Inefficient supporting effect in the tunnel by small pipe? Self-drilling anchor bolts (SDA) solve problems at once

 In the subsurface excavation section of Baishiqiao South Station of Dongdiaoyutai Station on Beijing Metro Line 9, the original design plan was the advance small pipe support,, and self-drilling hollow anchor rods were selected because of the inability to make holes. This interval is mainly located in the Quaternary pebble layer and the Tertiary strongly weathered conglomerate layer. Generally, the pebble has the characteristics of large particle size, high content, and good cementation. When excavated to the full conglomerate section, the outline of the tunnel is all in the conglomerate layer, and the groundwater is abundant. Due to the high hardness of the conglomerate layer, the small pipe cannot be penetrated, resulting in unsatisfactory grouting effect and continuous water leakage in many places in the tunnel. In order to ensure normal construction, after many tests, the self-drilling anchor rod was finally selected for support.
Combined with the previous self-drilling bolt diameter and wall thickness tests, self-drilling bolts of R25*3m and R32*5m were adopted for construction. The grouting slurry is made of cement-water glass double-liquid slurry: the anchor rod is grouted on the top of the section vault 140. A 1.5 m thick dense crust is formed in the range, so as to achieve the purpose of adding the same strata and stopping water.
After project verification, the comparison between the self-drilling hollow grouting anchor rod construction and the original small pipe construction effect is as follows:
1. Improve soil reinforcement effect
In the original design, the grout diffusion radius of the small duct on the vault is 0.5 m, and the maximum thickness of grout reinforcement is 1.0 m. The thickness of self-drilling hollow grouting bolt is 1.5m, which is significantly improved compared with the original plan.
2. Improve grouting leakage
Before the use of bolt grouting, there are many water seepage points in the vault, but the section vault of self-drilling hollow grouting 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 standard section vault with advanced small pipes can reach 2mm. When the settlement of the vault becomes stable, the accumulated settlement will reach 8-9 mm, and the maximum accumulated settlement will be 12mm; self-drilling hollow grouting anchor rod The daily settlement of the vault is controlled within 1 mm. When the settlement of the vault becomes stable, the cumulative settlement is 6-7 mm, and the maximum cumulative settlement is 9 mm.
In summary, through the construction analysis and comparison of the test section, the self-drilling hollow grouting anchor is used for the formation of small pipes such as pebbles and conglomerate that are not easy to form holes. Advantages such as reduced settlement.

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