About the author: Stephen C. Cooper is a storm water industry writer for SCA Communications. Cooper can be reached at [email protected] or 516.623.7615.
Stephen C. Cooper
undefinedIn the coal-rich areas of the southeast corner of Ohio at the West Virginia border, acid mine storm water presents one of the most severe challenges for drainage pipe. A 40-ft run of high-density polyethylene (HDPE) pipe, however, continues to substantiate the confidence Ohio Department of Transportation (ODOT) engineers had in its original selection. A recent inspection found that after 27 years of flowing storm water containing acid from coal mines and being hit with periodic floods, the pipe and roadway are still intact and performing as predicted.
Site Scenario
Installed under State Route 145 near Marietta, Ohio, in 1981, the 24-in.-diameter corrugated HDPE pipe from Advanced Drainage Systems Inc. (ADS) was the first use of HDPE by ODOT. It was used as a highway crossdrain and replaced Nexon-coated corrugated steel pipe, which had deteriorated after five years.
“The failure of the steel pipe was caused by the abrasive flow perforating the coating, and then the acidic water attacking the metal and undercutting the coating,” said ADS’ James Goddard, who was there at the time and visited the site recently.
“Actually, steel pipe had to be replaced several times at this site during the years, which prompted ODOT to use HDPE pipe, a material unaffected by low-pH flows and known for its resistance to abrasion and a wide range of chemicals. ADS had just started to produce 18-in. and 24-in. corrugated HDPE pipe in the spring of 1981. This was really a first for the industry.”
The pipe was installed under the two-lane highway, mainly used by laden coal trucks, oil tankers and logging trucks, plus other commercial and local traffic. Cover ranged from 15 to 18 in., and the trench was backfilled with a bankrun gravel.
Runoff in this area comes streaming off of the hillside from abandoned strip mines and carries crushed sandstone. The pH of the water varies from 2.5 to 4.9, which is a result of acid mine drainage occurring naturally during the mining process as pyrite reacts with air and water to form sulfuric acid.
Not only did the HDPE pipe have to maintain its integrity from the corrosive water, it also had to stand up to frequent floods and carry rapidly flowing water. In 1997, a 500-year storm dropped nearly 15 in. of rain in 36 hours, resulting in the worst flooding in the area’s history.
Inspection Data
One year after installation, the site was inspected by ODOT. The visual inspection showed no signs of pavement cracking or settling, and the pipe showed no signs of attack from acidity. The inlet invert had surface roughness due to abrasion, but no significant erosion of material.
Using a simple deflectometer, vertical deflection measurements were taken at every foot and analyzed. The highest deflection was located at the joint and most likely due to keeping one lane of the road open while the pipe was installed. The average deflection was 1.9%, well within accepted deflection values for flexible pipe in this application.
Since it was installed, the pipe has been inspected regularly, with little change found to the pavement or pipe. The most recent inspection was conducted in March 2007. During this inspection, deflection values were taken every 2 ft and compared with past deflection values; the comparison showed no change in deflection.
Data gathered over the past 26 years confirmed that:
- The deflection on the corrugated polyethylene pipe did not increase over time, despite regular traffic with 18 in. of cover;
- Acid mine drainage with a pH of 2.5 to 4.9 had no impact on the corrugated polyethylene pipe; and
- The corrugated polyethylene pipe was resistant to abrasives at low pH.
“In the early 1980s, the plastic pipe industry was in its infancy. I guess you could say this was the first toddler, and we are pleased to see how it has performed under some pretty challenging circumstances,” Goddard said. “At the time, this was a bold move. Now, HDPE pipe is found in thousands of miles of installations, but it first had its baptism of fire in this installation—which was also economically sound, as the pipe is still functioning like new.”
