THE FROST BEHAVIOR OF SOILS. I. VERTICAL SORTING

STUDIES IN PERMAFROST AREAS SHOW THAT A PARTICLE-SIZE SORTING OCCURS IN SATURATED GRANULAR SOILS DUE TO FREEZE-THAW CYCLING. THIS PHENOMENON IS EVIDENCED BY A GRADUAL CHANGE IN SIZE OF SOIL GRAINS, WITH THE FINES AT THE BOTTOM OF THE ACTIVE LAYER AND THE COARSE AT THE SURFACE. THIS HAPPENS IN WELL-GRADED MATERIALS WITH AND WITHOUT PARTICLES FINER THAN 0.02 MM. IN LABORATORY EXPERIMENTS SOILS SAMPLES WERE SUBJECTED TO ALTERNATE FREEZING-AND-THAWING CYCLES WITH THE TEST SPECIMENS COMPLETELY SATURATED. SERIES HAVE BEEN RUN IN WHICH FREEZING AND THAWING BOTH PROGRESSED FROM THE TOP DOWN, AND OTHERS IN WHICH FREEZING OCCURRED FROM THE BOTTOM AND THAWING FROM THE TOP. THE SOIL MATERIALS USED WERE WELL-GRADED SANDY GRAVELS LACKING THE 0.02-MM FRACTION. THE TESTS SHOW THAT ALL SAMPLES DEVELOPED VERTICAL SORTING AND CHANGES IN DENSITY (THAWED CONDITION) AS THE FREEZING-AND-THAWING CYCLES PROCEED, AND THAT THE DENSITY CHANGE IS A FUNCTION OF THE INITIAL DENSITY AND THE SPEED OF FREEZING. WITH A FREEZING RATE OF 0.6 MM PER HR, A SAMPLE THAT HAS 14 PERCENT FINER THAN 0.074 MM DEVELOPED SORTING AND INCREASED 10 PERCENT IN VOLUME AFTER 5 CYCLES OF FREEZE-THAW. THE RATE OF MOVEMENT OF THE 0.074-MM FRACTION IS 0.25 PERCENT PER CYCLE. A SAMPLE THAT HAS 0 PERCENT FINER THAN 0.074 MM ALSO PRODUCED SORTING AND INCREASED UP TO 6 PERCENT IN VOLUME AFTER 10 CYCLES. THESE TESTS WERE PERFORMED WITHOUT SURCHARGE ON THE SAMPLES. IT IS POSSIBLE THAT THIS DECREASE IN DENSITY MIGHT BE A RESULT OF THE MIGRATION OF SMALLER PARTICLES FROM THE VOIDS BETWEEN LARGER ONES. IT IS THEREFORE APPARENT THAT A MIXED SOIL SUBJECTED TO FREEZE-THAW CYCLES BECOMES VERTICALLY SORTED UNTIL A FILTER SYSTEM IS ESTABLISHED: THAT IS, THE MATERIAL IN ONE LAYER CANNOT PASS THROUGH AN INTERVENING LAYER INTO A THIRD. IN PRACTICE, THIS PRINCIPLE IMPLIES /1/ THAT A WELLGRADED GRAVELLY SOIL, WITH OR WITHOUT FINES, WILL BE SORTED AND A DENSITY CHANGE WILL TAKE PLACE WHEN IT IS SUBJECTED TO A SUFFICIENT NUMBER OF FREEZE-THAW CYCLES, AND /2/ THAT, ALTHOUGH A SOIL MAY NOT FOSTER FORMATION OF GROUND ICE DURING THE FIRST FREEZE CYCLE, THE CHANCE FOR SUCH A FORMATION WILL INCREASE DURING SUBSEQUENT CYCLES, AS MORE OF THE FINER MATERIAL IS CONCENTRATED AT THE BOTTOM OF THE FREEZE-THAW LAYER. MORE TESTS ARE BEING CONDUCTED TO DETERMINE /1/ THE SEGREGATION EFFECTS WHEN THE ZERO LINE MOVES VERTICALLY, INSTEAD OF HORIZONTALLY, INTO THE SOIL,/2/ THE EFFECT OF SPHERICITY OF THE PARTICLES, AND /3/ COMPOSITION OF THE FINER FRACTION. /AUTHOR/