Recycling waste sand from slurry shield tunneling: A sustainable filter aid for waste slurry dehydration

The slurry shield tunneling construction process produces waste slurry (WSL), which is often dehydrated using lime as a filter aid. However, a high pH limits the reusability of the mud cake produced, leading to environmental problems. Slurry shield tunneling also produces waste sand (WSA), which the present study evaluates as a substitute for lime via belt filter press testing. The ability of WSA to reduce the mud cake water content and porosity is found to be the same as that of lime, although double the amount is required. Specifically, the optimal amounts of lime and WSA addition are 2% and 4%, respectively, which produce similar total drainage rates and mud cake moisture contents and porosities. Both lime and WSA reduce mud cake macropores, increase small pores and improve compactness. The addition of >2% lime increases the bound water and moisture contents of the mud cake, while WSA reduces the bound water without affecting dehydration. Finally, the use of WSA instead of lime as a filter aid reduces the pH of the mud cake from strongly alkaline (about 12) to near-neutral (about 8). The results show that WSL can replace lime as a filter aid, providing a novel approach to WSL dehydration treatment and WSA reuse that has both environmental and economic benefits.

Automated summary

The slurry shield tunneling construction process produces waste slurry, which is often dehydrated using lime as a filter aid. However, a high pH limits the reusability of the mud cake produced, leading to environmental problems. This study evaluates the use of waste sand as a substitute for lime and finds that it can effectively reduce the mud cake water content and porosity. The addition of waste sand instead of lime can also reduce the pH of the mud cake, providing a novel approach to WSL dehydration treatment and WSA reuse that has both environmental and economic benefits.