Abstract: To meet the requirements of geotechnical material for engineering in loess area, a series of tests on loess with long age improved by lime or fly ash to investigate the effects of original soil type, admixture material and admixture amount on shear strength and permeability, and to reveal the microstructure and reaction mechanism. Some conclusions are concluded as follow: the strength development and the stable age of lime soil and lime-ash soil mainly owe to working time of lime and fly ash; the change of hydration reaction rate makes the influence of admixture amount on lime soil show age effect; the granularity of original soil has the most important influence on shear strength and permeability; the strength development is not synchronous with the development of permeability of same improved soil, so the standard age should be different and should be distinguished in practice work. According to the influence of various factors on shear strength and permeability, engineering design shall distinguish the type of original soil at first; and then select improvement mode and admixture amount based on demand nature on the premise of ensuring curing age.

Key words: long age, improved loess, shear strength, permeability