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Vinyl sheet piles in place of steel sheet piles i.e. how flexible approach and openness to innovation helped cut the investment costs of the flood protection project carried out in Cracow.
As the impact of extreme weather triggered by the ongoing climate change becomes more and more destructive, it is essential to maintain flood protection infrastructure in good working order. Flood embankments which constitute the first line of defense against flooding, must be especially taken care of since their infallible operation is particularly important where water resources, e.g. rivers are part of the urban landscape. For centuries, people have settled by rivers. Such has been the case with Cracow, one of the oldest cities in Europe, where the Vistula River is an invaluable landscape asset, but at the same time it is a source of potential threat.
Taking the project of flood embankments expansion in Cracow as an example, in this article, we describe how flexible approach and openness to innovation resulted in efficient modernization and expansion of the key flood protection infrastructure of the Vistula River, in a project where the use of our environmentally friendly vinyl sheet piles, not only turned out to be an ideal solution in terms of technology, but at the same time helped generate significant financial savings.
The challenge
The flood embankments of the Vistula river in the eastern part of Krakow required reconstruction to ensure adequate flood protection. Their technical condition was so bad that in the event of a flood, water seepage through the embankment could lead to a hydraulic rupture and, consequently, permanent damage to the embankment. Failure to intervene properly could result in a major disaster.
In order to protect the local residents, the investor, i.e. the state-owned company Polish Waters, decided to expand and modernize flood embankments within three districts of Cracow, located in the East: Podgórze, Czyżyny and Nowa Huta, as well as partially cover the Wieliczka commune. The project assumed the construction or modernization of a total of 21 kilometers of flood embankments and improve the safety of as many as 40,000 citizens of the Cracow region, inhabiting an area of over 31 km2.
The Section 1 of the project from the Bridge of Wanda to the intersection with the Zakarnie Street, the flood embankment has been simultaneously used as a busy road – the Podbipięty Street.
The original solution: why it failed
The initial design was to tighten the flood wall using the TrenchMix/CDMM technology. This technology consists in deep mixing of soil with cement using a special tool resembling a large chainsaw which is over 10 meters long. Work is being carried out on site and the TrenchMix/CDMM technology is characterized by a relatively fast pace of installation and low cost.
Unfortunately, just after the works started, it turned out that the core of the embankment was made using hard material obtained from steel works, which was too compacted to be efficiently processed by the TrenchMix/CDMM machine. Due to the impossibility of effectively mixing the soil material with cement, an alternative solution had to be found.
Technology switch – attempt no 1
Due to the factors mentioned above, which prevented the use of a cement-soil barrier, it was decided to replace the CDMM barrier with steel sheet piles 7-8 m long.
Steel sheet piles were to be driven into the base of the flood wall, and a bentonite mat was to be attached to the upper edge of the structure, covering the water adjacent side of the embankment to prevent the water from filtering through the embankment above the sheet piles.
Application of such alternative solution, would tighten both the base of the embankment to a depth of 7 meters and protect its body against seepage during potential floods that might occur in this area.
War in the Ukraine
In late February, when the works carried out at the flood embankment were advanced, the war in the Ukraine started with its direct and indirect consequences impacting many aspects of global economy and a many industry sectors, with the steel market as one of them. The supply collapse resulted in severe shortages in the availability of material and steel products. The completion of the works within the contracted deadline has become a big question mark and so was the safety of the inhabitants of Cracow with the flood risk season approaching. Following a series of consultations carried out by the investor, the chief designer and the contractor, the parties decided to replace the steel sheet piles with their vinyl alternatives.
As it turned out, the decision could not have been a better one.
Despite the challenges, the project was a success. A total of 7,840 m2 of vinyl sheet piles were installed over a length of approximately 900 meters.
The functionality of the vinyl sheet pile structure is identical as the one originally assumed for the wall made of steel sheet piles. The bentonite mattress was installed exactly in the same way as it was designed for steel sheet piles, i.e. a reinforced concrete cap was constructed behind the wall made of vinyl sheet piles, which was mechanically connected to the bentonite mat.
Vinyl sheet piles and measurable value added
The innovative approach to the problem not only ensured safety, but also brought significant investment savings.
The cost of a quare meter of steel sheet piles, which was applicable and installable in this task, would amount to approx. EUR 225 while one square meter of vinyl sheet piles used as an alternative equalled only EUR 57. This means that using vinyl sheet piles generated a saving of EUR 168 per square meter. For the total installed amount of 7,840 m2, the material costs were therefore reduced by approx. EUR 1.35 million.
Vinyl sheet piles are installed using exactly the same equipment as steel sheet piles, but with a much more simplified logistics on site. Thanks to the lower weight of PVC in comparison with steel, less equipment and man force is required to transport and handle the vinyl sheet piles at the construction site.
While the logistics savings are in this case difficult to calculate precisely, it has to be highlighted that that the investor spent approx. EUR 1.35 million less from the project budget. Such amount is much too significant to be ignored.
Summary
The project of extending the Vistula River flood embankments in Cracow teamed with challenges and plot-twists, both technical and those resulting from the geopolitical situation.
However, thanks to the investor's foresight, flexible approach to planning and the ability to quickly adapt to the unforeseen circumstances, it was possible to successfully complete the project using an alternative technology, achieving significant financial savings at the same time.
The above case has proved that even in the face of unexpected obstacles, openness to alternative solutions, innovative thinking and cooperation between the investor, contractor and designer can bring both technical, ecological and financial success.