General description of the flow phenomenon

Using the testing system above, in order to demonstrate the reliability of the testing system, first the permeable law of fine sand was studied. The relationship between the seepage velocity and hydraulic gradient is shown in Figure 5.17a. A linear relationship existed between the seepage velocity and the hydraulic gradient. For the clay specimen, the soil matrix contained a lot of clay minerals (e.g., montmorillonite, kaolinite, etc.) that was strongly hydrophilic. When water flows in a clay specimen, the minerals react with water, resulting in nonlinear seepage properties for the clay specimen. But the seepage velocity was approximately linear to the hydraulic gradient. In saturated clay, the seepage law is influenced by the pore characteristics, structure, and hydraulic gradient; actually, only at lower hydraulic gradients does the seepage law of clay obey Darcy’s law, and this is consistent with the results of Dixon et al. (1992).

The relationship between the seepage velocity and hydraulic gradient for typical SRM specimens with different rock block percentages is shown in Figure 5.18. As shown in Figure 5.18, the relationship between the seepage velocity and hydraulic

Relationship between the seepage velocity and hydraulic gradient for the sand and soil specimen, and the permeability properties of the sand and clay specimen obey Darcy’s law

Figure 5.17 Relationship between the seepage velocity and hydraulic gradient for the sand and soil specimen, and the permeability properties of the sand and clay specimen obey Darcy’s law. (a) Sand sample; (b) Soil matrix sample.

The plots of seepage velocity against hydraulic gradient for typical SRM specimens, (a-f) Plots for the specimen with a rock block percentage of 20%-70%, respectively

Figure 5.18 The plots of seepage velocity against hydraulic gradient for typical SRM specimens, (a-f) Plots for the specimen with a rock block percentage of 20%-70%, respectively.

gradient is not linear. The seepage velocity increased with the increase of hydraulic gradient, and the increment rate for SRM specimens with a rock block percentage of 70% is most evident. These results implied that the value of the permeability coefficient for SRMs is not constant, which varies with the hydraulic gradient. Soil and rock mixtures are a special kind of geomaterial, and the permeability law may not comply with Darcy’s law.

The plots of the permeability coefficient against hydraulic gradient for typical SRM specimens, (a-f) Plots for specimens with a rock block percentage of 20%-70%, respectively

Figure 5.19 The plots of the permeability coefficient against hydraulic gradient for typical SRM specimens, (a-f) Plots for specimens with a rock block percentage of 20%-70%, respectively.

As shown in Figure 5.19, under different hydraulic gradients, the permeability coefficient of SRMs with different rock block percentages w'as different, which further proved that the seepage law' of SRMs is different from that of fine sand and clay soil. The nonlinear relationship between seepage velocity and permeability coefficient is stronger than in clay specimens. The permeability law of SRMs does not comply w'ith Darcy’s law.

The plots of seepage velocity versus hydraulic gradient for SRM specimens with different rock block percentages, (a-d)

Figure 5.20 The plots of seepage velocity versus hydraulic gradient for SRM specimens with different rock block percentages, (a-d): Rock block percentage is 20%, 30%, 40%, and 50%, respectively.

Non-Darcy’s flow of SRMs

For a given rock block percentage of SRMs (i.e., 20%-70%), five specimens were used to perform the permeability tests. The plots of seepage velocity against hydraulic gradient are shown in Figure 5.20. As shown in Figure 5.20, the nonlinear relationships are obvious for the tested SRM specimens. Seepage velocity increases with the increase of hydraulic gradient, and the increasing trend becomes quick at a higher hydraulic gradient. In addition, because the spatial distribution and size distribution for specimens

Table 5.6 The curve fitting results of seepage velocity against hydraulic gradient for typical specimens

Rock block percentage %

20

7.897E-4

1.925

0.982

30

3.290E-4

2.055

0.975

40

I.27E-3

1.631

0.987

SO

I.604E-4

2.241

0.934

60

2.28E-3

1.677

0.971

70

S.I9E-3

1.520

0.982

with the same rock block percentage are different, this results in the discrepancy in the permeability coefficient. In spite of this, the permeability coefficient of the specimen with the same rock block percentage is almost the same.

The permeability law for SRMs is different from that of the sand and soil specimen; this is due to the existence of rock blocks in the soil matrix. When water flows in the SRM specimen, the flow direction and velocity are strongly influenced by rock blocks, especially at the rock-soil interfaces because the interface is the weakest position in the specimen, and the seepage is extremely unstable. Furthermore, a turbulent flow may form at those interfaces. The nonlinear curve fitting equations for the specimens are listed in Table 5.6. The correlation coefficients were all greater than 0.9, and the coefficients of the equations are quite good.

 
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