Connection of timber foundation piles to concrete extension piles

In many regions of the world, timber piles have been used for centuries for the foundation of buildings and bridges. For historical buildings, it is common that masonry foundation walls are placed upon timber piles and cross beams that remain below the water level. With the introduction of concrete foundation beams in the 20th century, the distance between the head of the timber pile and the concrete foundation beam was bridged by concrete extension piles. Originally made in situ, currently only prefabricated extension piles are used. In the Netherlands foundations of houses were widely realized with timber piles until the 1980s (Van de Kuilen, 1995). Currently, timber piles are mostly used for the foundation of industrial buildings like greenhouses. However, the lacking of relevant mechanical properties of timber piles and connections of parts of piles and of piles with pile extensions together with respective design rules, hinder the wider application of timber foundation piles. For the verification, the strength properties of timber piles and the connections have to be specified in standards or determined by tests, but still test data is available only to a very limited extent. In the future revised version of Eurocode 5, rules for the design of timber foundation piles will be included. The new Clause to prEN 1995-1-1 drafted by the Project Team SC5.T3 (Project team SC5.T3 ,2020) and by CEN/TC 250/SC 5/WG 3 (Working Group CEN/TC 250/SC 5/WG 3, 2021) covers general rules for timber piles, materials (wood species, grading rules), properties (compressive strength and MOE parallel to the grain in the fibre saturated state, shrinkage, shaft friction), durability, ultimate limit states (axial compression, stability) and execution. In order to guarantee for sufficient durability throughout the design service life of the foundation, the tops of softwood piles should be permanently located at least 500 mm below the lowest ground water table to be expected at the location of the foundation. If this cannot be met, the timber pile is driven to a level reliably beyond the lowest ground water table and extended by a pile extension made of material exhibiting sufficient durability when exposed to varying ground water levels (Figure 1). Extension piles are frequently made out of a concrete, either totally out of concrete or with a steel tube on the concrete. In tests in the Netherlands the strength and failure mechanisms of the connection of such a concrete extension pile was investigated, together with the influence of the stiffness of the connection on the overall behaviour of the pile in the soil. This paper presents elements of the draft section for the new EC 5 on timber foundation piles related to this connection and test results on a connection type. The failure mechanisms are analysed and the effect of the found strength and stiffness on the behaviour of the pile in the soil. Specific calculation rules are necessary because with the available rules according to the current version of EN 1995-1-1 [Eurocode 5, 2004)], engineers in practice have concluded that the timber would not fulfil the requirements for strength at the location of the connection.