Shotcrete is used to make ceilings and walls of mining or transport tunnels safe for operation and work. It can be produced either in an over-ground or underground mixing plant.
In case of underground production the provision of raw materials depends on the amount of shotcrete required as well as on the local circumstances.
There are different ways of getting raw materials respectively concrete down to a tunnel, with different impact on the production process and the operational costs.
1. Spiral road
A spiral road is a road going down in continuous bends. The length of the spiral road depends on the depth of the tunnel below ground. The spiral road is a quite practical choice, because it allows transports by trucks. It makes it quite easy to transport the modules of the mixing plant, binders, aggregates and other materials down to the tunnel.
Slickline is a metallic pipe, usually with a diameter of 100-150 mm (4-6”) that has been installed in a hole drilled through the rock. Such a pipe can be up to a kilometer long.
If the shotcrete is produced above ground, it can be dropped down through the slickline to a receiving hopper in the tunnel. From the hopper shotcrete is transported to the spraying area by suitable underground transmixer or agitator.
The disadvantage of using this solution is the possible segregation of the shotcrete mix by dropping and due to this negative influences on sprayability and quality.
If shotcrete needs to be produced underground, all raw materials need to be transported down to the plant in the tunnel. Bulk materials can be dropped through a slickline pipe. Cement though needs to be transported in big bags.
In an underground mixing plant all materials can be mixed as usual, with no negative influences on the sprayability or quality.
The most logistically impractical and expensive option is, to use a gage. If the size of the gage is only small, it will be difficult to get the plant modules and slow and impractical to drop materials or goods down to the mine.
Shotcrete plant on the ground surface in gold mine
How to produce shotcrete?
Concrete for shotcrete purposes can be mixed either on ground level or underground in a tunnel.
There are a few differences between these solutions as far as concrete quality and operational costs are concerned.
The maximum aggregate size for shotcrete usually is 8-10 mm. Bigger size aggregates might get stuck in the spraying nozzle.
Dry shotcrete mixed in a dry-mix plant is called “dry pre-blended shotcrete”.
With the production of this dry pre-blended shotcrete, cement, aggregates and eventually admixtures are mixed and the mix is discharged in big bags. Such big bags normally can take a load of a 1,000 kg. The big bags then are transported from the dry mix plant to the tunnel where the material gets mixed with water.
Big bags are easy to handle however, storing can be problematic as humidity can lead to pre-hardening of parts of the material.
Drying of aggregates requires a lot of energy and generates high CO2 emissions. The premixed material needs to be transported first to the mine sometimes several thousand kilometers and then down to the tunnel, probably over a distance of several thousand meters. These are the reasons why using dry pre-blended shotcrete is a very expensive method, leading to costs of even 500 $ per m³ and more.
Another possibility is to mix all the materials underground, close to the spraying area. In this case the shotcrete quality will be much better and less cement is required. Production costs will be a lot lower however, the costs of getting all materials down to the plant need to be considered.
Specially designed underground shotcrete plants are suitable for mixing shotcrete in tunnels. They can homogenously mix aggregates, cement, water and admixtures according to the actual mix-design. If necessary, additionally microsilica and fibres can be dosed automatically.
Benefits of underground mixing:
- Shotcrete is homogenous
- Less cement required
- Early strength is higher
- Short distance between mixing and spraying areas
- Fibres can be mixed in homogeneously
- Production of in place mixed shotcrete is much cheaper than production using “dry pre- blended shotcrete”
- Energy saving and reduced CO2 emsissons
Underground shotcrete plant with a planetary countercurrent mixer in zinc mine 800 m below the ground level
What types of mixers are used for mixing shotcrete?
For mixing shotcrete, either continuous auger (screw) or planetary countercurrent mixers can be used. Auger mixers especially are used to mix dry pre-blended shotcrete with water.
If using a continuous mixing plant, the consistency and quality, especially the early strength of the shotcrete might vary, because the water won’t be added to the mix equally. Some of the concrete even might have to be dumped due to poor quality. If using a continuous mixer fibres cannot be added to the mix, or, if so they cannot be mixed in properly. Clumps of fibres will lead to problems with the spraying machine and also with the sprayed concrete lining the tunnel walls.
If a planetary countercurrent mixer is used, aggregates, cement, water admixtures and fibres can be dosed separately and all ingredients can be mixed properly. Quality of the shotcrete will be better, early strength will be higher and more equal, much less cement will be required.
Requirements for an underground shotcrete plant
First of all, the plant modules have to get down to the underground area. As mentioned earlier, different methods to get access to the underground area can be used.
The limited space in a tunnel needs to be considered when designing a shotcrete plant. Some extra space around the shotcrete plant is required, so all materials can be loaded into bins, silos and tanks, and ready mixed shotcrete can be discharged.
Other relevant features for the plant are a sophisticated dosing and mixing system for fibres.
Fibres are used as equally spread reinforcement, which is why the fibre dosing system needs to work reliably. Proper mixing is important to ensure that the shotcrete is as homogeneous as possible. Low quality shotcrete not only reduces the efficiency of the construction work but also leads to higher costs and increases the risk of failures of the tunnel wall linings and with this the safety in the tunnel.
Underground shotcrete plant fits in a narrow tunnel in nickel mine.
Adding different admixtures to the mix is an important step with the shotcrete production.
In tunnels, it may take some time to get shotcrete delivered to the spraying area. Suitable admixtures ensure that shotcrete can be transported, pumped and sprayed easily, even after a longer transport time.
Investing in a good quality shotcrete plant with the right mixer is worth it. A wrong investment decision might cause huge financial losses. Here are a few examples:
- Poor quality shotcrete bears the risk of stones dropping from tunnel walls and collapsing of parts of the tunnel wall shotcrete linings
- Fibre clumps in the mix might get stuck in hoses or nozzles
- Areas spayed with shotcrete of poor quality might have to be taken down and to be replaced
Spraying of shotcrete in road tunnel
Tecwill underground shotcrete plants
Tecwill manufactures shotcrete plants for production over-ground as well as underground, using high quality countercurrent planetary mixers to ensure the best possible mixing result. Tecwill solutions improve the safety in tunnels, reduce cement and the carbon footprint of a project. Tecwill can provide fully automatic fibre dosing systems.
For answering of any questions or further information, please feel free to contact us.