMPERMEABLE SURFACINGS WITH THE CHANGES OF MOISTURE CONTENT IN THE SOIL BENEATH. DETAILS ARE GIVEN OF THE EXPERIMENTAL TECHNIQUES USED IN THE MEASUREMENT OF PORE-WATER PRESSURE, SUCTION, COMPRESSIBILITY FACTOR AND SOIL TEMPERATURE. FIELD EXPERIMENTS WERE CONDUCTED ON THREE DIFFERENT SOILS: A SAND, A SILTY CLAY AND A HEAVY CLAY. THE SEASONAL VARIATIONS OF PORE PRESSURE AND MOISTURE CONTENT WERE FOUND TO BE LARGE UNDER GRASS COVER, BUT WHERE THE VEGETATION WAS REMOVED, ONLY SMALL SEASONAL VARIATIONS OCCURRED. THIS SHOWS THAT TRANSPIRATION FROM VEGETATION IS MORE IMPORTANT THAN SURFACE EVAPORATION IN REMOVING WATER FROM THE SOIL. UNDER IMPERVIOUS PAVEMENTS ONLY SMALL SEASONAL FLUCTUATIONS OF PORE-PRESSURE AND MOISTURE CONTENT WERE OBSERVED, EXCEPT NEAR THE EDGES OF THE PAVEMENT WHERE THE SOIL WAS INFLUENCED BY THE EXPOSED VERGES. THE ULTIMATE PORE-WATER PRESSURE CONDITION UNDER AN IMPERVIOUS SURFACING WAS FOUND TO DEPEND LARGELY ON THE POSITION OF THE WATER- TABLE. RESULTS FROM THIS STUDY WILL BE APPLIED TO ROADS AND AIRFIELD PAVEMENTS SINCE THEIR DESIGN THICKNESS DEPENDS ON THE MOISTURE CONDITION OF THE SOIL AND ON THE CHANGES LIKELY TO OCCUR IN THE SOIL AFTER CONSTRUCTION OF THE PAVEMENT.