# A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in EPBs
## 3 Results and Discussion
### Slump Test
#### 3.2.2 Effect of the Injection Ratio and Particle Size of the Gravel on Slump  
The gravel slump is not only related to the type of bentonite slurry, but is also affected by the B/A modified slurry concentration and the size of the sand gravel sample. Therefore, the slump experiments of different sand gravel samples mixed with different injection ratios of B/A modified slurry were conducted and the results are shown in Figure 6. [PERSON] et al. (2013) proposed that the modification state of sand gravel can be classified into three categories according to the flow, suitable and stiff states, as shown in Table 8.  
As seen from Figure 6, the slump value of differently sized sand gravel samples without the addition of modifier is between 170 mm and 180 mm. When the injection ratio of B/A modified slurry is 5%, the slump value of sand gravel with a diameter of 0-2 mm sharply decreases to 132.5 mm, while the flow plasticity state almost does not change for the sand gravel samples of 2-20 mm in size. As the slurry injection ratio increases to 10%, the slump value of the sand gravel a size of 0-2 mm decreases to zero, and the flow plasticity state reaches a plastic non-flow state. For the sand gravel sample with a diameter of 2-6 mm, the flow plasticity state is transformed from flow plasticity to plastic flow, while the state of the sand gravel of 6-20 mm in size is still the flow plasticity state. When the slurry concentration continually increases to 15%, the 2-6 mm sand gravel sample changes from plastic flow state to medium plastic state, and the mixture of 6-20 mm sand gravel and B/A modified slurry changes from flow plasticity state to plastic flow state. Furthermore, the slump value of the sandy gravel sample with a diameter of 0-10 mm decreases to nearly zero, and the plasticity paste state is the plastic state with the addition of 20% B/A modified slurry.  
It can be concluded that, as the slurry concentration increases, the slump value of various sand gravel samples gradually decreases and finally the flow plasticity reaches the plastic state. However, the injection ratio of B/A modified slurry that makes sand gravel sample with different size reach plastic state is different. Specifically, the smaller the sand gravel diameter is, the lower the slurry concentration is. As for sand gravel sample with 0-2 mm size, the plastic state is reached when B/A modified slurry injection ratio is between 5% and 10%, while the injection ratio is 15-20% for 2-10 mm sand gravel samples. Sand gravel with a diameter larger than 10 mm reaches a plastic state with a mixture of 20-30% B/A modified slurry. In addition, the voids of the 0-20 mm sand gravel sample can be completely filled when the addition of B/A modified slurry injection ratio is 30%. For the earth pressure balance shield, some scholars believe that the suitable slope of the soil should be between 100 mm and 200 mm. But for the full-face water-rich gravel, how to maintain the stability of the excavation surface and reduce the penetration is the main consideration. Therefore, by enhancing the cohesion of sand and gravel to achieve a plastic (flow) state is an effective way of improving the self-stability of the excavation face and preventing the instability of the formation.