Parting the Water at Halsteren's Fortress in the Netherlands

 
     

Usually a bridge crossing water would take a route above the waterline, but not in this case. This entirely wooden bridge in Halsteren, Netherlands is designed to last at least 80 years. It spans the moat around a restored 17th century fortress which at the time would not have provided a bridge for the enemy to use. In keeping with the history of the fortress a novel, and sustainable solution was proposed.

           
 
     
   

In wet conditions wood will rot. The degree to which wood rots depends on its species. Generally a softwood will rot faster than a hardwood. Hardwoods like Iroko are well known for being naturally rot resistant and other woods high in tannic acid (tannins) like oak. The problem with hardwoods is they grow slowly, too slow for the demand people put on them.

Fast growing softwood can be modified in various ways making it more durable, but the methods used often involve toxins that leach out into the environment. However, the Radiata Pine used for this submerged bridge is preserved using a method called acetylation. The process, not without energy cost but benign, gives the pine the performance characteristics of a hardwood. Acetylation involves impregnating the pine with acetic acid (as in vinegar). The process has a long international development history of some 80 years but is finally in production by Accoya in the Netherlands where this bridge is their flagship project.

 
           
       
   

The architects RO&AD say, "... wood can now offer a very real alternative to other materials, such as steel... It is the first time that fast-growing, sustainably farmed wood can be used in such demanding applications.", and as you can see (left) the demands on this bridge in winter can be considerable.

You can read more about acetylation and take a look at the CO2 calculator for the product.

There are other, perhaps more natural, ways to improve the rot resistance of wood. A charred timber will last much better in damp conditions. So much so it has been used by Tony Wrench in his many roundhouses in some very damp places like Wales.