City planning of Hermanninranta – A combination of geotechnics, contamination, and climate change

Abstract

The Hermanninranta area presents one of the most geotechnically challenging urban planning areas in the City of Helsinki. The area located on the shore of Vanhankaupunginlahti (Old City Bay) is approx. 50 ha in size. Most of the area is reclaimed from the sea and only Kyläsaari island is a natural formation. Land reclamation in the area was performed in various stages between the 1840s to the 1980s. Natural very soft clay layers encountered in the area extend to a depth of over 40 m below ground surface. Today, the proximity to the city centre makes Hermanninranta very attractive for residential and commercial construction. Thick fill containing contaminated soil, slag and ash on top of soft deep clay, are a challenging combination. Variable fills have caused the settlement of soft clays and the displacement of the embankment towards the bay. Changing current land use for residential purposes will require changing of current ground surface elevation to approximately 2 to 2,5 m upward. This will be an issue impacting geotechnical engineering. Rising sea level caused by global warming shall also be considered. Settlement calculations show that within the next hundred years almost half of the area will be under water in case of no pile slab or soil improvement. The draft city plan has been updated based on these geotechnical, environmental and climate change studies.