<p>Previous studies have mainly focused on liquid-induced slope failures, whereas gas-induced failures, which are common in municipal solid waste (MSW) landfills, remain insufficiently understood. This study presents the 1 <i>g</i> model tests of slope failures caused by rising gas pressure. To reveal the triggering mechanism and improve the accuracy of stability assessment, pore liquid pressure and pore gas pressure during two-phase flow were monitored independently. Experimental and numerical comparisons were further conducted between foam flow (disconnected phase) and gas flow (connected phase) to examine their different effects on slope instability. The results show that cracks started to develop on the slope surface at the peak of the pore gas pressures, which were larger than the peak liquid pressures. Under the same injection pressure, foam and gas have significant differences in the failure mode and degree of slope. Foam could partially block the pore throat, thereby trapping a large volume of gas inside the model, causing the slope to fail through penetrating cracks. However, there was only some erosion failure occurred locally on the slope surface under gas flow. The critical gas pressure ratio (gas pressure/earth pressure) were determined to be 0.87 for foam flow and 0.99 for gas flow, indicating that slope failures were more likely to occur under foam conditions. By monitoring in-situ gas and earth pressures, the current gas pressure ratio can be evaluated as a practical safety early-warning indicator for MSW landfills.</p>