This is how we build tunnels for Stockholm's Metro extension project
On June 30, after the completion of the last blasting, it marked the completion of the tunnel excavation of Contract 87133, CRTG made a milestone breakthrough in the high-end market of northern Europe.
Rock tunnel construction has been a Stockholm specialty for over 100 years. Blasting of the Metro’s rock tunnels started in the 1930s. However, the major periods of Metro expansion were the 50s, 60s and 70s of 20th century. Now it is time for some more. The expansion of the Sickla-Järla-Nacka under construction by CRTG goes primarily through rock of good quality, where is injected with cement-based sealant before subsequent blasting. This pre-grouting technique prevents ground water from entering the tunnel. Then the tunnel is blasted out with a traditional drill and blast method.
Blasting and building a tunnel is called driving a tunnel or tunnel driving. The tunnel is driven forward in cycles consisting of eight steps in each cycle. We drive one tunnel forward 15-20 meters per week by repeating these steps in 87131/87132/87133 projects.
Here are the eight steps of building a tunnel.
1. Drilling for Grouting.
To avoid water leakage, the rock's natural cracks are sealed. This process is called Grouting.
Grouting holes at a specific diameter are drilled to the design length obliquely in a ring around the tunnel. A mixture of cement and water is forced into the bore holes under high pressure and out through the cracks in the rock. The cement hardens and thereby seals the rock around the future tunnel since water leakage can influence both the tunnel and the surroundings.
2. Drilling for Blasting.
The next step is to drill the actual blasting holes of specific diameter. Using a computer-guided drilling rig, the bore holes are positioned precisely with the correct length.
Blasting agent is charged into the bore holes by Boomer. The blasting agent consists of multiple components that mix with the charge. This renders the blasting agent explosive. The charges are connected in a specific sequence to detonate in right order. This controlled blast makes it possible to blast the surrounding rock carefully and gently.
The rock is blasted out after the charging. The amount of blasting agent is adapted to the surroundings. For example, the blasting salvoes are smaller in densely populated areas. Small blasting salvoes reduce vibrations, while they make the tunnel driving process slower.
6. Mucking out
The process of knocking off the loose rock from the tunnel ceiling and walls is called scaling. The scaling is executed firstly by mechanical excavators with a hydraulic hammer and then in a manual way.
The tunnel is reinforced with rock bolts. The need for rock reinforcement varies along the tunnel route and depends on the quality of the rock. After rock reinforcement, concrete is sprayed on the tunnel walls and ceilings.