Reducing Risks in Unfavorable Ground Conditions during Horizontal Directional Drilling

The presence of gravel and cobble during horizontal directional drilling (HDD) operations is one of the most challenging situations encountered by contractors. An unexpected encounter with this soil condition often results in difficulty or even failure in completing the bore. This paper presents the results of field research in which three 61 m (200 ft) installations (bores) of 100 mm (4 in.) high-density polyethylene pipe were performed in gravel/cobble conditions. Each of the installations was completed using various combinations of drilling fluids to demonstrate the effectiveness of using fibers to increase the chances of success. The first installation was completed using a traditional bentonite/polymer drilling fluid mixture, while the second and third installations were performed with fiber additives mixed into the drilling fluid to provide mechanical suspension of the cobble/gravel formation. The results demonstrated that increased productivity and flow of returns could be achieved with proper drilling fluid considerations. Overall, the third installation experienced a 25% increase in productivity from the second installation and a 67% increase from the first installation through the use of two fiber additives in combination. The results of this research should assist HDD contractors in improving the success rate of installations in gravel/cobble conditions, resulting in significant cost savings in fluid usage, mixing time, and problems associated with inadvertent returns.