EFFECT OF STRESS HISTORY AND FREQUENCY OF STRESS APPLICATION ON DEFORMATION OF CLAY SUBGRADES UNDER REPEATED LOADING

Abstract

THE DEFORMATION OF A COMPACTED CLAY, EVEN AT CONSTANT COMPOSITION, RESULTING FROM A GIVEN SERIES OF STRESS APPLICATIONS MAY VARY WIDELY DEPENDING ON ITS PREVIOUS STRESS HISTORY, AND IN GENERAL IT APPEARS THAT A GRADUAL INCREASE IN THE MAGNITUDE OF THE APPLIED STRESS MAY OFTEN CAUSE LESS DEFORMATION THAN THE DIRECT APPLICATION OF A SHORT SEQUENCE OF WHEEL LOADS. PREVIOUS STUDIES INDICATE THAT IF TRAFFIC COULD BE CONTROLLED ON A MODERN HIGHWAY IN SUCH A WAY THAT THE WHEEL LOADS WERE GRADUALLY INCREASED OVER A PERIOD OF YEARS, REDUCTIONS IN PAVEMENT THICKNESS MIGHT WELL BE ACHIEVED. THE PRACTICAL DIFFICULTIES OF SUCH A CONTROL SYSTEM SEEM INSURMOUNTABLE SINCE, IF PAVEMENTS ARE DESIGNED FOR SATURATED CONDITIONS IN THE SUBGRADE, THERE IS NO NEED FOR CONTROL UNTIL THE WATER CONTENT OF THE SOIL INCREASES TO SUCH AN EXTENT THAT A HIGH DEGREE OF SATURATION IS ATTAINED AND THE LENGTH OF TIME REQUIRED FOR THIS TO OCCUR IS LIKELY TO VARY WIDELY. LABORATORY TESTS REPORTED IN THIS PAPER AND PREVIOUS ONE BY THE SAME AUTHORS SHOW THAT IN SOME SOILS THE FREQUENCY OF STRESS APPLICATION MAY HAVE A MARKED EFFECT ON THE RESULTING DEFORMATION, AND THIS FACTOR SHOULD NOT BE OVERLOOKED IN ATTEMPTS TO RELATE THE RESULTS OF ACCELERATED TESTING PROGRAMS TO ACTUAL FIELD CONDITIONS. THE EFFECT OF THIXOTROPY ON THE DEFORMATION CHARACTERISTICS OF COMPACTED CLAYS REQUIRES CAREFUL CONSIDERATION IF LABORATORY TESTS ON COMPACTED SAMPLES ARE TO BE USED TO MAXIMUM ADVANTAGE FOR PAVEMENT DESIGN PURPOSES. SOME JUSTIFICATION FOR TESTING SOIL SPECIMENS COMPACTED TO INITIALLY HIGH DEGREES OF SATURATION MAY POSSIBLY BE CLAIMED BY THE FACT THAT THIS IS THE CURRENT PRACTICE OF A NUMBER OF STATE HIGHWAY DEPARTMENTS, AND IT IS HOPED THAT THE LIMITATION OF THE TEST DATA TO ONE PARTICULAR SOIL IS JUSTIFIED BY THE FACT THAT THE PRINCIPLES INVOLVED ARE OF SUFFICIENT IMPORTANCE TO WARRANT AN EARLY PRESENTATION. /AUTHOR/