A series of tests on free-headed rigid piles were carried out in order to explain the instability mechanism of rigid piles under lateral loads. The soil used was unsaturated remolded silty clay. The water content of soil was between 9.85% and 13.85%. The total process load-displacement curve indicated that rigid piles would be instable when lateral loads attained to a certain amount. Visual records and soil sections showed that one shear band and one wedge were formed in the soil behind a pile with the softening failure of soil

along with one incomplete elliptic bulging and one tension crack parallel to the load direction on the soil surface. In the soil before a pile

one penetration groove was formed owing to the extrusion of the pile. The formation of the shear band is the essential reason of the instability of a rigid pile. And the extrusion failure of the soil before pile can’t play a decisive role in the instability of a rigid pile. As to rigid piles with large displacement

the essential reason of the instability is the failure of the soil mass behind piles.