*2.4. Flocculation-Settling Experiment*

The water content of slurry was initially adjusted to 300% by adding tap water, which was used to simulate the initial state of slurry produced by pipe jacking construction. Afterwards, flocculation tests were carried out. The flocculation scheme is shown in Table 3. The flocculation test was carried out in a 500 mL beaker. After adding flocculant, the slurry was stirred at 450 rpm for 2 min. The slurry after flocculation was then subjected to a sedimentation test. The clear sediment–water interface was used to evaluate the result of separation during settling, which was recorded by the scale value on the side wall of the beaker. The sediment below the sediment–water interface is considered to be the product after slurry separation. The sedimentation test lasted for 600 s, representing a short period of concentration. As a comparative test, the sedimentation test of the slurry without adding flocculant was also carried out.



<sup>1</sup> The dosage is calculated as the ratio of the dry mass of conditioner to the dry mass of slurry. The dosage is determined by pre-experiment. <sup>2</sup> 1%, 3%, and 5% are the dosage of FeCl3·6H2O, which are determined by pre-experiment.

During the pre-experiment, it was found that the flocculation-settling effect of Type II slurry was very poor. Thus, two types of composite conditioning experiments were designed, as shown in Table 3. Type II slurry was first added with FeCl3·6H2O and was stirred at 450 rpm for 2 min, and then added with PAM and stirred at 450 rpm for 2 min to test sedimentation results.
