Customer situation and background
The Ilmala railway depot, located in Helsinki near Finland’s largest and busiest railway station, had problems with soil subsidence.
The Ilmala depot is located in an area that was originally a swamp but was later turned into a landfill. The landfill was in use until 1963, and after several years of preloading the railway embankment, the first tracks were built in the area in 1968. Today, the area has a total of 65 kilometres of tracks.
The history of the depot area, where the tracks were built on top of a 20-meter layer of waste, peat and clay, has contributed to the creation of the current depressions. The soil sinks in some places at an annual rate of 5 to 30 millimetres, which means that in 30 years the soil has sunk by as much as two meters at these points.
The buildings and foundations in the area are piled, but the tracks rest on an embankment formed up to two meters thick.
The reason for the extensive depression problems is the differing foundation methods and soils in the area. In this case, the column foundations themselves stand stable, but the soil around them sinks. This creates cavities, which also increases the risk of accidents, as those working in the area can, in the worst case, fall into these cavities.
Since the problem, in this case, is that the soil around the foundation’s sinks and cavities form in the soil, Ilmala workers had to ensure that the cavities were filled and preferably also prevented from forming completely.
One alternative to geopolymer injection would have been to fill the cavities with sand, gravel, or concrete. However, the disadvantage of the method is that these materials are heavy, which would thus further accelerate the compaction of the soil.
Another option was to surround the foundations with problems with tongue walls, which would prevent depressions from spreading. This would mean that the pillar would first have to be excavated before installing the tongue wall. The cavities under the foundation can then be filled with pipes drilled through the foundation. Finally, the area should be restored to its original state.
Filling the cavities with sand, gravel or concrete would have been a slightly cheaper solution in the short run, but since the subsidence would have accelerated further, it was not a good option in the long run.
Why did the customer choose Geobear over other options?
The solution chosen was the Geobear method, which utilised geopolymer injection and Leca gravel, as it was a cost-effective and long-lasting solution. Thanks to the Geobear method, disruptions to rail traffic were also kept to a minimum.
Geobear solution and result
The work began by mapping the cavities in the area using the available geotechnical information from the railway depot. Based on these, it was estimated that the cavities formed were 0.5–1 m in size.
The existence of the cavities was then confirmed with a 3D scanner, which allowed the detection of “anomalies”, i.e. differences between different depths. Three foundations were selected for closer examination.
Further investigations – phase 1
The voids found using the 3D scanner were verified by diamond drilling, after which approximately 5 cubic meters of Leca gravel was blown under these three pillar legs.
Further investigations – phase 2
In the next step, geopolymer was injected into the voids filled with Leca gravel. In this way, the voids in the soil were filled and the Leca gravel compacted to achieve a 100% filling level.
Injection holes were drilled around the outer edge of the foundation, and tomato-drilled holes were also utilized for injection. Once the project was completed, no cavities were found under or around the foundations. The success of the work was confirmed by penetrometer tests and drilling through pillar sensors. A successful end result is also ensured by a new 3D scan.
Geopolymers and Leca compared to concrete
Because Leca gravel and geopolymer together form an expandable product, it is possible to achieve a 100% filling rate. The material is also very light, only 10% by weight of concrete. Another significant advantage is that geopolymers have a very fast reaction time, which means that the material reaches 90% of its durability and strength in 15 minutes, while in concrete the same takes days.
Other benefits of geopolymer injection
The geopolymer is long-lasting as a material, the material is environmentally neutral and its properties are not affected by various physical, chemical or biological phenomena.
The work did not disturb the rail traffic – thanks to the fast work process and the short reaction times of the material.
The voids can be filled with very high accuracy. This is because the process can be utilised exactly where it is needed and the reaction times can be adjusted to suit just one range.