The underground excavation geology condition of the Diaoyutai station to Baishiqiao south station of Beijing Metro Line 9 generally presents the characteristics of large particle size, high content and good cementation of pebbles. When excavated to the whole conglomerate section, the tunnel contour is all in the rock strata and has abundant groundwater. The original design uses advance small conduit, but due to the high hardness of conglomerate layer, small conduit cannot be driven. This leads to unsatisfactory grouting effects and continuous water leakage in many places in the tunnel. To ensure normal construction, after many tests, self-drilling hollow grouting anchor bolt support is finally selected.

Based on the previous experiment of bolt diameter and wall thickness of self-drilling anchor bolt, the bolt with φ30*5.0mm, 2.5m length and TC cross cut drill bit was adopted for construction. The cement and sodium silicate are used as the grouting slurry. After grouting, a 1.5m thick dense hard shell is formed within 140° of the cross-section vaults, so as to achieve the purpose of strengthening the stratum and stopping water.

The construction effect comparison between self-drilling hollow grouting anchor bolt and original small conduit is as follows:

1)Improve the soil reinforcement effect

In the original design, the grout diffusion radius of small conduit in the vault is 0.5m, and the maximum grouting reinforcement thickness is 1.0m. The grouting reinforcement thickness of self-drilling hollow grouting anchor bolt is 1.5 m, which is significantly improved compared with the original plan.

2)Improve grouting leakage

There are many water seepage points in the arch before anchor bolt grouting, while the cross-section arch with self-drilling hollow anchor bolt grouting is relatively dry, and the water leakage phenomenon has been well improved.

3)Reduce settlement

According to the monitoring data, the maximum daily settlement of the vault with advance small conduit is 2mm. When the vault settlement tends to be stable, the accumulative settlement reaches 8-9 mm, and the maximum is 12mm. The daily vault settlement is controlled within 1 mm by the self-drilling hollow grouting anchor bolt. When the vault settlement tends to be stable, the accumulative settlement is 6~7 mm, and the maximum is 9 mm.

To sum up, through the construction analysis and comparison of the test section, for pebbles, conglomerates and other formations where small conduits are difficult to form holes. The construction of self-drilling hollow grouting anchor bolt can improve the soil reinforcement effect, improve the water leakage phenomenon, and reduce the settlement.