Horizontal Directional Drilling (HDD) contractors are currently facing heightened environmental scrutiny due to concerns regarding hydraulic fracturing and subsequent migration of drilling fluids to the surface. To address these concerns, a research program was undertaken based on two critical assumptions: 1) hydraulic fracturing is likely to occur if the yield point, plastic viscosity, and gel strength of the fluid are not maintained within flowable limits; and 2) drilling fluid design matched to soil type is critical in prevention of hydraulic fracturing. Studies have also revealed that the two primary factors affecting hydraulic fracturing in soil are borehole pressure and depth of cover. Borehole pressure in an open borehole is determined by the weight and resistance to flow of the drilling returns. While empirical models such as the Bingham and Herschel-Bulkley fluid models have been employed to predict borehole pressures, their use has been limited by the lack of information on fluid return properties, This paper presents the results of a laboratory testing program that was undertaken to evaluate rheologic properties and flow characteristics of fluid returns in an attempt to predict and control hydraulic fracturing during HDD installations. Three collected soil samples were analyzed including: 1) 50/70 Sand Mixture (SP); 2) Lean Clay (CL) from Ohio; and 3) Merchantville Clay Marl (MH) from Mount Laurel, New Jersey.