*3.1. Sedimentation Results without Flocculation*

The pipe jacking waste slurry with an initial water content of 300% was subjected to a self-weight sedimentation test. Figure 2 demonstrates that the sedimentation and separation of two types of slurry without pretreatment are very slow, and the water content of slurry drops from 300% to 292% and 296% respectively within the settling time of 600 s. It can be inferred that if the pipe jacking waste slurry is not treated, it could occupy a large area of the site for storage. Most pipe jacking projects are in the underground areas of a city. It is obviously difficult to find a large storage space in the city to store the slurry. Therefore, it is meaningful to quickly reduce the amount of slurry.

**Figure 2.** Sedimentation results of pipe jacking waste slurry without flocculation.

## *3.2. Sedimentation Results with PAM Pretreatment*

Figures 3 and 4 show the sedimentation results of the waste pipe jacking slurry after adding CPAM. Compared with the slurry without pretreatment, the addition of two types of CPAM can make the waste slurry settle and separate rapidly.

**Figure 3.** Sedimentation results of slurry (Type I) after pretreatment with CPAM. (**a**) 412VS. (**b**) 611HN.

**Figure 4.** Sedimentation results of slurry (Type II) after pretreatment with CPAM. (**a**) 412VS. (**b**) 611HN.

Figure 3 shows the settling results of Type I slurry. The optimal dosage of CPAM 412VS was 0.15%, and the water content of the sediment dropped rapidly from 300% to 159% within the settling time of 600 s. The CPAM 611HN also obtained good treatment results. The optimum dosage was 0.15%, and the water content of the sediment finally dropped to 185%.

Figure 4 reveals the settling results of the Type II pipe jacking waste slurry. Compared with Type I slurry, the sedimentation effect of Type II slurry containing bentonite was worse, and a higher dosage of CPAM was required for pretreatment. Using the same type of CPAM, the optimal addition amount was increased by three times, and the optimal dosage of 412VS and 611HN were both increased to 0.60%. In addition, in the same settling time, the water content of the sediment was higher, and the water content of the sediment after pretreatment with the two types of flocculants was 194% and 208%, respectively.

Comparing two types of CPAM, 412VS has a better pretreatment effect than 611HN, and the water content of the sediment can be reduced to a lower level under the same addition amount.

Figures 5 and 6 show the sedimentation results after pretreatment of waste pipe jacking slurry using APAM. The addition of APAM also increased the speed of soil–water separation. Unlike CPAM, the optimal dosage of APAM was much lower than that of CPAM, no matter for Type I slurry or Type II slurry.

**Figure 5.** Sedimentation results of slurry (Type I) after pretreatment with APAM. (**a**) 7126. (**b**) 720VJ.

**Figure 6.** Sedimentation results of slurry (Type II) after pretreatment with APAM. (**a**) 7126. (**b**) 720VJ.

Figure 5 reveals the sedimentation results of Type I slurry. The optimal addition amount of APAM 7126 was 0.06%. After 600 s of sedimentation, the water content of sediment finally dropped to 166%. The optimum dosage of APAM 720VJ was 0.07%, and the final water content of the sediment was 183%.

Figure 6 shows the settling results of bentonite-rich slurry (Type II slurry). Similar to the results of CPAM pretreatment, the treatment of Type II slurry with APAM requires a larger amount of flocculant than Type I slurry. The optimum addition amount of APAM 7126 reached 0.25%, and the final water content of the sediment dropped to 201%. The optimum dosage of APAM 720VJ reached 0.25%, and the water content of the sediment was 208%.

## *3.3. Sedimentation Results with Compound Pretreatment*

Figure 7 reveals the sedimentation results of Type II slurry after compound pretreatment. Comparing Figure 7a with Figure 4a, after the compound treatment of FeCl3·6H2O and CPAM 412VS, the settling and separation effect became better. After adding 3% FeCl3·6H2O, the optimal addition of CPAM 412VS decreased from 0.60% to 0.25%. The water content of the sediment can be reduced to 169% in 600 s, which is lower than the result of the single addition of CPAM 412VS.

**Figure 7.** Sedimentation results of slurry (Type II) after compound pretreatment. (**a**) FeCl3 + 412VS. (**b**) FeCl3 + 7126.

Similarly, Type II slurry pretreated with FeCl3·6H2O and APAM 7126 also obtained better separation results, and the optimal combination was 3% FeCl3·6H2O and 0.10% APAM 7126. Comparing Figure 7b with Figure 6b, after the compound pretreatment, the water content of the sediment can be reduced to 165% in 600 s, and the added amount of the APAM 7126 used was also reduced from 0.25% to 0.10%.

#### *3.4. Solidification Results*

In experiments where the slurry was only pretreated with PAM, the slurry treated with APAM 7126 achieved the best settling effect. The sediment obtained by settling had a lower water content, so the sediment was used for the solidification experiment. The cured samples were tested for unconfined compressive strength and the results are shown in Figure 8. Figure 8a shows the unconfined compressive strength of cured sediments with different dosages of OPC at different curing ages. With the increase of curing age, the unconfined compressive strength increased continuously. When the OPC dosage was 30%, the strength was significantly improved, the 7-day strength was 49.23 kPa, and the 28-day strength was 83.09 kPa. In the initial stage, the strength of the 3-day was only 8.95 kPa.

**Figure 8.** Unconfined compressive strength of the solidified sediment after APAM pretreatment. (**a**) OPC. (**b**) SAC.

Figure 8b shows the unconfined compressive strength of sediments added with SAC at various curing ages. Compared with OPC, SAC could improve the strength of the cured sediment in less curing time. For the solidified sediment with 30% SAC, the 3-day strength was 33.70 kPa, and the strength nearly reached the peak value of 51.46 kPa after curing for 7 days, and the strength was 55.30 kPa at the age of 28 days.

The slurry had better sedimentation results after the compound pretreatment of FeCl3·6H2O and APAM 7126, and the sediments were also subjected to solidification experiments. Figure 9 shows the unconfined compressive strength of the sediments. Compared with the samples obtained after the pretreatment with APAM 7126, the samples with the compound pretreatment have higher strength under the same amount of curing agent. For example, when OPC was used as the curing agent, the unconfined compressive strength at 30% was 15.2 kPa at 3 days, 55.8 kPa at 7 days, and 100.44 kPa at 28 days. In contrast, SAC can make the sample have a certain strength earlier under the same addition amount, as shown in Figure 9b. When the dosage of SAC was 30%, the strength reached 45.80 kPa, 64.2 kPa and 64.1 kPa at 3 days, 7 days, and 28 days, respectively.

**Figure 9.** Unconfined compressive strength of the solidified sediment after compound pretreatment. (**a**) OPC. (**b**) SAC.

In summary, the sediment separated by sedimentation has a certain strength after solidification. Both types of curing agents can improve the strength of sediment and modify the sediment into a soil material with a certain strength. The use of SAC as a curing agent can rapidly improve the mechanical properties of solidified sediment in a short period of time. The unconfined compressive strength of cured sediment is greater than 30 kPa in 3 days.
