Probabilistic Assessment of Pile Drivability in Swedish Soils

Abstract: Site investigations are often performed prior to the design of pile foundations with the aim to collect data regarding soil properties including boulder content. The obtained data is typically limited due to non-homogeneous characteristic of the soil. The geological conditions of the Mälardalen region are characterized by glacial and post glacial clay overlaying on the layer of moraine containing boulders on a bedrock. Thus, pile refusal in results of encountering boulders is a common issue during pile driving in this region. The current methods to assess the pile refusal risk are mostly qualitative and relies on the expertise gained from experience.This thesis aims to develop a numerical model to quantify the boulder content in a soil strata with a confidence interval based on the site investigation results. Furthermore, this study estimates the probability and the consequences of hitting boulders while installing piles.The model simulations show that an increase in the boulder content raises the probability of hitting a boulder, but it is not proportional to the diameter of piles. It means that even in low rate of boulder content with small piles there is a high probability to hit a boulder. Results from simulations using simplified quantitative method show that slender piles have substantial consequences in case they encounter boulders. Thus, these piles are more prone to pile refusal or breakage. It is suggested that in projects where small piles are preferred due to the superstructure design, enough tests be performed to have an estimate of boulder content with tighter interval and more precise design. On the other hand, in projects with low pile counts, it is recommended to have a conservative design with large diameter piles instead of performing numerous in-situ tests. This is because the price for piling becomes relatively low compared to site investigations’ cost.