Abstract: The typical area was chosen on the tidal flat of the Yellow River estuary where hard crust develop, simulating wave load test with a light compactor was carried out to study the repeat liquefaction and change law of crust layer within one meter through the monitoring of pore water pressure and in-situ soil strength measure. The results revealed that the dynamic response varies a lot along depth and the strength of silt increases after repeat liquefaction; the upper crust layer increases more obviously than the down. The undisturbed samples were taken before and after vibrating; and quantitative analyzing of SEM structure images is carried out on the samples of different depths. The results revealed that some indexes of microstructure change evidently; such as asymmetry coefficient descended, granularity direction strengthed, granulometric fractal dimensions improved and figure coefficient improved and ratio of pore will descended etc. The transformation varied a lot along depth with the range mainly concentrated above the upper site. The relation between the dynamic response character and their microstructure changes has been discussed; and the character and formation mechanism of hard crust under the control law of microstructure are grasped primarily on this base.

Key words: silt, hard crust, cyclic load, dynamic response, microstructure