Concrete pipe re-used on coal tar site

By Peter O'Connor,
Lafarge Pipe & Precast, Ottawa

Horizontal elliptical pipe, produced in 1959 and 1964, was excavated, cleaned, and then reinstalled on a property owned by the City of Ottawa. Not only was the pipe re-used, but it was returned to continue its function as a stormwater system through a site that had been contaminated with coal tar.

The backhoe operator turns each pipe over so the coal tar can be removed easily.

Coal tar is a heavy, dark, viscous liquid obtained from the destructive distillation, or carbonization of coal. The primary product of the carbonization of coal is coke, while the secondary products are coal tar, light oil, ammonia liquor, and coal gas. Over time, following its appearance in 1681, it was realized that important solvents and wood preservatives could be obtained from coal tars. The residue could be used for highway pavement binders and for specialized coatings. Coal tar was a common product used by cities and towns in Europe and North America during their early development.

The Ottawa project was located on a property on Lees Avenue. The original tender called for replacement of 110 m of 1090 mm x 1725 mm horizontal elliptical pipe (ASTM C507 H.E. III Mortar Joint) and maintenance holes, and removal of the coal tar-contaminated soil to a designated disposal site.

Taggart Construction Ltd,. of Ottawa, was awarded the work, and excavation commenced in January 1997. But when the pipe was removed and examined, it appeared to be in excellent condition ­ after 33 years in soils containing coal tar. The City and contractor then came to an agreement to remove the pipe, steam-clean each piece, and return it to the trench after inverting the pipe, so that the obvert became the invert. In addition, pipes were joined with single offset gaskets. The decision was taken to install new pipe and maintenance holes only where needed. The cost saving to the City was estimated at $54,500.

When the pipes were excavated for cleaning, it was discovered that two of the six foot lengths had been manufactured in 1959, and the others in 1964. The pipe had been manufactured by Concrete Pipe Company at their Caledonia Road plant in Toronto, Ontario.