Application of Flax Straw in Subgrade Strengthening

Due to the recent developments affecting the transportation systems in Saskatchewan such as rationalization of the grain handling facilities, economic diversification, increased value-added production, abolishment of the Crow Rate, consolidation of railway branch lines and introduction of bigger truck configurations, Saskatchewan Department of Highways and Transportation (DHT) has been struggling to provide a desirable level of service on its thin membrane structure (TMS) road network. This has occasionally resulted in less than acceptable and unsafe road surface conditions. In order to address some of these problems, DHT began researching innovative TMS strengthening techniques. One such technique involves rotomixing the existing asphalt oil surface and incorporating mulched flax straw to increase the subgrade tensile strength. Flax straw was chosen primarily because of its tough fiber properties and slow biodegradability caused by the fiber oils that act as decomposition inhibiting agents. In the summer of 2000, DHT installed three test sections using flax straw as a primary subgrade strengthening material on a section of the provincial Highway 19 near Strongfield, approximately 100 kilometers south of Saskatoon. The main purpose of this paper is to describe the construction procedures used in incorporating flax straw into the subgrade on Highway 19 and share DHT experience in using this agricultural waste by-product as a subgrade strengthening material. The field test results and performance findings are also discussed. The research methodology used considers three main elements: 1. pre-construction investigation including road surface conditions, preliminary field and lab tests; 2. construction procedures used in installing the flax straw test sections; and 3. discussion of post-construction field and lab test results. Key terms: thin membrane structure (TMS), desirable level of service, rotomixing, flax straw, subgrade strengthening, construction procedures, field and lab tests, performance findings.