When a 12-inch water main on the main campus of Texas A&M University ruptured at about 3:30 AM on a June morning, the effects included the loss of all water from the school’s elevated water tank and an unplanned shutdown of the central utility plant and all air-conditioning on the main campus. Full service was restored within a few hours, but the incident was a wake-up call to university facilities managers, who quickly arranged for a thorough survey and assessment of their water-supply system.

Texas A&M is a well-known land-grant, sea-grant, and space-grant university located in College Station in central Texas. It has 44,000 students and is bigger than many small towns. Drinking water for this large institution comes from the Carrizo-Wilcox Aquifer, via wells located north of TAMU’s Riverside campus in the vicinity of Sandy Point Road and Old San Antonio Road (OSR). Sixteen miles of 18-inch and 24-inch parallel transmission pipelines carry potable water to hundreds of facilities on the main campus. By the time this project began, the pipelines had developed significant leaks on numerous occasions, and excavations and repairs on an emergency “site-by-site” basis had become an almost routine occurrence.

Corrosivity Survey
V&A Consulting Engineers was retained for two purposes. As corrosion consultants, V&A conducted an above-grade corrosion survey and hydraulic investigation along 2,000 feet of the pipeline alignments, identified the sections of the concrete-coated steel cylinder pipeline where below-grade corrosion was likely, and made recommendations to repair the existing problems and prevent future pipe failures. The project was also intended to assist the Texas A&M physical plant and maintenance staff on how to evaluate the remaining 15 miles of pipeline for corrosion.

Over a five-day period, V&A personnel conducted a cell-to-cell survey (aka an “over-the-line” survey) to locate anodic areas, i.e., to identify areas where corrosion of the pipe was most likely to be occurring. The soil environment was evaluated for field soil resistivity using the Wenner 4-pin test method (ASTM Standard G57) and for saturated soil resistivity using Caltrans (California Department of Transportation) Test Method 643.

The testing was performed with copper/copper sulfate reference electrodes (half-cells) placed along the alignment at the ground surface. The reference electrodes were connected to a recording voltmeter to measure the half-cell potentials at either side of the pipe. The difference in potential was taken between a cell placed over the center of the pipe and cells placed 10 feet to the left and right of the pipe. Readings were taken from cell-to-cell at 2.5-foot intervals along the alignment.

click to view enlarged image

click to view enlarged image

This technique is an effective tool for locating areas along an electrically discontinuous pipe that are most likely to be corroding. Areas of possible corrosion activity are indicated by relatively large-magnitude differences (spikes) in the cell-to-cell potential measurements along the line, fluctuations in the readings taken at either side of the pipe, and changes in sign of the readings (see graph, right). Field and saturated soil resistivities were generally below 2,000 ohm-cm, which is considered corrosive to metal structures.

Based on spikes in the survey data, the engineers recommended five specific sites for excavation, inspection, and—if necessary—repair of the pipelines. Field inspection showed corrosion of the concrete-coated steel cylinder water pipeline. The 18-inch line was in better condition than the 24-inch pipeline.

18-inch and 24-inch Water Pipelines .	Corrosion on 24-inch Pipeline
Corrosion on 18-inch Line	Corrosion on 24-inch Pipeline

V&A evaluated all options for cathodic protection and strongly recommended installing an impressed current system on both pipelines. Galvanic anode systems were not recommended because it would require too many anodes at a high cost. Another option was to simply repair leaks as they occurred but that would only provide a quick short-term fix, but would do nothing to prevent future leaks. The most effective—and most costly—solution would be to replace the entire 16 miles of pipeline and install an impressed current system for corrosion protection. That alternative, however, would cost an estimated $6-7 million, while the cost of installing the impressed current system on the existing pipelines alone would only be approximately $1 million.

At the same time, another consultant found that the pipelines’ existing air and vacuum relief valves were not functioning correctly to provide protection against high internal surge pressures. As a result, surge pressures were causing cracking in the pipes’ exterior cement mortar coating, thereby exposing the bare steel to the corrosive soil. V&A recommended installation of a storage tank for the waterline to protect against the surge pressures.

Pipeline leaks like those at Texas A&M may not be the result of poor construction, the age of the pipe, or inadequate maintenance. In many cases, the culprit is simply corrosive soil. That is where the services of a corrosion engineering consultant can be of special value. In the case of an existing pipe, the consultant may be able to identify the cause of a pipe failure or failures by evaluating soil corrosivities. A successful above-ground corrosion survey of the buried pipe can help identify areas where future leaks are most likely to occur, allowing for immediate repair.

The benefits of doing the cell-to-cell survey are that it can locate areas of possible corrosion without having to dig up the entire pipe. It is especially cost effective for pipes buried at depths greater than 10 feet. Such buried pipes would normally require a greater crew size and heavy equipment for excavation depending on the repair materials and pipe diameter.

Even better, of course, is the “ounce of prevention” approach. Ideally, any organization or agency should take the possibility of corrosive soils into account whenever they are planning or designing a water or other utilitiy service line. It is a very cost-effective way to avoid having to fix the pipe sometime in the future.