# The Curing and Strength Properties of Highly Moist Waste Mud from Slurry Shield Tunnel Construction
## 1 Introduction  
In recent years, the progressive utilization of underground space has gradually become a new trend in energy conservation worldwide. Due to urban development, many underwater tunnels crossing rivers, lakes, and seas have been constructed in major cities. The slurry shield method has become the first choice for underwater tunnel construction due to its high efficiency and security [1, 2]. During slurry shield tunnel construction, the sludge-water separation equipment separates the large mud particles but not the small particles promptly, resulting in increased mud gravity and viscosity. As part of the waste mud meets the construction requirements, a proper amount of water or additive is added to produce a lot of waste mud with high specific gravity and high viscosity. The waste mud contains much cellulose, sodium carbonate, bentonite, and other auxiliary additives and chemical substances, which could cause environmental pollution and land occupation [3]. Currently, engineering mud generally refers to the treatment methods of oil drilling, i.e., adding specific curing agents and water to the mud to initiate the hydration reaction and reduce the moisture of the mud. Additionally, hydration reaction products can improve the strength of the solidified soil, which is conducive to waste mud recycling [4, 5]. Sewage sludge ash for manufacturing building materials has been investigated, such as bricks and tiles, raw materials for cement production, aggregates for concrete and mortar, and substitutes for sand and cement in cement solidified bases, subbases, and subgrades in road constructions [6]. Based on the highly moist waste mud from a tunnel project in Wenzhou, [PERSON] et al. [7] performed a vacuum preloading model test to investigate the effect of different inorganic reagents on the vacuumpreloading of waste mud with high moisture content. Their results indicated that the reagent treatment could cause the small mud particles to aggregate into larger particles, thus improving mud permeability and accelerating drainage. The time required to reach the same water discharge was reduced to nearly half of that without adding the reagents. [PERSON] et al. [8] used steel slag, cement, and metakaolin (SCM) as composite curing agents to solidify the soft marine soil of Lianyungang and studied the effects of moisture content, plasticity index, and age on the strength of the solidified soil. The results showed that SCM could effectively solidify the soft marine soil. [PERSON] et al. [9] discussed the effects of nitrate, phosphate, and chloride on the mechanical properties of lime solidified soil. Compound concentration, soil properties, cement, and curing conditions could reduce the mechanical strength of the solidified soil as the added compounds delayed the hydration of the curing agent. [PERSON] et al. [10] suggested that the chemical additives improved the permeability of the mud and accelerated its consolidation. [PERSON] et al. [11] studied the changes in physical and mechanical properties of cement solidified soil as a flexible subgrade material after adding lime and fly ash. [PERSON] et al. [12] studied the effect of Portland cement on soil stability and measured soil moisture content, maximum dry density and pore pressure, and other characteristic parameters under different cement content. [PERSON] et al. [13] analyzed the effect of curing age, lime content, and periodic freezing-thawing cycles on the mechanical properties of lime solidified soil. Moreover, the liquid limit, plastic limit, and strength of the soil could be significantly enhanced. With the increase in fly ash and slag contents, the moisture content of solidified sludge first decreased and then increased, whereas the shear strength first increased and then decreased, thus enabling an optimal combination of curing agents [14; 15]. The research on curing agents mainly focuses on the curing mechanism of soft soil, such as drilling mud and mud soil, while few scientists have studied the curing mechanism of waste mud.