Large scale driving of concrete piles in stiff to very stiff clay
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BAMA Gruppen AS have constructed a new 44 000 m2 warehouse, office and distribution centre in Groruddalen in northern parts of Oslo, Norway. Norwegian Geotechnical Institute (NGI) were employed as geotechnical consultants, contributing to evaluation of different foundation methods, design of pile foundations in cooperation with structural engineers and installation of piles in cooperation with the piling contractor. Within the perimeter of the warehouse, the depth to bedrock is between 30 to 50 meters. The typical soil profile is about 2 m of various fill material over 5 to 10 meters of stiff to very stiff clay over a slightly overconsolidated quick clay. Because of the extent of piling, concrete piles were very cost effective for the client. The stiff clay layer however could in theory lead to damaging tension stresses in the concrete piles during driving. For concrete piles up to 50 m length, the total number of blows is also considerable. NGI performed pile-driving simulations, and the contractor performed test-driving at the site. During test-driving, PDA equipment was mounted, monitoring pile stresses during driving of the whole pile length. Test-driving was successful, and pile stresses observed within acceptable limits. During the early phases of production driving, the number of piles broken during driving was higher than expected. The large number of piles did however give good statistical data to interpret combinations of pile cross section and hammer weight and energy (fall height) which were unfavourable. In cooperation with the piling contractor, NGI continuously evaluated the piling production procedure and stop criteria. The aim was to reduce the number of broken piles, and to establish documentation of the bearing capacity for all piles. In total, the contractor drove about 1600 functional concrete piles, with a combined length of about 71 km.