Study on the service performance of heterogeneous cutoff walls and three-dimensional transport of pollutants considering compression effect

Abstract

Vertical cutoff walls are widely used as an effective in-situ remediation technology for non-point source contaminated sites. In this study, considering the arch effect and lateral extrusion effect of the wall, a method for calculating the heterogeneous parameters of the cutoff wall is proposed based on the horizontal spring model. A three-dimensional model of unsteady pollutant transport in the unsaturated zone (internal aquifer-cutoff wall-external aquifer) is developed, incorporating the coupled convection-diffusion-adsorption-degradation processes. The research demonstrates that the shallow part of the wall is prone to forming a dominant migration channel. As the position ${\mu }_1$ of the peak concentration of the pollution source and the distribution range ($\sigma$) of high-concentration pollutants increase, both the breakdown time t_b and the depth h_b of the breakdown point increase. When the buried depth h_b of the breakdown point is small (with ${\mu }_1$ and $\sigma$ also small), t_b is less affected by the wall effective angle of internal friction ${\phi }_{\mathrm{cw}}$ and wall shear strength reduction factor R_r. Conversely, as the buried depth h_b increases (with larger values of ${\mu }_1$ and $\sigma$), the influence of ${\phi }_{\mathrm{cw}}$ and R_r on t_b becomes more significant. Finally, a design method for determining the depth h_r of the shallow enhanced cutoff wall under different pollutant concentration distributions is proposed. This study offers valuable insights for evaluating cutoff wall performance, optimizing backfill mix ratios, and improving wall structure design.