ON THE PARTICLE SIZE DISTRIBUTION OF A BASALTIC TILL

抄録

Stress and strain history play a major role in defining the final grading of a soil, and by studying how the particle size distribution of a soil evolves with stress and strain it may be possible to highlight some important aspects of the sediment past history. In this paper, the characteristics and evolution of the grading of a till from Iceland, so-called Langjökull sediment, are examined. Specimens of the till were tested in compression (triaxial, oedometer tests) to very high pressures, and in shearing (triaxial, ring shear tests) to very high strains. The change in grading corresponding to particle breakage during compression and shearing was monitored using sieve analysis and scanning electron microscopy. The results show that the particle size distribution of the till, which is well graded, does not evolve when the till is subjected to high stresses or strains. Additional tests on specimens made of the larger-sized till particles did however show signs of breakage during testing. This suggests that the current natural grading of the till was achieved by incessant shearing during its past history, and corresponds to an ultimate, “critical” grading. This is confirmed by the mineralogical analysis of the sediment, which highlights a single parent rock and weathering of the particles by mechanical processes only.