FALL RIVER — Sometimes you’ve got to dig it to make it better. Such is the scenario for Liberty Utilities, which since April has been digging up Fall River streets to replace 18 miles of low-pressure “leak-prone” distribution pipelines, or “mains,” with new high-pressure, polyethylene plastic tubing. The subsidiary of Canada-based Algonquin Power & Utilities Corp., whose legal name is New England Natural Gas Company, for a third year in a row has contracted Ferreira Construction Company of New Jersey to install the new polyethylene gas mains. The street work has led to an unavoidable series of traffic detours as workers dig into streets to install 40-foot lengths of so-called sticks of yellow-striped, black polyethylene piping.