Here you can read about how we are working with the geology in the tunnels.
Before we start construction, we always conduct preliminary studies and analyses of factors including rocks and geology. The scope of the surveys we conduct is weighed against time and cost. Projects on this scale are complex in nature, and despite preliminary studies there are uncertainty factors. This means that we must constantly be prepared to change our approach based on changing conditions, such as changes in the quality of the rock.
What lies beneath the ground?
Now, three years into the construction phase, we have a clearer picture of what the rock is actually like, and this picture does not quite correspond with the preliminary studies we conducted at the outset. When we drill cores to examine the rock and conduct other studies at an early stage, we only acquire knowledge of a section of the rock mass. We have now acquired more knowledge of the structure and properties of the rock, and we are continuously adapting our production methods accordingly. The Swedish Transport Administration gives priority to tunnelling being carried out in a safe way, and to us keeping us within the levels, for example for groundwater, that are specified in the environmental rulings and that govern our work.
In the southern section of the tunnel, at what is known as the Mälaren Passage (between Sätra and Kungshatt), we have had to reinforce the rock because of a regional weakness zone that extends under Lake Mälaren.
In the northern section of the tunnel, it has become evident that the rock needs to be sealed to a greater extent than planned. Here we have upgraded the technical solutions for how sealing will take place.
Fact box: Before we start construction, we need answers to certain questions.
- What is the rock like? (forecast): We produce a forecast of the geological conditions that exist, i.e. how the rock is behaving and what it looks like, what kinds of cracks there are, the extent of the cracks, in which direction they go and how they affect the groundwater. And how much uncertainty there is believed to be in the forecast.
- How are we going to work with the rock? (choice of work method and projected solution): Based on the forecast, we select the work method and technical solution. This involves, for example, what kind of sealing concept we choose, at what pressure we inject the sealing compound, into how many boreholes, and so on. The greater the uncertainty contained in the forecast, the greater the opportunity has to be to make changes to the selected working method and technical solution.
- How effective is the selected work method? (how well does the execution match the projected solution):
A forecast that corresponds well with reality requires less of the party that has to build the tunnel. The more uncertain the forecast, the greater the need for follow-up on geology, active decisions, follow-up and measurement of results.