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Petri Lintinen, Geological Survey of Finland (Finland)
Antti Ojala, Geological Survey of Finland (Finland)
Heikki Vanhala, Geological Survey of Finland (Finland)
Jukka-Pekka Palmu, Geological Survey of Finland (Finland)
Ossi Ikävalko, Geological Survey of Finland (Finland)
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Stratigraphy, sediment properties and their correlation with geotechnical properties were studied at the Suurpelto construction site in the City of Espoo, southern Finland.
The city of Espoo has started planning of the Suurpelto aiming to construct working, housing and recreational facilities for 20 000 persons. The aims of the study are to (1) provide information for the use of construction suitability planning of the Suurpelto area, (2) improve understanding of composition, structure and geotechnical properties of thick fine-grained deposits in southern Finland and (3) use the Suurpelto area as a test site of multi-proxy analysis (i.e. geophysics, geotechnical engineering, sedimentology and geochemistry), and as an example of construction location with the most challenging soft sediment conditions.
Airborne geophysical techniques, including electrical conductivity, magnetic and radiometric measurements, were used to characterize sedimentary basin (100 m line spacing, 30m nominal flight altitude, frequency 3.1 and 14.4kHz. Ground EM and gravity measurements were made at selected lines in order to map continuities of the sedimentological units between the drilling locations. Within the fine-grained sediments, the electrical conductivity increases from bottom (10 Ohmm / 100 mS/m) towards the top layers, maximum in the Litorina clays (3 Ohmm / 300 mS/m). The conductivity of the uppermost lake-phase sediments varies between 250-20 mS/m (4-20 Ohmm) decreasing upwards.
Stratigraphy and measurements mineral magnetic properties from core samples indicate that there is a good correspondence between different cores. The same sedimentary units, representing different phases of Baltic Sea, are found throughout the Suurpelto basin allowing a construction of 3D-model. Based on sedimentological analyses, Baltic Ice Lake, Yoldia Sea, Ancylus Lake and Litorina Sea phases were identified in sediment stratigraphy. 12 different sedimentary units with different composition and structure were identified from the 21 meters long sediment section. Baltic Ice Lake clay unit contains coarser sub-layers within varved structure and has a very low LOI (i.e. humus content) and highest dry density values. Yoldia sediment unit contains clay fraction in the highest proportions, LOI and WC (water content) are 10 and 5 %, respectively and shear strength is highest. Ancylus Lake sediment unit can be distinguished having lower LOI and WC and stable shear strength. These units contain sulphide material and sub-layers of coarser silt and sand. The earliest stage of the Litorina Sea is very rich in sulphide material (1-2%). LOI and WC increase upwards in Litorina sediments and shear strength decrease. Laguna and lake phase sediments are rich in humus content and have lower shear strength than the underlying deposits. Also the Swedish weight-sounding test shows lower values.
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