Assessing the effectiveness of an airshaft for dropshaft air re-circulation and depressurization

Dropshafts are commonly used to convey water to deep tunnels. Plunging flow dropshafts have been reported to cause downstream air pressurization and subsequent sewer odor issues. A retrofit by connecting the dropshaft to an airshaft with several horizontal pipes for air re-circulation has been implemented in Edmonton, Canada. Tools for predicting the performance of this retrofit are needed. In the current work, physical model study and prediction were conducted to assess the effectiveness of the retrofit. Experiments were run with the dropshaft connected to the airshaft with different numbers of horizontal pipes. Predictions of air pressure and air entrainment of the retrofitted dropshaft were in good agreement with the experimental data. The effect of the airshaft became more important under a pressurized downstream condition with a larger sized airshaft. The prediction was also applied to simulate the performance of the retrofitted prototype dropshaft in Edmonton, regarding the bottom air pressure and the air flow rates in horizontal pipes. The prediction compared well with the field monitoring results, and the airshaft was found to be effective in depressurizing the downstream sewers, which helped to reduce the downstream pressure by about 60%.