A Balancing Act

As engineers in the pipe industry, we see the results of a lot of pipe projects.  Fortunately, the vast majority of the installations are successful and result in round pipes that are well supported by the backfill around them.  However, occasionally pipe installations don’t follow the recommended techniques and result in distorted pipes.

Although unusual, one of the most common installation errors is not maintaining a balanced load condition while backfilling the pipe.  Backfill is placed and compacted in lift heights that are generally 6 to 8 inches in depth before compaction.  It is important that the backfilling operation is alternated from one side of the pipe to the other to minimize the unbalanced load condition created when more fill is placed and compacted on one side of the pipe as compared to the other.  It is generally recommended that the differential in backfill height from one side to the other not exceed more than two lift thicknesses.  If this procedure is maintained throughout the backfilling process, the pipe ring stiffness will support the limited differential pressure and maintain its round shape.  However, if the imbalance across the pipe centerline becomes too great, the pipe can become distorted with the top centerline of the pipe moving away from the higher fill pressure, flattening the radius of the side of the pipe and resulting in distorted pipe.  In extreme cases, the pipe geometry can be distorted to the point of wall buckling and even collapse.  Care should be taken to maintain a balanced load condition until the minimum cover depth over the pipe is achieved.  Minimum cover is generally equal to 1/8 of the pipe diameter, not less than 12” although it can be greater in some cases.

Compacted sidefill should be completed on both sides of structure before fill is “carried over” for construction traffic.

The good news is that this sort of pipe distortion is easy to avoid using recommended installation techniques and occasional monitoring of the pipe geometry during the backfilling operation.  As always, consult with your Contech representatives and refer to the installation guidelines available for whichever pipe product is being used on the project.