Horizontal Directional Drilling (HDD) is an established trenchless construction method for the installation of underground utilities and pipelines. Subsequently, the method is becoming widely accepted as a cost-effective alternative to traditional open-cut construction. However, the occurrence of hydraulic fracturing, resulting in the migration of drilling fluid to the surface has placed the HDD process under scrutiny, especially when being considered for environmentally sensitive projects. Hydraulic fracturing results from an excess buildup of fluidic pressure within the borehole. Models have been developed to predict borehole pressures; however, there is limited information available on the relationship between drilling returns and fluid composition to these pressures. A research program was undertaken to model and determine flow characteristics for drilling returns under a variety of soil conditions and bore penetration rates. Nine soil samples were gathered based on the Unified Soil Classification System (USCS) and their respective rheological properties were obtained for different drilling fluids and target slurry densities. This paper presents, as an example, a comparison and analysis of the predicted borehole pressures of clayey-sand (SC) soil in a large directional drill rig application and provides recommendations for contractors when attempting installations in various geological formations. The pressure effects of pipe eccentricity within a borehole were analyzed using a computer model. The result of this research is a simplified approach for predicting downhole fluid pressures for a wide range of project parameters that can be used as a guide to minimize the occurrence of hydraulic fracturing.

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