Experimental procedure for evaluating ground displacement factors in directional drilling

As the utilization of horizontal directional drilling increases in urban areas, damage resulting from surface heave during pullback is becoming a greater concern for municipalities, regulatory agencies, and contractors alike. The magnitude and extent of surface deformations are influenced by several factors, including borehole pressure, depth of cover, soil density, backream rate, soil composition, annular space, and reamer type and diameter. With any complex system, understanding how these factors relate to each other in the development of surface deformations is beneficial in the planning of future installations. This paper outlines work towards the development of a model to understand how these factors contribute to surface heave during the backream phase of a horizontal, directionally drilled installation. Initial identification and selection of factors to be used are facilitated through the analytical hierarchy procedure (AHP). Using the ranking identified by the AHP, a full factorial experiment was designed and implemented in field conditions, with ground movements measured at various stages along four uniquely designed borepaths. With a better understanding of the factors associated with directionally drilled installations, contractors and engineers may better select tooling and techniques to minimize the magnitude and extent of surface heave.Key words: horizontal directional drilling, analytical hierarchy procedure, data collection, modeling, experimental design, surface heave, trenchless technology.