**1. Introduction**

As a trenchless technology, pipe jacking is increasingly used in the construction of urban sewage and rainwater pipe networks [1,2]. Slurry is an indispensable material in pipe jacking projects, which plays the role of balancing the pressure of the excavation surface, discharging the slag, and lubricating [1,3]. During the construction process, a large amount of waste slurry is continuously produced, which is likely to contain bentonite, carboxymethyl cellulose (CMC) and other components [1,4,5]. These components may affect the separation results, thus requiring more site on which to dispose of the slurry. With the development of urbanization, there are fewer sites that can be used as slurry storage. It is necessary to progress the rapid reduction and resource utilization of waste slurry.

Flocculation was initially used in water treatment [6–8], and was gradually applied in the dewatering of slurry, such as waste dredging slurry [9,10] and waste tailings slurry [11,12]. Suitable flocculation pretreatment can increase the floc size in slurry and improve the separation efficiency [9]. Wang et al. [13] used various types of polyacrylamide (PAM) to treat construction waste slurry. After 7 days of settlement, the water content of the slurry can be reduced to about 80%. He et al. [14] used cationic PAM to treat waste slurry and, after 10 min of setting, the volume of slurry can be reduced by 60%. However, the properties of pipe jacking slurry are special, and it is likely to contain bentonite, CMC, and so forth. These special components may have a potential impact on the flocculation and separation of pipe jacking slurry, and corresponding research needs to be carried out.

**Citation:** Jiang, L.; Zhen, L.; Wang, J.; Zhang, T.; Huang, X. Research on Dewatering Characteristics of Waste Slurry from Pipe Jacking Construction. *Materials* **2022**, *15*, 2242. https://doi.org/10.3390/ ma15062242

Academic Editors: Rossana Bellopede and Lorena Zichella

Received: 6 February 2022 Accepted: 15 March 2022 Published: 18 March 2022

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In addition, there are many types of commercial flocculants on the market—it is unclear which flocculant has the best flocculation effect and needs to be studied.

During the flocculation process of slurry, soil particles aggregate to form flocs [15], which increases the soil–water separation rate. However, due to the limitation of storage site space, the time of settlement cannot last too long. The water content of sediment remains high after a short period of settling [13,16], resulting in poor mechanical properties and cannot be used as soil materials. In some projects, plate and frame filter presses have been used to dewater the slurry, which can greatly reduce the water content of slurry [17,18]. However, the dewatering efficiency of the filter press is low in some cases, and the problem of untimely treatment often occurs [19]. Untimely reduction leads to no place to store excess waste slurry, resulting in the stagnation of construction. Solidification technology is often used to increase the strength of soft soil [20–23] and has been applied in some projects [24]. The curing agent converts the excess free water into mineral water through hydration, which improves the mechanical properties of soft soil. It may be possible to further treat the flocculated sediment by means of solidification, which could increase the strength of sediment to make it a reusable soil material. The solidification characteristics of sediment obtained by slurry flocculation and settling are still lacking, and need to be studied further.

Based on the problem of unclear flocculation, the sedimentation characteristics of waste pipe jacking slurry and the unclear strength growth law of the solidification sediment after sedimentation, this paper carried out relevant laboratory experiments. The waste slurry produced in two different stages of pipe jacking construction was selected as the experimental material. Five different conditioners were used to pretreat pipe jacking slurry, and the sedimentation characteristics were studied. For the sediment obtained by settling, two solidification agents were used to conduct solidification experiments to study mechanical properties of the solidified sediment. Based on the above tests, the flocculationsettling characteristics of pipe jacking waste slurry were studied, and the feasibility of using a flocculation–sedimentation and solidification combined method to treat waste pipe jacking slurry was discussed, to provide a reference for related projects.
