###### Abstract  
The slurry-earth pressure balance dual-mode shield has an earth pressure balance (EPB) and slurry shield functions. Based on a shield tunnel project of Guangzhou Metro Line 12 in China, this study investigates the clogging prevention of a slurry-earth pressure balance dual-mode shield in a composed stratum with medium-coarse sand and argillaceous siltstone. The results show that the slurry mode was not applicable to the composed stratum with medium-coarse sand and argillaceous siltstone. The excavated soil accumulated easily in the slurry chamber, causing shield clogging. The total thrust force of the shield increased significantly, the tunneling speed gradually decreased to 0, and the torque of the cutterhead increased slightly after the slurry shield was clogged. The fluctuation in the total thrust force, the cutterhead torque, and the tunneling speed also increased significantly. The EPB mode is recommended for composed strata with medium-coarse sand and argillaceous siltstone. The dispersible foam agent and water needed to be used for soil conditioning. The injection amount of foam and water was determined according to the status of the mud discharged by the screw conveyor. Water absorption can be used to characterize the water absorption capacity of particles larger than 0.15 mm. The ideal soil state was that the consistency index of the particles smaller than 0.15 mm was less than 0.5 to prevent the EPB shield from clogging. The water absorption of soil with a particle larger than 0.15 mm should be removed when calculating the consistency index. composed strata; dual-mode shield; shield clogging; soil conditioning; soil softness +
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requirements. However, it is unclear which mode should be used in composed strata with medium-coarse sand and argillaceous siltstone. The properties of the composed strata with medium-coarse sand and argillaceous siltstone are very complex. The upper medium-coarse sand layer is unstable. When shield tunneling occurs in a stratum, the tunnel face easily loses stability, causing excessive ground settlement and threatening the safety of the surface structures. The lower weathered argillaceous siltstone has strong adhesion. When shield tunnels are present in this stratum, the soil easily adheres to the cutterhead, cutter, and bulkhead of the soil chamber, thereby significantly reducing the shield tunneling speed and affecting the shield construction efficiency [8]. A typical example is the construction project of Nanchang Metro Line 1, China. The EPB shield passed through a strongly weathered argillaceous siltstone stratum. The maximum particle diameter exceeded 40 mm. The tunneling speed decreased during the shield tunneling. It was found that a large amount of muck adhered to the cutterhead and the chamber wall after opening the soil chamber [9]. The soil state is one of the main factors that affects shield clogging when the EPB shield tunnels in a high-adhesion stratum. Soil conditioners are often used at construction sites to change the properties of soil. Several researchers have conducted related studies on soil conditioning to prevent shield clogging. [PERSON] et al. [10] determined the critical particle size of soil with clogging potential in shield tunneling through large-scale rotary shear tests. The authors also pointed out that the main reason for shield clogging was particles smaller than 0.15 mm in the soil. [PERSON] and [PERSON] [11; 12] studied the effect of dispersants on the adhesion of clay by mixing tests. The results showed that when the consistency index of the soil was greater than 0.8, the effect of the dispersant was not obvious. When the consistency index of muck was less than 0.8, the dispersant significantly reduced the adhesion of the soil. The consistency index is the ratio of the difference between the liquid limit and the water content of the soil specimen and the plasticity index.