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After ruling out internal spikes in pressure (also known as waterhammers), pipe corrosion, differential settlement, surface loading, cathodic exposure, thermal expansion/contraction, and numerous other prospective causes of pipe failure, the engineering study discovered that the sulfur based joint sealant (noted in pink) that was standard for pipes built in the 1930s 40s was reacting with water in the soils and with the cast iron pipe which created expansion pressure in the joints.
When this joint swelling pressure exceeded the threshold of the cast iron pipes to flex, it caused the pipe to blow-apart as the cast iron snapped under extreme joint pressures.
This joint problem was subsequently confirmed in research studies from other cities around the country that used the same joint compound as Kingsport.
The use of this compound ended in the early 1950s based on the failure of many of the newly installed pipes.
If there is any good news, it is that the pipes in Kingsport lasted much longer than most communities that used this product.
This simulation of the signature break pattern reflects the origin of the pipe break in the bell-end of the joint. As the chemical reaction occurs in the joint, the joint sealant compound expands and creates a hoop stress on the ring of the joint. Cast iron is a very brittle material and has a low tolerance for flex under pressure. As a result, once the pipe flex limits are breached in the joint the pipe bursts rapidly under the high water pressure in the pipe and causes the top to blow off.
The conclusions of the April study recommended complete pipe replacement using state-of-the-art ductile iron pipes and rubber gasket joints.
Cast iron has been obsolete for over 40 years in the water industry and because the problematic joint sealant was used at every joint (approximately one joint every 18, over 480 joints total in the water main) it is impractical to try to dig out and replace each joint.
As a result, the study recommended full pipe replacement at an estimated total cost of $6.3 million over 6 years in a 4 phase construction project.
In addition the study recommended relocating the water main out of the core business district to minimize construction impacts on traffic, business and residences.
This report provided an explanation of the new route selected for the water main replacement and corresponding construction schedules.