SUBGRADE MOISTURE VARIATIONS STUDIED WITH NUCLEAR DEPTH GAGES

ESTABLISHMENT OF RELATIONSHIPS BETWEEN SUBGRADE MOISTURE VARIATIONS AND HIGHWAY PAVEMENT PERFORMANCE WOULD HELP TO SOLVE MANY HIGHWAY DESIGN PROBLEMS. IN JUNE, 1964, A SIX- YEAR STUDY WAS BEGUN TO RELATE SUBGRADE MOSITURE VARIATIONS TO SOIL, CLIMATE, AND PAVEMENT CONDITIONS. CORRELATIONS AFTER 18 MONTHS OF DATA COLLECTION HAVE ESTABLISHED QUANTITATIVE RELATIONSHIPS BETWEEN THE FACTORS, AND INDICATIONS ARE THAT QUALITATIVE DATA WILL BE DEVELOPED BEFORE PROJECT TERMINATION. SUBGRADE MOISTURE DATA WERE COLLECTED PERIODICALLY BY NUCLEAR DEPTH GAGES. FIFTY FIELD TEST SITES, ON TYPICAL HIGHWAY SECTIONS, WERE LOCATED NEAR CLIMATOLOGICAL RECORDING STATIONS IN NORTH CENTRAL AND NORTHEASTERN OKLAHOMA. SUBGRADE SOILS DATA WERE OBTAINED FROM SAMPLES TAKEN DURING SITE INSTALLATION. PAVEMENT RATINGS, ORIGINAL HIGHWAY DESIGNS, MAINTENANCE HISTORY, TRAFFIC COUNTS, AND SIMILAR DATA WERE OBTAINED FROM THE OKLAHOMA DEPARTMENT OF HIGHWAYS. CORRELATIONS INDICATE THAT MOST SUBGRADE MOISTURE VARIATIONS OCCUR ON AN ANNUAL CYCLE WITH MAXIMUM MOISTURE CONTENTS OCCURRING DURING WINTER MONTHS. MAGNITUDE AND FREQUENCY OF VARIATIONS ARE GREATLY AFFECTED BY INFILTRATION OF RUNOFF WHERE PAVEMENTS ARE NOT IMPERVIOUS. VARIATIONS BENEATH PAVEMENTS IN GOOD TO EXCELLENT CONDITION WERE FOUND TO BE TEMPERATURE DEPENDENT. DESIGN FEATURES SUCH AS WIDE SEALED SHOULDERS AND GOOD SURFACE DRAINAGE HAD SIGNIFICANT EFFECTS ON SUBGRADE MOISTURE VARIATIONS. RIGID PAVEMENTS ON WELL-GRADED SAND BASE COURSES GAVE BETTER PAVEMENT PERFORMANCE THAN ANY OTHER TYPE OF DESIGN INVESTIGATED. /AUTHOR/