## 5 Soil Conditioning and Variation in the Tunneling Parameters  
### Determination of the Type of Soil Conditioner  
A dispersive foam agent is used to condition the cohesive soil. The effective components of the dispersive foam agent are dispersants and surfactants. The role of the dispersant is to disperse the cohesive particles and reduce the adhesion of the soil to the cutterhead, cutter, and bulkhead of the chamber. The role of the surfactant is to reduce the surface tension of the water and produce foam after mixing with air. The dispersant can enhance the negative charge of clay particles, release the bound water between clay particles, and reduce the liquid limit of the residue. Thus, the liquid limit reduction in the soil can be used to evaluate the effect of dispersants [22]. The performance of the foam agent mainly includes the foaming performance and stability. The evaluation indexes are the foam expansion ratio (FER) and half-life [23; 24]. Figure 9: Determination of the water absorption of particles with sizes of 0.15–2 mm. Evaluation of the dispersant performance of soil conditioners  
The critical particle size of the soil with clogging potential in the shield tunneling was 0.15 mm [10]. Particles less than 0.15 mm in the soil are the main reason for shield clogging. Thus, the dispersion performance of the soil conditioners can be evaluated by determining the variation in the liquid limit of particles less than 0.15 mm in the soil. The test was carried out step by step as follows. The soil was placed in an oven at 105 \({}^{\circ}\)C for at least 24 h. Then, the dried soil was crushed by using rubber. The crushed soil was passed through a 0.15 mm sieve. Particles less than 0.15 mm were collected for the liquid limit test. The liquid limit was determined by the fall cone method according to Chinese standards [17]. Four foam agents, namely A, B, C, and D, were selected for testing. The injection ratios of the soil conditioner, i.e., the mass ratio of the soil conditioner to the dry soil, were 2%, 4%, and 6%, respectively. Figure 10 shows the variation in the liquid limit with the injection ratio of the foam agents. The liquid limit of the soil specimen remained constant after adding foam agent A, which indicated that the dispersing effect of foam agent A on the soil specimens was poor. After adding foam agents B or C, the liquid limit of the soil specimen decreased with an increase in the injection ratio of less than 4%. The liquid limit did not change obviously when the injection ratio was greater than 4%, which indicated that the effect of the soil conditioner remained almost constant. The liquid limit of the soil specimen gradually decreased with an increase in the injection ratio after adding foam agent D. The liquid limit reduction in the soil specimen with foam agent D was the maximum at the same injection ratio, which indicated that the dispersion effect of foam agent D was the best. Therefore, foam agent D was selected as the soil conditioner for the project. Evaluation of the foaming performance of the foam agent  
The foam properties varied with the foam concentration. To determine the optimal concentration of the foam agent, the foam generator shown in Figure 11 was used to generate the foam. The compressed air generated by the air compressor was passed into the liquid storage tank containing the foam solution. After the air and foam solution were mixed, the foam was obtained at the air outlet. The foam was put into the measuring cylinder. The foam expansion ratio was determined by calculating the volume ratio before and after the foam dissipation. Figure 12 shows the foam stability test device. The device contains a self-made funnel, a bracket, a beaker, and an electronic scale. The dispersed foam agent solution flowed into the beaker along the container. The reading on the electronic scale gradually increased as the foam burst. The dissipation curve and the half-life of the foam were obtained by recording the readings of the electronic scale at different times.