Failure Characteristics of a River Embankment on Soft Ground Improved with Combined Columnar and Grid-Form Deep-Mixed Columns: Three-Dimensional Centrifuge and Numerical Modeling

Grid-form reinforcement is a composite structure formed by overlapping cement deep mixing (CDM) columns. Under embankment loading, CDM columns may experience instability and failure. Most existing calculation methods assume simultaneous shear failure of the columns, which can lead to an overestimation of embankment stability in engineering design. Therefore, accurately identifying the failure characteristics of CDM columns is crucial for improving the accuracy of stability assessments. The research is conducted with reference to the soft ground improvement project of the controlling channel in Baishan Bay, which serves as a waterway linking the Yangtze River and the Huai River. On the basis of the centrifuge model test, the corresponding numerical model was established in this study by the three-dimensional FEM to analyze the effect of combined columnar and grid-form columns on river embankment stability. The rationality of the similarity design in the centrifugal model test was also validated. Furthermore, this study utilized numerical analysis methods to investigate the effects of parameters such as the degree of cement soil cementation damage and column strength on the failure characteristics of CDM columns and the stability of the river embankment. The results indicate that considering the cementation damage of the CDM columns can accurately assess the overall stability of the river embankment. The failure modes of the CDM columns under the embankment mainly include shear and bending-tensile failures. During embankment failure, multiple three-segment wedge-shaped plastic shear planes are present, each divided into three zones: compression zone, bending-shear zone, and tension-bending zone. However, the CDM columns under the river embankment are not destroyed simultaneously; instead, they fail progressively with a failure order that is related to the strength of the columns. Shear failure in CDM columns initiates at the center of the slope and propagates toward the slope toe until failure. Reinforcement at each slope toe of the river embankment is crucial for ensuring overall stability.