Mechanical properties and microstructure of SWGA-BF improved rock muck discharged from slurry shield tunnels

Abstract

The waste muck discharged during shield tunneling has become the main source of urban construction waste. The modification of discharged muck from shield tunnels into roadbed materials can significantly improve resource utilization efficiency, but freeze–thaw environments can pose safety hazards. In this study, solid waste gelling agents (SWGAs) (50 % cement, 36 % blast furnace slag, 6 % calcium aluminate, 5 % semihydrate gypsum, 2 % calcium oxide, and 1 % alum) and basalt fiber (BF) were used as reinforcing materials for the waste muck discharged from the construction of slurry shield tunnels in rock formations. By conducting freeze–thaw cycle tests, unconfined compression tests, scanning electron microscopy (SEM) analysis, and image analysis by the ImageJ software, the effect of the freeze–thaw environment on the mechanical properties and microstructure of the improved tunnel muck by SWGA-BF composite was studied. The results show that the damage to the unconfined compressive strength (UCS) of the SWGA-BF improved tunnel muck caused by freeze–thaw cycling can be divided into three stages: rapid damage stage, slow damage stage, and basically stable stage, corresponding to N = 0 ∼ 3, N = 3 ∼ 9 and N > 9, and the damage degree generated by freeze–thaw cycle in each stage is 33.9 %, 16.7 %, and 5.0 %, respectively. The first freeze–thaw cycle causes the greatest damage. The variation characteristics of the elastic modulus of improved tunnel muck were similar to those of UCS, and the value of elastic modulus is linearly related to the UCS. The optimal content of BF is 0.5 %. As the fiber content increases, the yield strain and failure strain increase in a wavy manner. SEM tests reveal that the freeze–thaw environment increases the pore area, the size, and the number of fissures of the improved tunnel muck. However, after 12 freeze–thaw cycles, the particle diameter and abundance decreased by 20.46 % and 4.89 %, indicating that the freeze–thaw cycle effect has a greater impact on particle diameter than particle abundance. The deterioration of the microstructure eventually decreased the macro tunnel muck strength. These results have important reference values for the reutilization of waste muck discharged from shield tunneling for roadbed backfill materials in seasonally frozen areas.