Approaches in the design of softwood sheet pile-vegetation system as an alternative to twin-wood sheet pile

Abstract

Tropical hardwood or treated wooden sheet piles have been used historically to protect stream banks in countries like the Netherlands. However, there is an imminent need for alternative as exploitable tropical hardwood forests available in the country is limited. Locally available softwoods are less resistant to decay and in extreme conditions like in a stream bank with high moisture content, softwoods show minimal durability. The section of the sheet pile at air-water-soil interface is the most prone to decay. Hence twin-wood sheet piles have been developed in industrial scale, which is a combination of hardwood and softwood. Hardwoods which have high resistance to decay is used above the water table while softwood is used in the section which is always below water. An alternative stream bank protection system using vegetation and softwood sheet pile was proposed recently. In such a system the effect of vegetation can be included as an increase in cohesion of the soil or an increase in internal friction angle. In this article two different approaches (System approach (SA) and Discrete approach (DA)) for the design of wooden sheet pile-vegetation system are examined. The SA shows that the presence of vegetation increased the time for complete damage of the wooden sheet pile-vegetation system to occur. In DA approach a change in slope of the face of the stream bank is required if the effect of vegetation is considered as an increase in friction angle of the soil.