Experimental Study on Instant Grouting with Formwork for Tunnels

Abstract

Voids between primary support and secondary lining of existing tunnels pose a serious threat to safety and have become a key challenge for tunnel engineering. Due to the poor bonding performance between concrete slurry backfill and lining concrete, the integrity of supporting structures cannot be guaranteed. In recent years, vault grouting with formwork has proved to be effective in solving this problem. However, few studies have focused on the selection of grouting materials and associated grouting process parameters. In this paper, a slight-expanding concrete slurry is selected as the grouting material. The early strength growth trend of the slight-expanding slurry is explored via laboratory tests. The compatibility of the slight-expanding slurry and the lining concrete is analyzed by comparing the slump, expanding diameter, water retention and compressive strength. Bonding-body specimens were fabricated using a mold developed for this work. Furthermore, we compare the compressive strength and failure mode in order to investigate the bonding properties between the sight-expanding slurry and lining concrete at different grouting intervals and pressures. A reasonable set of operational parameters is subsequently proposed. Finally, field tests undertaken in an expressway tunnel in south China are performed in order to determine the impact of grouting with formwork on tunnel lining, while the grouting effect at the vault is evaluated using a ground penetrating radar. The slight-expanding slurry exhibited a high early strength and rapid hardening. Adding slight-expanding slurry to lining concrete at a volume ratio of 1:0.2 increased the slump, diameter and compressive strength by 13.4%, 15.6%, and 25.0%, respectively. When a grouting interval is less than 6 h, the pressure of grouting with formwork should be no less than 0.2 MPa, while for a grouting interval of 8 h, the grouting pressure should be no less than 0.4 MPa. The grouting pressure of 0.4 MPa exerts a limited effect on the existing secondary lining concrete, and the stress increment produced by grouting accounts for 6–17% of the total increment. The results presented can provide a reference for future tunnel construction.