Post-installation of high density polyethylene pipe submerged in saturated silty soils

The thesis examines how high density polyethylene (HDPE) pipe installed by horizontal directional drilling (HDD) and traditional open trench (OT) construction techniques behave differently in saturated soil conditions typical of river crossings. Design fundamentals for depth of cover are analogous between HDD and OT; however, how the product pipe is situated in the soil medium is vastly different. This distinction in pipe bedding can produce significant differences in the post installation phase. The research was inspired by several incidents involving plastic pipe installed beneath rivers by HDD where the pipeline penetrated the overburden soil and floated to the surface after installation. It was hypothesized that pipes installed by HDD have a larger effective volume due to the presence of low permeability bentonite based drilling fluids in the annular space on completion of the installation. This increased effective volume of the pipe increases the buoyant force of the pipe compared to the same product diameter installed by OT methods, especially in situations where the pipe is installed below the ground water table. To simulate these conditions, a real-scale experiment was constructed to model the behavior of buried pipelines submerged in saturated silty soils. A full factorial design was developed to analyze scenarios with pipe diameters of 50, 75, and 100 mm installed at varying depths in a silty soil simulating an alluvial deposition. Contrary to the experimental hypothesis, pipes installed by OT required a greater depth of cover to prevent pipe floatation than similarly sized pipe installed by HDD. The results suggested that pipes installed by HDD are better suited to survive changing depths of cover. In addition, finite element method (FEM) modeling was conducted to understand soil stress patterns in the soil overburden post-installation. Maximum soil stresses occurring in the soil overburden between post-OT and HDD installation scenarios were compared to understand the pattern of total soil stress incurred by the two construction methods. The results of the analysis showed that OT installation methods triggered a greater total soil stress than HDD installation methods. The annular space in HDD resulted in less soil stress occurring in the soil overburden. Furthermore, the diameter of the HDD annular space influenced the soil stress that occurred in the soil overburden, while the density of drilling fluids did not vastly affect soil stress variations. Thus, the diameter of the annular space could impact soil stress patterns in HDD installations post-construction. With these findings engineers and designers may plan, design, and construct more efficient river-crossing projects.