Rebild Forsyning lays out the outlet line from basins, where bog terrain without bearing capacity makes conventional piping impossible. The solution was found in a research project with screw foundations.
In connection with the laying of an outlet line from two rainwater basins at a new residential area in Støvring, Rebild Forsyning had to pull the line through a soft bog area. Unfortunately, a geotechnical investigation by the collaboration partner, Andreasen & Hvidberg, revealed soft layers down to a depth of 10 meters, where bearing capacity could first be established. Therefore, thinking outside the box had to be done.
– Normally, you dig up the old soil and replace it with sand, but it would be impossible to do with the depth conditions. On top of that, we have experienced that iron sheets have sunk into the area over a weekend, so we were worried if the bog soil could support the weight of a ram for driving concrete piles, although we had that in mind at first. Therefore, there was uncertainty about the use of concrete piles, says Michael G. Jørgensen, civil engineer at Rebild Forsyning.
Screw foundations became the solution
To meet the request for pile foundations from Rebild Forsyning, Jens J. Porsmose, director at Andreasen & Hvidberg A/S, suggested using experience from a research project with screw piles. A project such as the geotechnical expert and BAYO.S Screw foundations is a joint effort together with Aalborg University and the Innovation Fund.
– We first turned the geonet, which is used to create a stabilized layer that increases the load-bearing capacity of the subsoil, but Rebild Forsyning requested a pile foundation as a solution, as it does not settle. That’s why I presented the screw foundations, as they corresponded well with the criteria where we can secure the outlet line – both against future buoyancy and settlements. We carried out a geotechnical assessment and made dimensioning of the piles based on the experience gained from the research project for the design, says Jens J. Porsmose. The advantages of screw foundations compared to concrete piles or the geonet are that there is no digging, no driving away of soil, no vibrations and quick installation. On top of that, the materials and equipment for establishment are light and do not sink into the soft bog terrain.
Fixed several issues
Thanks to the new option, the outlet line will be laid out through the bog area outside Støvring out to Lindenborg Å. This ensures efficient diversion of water from the two rainwater basins in the residential area in Støvring. However, it could have ended differently with a new, huge rainwater basin that would not help the discharge of water from the reservoir and would require more resources for the establishment.
– We can draw eight times as many liters of water out of the two rainwater basins now than if we had ended up with the other solution. On top of that, the installation was relatively simple. It saves time and resources to be able to adapt the screw foundations immediately and mount our own materials directly on without waiting for hardening or additional mounting as with, for example, concrete piles, explains Michael G. Jørgensen.
As the executing contractor, HMC A/S states that the subsoil is one of the more difficult types to work with. The construction company, which has been responsible for coordinating the laying of the pipe and the placement of the screw foundations, also sees the new screw foundations as a smart solution for the future.
Geotechnical investigations from Andreasen & Hvidberg A/S showed that bearing capacity only occurred at a depth of 10 metres.
– From the point of view of the client and construction consultant, the screw foundations seem like an environmentally sound solution in the construction of the future. It is light equipment compared to concrete piles and relatively quick installation in lowlands, says Nicolai Føhns Pedersen, head of the Department for Construction at HMC A/S.
