One of the largest assets water utilities have to manage is buried water mains, which vary in diameter, material and age. It is not uncommon for water mains to be over 100 years old. Often, water mains are replaced based on their age, or failure rate. However, rehabilitation instead of replacement can be cost-effective and extend useful life if applied to a good pipe candidate. But, what makes a good pipe candidate for rehabilitation? There are several items to consider:
Pipe class refers to pipe thickness for a certain manufactured date. The higher the class, the thicker the pipe. Class can be obtained from submittals, record drawings or physical examination. Tap coupons or pipe samples retrieved from a water main failure can be measured for pipe thickness, and based on installation date, pipe class can be determined. The American Water Works Association (AWWA) has documentation on pipe class for cast and ductile iron water mains.
Corrosive soils can destroy water mains from the outside in. Again, physical examination of tap coupons and pipe samples can determine extent of external corrosion. Externally corroded water mains should be replaced with poly-wrapped ductile iron pipe or materials suitable for corrosive soils. Existing water mains with little to no external corrosion are good candidates for rehabilitation.
Hydraulic analysis should be conducted as part of an asset management program to identify water mains that are undersized and do not provide adequate fire flow. Typically water mains smaller than 4-inch diameter are not rehabilitated and in some cases this extends to 6-inch diameter. Existing water mains 6-inch diameter and above with adequate hydraulic capacity are good candidates for rehabilitation.
Main Failure Rate
Water mains with a history of failure should not be rehabilitated, but should be replaced even if they are found to be a good pipe class, in non-corrosive subsurface environment, and have adequate hydraulic capacity. Information on the cause of previous main failures can assist in the decision to replace or rehabilitate. Once there is a failure on a particular section of pipe, the probability of a subsequent failure rises.
One-foot long pipe samples can be crushed to measure the load applied at failure. This value is compared to the minimum factory acceptance failure load with a 2.5 factor of safety. From these calculations the remaining factor of safety can be estimated.
The oldest pipe in a water distribution system is not necessarily the weakest pipe in the system. the contrary many times the oldest pipe is found to be high class and thickness depending on installation date. Pre-1952 cast iron pipe classes are particularly thick and are prime candidates for rehabilitation if they adhere to the points listed above.
Make an Educated Decision
Asset management programs, field examination, and collection of tap coupons can provide data used to make educated decisions on whether to rehabilitate or replace.