Biodynamic timber sheet pile – vegetation retaining structure

Timber sheet pile walls are widely used for the protection of stream banks in different parts of the world. However, there is tendency of creating more sustainable types of stream banks not only because exploitable wood is more difficult to obtain, but also because of disturbance to the natural habitat of plants and animals due to hard embankments. In The Netherlands alone, about 2500 km of engineered timber sheet pile wall embankments exist, primarily made with tropical hardwood, apart from an even much larger amount of ´non-engineered´ small size timber based embankments. As an alternative, the authors propose to use a mixed timber sheet pile-vegetation system, where locally available timber can be applied in combination with natural vegetation. Unlike the usual bioengineering scheme, vegetation is not seen as an element, which could replace the timber sheet piles. Instead a new perspective is tested, where the vegetation is included as a ´structural´ element which will reduce or even counteract the consequences time dependent biological degradation of the timber sheet pile. By doing so, both long term durability as well as reliability of the stream bank are improved. We have developed a comprehensive design model, based on well-established sub- models from the literature on plant growth as well as timber service life. The timber sheet pile wall-vegetation system is illustrated in an example case study. Preliminary analysis including only the mechanical reinforcement of vegetation shows that the durability of timber sheet piles is enhanced. Thus, using vegetation in combination with highly degradable timber could possibly negate the need for using hardwood timber, or more generally, save resources that are currently used for these structures.