Karst disposal of Big diameter self-drilling anchoring applied in tunneling

Abstract

The karst development of National Highway G5 Gujia Mountain Tunnel is strong. It is characterized by many deep and narrow vertical karst fissures, karst troughs, and karst ditches. Such measures as backfilling, reinforcement, and bridging were adopted for this project, and advance support with T75mm large diameter self-drilling IBO rock bolts were widely applied in the construction of this tunnel.

Big diameter IBO Anchoring Project Overview

The 2084 m long highway tunnel has 2 lanes. The maximum buried depth is 191m.

The tunnel had to pass through over 80 karst caves since they are strongly development.

Anchoring Project Design

Small Diameter IBO rock bolts are normally used as advanced support. But its strength is limited, and total laying length is less than 6m. Moreover, the grouting effect is not as good as bigger diameter ones. If the gouge is big, the small diameter anchoring support can’t ensure safety, collapse might occur.

After investigation and test on site, we decided to use T75 Large diameter self-drilling IBO Rock Bolts in the application of tunneling. The IBO rock bolts were assembled with alloy drill bits, high strength plates and nuts. Since the big diameter and higher strength, the T series IBO rock bolts can prevent the mass of earth-rock from gliding.

Anchoring and grouting of self-drilling IBO rock bolts

Fig. Anchoring and grouting of T75 self-drilling IBO rock bolts

Solution approach

1. Spraying concrete to the landslides and hole face. The concrete thickness should be more than 20cm, to assure safety operation.
2. Place the large diameter self-drilling IBO rock bolts 5m from the tunnel face. The layout of rock bolts is according to tunnel’s geological forecast and caves locations.
3. The nominal diameter of T75 self-drilling anchor IBO is 75mm, thickness is 7mm. the ultimate strength is higher than 750Mpa. ISO 10208 left hand thread. The designed grouting hole is 6mm in diameter.
4. The injection of cement-water glass parameters:1). Injection pressure:1.0-1.5Mpa.

   2). Water-Cement Ratio W/C=1:1, cement-water glass volume ratio=1:0.5.

   3). Cement Grade:32.5.

   4). Glass water baume degrees=35, module=2.4.

5. V Grade tunnel strut protection and builds method. Anchor bolt-spray support in the preliminary stage.
6. Applying backfilling, reinforcement, or bridging methods if there are no more collapsed caves.

Conclusion

Large diameter self-drilling IBO rock bolts were widely used in the construction of karst tunnels. With the reinforcement support by T75 big diameter IBO anchor bolts, the tunnels can across through the 10-20m karst caves successfully. The big diameter self-drilling IBO anchors have features:

1. Higher strength to avoid tunnel collapse.
2. Self-drilling methods improve the drilling speed and operation efficiency.
3. Steel materials saved.

Now the highway has been opened to traffic. And Gujia Mountain Tunnel also functions well. It has been approved that large diameter self-drilling IBO rock bolt anchoring is a kind of reliable solution for tunneling.