## 3 Mechanical Characterization of Tailing Sand
### Results of Tailings Sand Penetration Test
#### 3.8.2 Test Method  
According to the geotechnical test specifications, the ring knife sample with dry density \(\rho=1.68\) g/cm\({}^{3}\) was prepared and put into the three-axis osmometer after saturation, testing the seepage through linear saturation, B-value detection, consolidation and normal head infiltration in the Geosmart infiltration module. Linear saturation and B-value detection module  
By setting effective axial pressure and back pressure, the ring cutter sample was saturated step by step. This test adopted seven stages of saturation. The loading scheme is shown in the table. After the saturation is completed, the B-value test is carried out, which is used to test whether the saturation is complete. When the B-value reaches 0.95 or above, the sample is considered to be completely saturated, and the next test can be carried out. Otherwise, continue to apply effective axial pressure and back pressure to further saturate the sample. Consolidation module  
The effective consolidation stress was achieved by applying effective axial pressure while the back pressure remained unchanged. In this test, 100 kPa effective consolidation stress was applied to observe the end of consolidation when the axial displacement did not change. Seepage module  
The method of constant head seepage was adopted, and the osmotic pressure was set at 30 kPa. The change in the law of back pressure volume and bottom pressure volume was observed, and the flow rate Q was calculated. Analysis of Results  
Permeate specimens of the three graded tailing grains of sand were prepared according to the specification and placed in the triaxial permeation test system. The pressurization and percolation operations were carried out. Figure 19 shows the triaxial permeation loading frame and permeation pressure chamber. Figure 20 shows the morphology of each specimen after the end of the permeation test. Figure 18: Grading curves of tailing sand. The specimens of different grades showed different morphology: C\({}_{\rm u}\) = 3.934, C\({}_{\rm c}\) = 1.042, no obvious damage and defect occurred on the surface of the specimen; C\({}_{\rm u}\) = 5.5, C\({}_{\rm c}\) = 1, observed from the bottom of the sample, it was found that the bottom was damaged; C\({}_{\rm u}\) = 5.5, C\({}_{\rm c}\) = 2.5, it was found that damage marks appeared in the middle and bottom of the sample. To quantify the influence law of permeability coefficients for different grade pairs, Figure 21 shows the curves of seepage volume versus time for specimens under the conditions of 100 kPa consolidation of effective axial pressure and 30 kPa permeability pressure. In [PERSON]'s seepage theory, the amount of seepage per unit of time through the area is proportional to the difference in head upstream and downstream, and inversely proportional to the length of the seepage sample. The calculation equation is as follows. \[Q=KA\frac{\Delta h}{L}=KAi \tag{4}\]  
In the formula: Q--Seepage flow/(mm\({}^{3}\)/s)  
Figure 19: Triaxial Penetration System. Figure 20: Morphology of the specimen after damage. \(A\)**--**Fracture area of the specimen/cm\({}^{2}\)  
\(K\)**--**Hydraulic conductivity/(cm/s)  
\(\Delta\lambda\)**--**Delivery head/cm  
\(L\)**--**The distance of penetration/cm  
\(i\)**--**Hydraulic gradient  
This can be seen in Figure 21 and Table 6, it can be found that the percolation volume of tailing sand shows a good linear relationship with time. Its percolation law is consistent with [PERSON]'s law. Under the action of permeation pressure of 30 kPa, the flow rate of the bottom pressure and back pressure is very consistent. Because of the good mechanical properties of the three gradations, the fine particles fill the pores between the coarse particles obviously, so the numerical difference is not obvious. The permeability coefficient of the specimen was further calculated by the flow rate of the bottom pressure.