Over the past few years, the Horizontal Directional Drilling (HDD) market has become more competitive as it has matured and, in some regions of North America, become the most cost-effective option for installation of underground utilities and pipelines. In order to remain competitive in today's marketplace, HDD contractors must maximize production rates. Increased penetration rates can lead to frac-out condition that may require cleanup and result in delays that consume profit. A better understanding of the causes of hydraulic fracturing can lead to a reduction in delays caused by frac-outs and, in many cases, an increase in penetration rates. Studies have revealed the two primary factors effecting hydraulic fracturing in soil to be borehole pressure and depth of cover. The borehole pressure in an open borehole is determined by the weight and resistance to flow of the drilling returns. While models have been developed to predict borehole pressure, their use has been limited by the lack of information on drilling return properties. A laboratory research program was undertaken to determine flow characteristics of drilling returns. Samples representing five soil types were mixed with a bentonite-based drilling fluid with flow characteristics of the resulting slurries measured. This information was used to predict borehole pressure at different penetration rates. The results of this testing can be used to optimize penetration rates, while minimizing hydraulic fracturing. This paper describes the testing procedures used in the research and presents some of the results.