The object is to stabilise the land to permit the passage of heavy machinery, required to drive concrete piles to depths of 25 m. Both vertical and raked piles are driven to very close tolerances. A ground plate is then cast onto the top of the piles, providing a solid foundation for the railway line which will carry axle loadings of 30 tonnes.
A 35 tonne Volvo excavator is fitted with the Allu Power Mix attachment. Its rotating blades dig into the earth so that the binder agent is mixed evenly. The usual binder agent is a mixture of lime and cement, which comes from the Allu Pressure Feeder into the Power Mix and is then injected into the ground.
The Power Mix is moved evenly through the material, in order to get a consistent mix of the binder agent into the peat. The Power Mix penetrates to a maximum depth of five metres. The Pressure Feeder must be re-filled about once an hour on this application due to the high percentage of binder agent being introduced into the material. The Pressure Feeder is on tracks, with remote control. A small display and control panel in the cabin of the excavator tells how much material is going into each area.
The peat stabilisation proceeds at a rate of approximately two linear metres an hour across the 100 m width of the site, depending on the softness of the soil. On this particular application it is necessary to mix a ratio of 20 per cent binder agent into the peat, due to the high specification of the contract, but for most other jobs half this amount is usually sufficient on peat.
Finally a 400 mm layer of aggregate is spread over geotextile across the site and lightly compacted. This provides a clean and stable surface for the piling machinery and provides lateral support for the piles while the ground plate is being constructed.
Contractor Valto Tikkanen believes that stabilisation will become increasingly popular. “This is much more sensible than would have been hauling the soil off and bringing in new crushed gravel. That would definitely have been much more expensive. In that case we also would have had to ponder where to tip the peat, environmental permission would have been required and waste taxes would have to be paid.”