Foam Generation Performance of Earth Pressure Balance Shield Soil Conditioning

The performance of foam plays a pivotal role in the soil conditioning effectiveness of earth pressure balance (EPB) shield tunneling. This study presents a comprehensive set of foam performance tests conducted using a laboratory-designed foam generation system. The study primarily investigated the effects of gas flow rate, liquid flow rate, and foaming agent concentration on the flow rate, foaming expansion ratio, half-life, and mesoscale morphology of foam. Moreover, the correlation between macroscopic foam performance and mesoscale particle size was analyzed. The study results show that, under a constant gas-liquid ratio, the foam flow rate initially increases and subsequently decreases as the gas flow rate increases. Simultaneously, the foaming expansion ratio gradually decreases with increasing gas flow rate, and the foam half-life initially increases and subsequently decreases with the increase of gas flow rate. When the gas flow rate is constant, the foam flow rate increases approximately linearly with the increase in liquid flow rate. The foaming expansion ratio slightly decreases and tends to stabilize with the increase in liquid flow rate, while the foam half-life initially increases and then decreases with increasing liquid flow rate. When the foaming agent concentration is below 2 %, the foam flow rate, foaming expansion ratio, and half-life increase rapidly with increasing foaming agent concentration. However, when the foaming agent concentration exceeds 3 %, the foam performance is less affected by changes in foaming agent concentration. The foam generation system performs optimally under the following conditions; gas flow rate of 21 L • min~', liquid flow rate of 0. 3-0. 5 L • min~', and foaming agent concentration of 3 % . The initial particle size distribution of foam bubbles is most affected by the gas flow rate compared to the liquid flow rate and foaming agent concentration. As the gas flow rate increases, the foam becomes finer and more uniform. A significant correlation is observed between foaming expansion ratio and the foam bubble size, with relatively fine foam resulting in relatively low foaming expansion ratio. Nevertheless, the correlation between foam half-life and bubble size is not as readily apparent as the relationship between the foaming expansion ratio and foam bubble size. In the practical soil conditioning during shield tunneling, emphasizing the matching relationship between the foam bubble size and soil particle pore structure is important. Foam conditioning techniques are more suitable for well-graded soils. Under the same geological conditions, different foam gradings require different foam-injection pressures. © 2024 Xi'an Highway University. All rights reserved.