Abstract:  Expansive soil belongs to special rock soil, and its force and deformation is complex. Based ona deep foundation pit projeet in Honghe Prefecture, this article studied on the collapse instability anddeformation damage in the northeast side of the foundation pit during the construction process ; The softsilty clay layer at bottom of foundation pit with the expansive soil pit has been subjected to waterimmersion for a long time , which is the main cause of pit collapse and destabilization through the on-siteinvestigation ; Finally, it carried out the targeted emergency treatment and put forward the comprehensivetreatment scheme of adding row piles at the bottom of foundation pit to reinforce it. The implementation ofthe program is effective, suecessfully preventing the further deformation of the supporting structuredamage , and finally ensuring the safety of foundation pit project.

Key words: foundation pit;expansive soil, weak layer, instability and deformation