Research on the Influence of Meso-Mechanical Characteristics of Medium Sand on the Stability of Cofferdam Foundation Pits

The microparticle characteristics and physical and mechanical properties of medium sand vary significantly with deposition depth. Studying how these micromechanical characteristics influence cofferdam stability in sand layers can provide valuable insights for foundation pit design and safety assessment. Taking a bridge foundation cofferdam project in the Wei River medium sand formation in Xi’an as an example， this study investigated the microstructural and mechanical characteristics of Wei River medium sand through direct shear tests， permeability tests， and SEM tests. Meanwhile， a foundation pit construction model for the medium sand stratum was established using FLAC^3D software to systematically analyze the influence of sand particle length-to-width ratio， roundness， and roughness on internal friction angle， permeability coefficient， and cofferdam stability deformation. The results showed that： 1） the sand grains of the Wei River medium sand were irregular in shape with non-smooth surfaces； particle length was mainly distributed between 0.1 mm and 0.3 mm， and the width was mainly distributed from 0 to 0.2 mm. 2） As the length-to-width ratio and roughness of the medium sand increased， the internal friction angle gradually increased while permeability gradually decreased； conversely， with an increase in roundness， the internal friction angle gradually decreased and the permeability gradually increased. 3） The horizontal displacement of the cofferdam decreased with increasing length-to-width ratio and roughness of the medium sand， but increased with increasing roundness. This study revealed the relationship between the microstructure of Wei River sand and its physical and mechanical properties， aiming to provide a scientific basis for the design of foundation pit cofferdams in similar projects.