Behavior of a CDM System as a Retaining Wall and Ground Improvement for Bottom-Up Excavation in Soft Clay

Abstract This study investigates the behavior of a Cement Deep Mixing (CDM) pile system serving a dual function as a retaining-earth structure and ground improvement for a deep, bottom–up excavation in the soft clay of Ho Chi Minh City, Vietnam. The project site is characterized by challenging ground conditions, primarily soft clay interbedded with sand layers, posing significant stability risks to the deep excavation (-4.7 m to -6.27 m). To address these challenges, a retaining–earth system composed of 1000 mm diameter CDM piles, with lengths varying from 9.7 m to 17.0 m, was implemented. Using the Plaxis finite element software, a parametric study was conducted to quantify the influence of two key design parameters-CDM pile length and area replacement ratio-on the primary performance indicators: lateral retaining–earth displacement and the overall Factor of Safety (FoS) of the excavation. Results reveal a strong correlation between these parameters and the system’s behavior. The findings confirm that an optimized combination of pile length and replacement ratio is critical for controlling lateral deformations and ensuring an adequate Factor of Safety. This research offers valuable quantitative insights for optimizing the design of CDM retaining-earth systems in soft clay, leading to safer and more cost-effective solutions for deep urban excavations.