Test piles have been installed in Oostwoud and Werkendam to determine nationwide pile classification factors for the DPA and DPA PLUS screw piles.
The pile toe strain for all piles in the Netherlands was reduced by around 30% in 2017. The result is that piles are made to be longer and reach deeper depths. Moreover, design and regulatory bodies in the Netherlands are sometimes hesitant to approve pile systems or are unfamiliar with the systems we use; such bodies tend to take a conservative approach to designs using our pile systems. This results in very heavy-duty drilling conditions which would occasionally be impossible without additional measures, such as rough boring or load testing during the work.
Collateral material damage, high levels of wear and tear, dangerous situations and finite (non-sustainable) design are why it is necessary to determine the actual load-bearing weight of our drilled soil-displacement pile systems. In 2022, for instance, we worked on five projects in which a rapid load test after installation proved that the piles were able to bear the load. Dynamic load testing, applied after the installation of piles, is not always desirable so it was important to find a solution to demonstrate the bearing weight of the piles in advance.
Geotechnical failure
Working in partnership with Gebrs. Van ‘ Hek and Deltares, we configured two test sites to carry out a total of 16 load-testing operations using a static reaction frame. Measuring tubes are cast in the piles to determine the load-bearing capacity of each separate stratum of soil. The test aims to achieve geotechnical failure of the test pile within 20 hours by continuously increasing the force on the pile using a jack. The transfer of the force in the pile to the soil is monitored by sensors within and outside the pile.
This makes it possible to determine how much of the force in weak strata of clay is transported to the hard stratum. More important are the load-bearing strata of sand in which both the DPA and DPA-PLUS piles compact tense soil. Ultimately, the pile toe is able to absorb the remainder of the load-bearing force. The pile toe is viewed as being the most unreliable feature when working with screw-pile systems, so the focus in the pile testing is on the pile shaft.
Movement of just a few centimetres in the pile head leads to the assumption that the pile has failed geotechnically. Geotechnical failure of the pile often happens quite suddenly, when the pile toe is no longer able to absorb the force. So at the end of each pile test, piles are subjected to a further 8-12 hours of loading during which the force in the pile is gradually increased; however this results in greater movement.
The results of these tests are submitted to the geotechnical standards committee (normcommissie Geotechniek). The standards committee assesses the installation of the piles, the test set-up and actual results. Based on these results, the load-bearing factors that apply for Vroom for DPA and DPA-PLUS piles can be determined. Each design and regulatory body in the Netherlands must comply with this. The load-bearing capacity of piles is safeguarded in this way and there can be no doubt about it when installing the pile in question.
Another aspect of this is that it may be possible to create a more sustainable design. The requirement is to create at least two test sites; at 3 or more sites, the result will be even more accurate. We may carry out more static pile testing in future.