Full-scale static compression pile load test in sand with post installation pre-stressing to engage toe resistance

Abstract

A full-scale static compression pile load test was conducted in connection with construction of a new road bridge in Silkeborg, Denmark. The new road bridge crosses a river between two existing bridges, one for pedestrians and one for an existing rail way. An existing pedestrian bridge was moved five meters to the north to give space for the new road bridge. The western part of the existing bridge was therefore to be founded partly on the existing footing of the pedestrian bridge and partly on a new pile-based footing. Bored cast-in-place piles were chosen to minimize vibrations during construction. The pile load test was necessary to document the bearing capacity of the bored piles because the Danish National Annex to the Eurocode 7, part 1 (2015), Annex L (DS/EN 1997-1 DK:NA) obligates the designer only to regard 30 % of the shaft resistance determined by geostatic calculation for the corresponding driven pile. Furthermore, it is a common design practise to disregard the toe resistance, as mobilization only occurs after deformations larger than normally acceptable for the superstructure. To overcome the latter, the test pile was constructed with a flat-jack device to allow post installation prestressing by grouting underneath the pile toe. Hence, it was possible to include toe resistance with only minimal displacements of the pile toe, as the grouting was conducted successfully, and post installation pre-stressing was obtained. The results of the pile load test show a bearing capacity up to 50% larger than estimated by geostatic calculation, even when including toe resistance and disregarding the 70 % shaft reduction in the geostatic calculation for the corresponding driven pile. An estimate of the displacement of the new pile-based footing was made based on load-displacement curves for the test pile, and the subsequent production piles were installed using the same principles due to the flat-jack device’s effective post installation pre-stressing.