Importance of Haunching

Most pipeline designs assume uniform haunch support for the pipe. However, during installation good haunch support is difficult to obtain and to inspect. Research has shown that dumping Class I and Class II soils in beside a pipe creates a void in the lower part of the pipe haunch. Compaction is essential to move the material into this void area and provide the needed uniform support. The current methods of compacting soil into the haunch area are shovel slicing (rodding) or vibration for Class I and Class II soils and mechanical compaction of Class III and Class IV soils. Experiments at the U.S. Bureau of Reclamation demonstrated that the amount of support the haunch provides a pipeline depends on the relative stiffness of both the bedding material under the pipe and the haunch soil. The weight of the pipe, water, and backfill compresses uncompacted bedding so the pipe is effectively supported by the material around the haunch reducing any concentrated load on the bottom of the pipe. Early research by the National Clay Pipe Institute demonstrated the importance of shovel slicing. Recent research confirmed the significance of shovel slicing during installation. In this 2013 work, the relative stiffness of crushed rock placed into pipe haunch areas was measured by both pushing rods and pulling straps at the pipe-soil interface. Significant differences in circumferential support were seen between dumped rock and shovel-sliced rock. In these experiments, shovel slicing increased the support for the pipe about tenfold. For Class I and Class II soils, lifts up to 8 ft thick can be compacted using water and internal vibration. Impact compaction of Class III and Class IV is time consuming, labor intensive, and inherently inconsistent. To achieve a high degree of compaction, the soil should be placed in 6-in. lifts and constantly tested. Because of the difficulty of obtaining uniform haunch support, flowable fill (CLSM or controlled low-strength material) has been used in the haunch areas. For large-diameter pipe (6 ft and greater), flowable fill is the only viable option.