The use of observational method to substitute deep foundation to shallow foundation on a deep marine fill

Helsinki, the capital of Finland, is constantly expanding towards the Baltic Sea. Therefore, extensive marine fills are needed to create new ground for buildings. Seabed consists mostly of soft, normally consolidated clay, the thickness of which can be over 10 m. In such conditions the buildings are normally founded on drilled piles, which are installed through thick rock fills. Piles can be up to 40 m long and hence the foundation costs are significant. Instead of using drilled piles, a cheaper type of foundation was desired. This case study presents instrumentation, measurements and back analysis of a preloading embankment which was built to enable shallow foundations for the forthcoming buildings. In this case, a 10 m thick soft clay layer was first dredged from the seabed. Then a 20 m thick rock fill was built and deep compacted to create stable ground for the four, 7-storey residential buildings. Additionally, a preloading embankment was constructed and instrumented on the site early in the year 2013. After the embankment construction, considerable movements have been observed deep in the subsoil and in the rock fill. Extensive analyses of the embankment and forthcoming buildings were hence conducted to verify the serviceability of the shallow foundations. Calculation analyses have been conducted with traditional settlement and stability analyses and both 2D and 3D finite element codes (PLAXIS). These analyses have indicated that the shallow foundations are conceivable in this project case.