Utilization of high fine-grained shield tunnel spoil in CLSM and effect of foam agent content on properties

Abstract

The volume of shield tunnel spoil (STS) is very large, its effective management is difficult, and it even causes environmental pollution. In this study, to achieve its recycling, a novel controlled low strength material (CLSM) was prepared by utilizing high fine-grained STS as partial aggregates instead of sand, and its engineering performance was thoroughly evaluated. In the process of mix proportion design, key parameters such as the STS-to-total aggregate ratio (TS/TA), foam agent content (F), water-to-binder ratio (W/B), binder-to-total aggregate ratio (B/TA), and fly ash-to-cement ratio (FA/C) were employed. Workability aspects (i.e., flowability, bleeding rate, and setting time) and physical and mechanical properties (i.e., unconfined compressive strength and density) were evaluated. Additionally, the pH of bleeding and leachate, as well as the impact of foam agent content on CLSM properties, were examined. The findings revealed that an increase in the TS/TA ratio was associated with a decrease in flowability, density, and compressive strength, as well as an extension in setting time. The CLSM, with a flowability range of 150–300 mm, exhibited a bleeding rate below 2%, setting times between 3.6 and 6.1 hours, 28-day strength ranging from 1.06 to 3.24 MPa, and fresh density ranging from 1810 to 2060 kg/m³. Generally, these results met the required specifications, although the fresh density was slightly lower. The pH results indicated that the CLSM is non-corrosive. Furthermore, our investigation highlighted the substantial influence of foam agent content on flowability and setting time. An increase of 0.1‰ in the proportion of foam agent within the total aggregates resulted in a flowability increase of 2.1–2.6 mm and a setting time increase of 4.25–4.99 minutes. Therefore, it is feasible to utilize high fine-grained STS in the production of CLSM.