Analysis on Support Technology in Expressway Tunnel Construction

In recent years, the transportation industry of the BRICs (Brazil, Russia, India, and China) is growing rapidly. The construction period of a highway is shorter than ever. And more vehicular tunnels are designed. Therefore, forepoling technology is applied in all kinds of tunnel construction.

1 Primary supporting techniques

1.1 Hollow injection rock bolting / IBO / SDA technique

General speaking, the injection hollow bar anchor is in the diameter of 25mm to 38mm. the length is 3.0m to 4.0m. The hollow rock bolts are mainly installed into the crown section of the tunnel. Drilling rigs and grouting platforms provide power and support.

As the self-drilling anchor bolt is tubular, grouting is commenced from the bottom of the hole and the cement grout fills all the fissures and cracks on its way back, therefore achieving a huge pull out force, which is an important factor for the NATM / umbrella fore piling design.

1.2 Grouted mortar anchor bolting technique

The installing process: Boring → Declogging → Capsule resin → Insert anchor rod. Operate rock-drilling machine to drill the hole.

During this process, the anchor bolt should be welded to the steel reinforcing mesh frame.

1.3 High-performance ferrocement technique

Geogrid shotcrete and rock bolt net are applied in the premier reinforcement of tunnel construction. It features a flexible, airtight, timely, intensive and strong adhering.

2 General Provisions

• Primary supporting and tunnel excavation should proceed at the same time.
• Spray anchor bracing technique (shotcrete) should be applied in primary supporting in a tunnel. According to surrounding rock condition, section size and location excavation point, choose shotcrete, anchor bolt, reinforcing steel mesh and frame for the forepoling supporting method.
• Before the construction of a tunnel, confirm the time of excavation and reinforcement, distance and procedures.
• When constructing in wintry weather, the temperature of operation area and the mixture in the concrete injector should be higher than 5℃.
• There should be good illume facility and ventilation in the working areas. Dust level should be less than 2mg/m³.

3 Process flow

1. General process: Shotcrete after excavation → Construction of support (Anchor bolt, reinforcing mesh and steel frame) → Re-shotcrete until reaching the design thickness.
2. Clear the pumice stones after blasting, then sealing the surrounding rock with shotcrete. Install forepoling anchor bolt, net and arches, and re-shotcrete.
3. Inspect the thickness of concrete. The average thickness should exceed the design value, and the minimum thickness should exceed the two-thirds of the design value.

4 Forepoling (spiling) Anchor Bolt

4.1 Reinforcement effect of rock anchor bolt

Rock anchor bolt is a long anchor bolt, for stabilizing rock excavations, which may be used in tunnels or rock cuts. IBO self-drilling anchors made by Onton is a kind of rock anchor bolt. It provides high tensile property and good ductility performance. In the following figure, you can see how the rock anchor bolt performs in the rock body.

• Support surrounding rock
• Reinforce surrounding rock
• Raise the friction between the soil layers, to form the structure of composite beam.

Rock Bolt Reinforcement

4.2 Rock anchor bolt types

• Rebar rock bolts

  

  Rebar rock bolts

Installation Procedure:

1. Borehole drilling
2. Injection grout
3. Insert the rebar rock anchor bolt
4. tensioning of the bolt’s head by tightening the nut and plate

• IBO Hollow bar self-drilling spiles

The self-drilling, hollow ONTON rock bolts, facilitate and extend the use of the NATM method to gravelly, shifty, sandy and even coarse clay soils.

Rock Bolt @ibodrill.com

Installation Procedure:

1. Install the IBO hollow bar rock anchors by using rotary percussive drilling rigs.
2. The drilling and grouting are simultaneous.
3. Adding a coupling to extend the rock bolt with another hollow bar anchor.
4. Allowing the grout to cure.

5 Sprayed concrete (shotcrete)

The supporting system, including both temporary support and permanent support, consists of bolting and shotcrete. Shotcrete is a common reinforcing measure in civil engineering.

Shotcrete in Tunnel Construction

• Shotcrete process

1. Spraying surface treatment
2. Check steel frame
3. Mark the laying of shotcrete layer thickness
4. Spray concrete
5. Curing

• Main advantages

1. Support surrounding rocks
2. Transfer and unload the deformation energy
3. Fill and reinforce the surrounding rocks
4. Cover the surface of surrounding rocks
5. Prevent from the surrounding rock loose
6. Distribute the external force

Sprayed concrete

6 Forepoling section-steel and grid-steel

J-channel beams, wide flange beams (W beams) can be used as per customer’s individual needs. A grid-steel frame can be fabricated on site.

Forepoling Tunnel Steel Frame