Modelling of surface heave contributing factors in directional boring operations

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, back ream rate, soil composition, annular space, 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 the development of a model to understand how the aforementioned factors contribute to surface heave during the back ream phase of a horizontal directionally drilled installation. Initial identification and selection of factors to be utilized in the model were facilitated using a panel of experts and the Analytical Hierarchy Procedure (AHP). Using the ranking identified by the AHP, a full factorial experiment was designed and implemented under actual field conditions with ground movements measured at various stages along four uniquely designed borepaths. Results from this experiment are utilized in the development of a model to further understand how these factors relate to each other in the propagation of surface displacements. 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.