It is well known that the primary mechanism of the electroosmotic improvement without injection of solutions is the attraction of cations towards the cathode1,26. As a result, water moves towards the cathode along with the cations, driven also by the electric potential.
As for electroosmosis with injection, in addition to the pure electroosmotic effects, the increase of cation in the soil induced by the injection of saline solutions will result in an increase in electric conductivity and hydration of cation, which causes more absorbed water, along with the cation, to migrate towards the cathode. The water content thus decreases with the increase of distance from the anode. The electroosmotic effect is certainly enhanced, compared with the electroosmosis without injection27. Furthermore, the exchange of ions on the surfaces of soil particles will lead to flocculation and coagulation of soil particles, resulting in larger colloids and an overall increase in soil strength. Although electroosmosis with injection through the anode is effective in strengthening soil, the treated soil is, however, obviously limited to the anode region4,17.
As stated, it was confirmed by the fact that injection of CaCl2 through both the anode and central tube could effectively expand the treatment region. This may be primarily due to the following two reasons: first, water can be drained simultaneously in both anode and middle section of the soil, thereby expanding the consolidation area, and second, the formation of reaction products and precipitations were accumulated and thus clogged the pore spaces near both the anode and middle section of soil matrix, and therefore the area of cementation would be expanded. However, this clogging effect would also hinder the flow of water and reduce the overall permeability of the soil, resulting in a progressive decrease in drainage efficiency26,28. In addition to the clogging effect mentioned above, the electric resistance of the entire sample of S3 and S4 should be further discussed, as the efficiency of electroosmotic treatment is substantially controlled by the electric resistance of system1,29.
Figure 10 shows the electric resistance (Ω) vs. time (h) during the electroosmosis process for S3 and S4. According to the electrical current and voltage data, the electric resistance of part 1 (the soil near anode), part 2 (the soil near the left side of the central tube), part 3 (the soil near the right side of the central tube) and part 4 (the soil near cathode) of soil matrix with regard to time can be achieved, as shown in Fig. 10(a). Generally speaking, the whole soil sample could be separated to two regions by the central axis, i.e. upper regions including part 1 and part 2, and lower regions including part 3 and part 4. In the first period of 12.5 h, it is observed that the electrical resistance in S3 is obviously lower than that in S4. This is because the volume of CaCl2 solution injected in S3 is twice that in S4 in the first 12.5 h, and consequently resulting in a higher electric current (as shown in Fig. 5)27. After that, in the second period of 12.5 h, a continuous and substantial increase in electric resistance was observed in the anode section (part 1) of S3, reaching a final value of 345 Ω at the end of treatment, which is approximately 5 times higher than that of S4. The occurrence of cracks should account for the sharp increase in electrical resistance near the anode in S3. During electro-osmosis, water close to the anode will be heavier electrolyzed and therefore a volume shrinkage appeared in the soil in the anode section (part 1) (as shown in Fig. 11a). These cracks caused significant voltage loss, which further led to decrease in electric current and drainage rate24. It is apparent from Fig. 11b that the injection of CaCl2 solution through the anode followed by injection of the CaCl2 solution through the central tube after 12.5 h can effectively mitigate the formation of cracks in the anode section, and therefore considerably reduce the increase in the electric resistance. Additionally, appropriately prolonging second CaCl2 injection interval through the central tube can re-establish the electrochemical gradient after the first injection, causing an increase in electric current and a decrease in electric resistance. Therefore, to improve the effect of electroosmosis on soil, the shrinkage of soil should be restrained by methods such as prolonging second CaCl2 injection interval through the central tube during electroosmosis.