High-strain dynamic testing: an opportunity for efficiency gains and risk mitigation under variable ground conditions

Risk is ever-present when designing deep foundations for structures and is exacerbated when variable ground conditions are experienced on-site. Throughout a 2400 m2 Brisbane site, a total of six boreholes, eight Cone Penetration Tests (CPTs) and three test piles (which achieved a practical refusal set) were conducted, which identified expected pile founding depths of between 6.5 m and 27.0 m. The expected pile founding depths were extrapolated between CPT locations using contour plotting software. However, during the installation of the piles, only 42% of piles were pitched at the original design length due to the variability encountered. One third of piles were driven to a practical refusal set (an installation set of less than 2.5 mm per blow) with others achieving an ultimate geotechnical capacity prior.

The as-built pile penetration depths were used to create a second contour plot. This identified a deep trough running diagonally through the site which was not identified in the geotechnical investigation. High-strain dynamic testing using the Pile Driving Analyzer® (PDA) was performed to confirm the bearing capacity even when a refusal set was not achieved. As such, new driving criterion were developed across the site and confidence was given to new pitched pile lengths which relied on the benefits of soil set-up effects rather than solely end bearing capacity. Despite these differences in pile instillation behaviour, this project was completed on-time with an increased testing rate and approximately 200 m more pitched pile length than designed.