SOIL RESISTANCE TO A SLOWLY MOVING PENETROMETER

THE RELATION WAS STUDIED BETWEEN PENETRATION VELOCITY AND FORCE WHEN A METAL PENETROMETER WAS FORCED INTO SOIL AT RATES APPROXIMATING THOSE OF ELONGATING ROOTS. NO PRIOR ASSUMPTIONS WERE MADE ABOUT THE PROPER RHEOLOGICAL OR CONSTITUTIVE EQUATIONS TO DESCRIBE THE SOIL. SLOW, SMALL OSCILLATIONS OF A 2 MM. DIAMETER CYLINDRICAL PENETROMETER MATERIALLY REDUCED SOIL RESISTANCE TO SLOW PENETRATION, PROBABLY BY REDUCING THE SOIL-METER COEFFICIENT OF FRICTION. IMPEDANCES DEPENDED ON PENETRATION RATE, BUT PENETRATION VELOCITY IS NOT PROPORTIONAL TO APPLIED FORCE, PARTICULARLY AS VELOCITY DECREASES TO ABOUT THAT OF ELONGATING ROOTS. FORCE REQUIRED TO PRODUCE THE USUAL RAPID FAILURE IN PENETRATION TESTS MAY EXCEED THE FORCE REQUIRED TO KEEP THE SAME PENETROMETER MOVING AT RATES LESS THAN .035 MM/MIN. (5 CM/DAY). A POWER LAW IS GIVEN RELATING PENETROMETER RESISTANCE AND VELOCITY. IT IS USED TO ILLUSTRATE THAT THE OBSERVED DISCREPANCIES BETWEEN ROOT AND PENETROMETER RESISTANCE MAY ARISE FROM VELOCITY DIFFERENCES.