## 3 Results
### Analysis of Settlement and Cracks  
After the test, the settlement at each location is measured, and the settlement distribution is shown in Figure 7. It can be seen from the figure that the settlement distribution shows a distribution of high in the middle and low on both sides, regardless of the presence or absence of copper contaminants. Settlement range from 1.58 to 7.00 mm in the middle position, from 1.40 to 3.00 mm at the anode, and from 1.45 to 2.80 mm at the cathode. Settlement increases with the increase of distance from the middle position, and the whole shows saddle-shaped distribution with obvious sinking in the middle position. The cause of saddle-shaped inhomogeneous distribution is due to soil consolidation and shrinkage after the transport and discharge of pore water, along with the significant reduction in soil volume. The inhomogeneity of settlement under vacuum is more obvious, and the difference between the settlement in the middle and on both sides is about 3 mm, and that of electro-osmosis is smaller. Under the vacuum action, the vacuum membrane shrinks inward to squeeze the soil sample, and the outer vacuum membrane is obstructed by the rigid boundary of the sample tank leading to a larger squeezing effect in the middle than in the sides. Moreover, the electric field effect and cathode direct discharge cause high moisture content and low shear strength in the middle position, which makes the middle position easier to compress and sink. Thus, the settlement is greater than the two sides, and the frictional resistance between the soil sample and the side wall hinders the sinking of the soil sample to a certain extent. For the contaminated soil sample, the magnitude of its saddle-shaped distribution is smaller than that of the pollution-free soil, which is due to the smaller drainage volume of the soil sample on the one hand. On the other hand, the precipitation generated by the reaction of copper ions inside the contaminated soil  
Figure 6: Vane shear strength of soil. filled the soil pores and encroached on part of the pore volume, reducing the compressible volume of the soil volume. The copper ions, which are co-transported with the pore water, precipitate and accumulate in the middle position, increasing the volume of the soil sample in the middle, and enhancing the soil strength to some extent. Thus, the settlement in the middle position of the contaminated soil is smaller than that of the pollution-free soil, resulting in a smaller magnitude of the saddle-shaped distribution. Figure 8 shows the figure after the test for the T1-T6 groups, where the soil all cracked to different degrees, with the more obvious cracks marked in red. The cracks in groups T1-T3 and T4-T6 are gradually expanding. The electro-osmosis method produces the least cracks, due to the small drainage volume and uniform drainage rates. The directional flow of pore water occurs under the action of the electric field, and the moisture content of the soil changes more uniformly. Comparing T1 and T4, it can be seen that the longitudinal cracks are both relatively small, while the transverse cracks of the contaminated soil are larger and the detachment of the contaminated soil from the drainage board is more obvious. As for the vacuum method, it is evident that the longitudinal cracks are wider and longer. Comparing T2 with T4, it can be found that there are longitudinal penetration cracks on both sides of the contaminated soil, i.e., the length of the cracks is approximately equal to 30 cm of the length of the sample tank. The cracks are wider, while the detachment from the drainage board is not obvious. The vacuum electro-osmosis method has the most extensive cracks, and a comparison of T3 and T4 shows that the same wider and larger longitudinal penetration cracks exist in the contaminated soil and that detachment cracks from the drainage board exist in the transverse direction.