Upward Pipe Deflection

Published: November 13, 2019 | Last updated: July 5, 2023

What Does Upward Pipe Deflection Mean?

Upward pipe deflection occurs in buried pipes when the buoyancy force created by the pipe below the water table exceeds the total downward weight of the pipe and soil above it.

During spells of dry and hot weather, the moisture content in the soil decreases and the pipe has a loss of support. During wet and cold weather, the soil regains the moisture content and swells up exerting pressure in the upward direction. If the upward deflection exceeds the bearable stress, the pipe can rupture.

Proper geotechnical investigation should be carried out to determine the moisture content of soil and other factors prior to choosing the type of pipe material to be used.


Trenchlesspedia Explains Upward Pipe Deflection

The upward deflection in buried pipes is a result of the swelling pressure acting just below the pipe. For a pipe to be deflected upward, the force of lift below the pipe has to be greater than the resistance to lift acting from above.

Excess deflection can increase the strain in a pipe leading to failure or rupture. Improper backfilling and compacting is one of the reasons why pipe deflection takes place. Care should be taken to ensure that right pipe materials are utilized and pipelines are adequately supported and compacted especially in reactive clay soil where moisture content varies significantly with temperature.

Lighter pipe materials such as high density polyethylene (HDPE) and polyvinyl chloride (PVC) have a greater tendency to deflect than reinforced concrete or glass reinforced plastic (GRP) pipes. Where pipe deflection has damaged the pipe, trenchless rehabilitation methods may not work and trenching may have to be done in the affected area to rectify the problem.


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