Canadian architecture and engineering firm Stantec has plans to acquire Burlington-based engineering and planning firm Fay, Spofford & Thorndike by October. FST was founded more than a century ago and specializes in large-scale transportation and water infrastructure projects, recently completing the design of the Interstate 93/Interstate 95 interchange in Canton for the Massachusetts Department of Transportation and design of the $33 million Logan Express Garage and Terminal for Massport.