Direct Pipe in Three Case Histories- Overcoming Challenging Ground and Site Conditions

Abstract

Direct Pipe® (DP) is a relatively new trenchless technology by which a steel pipe is installed in compression through curves and sometimes tangent sections. The ground in the heading of the pipe is excavated through conventional microtunneling methods while a Pipe Thruster machine provides the motive force. Once the crossing is completed and the microtunnel boring machine is recovered, the installed pipe may be subsequently used as a casing for another pipe or as the product or carrier pipe. From a practical standpoint, DP is most often compared to horizontal directional drilling (HDD) because the method installs steel pipe in curved geometry and utilizes long sections or strings of prefabricated pipe. This paper highlights three applications of the method from the authors’ experience with challenging geotechnical conditions and tight workspace geometry where HDD was considered but deemed infeasible. Specifically, examples of successful projects are shared for the following applications: (1) short and shallow crossing (relative to HDD); (2) coarse-grained ground crossing that would be high risk with HDD due to hole collapse and difficulty of cuttings return; and (3) risk avoidance for sensitive surface features with significant potential impact from hydraulic fracture and inadvertent returns from HDD. The referenced project examples are used to introduce and discuss engineering aspects of DP crossings such as geologic risk, methods for the assessment of steel stress, buckling risk, and hydraulic fracture risk.

Techniques