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Town Controls Pipeline Replacement Need
With Detailed Inspection & Analysis

The 22-mile Incline Village Effluent Export Pipeline (EEP) was built around 1970 to serve the rapidly growing, 22-sq mi Nevada resort town, situated at the northeast corner of Lake Tahoe. The pipeline consists of approximately 45,700 feet of cement mortar-lined steel pipe coated with a coal-tar enamel and approximately 8,900 feet of polyethylene-encased ductile iron pipe. Segment 4 consists of 36,500 feet of asbestos cement pipe.

By 2002 the Incline Village General Improvement District (IVGID) had been forced to make emergency repairs following a number of leaks in the pipeline, which is typical for metallic pipe of that age. Rather than continuing to conduct emergency repairs at a high cost, IVGID concluded that a condition assessment was needed to determine if the pipeline would have to be temporarily rehabilitated or, in the worst-case scenario, replaced immediately. Another driving force for the project was the need to protect the Lake Tahoe watershed which meant transporting all wastewater out of the basin. IVGID retained national engineering firm HDR (Omaha, Nebraska) to conduct a condition assessment and rehabilitation of the EEP. V&A was retained as HDR’s subconsultant.

For most of its length, the EEP runs through some of the most environmentally sensitive areas in the U.S. For 10 miles or so it follows State Highway 28, which rings the Nevada side of the lake. It then passes through Lake Tahoe State Park and crosses U.S. 50 near Spooner Summit, continues east through Toiyabe National Forest and crosses U.S. 395 before discharging at the IVGID wetlands 6 miles southeast of Carson City. The map (right) shows the alignment schematically.

Most of the condition assessment study was conducted by HDR and included an evaluation of the water chemistry, hydraulic and surge analysis, and rehabilitation/replacement development. V&A was responsible for the corrosion evaluation, which included:

• collecting and testing soils along the alignment;
  
• testing the mechanical properties of the steel pipe;
  
• reviewing the video inspection of the internal surfaces; and
  
• evaluating the condition of the cement mortar lining.
  
  

The biggest physical challenges in the condition assessment were gaining direct physical access to the pipeline, since the force main had been constructed with no manholes, and its location along Highway 28 in the environmentally sensitive Lake Tahoe Basin area. Thanks to close cooperation and teamwork with the owner, the prime consultant, and the contractor, the V&A team was able to access the pipeline at five locations, insert the camera, and complete the inspections within the very brief time permitted. Because of these conditions, and the limited (5-month) construction season at that latitude and elevation, nearly 8 months of planning was required in order to prepare for the actual inspection.

With coordination between the Nevada Department of Transportation, local environmental regulatory agencies and the inspection teams, the pipe was accessed through new access points made during excavations (Photo 1).

Photo 1. Saw cut made to investigate the interior of the pipe.	Photo 2. Metal surface exposed at pipe joint

There was concern that joints of the pipe had deteriorated past the point of repair and were believed to be the reason for most of the leaks that had been experienced to date. However as seen in Photo 2, the interior surfaces appeared to be in good condition and were not corroded. Additionally, V&A found that the pipe was originally installed with 10- and 12-gauge steel (0.135 inches and 0.105 inches thick, respectively).

Based on that information, V&A calculated the minimum wall thickness required to withstand the surge pressures found during HDR’s hydraulic analysis. The investigation revealed that the steel had retained most of its original strength but the wall thickness could not withstand pressures less than -5 psi and greater than 150 psi for certain pipe segments. The minimum wall thickness calculations accounted for a 2:1 safety factor.

The evaluation of the soil revealed that the majority of the pipe was in negligibly corrosive soils with soil resistivities of 10,000 ohm-cm and above. One area near the end of Segment 3, near a marshland, contained high concentrations of chlorides, which prompted a recommendation to replace 100 feet of pipe.

At the discharge area in the IVGID wetlands, the soils contained more than 2,000 ppm of sulfates, a level that is considered highly corrosive to metallic pipes. Inspection of the ductile iron pipe at the wetlands revealed that some of the pipe was actually submerged in water due to the shallow water table, while another section had severe pitting on the exterior surface due to the high sulfate concentrations.

The evaluation of the pH of the cement mortar lining revealed that some of the lime had been leached out by the wastewater. However, it had retained most of its original alkalinity. V&A calculated the Langelier Index (LI, a standard measure of water aggressiveness) for the wastewater at different points of the distribution wastewater system. The results showed an average LI of -1.4, indicating that the water did not have a tendency to deposit a protective layer of calcium carbonate (CaCO₃) on the metal surfaces exposed to the wastewater.

However, the video inspections of the interior surfaces did not reveal any significant signs of corrosion except in those areas where large sections of the cement mortar lining had been removed accidentally. The good condition of the interior surfaces of the pipe may be due to IVGID’s practice of adding a corrosion inhibitor to the wastewater to raise its pH and alkalinity.

As a result of the investigation, V&A recommended that an approximately 100-foot section of the steel pipeline be replaced due to the presence of high concentrations of chlorides near the marshland. Due to the considerable difficulty of excavating the ductile iron pipe in the shallow water table at IVGID’s Wetlands, V&A recommended that IVGID replace the pipe at the end of its design life rather than install a cathodic protection system on the existing pipe. The installation of the cathodic protection system would have been more expensive due to the joint bonding requirements to attain electrical continuity in the pipe. The investigation determined that most of the damage to the pipe was a result of third party damage during road and utility improvements. Most of those damages resulted in the removal of large sections of the interior cement mortar lining which exposed the steel surfaces to the wastewater. V&A recommended that the damaged areas be repaired immediately. Segment 4 was found to be in very good condition based on the video inspection and soil resistivities. V&A also determined that most of the existing pipe was in overall fair condition, except those areas in need of immediate repair, and recommended that IVGID continue to add corrosion inhibitor to the wastewater.