Abstract: A significant volume of waste slurry with high water content and ultrahigh clay particle content is produced during slurry shield tunneling in the second Jiangyin crossriver tunnel project. The water and slurry are hard to separate, resulting in low separation efficiency. Hence, various flocculants are used for slurry flocculation experiments to investigate the effect on flocculation and dehydration. The equivalent water content of sediment as well as interface sedimentation rate, supernatant turbidity, slurry dehydration rate, and Zeta potential are selected as evaluation indices to study the influence of flocculant type, dosage, and composite form on its flocculation effect. Finally, the flocculation mechanism of different substances is preliminarily discussed. The results reveal the following: (1) The slurry dehydration effect of different flocculants is in the following order: APAM＞PAFC+APAM≥PAC+APAM＞PAFC≥PAC, whereas the order of turbidity removal effect is as follows: PAFC＞PAC＞PAFC+APAM＞PAC+APAM＞APAM. (2) The optimal dosage of organic flocculant APAM is closely related to its molecular weight, and the maximum rate of dehydration varies for APAMs with different molecular weights. The best dehydration effect is achieved when the molecular weight of the agent is maintained at 12-14 million. (3) The PAFC+APAM composite flocculant exhibits the best conditioning effect on the waste slurry with high water content and ultrahigh clay particle content. In this research, the optimal dosages for PAFC and APAM solutions are 33 and 233 g·L-1, respectively, and the corresponding dosages of dry matter for PAFC and APAM are 1.6 and 0.466 g·L-1, respectively. Under this condition, the dehydration rate of the slurry is 29.6% after 90 min, and the turbidity of the supernatant can be reduced to 62.0 NTU. (4)The primary mechanism responsible for the dehydration effect of composite flocculants is the adsorption bridging effect of the long chain of the organic flocculant APAM. Meanwhile, turbidity removal is mostly associated with the electrical neutralization of the inorganic flocculants PAC and PAFC.

Key words: slurry shield, flocculation, waste slurry, ultrahigh clay particle content, high water content, dehydration performance