Direct Pipe Construction: An Engineering Perspective on Quality Assurance

Direct Pipe (DP) is a recently developed trenchless technique used for steel pipeline installations. DP combines the benefit of a continuous pipe fabrication and stringing area (similar to the horizontal directional drilling [HDD] method) with the continuously supported and earth pressure balancing capabilities of a curved microtunnel. Additionally, as opposed to traditional microtunneling, launch and retrieval shafts can often be of minimal depth or even at the ground surface. Since the first DP installation in Germany in 2007, the method has gained relatively rapid acceptance among engineers and owners. As of this writing, there have been 67 DPs successfully completed worldwide, of which 25 were completed in North America. Good documentation and timely analysis are critical to the successful and safe completion of DP crossings. In particular, during construction the alignment and profile of the microtunnel boring machine (MTBM) should be closely monitored along with the thrust forces required to complete the drive. If annular pressure is a design parameter (as with a levee crossing), it should be closely monitored and recorded to confirm the annular pressures related to the minimum factor of safety against soil hydraulic fracture are not exceeded. Engineers with a comprehensive understanding of axial, bending, hoop, and combined stresses; buckling; and hydraulic fracture risk should review the construction data and provide timely input into the construction process to reduce the likelihood of steel pipe yielding issues, hydraulic fracture of soil, or other undesirable outcomes.