Abstract: A deep excavation site in Chengdu City is located in expansive soils and large deformations were occurred in the norther supporting structures. Based on the analysis of the shape of the large deformation and the adjacent environment of the excavation site, it was concluded that the insufficient geotechnical investigation data, lack of layered and segmental excavations, presence of welldeveloped expansive soils and seepage of groundwater were the major causes. The mitigation measures should consider the safety as well as performance of adjacent building structures. One option included unloading soils, displacement micropiles plus daylighting weep holes and another option was corner and raking supporting plus daylighting weep holes. The monitoring data indicated that the excavation induced deformation was successfully controlled.

Key words: deep excavation； expansive soils, deformation monitoring, real time monitoring and construction, mitigation measures