Abstract: Laboratory tests on compaction behavior for middle expansive soil and flyash-treated soil have been carried out. And the dry density and degree of compaction changing specificity experiment after immersion , the varying character of unconfined compressive strength and modulus of compressibility in different soak periods and different wetting and drying cycles have been carried out in the state of optimal water content and different degrees of compaction. so as to study the water stability specificity of middle expansive soil and flyash-treated expansion soil. Test results show that: the range of compaction water content for flyash-treated soil is larger than middle expansive soil; this has brought great convenience for flyash-treated soil roadbed’s execution. And after absorbent, the fly ash-treated soil’s dry density and degree of compaction changes is smaller than middle expansive soil; less full water capacity, the variation of strength is small also. After different soak periods and different wetting and drying cycles, the strength of modified soil generate dramatic decay in the initial, and drive to stability of status in the last. But the property of strength decay have great improvement than the without modified soil; the water stability of flyash-treated soil is good.

Key words: flyash, expansive soil, effect of improvement, water stability