Trenchless Technology & Subsurface Construction

Chapter 5

Horizontal Directional Drilling (HDD)

Introduction

Directional or horizontal directional drilling (HDD) methods involve steerable tunneling systems for both small and large diameter lines. In most cases, it is a two-stage process. The first stage consists of drilling a small diameter pilot hole along the desired centerline of a proposed line. The second stage consists of enlarging the pilot hole to the desired diameter to accommodate the utility line and pulling the utility line through the enlarged hole. Figure 8.1 illustrates the two-stage process in the horizontal directional drilling method. These methods are so termed because of their unique ability to track the location of the drill bit and steer it during the drilling process. The result is a greater degree of precision in placing the utilities.

Basically all directional methods consist of a drilling unit to form the borehole and a survey system to locate the drill head. The drilling process is accomplished either by mechanical cutting using a drill bit or by fluid cutting with high pressure jets. There are a variety of survey systems which have been patented by different manufacturers. The choice of a particular system will largely depend on the type of job, the site conditions and accessibility, operator skill, finances available, etc.....................for more details