Automatic Identification of Local Instability in Shallow-Buried Thick Sand Strata during Diaphragm Wall Construction

Shallow-buried thick sand strata present considerable local instability risks during diaphragm wall trenching construction. However, this critical issue has not been extensively studied, despite its serious safety consequences. This paper proposes an automatic identification model for shallow-buried thick sand strata, integrating three-dimensional limit equilibrium theory with a genetic algorithm to precisely identify the most potentially dangerous local instability mass and determine its minimum safety factor. The model establishes three undetermined parameters: failure angle, upper boundary, and thickness of the local instability mass. These parameters define the search space for the local instability mass. The effectiveness of this approach was confirmed through a diaphragm wall engineering case near the Rhine River in France, where the predicted instability location closely aligned with field observations. A systematic analysis of the model indicated that the difference in slurry-groundwater levels and the friction angle are the most significant factors affecting local instability in shallow-buried thick sand strata. The model indicated that the location of the most potentially dangerous instability mass changes depending on geological conditions, and larger instability masses do not always relate to lower safety factors. Additionally, exploratory experiments revealed that support pressure losses caused by slurry infiltration significantly influence local instability calculations in sand strata. This points out the importance of considering these support pressure losses in the stability evaluations of high permeable sand strata. The results improve the evaluation of safety and the optimization of design for diaphragm wall construction in shallow-buried thick sand strata.