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Pipe failed during installation

Eventually the entire Poudre Canyon line must be rehabilitated. This project was limited to a 1,400-foot segment that went around a corner, paralleled a state highway for several hundred feet, crossed the highway, and went under a large irrigation ditch beside the river.

The HDPE liner pipe was fused together. A pit was excavated, shored, and set up with a TT Technologies Inc. Grundoburst 1250 CP static pulling machine. The prime contractor was Hydro Construction of Fort Collins, Colo.

After taken out of service, a CCTV was run through to confirm the line was clean and there were no protrusions into the pipe that needed to be addressed before the pull. Unfortunately, televising failed to reveal an 11-degree bend.

A pipe that’s not straight presents problems in terms of additional friction on the pulling rods and liner pipe. The pulling head was pulled off the liner pipe twice during installation.

The friction between the host pipe and the HDPE, as well as the pulling rods, caused the pipe to “jump” as the HDPE liner was being pulled through the host pipe. The pipe would move 6 inches to 18 inches, stop, and then jump forward again as the pulling force overcame the friction. This would cause the pipe to stretch every time before it would release and move forward.

It was thought that this repeated stretching and relaxing was fatiguing the pipe and making it susceptible to failure. However, the bend in the host pipe may have been the straw that broke the camel’s back.

The head of the HDPE pipe had just been pulled through the bend when it broke about 1,100 feet into the pull. This necessitated another entrance pit and all the setup to be done again 300 feet from the end of the pipe segment. We were very fortunate the break occurred right before the pipe went under the highway where we could access the pipe without closing the road.

The pipe failed because of the difference between estimated and actual force required. It was estimated that 64,000 pounds of force would be needed. The force on the machine was determined by the pressure readings; during installation, readings exceeded 2,000 psi. This equated to more than 161,000 pounds of force. The tensile strength of 24-inch HDPE pipe is just over 150,000 pounds.

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