MEASUREMENT AND INTERPRETATION OF SURFACE VIBRATIONS ON SOIL AND ROADS

Abstract

THE ROAD RESEARCH LABORATORY IS DEVELOPING NONDESTRUCTIVE TECHNIQUES FOR MEASURING THE DYNAMIC MECHANICAL CHARACTERISTICS AND THICKNESSES OF THE LAYERS FORMING A ROAD. THE MATHEMATICAL THEORY FOR COMPUTING STRESSES AND DEFORMATIONS REQUIRES KNOWLEDGE OF THESE DATA AND THE DEVELOPMENT OF THESE TESTING TECHNIQUES IS A NECESSARY STEP TOWARDS A SYSTEM OF PAVEMENT DESIGN BASED ON THE STRESSES ENCOUNTERED IN THE ROAD AND THE MECHANICAL PROPERTIES OF THE MATERIALS. APART FROM THIS, THE TECHNIQUES ARE ALREADY ABLE TO PROVIDE INFORMATION OF IMMEDIATE VALUE IN THAT THEY PROVIDE DATA OF ASSISTANCE IN APPRAISING THE PERFORMANCE OF EXPERIMENTAL AND OTHER ROADS UNDER TRAFFIC, THEY CAN BE USED TO LOCATE AREAS WHERE VARIATIONS OF MECHANICAL PROPERTIES OR THICKNESS OCCUR, AND THEY CAN BE USED TO STUDY THE CHANGES PRODUCED BY TRAFFIC AND WEATHER. THE FIRST PART OF THIS PAPER DEALS WITH THE EXPERIMENTAL TECHNIQUE FOR MEASURING THE WAVE LENGTH AND PHASE VELOCITY OF MECHANICAL VIBRATIONS PROPAGATED ALONG THE SURFACE OF SOIL OR ROAD CONSTRUCTIONS. THE VIBRATIONS ARE PRODUCED ELECTRO-MECHANICALLY BY APPARATUS WORKING WITHIN THE FREQUENCY RANGE FROM 40 TO 60,000 C/S AND HAVE WAVE LENGTHS RANGING FROM A FEW INCHES TO SEVERAL FEET. THE RESULTS ARE NORMALLY EXPRESSED GRAPHICALLY AS THE RELATION BETWEEN PHASE VELOCITY AND THE WAVE LENGTH OBTAINED AT SELECTED FREQUENCIES. THIS CURVE HAS A NUMBER OF CHARACTERISTICS WHICH DEPEND ON THE ELASTIC PROPERTIES AND THE THICKNESSES OF DIFFERENT PARTS OF THE CONSTRUCTION: THE SECOND PART OF THE PAPER DISCUSSES THEORETICAL ANALYSES TO CALCULATE THESE PARAMETERS. SO FAR, MOST OF THE WORK HAS BEEN LIMITED TO EXPERIMENTAL CONSTRUCTIONS AND ALL THE RELEVANT DATA CONCERNING THICKNESSES AND TYPE OF MATERIAL HAVE BEEN KNOWN, WHILE VIBRATORY EXPERIMENTS HAVE ALSO BEEN MADE, WHERE NECESSARY, ON LABORATORY SPECIMENS OF THE MATERIALS TO DETERMINE THEIR ELASTIC PROPERTIES. THESE DATA HAVE ENABLED CHECKS TO BE MADE OF THE VALIDITY OF PREDICTIONS FROM THE VIBRATIONAL EXPERIMENTS. /AUTHOR/