Designing and Interpreting Geotechnical Investigations for Horizontal Directional Drilling

Abstract

Horizontal Directional Drilling (HDD) is a trenchless construction method which has gained rapid acceptance in the pipeline engineering and construction industry. Once considered a small, specialized segment of the industry, HDD is increasingly being used for a broad spectrum of pipeline projects, including both pressure and gravity installations, and under difficult and diverse surface and subsurface conditions. Like many other trenchless construction methods, the feasibility and operating parameters for HDD are determined in large part by subsurface conditions along the pipeline alignment. While the pipeline industry has generally accepted the need for thorough geotechnical investigations on HDD projects, the quality of geotechnical reports is quite variable as is the use of geotechnical reports by both design engineers and contractors. Commonly, geotechnical reports do not provide the basic data necessary to make critical design and construction decisions. Similarly, designers and contractors often do not know how to interpret the findings of a geotechnical investigation and use them effectively for their HDD projects. This paper presents general guidelines for geotechnical investigations for HDD projects in soil environments, including a discussion of the subsurface information that is most useful for design engineers and contracto rs such as soil grain size distribution, soil plasticity, soil density/consistency, soil strength, vertical and lateral variability of soil deposits, presence of natural or man-made obstacles, etc. This paper also explains how the findings of a geotechnical report can be interpreted and put to practical use for a HDD project, including an examination of how subsurface conditions effect bore path desi gn, risk of hydrofracture, drilling fluids design, drilling rates, tooling selection, etc.