Targeted HDD Design under Critical Structures to Reduce the Potential for Hydraulic Fracture

The design of horizontal directional drilled (HDD) pipeline installations often requires an evaluation of the potential for hydraulic fracture of the soil layers through which a drilled crossing passes. This potential can be an important design factor if the crossing passes beneath a critical structure such as a regulated levee. If hydraulic fracture of the underlying soils causes the drilling fluid to migrate to the ground surface, the resulting inadvertent return may create preferential pathways for water seepage or cause damage to foundation soils. As a result, some regulating agencies require a very detailed evaluation of the hydraulic fracture and inadvertent return potential for HDD crossings. In developing a hydraulic fracture model for an HDD crossing, one of the important contributing factors to the factor of safety against hydraulic fracture and inadvertent returns is the shear strength of the soils. If an HDD crossing targets a zone of higher shear strength soils, the potential for hydraulic fracture will often be reduced. This study compares the results of the hydraulic fracture models for two HDD crossings. The subsurface soil conditions of the first crossing consist of soft to medium stiff clay for the full depth of the HDD crossing. The subsurface conditions of the second crossing consist of medium stiff clay overlying a significant layer of medium dense to very dense sand. By targeting the dense sand, the results of the hydraulic fracture and inadvertent returns models indicate significantly higher factors of safety.