Influence of metal cations on the yield stress of montmorillonite debris flow: Double layer analysis and novel model establishment

Yield stress is an important parameter determining the transport of debris flow. The main metal cations (Na⁺, Ca²⁺, and Al³⁺), of debris flow, were seldom studied on the yield stress. In this paper, clay, standard sand and gravel are configured in the ratio of 1:2:4 by mass to form the solid-phase portion of the debris flow respectively. Various metal cations (Na⁺, Ca²⁺, and Al³⁺) with different concentrations were used to investigate the microstructure, bound water content, zeta potential, and yield stress of montmorillonite debris flow. Metal cations had little effect on the microstructure of montmorillonite particles, but they could significantly compress the thickness of the double layer. When the concentration of Al³⁺ reached 0.039 mol/L, the weakly bound water content decreased to 136.8 %. This led to a dramatic decrease in the yield stress of the debris flow from 189.3 Pa (without metal cations) to 1.6 Pa (0.039 mol/L Al³⁺). Furthermore, the higher the valence state of the metal cation, the more significant the weakening effect on the yield stress of the debris flow. When the water content of the debris flow increases from 26.3 % to 37 %, the yield stress drops precipitously from 1727.5 Pa to 119 Pa. In the debris flow with a water content of 26.3 %, the yield stress even drops below 119 Pa, reaching 98.3 Pa under the action of 0.019 mol/L of Ca²⁺. By introducing Avogadro's number (Na), the shielding coefficient ($k_d=\frac{V_f}{\mathrm{Vs}}\ast {\theta }^{\prime }$), and further revising the size of montmorillonite particles, comparing the novel model results with the measured data, the accuracy of the model was verified from three aspects: zeta potential, metal cation concentration, and water content. Metal cations reduce the surface charge of montmorillonite, resulting in a decrease in zeta potential and a weakening of the adsorption capacity of bound water. Within the range of zeta potential from −9.3 to −8.5 mV, metal cation concentration from 0 to 0.02 mol/L, and water content from 26.3 % to 37 %, the yield stress decreases significantly. This study provides a theoretical reference for subsequent research on the initiation mechanism and transport laws of debris flows.