Abstract
Foam as a soil conditioner can transform the mechanical properties of the excavated natural muck and lubricate the interface between the cutting tools and muck, thus reducing the tools’ wear and promoting the efficiency of earth pressure balance (EPB) shield tunneling. This paper aims to explore the meso-mechanism of foam in improving the workability of sand by combining discrete element modeling (DEM) with experimental investigations of slump tests. A “sand-foam” mixture DEM model was generated by simplifying the sand grains and foam as individual particles with different properties. The particle-scale simulated parameters were calibrated based on a series of experimental observations. The effects of foam on the inter-particle contact distribution and the evolution of contact forces during the slumping process were investigated in detail through numerical modeling. It was found that injecting foam into sand specimens could increase the coordination number and the contact number around sand grains. Although the force transmission pattern changes from “sand-sand” into the coexistence of “sand-foam”, “sand-sand” and “foam-foam” contacts, the magnitude of contact forces transferred by foam particles is significantly lower than that by sand particles. The presence of foam reduces contact-scale frictional strength and thus reduces the stability of the microstructures of sand. In addition, the normal direction of inter-particle contact force deflects from the vertical to the horizontal and the magnitude of contact force decreases significantly with the influence of foam.
Highlights
Earth pressure balance (EPB) shield machines mainly cut the stratum by cutterhead to realize the full section excavation of tunnels, but the cutting tools are prone to wear when the shield tunneling through the hard stratum, leading to a decrease in cutting efficiency and even the severe damage of the cutterhead
The results show that a water content of 10% could make the foam work well in the sand
It was found that the slump value of foam-conditioned sand decreased from 5.7 cm to 4.9 cm and increased to 10.8 cm with increasing foam injection ratio (FIR), and the turning point appeared at the FIR of 15%
Summary
Earth pressure balance (EPB) shield machines mainly cut the stratum by cutterhead to realize the full section excavation of tunnels, but the cutting tools are prone to wear when the shield tunneling through the hard stratum, leading to a decrease in cutting efficiency and even the severe damage of the cutterhead. The cutterhead wear and the EPB shield tunneling process are significantly influenced by the workability of soil, which refers to the overall state, in which the muck has suitable fluidity and plastic properties and is helpful for smooth EPB shield tunneling. The “low plastic flow” state of muck is required to ensure the effective generation of chamber pressure and smooth discharging of muck [5,6,7]. To transform the muck into a “low plastic flow” state, foam is one of the most common conditioning agents to promote the workability of muck before cutterhead, in the pressure chamber and screw conveyor, resulting in an enhancement of the muck plasticity and fluidity [8,9]. The foam is mixed with the muck as the cutterhead rotates, and the workability of the muck becomes a kind of plastic flow state
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