Date of Award

Winter 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials and Infrastructure Systems

First Advisor

David Hall

Abstract

Thin-walled polymeric liners are routinely used to rehabilitate deteriorated underground pipe lines that are structurally sound but have lost their hydraulic integrity. These host pipes are said to be "partially deteriorated" since they can support soil and surface loading. For this case, the pipe liners must only be designed to withstand the pressure of the groundwater that seeps through openings in the host pipe and migrates through the small annular gap between the liner and the host pipe. This external pressure will cause the polymeric liner to slowly deform inward toward the center of the pipe, and the compressive hoop stresses in the pipe wall may cause sudden collapse or buckling when the liner deflections or stresses reach critical values. A significant body of research exists that details the structural response and design of these pipe liners assuming constant groundwater pressure. This dissertation focuses on the influence of varying groundwater pressure on the response and design of pipe liners. The work involves material characterization of two PVC liner materials, finite element simulation of liner response under varying groundwater loading, and development of a correction factor that effectively accounts for water level variations on 50-year liner design.

Material characterization testing of 16 specimens cut from higher compliance and higher stiffness PVC liner materials was conducted continuously over one year. The specimens were subjected to three-point bending to understand the accumulation of creep deformation and the recovery of this deformation due to partial unloading of the specimens. Two loading and two partial unloading cycles were applied to the specimens to model seasonal groundwater variations. A five-unit generalized Kelvin Model was employed to fit the creep and recovery data resulting from the material characterization data. The 12 retardation constants resulting from this initial fitting were converted to relaxation constants suitable for use with the viscoelastic material models available in the ABAQUS© finite element software.

A two-dimensional finite element model was constructed for a pipe liner having 5% ovality, 0.4% annular gap and a diameter to thickness ratio of 32.5. A trapezoidal groundwater loading pattern was adopted to simulate seasonal groundwater variation. The results indicate that liners designed for a 50-year life can withstand higher peak pressures when groundwater levels vary seasonally due to recovery of deformation during the partial unloading period. The results also show that pipe liners have a longer life when they are subjected to more frequent recovery periods. A correction factor was derived and directly applied to the ASTM F1216 design equation for the partially deteriorated case. The correction factor allows a designer to quantitatively estimate the influence of variable groundwater loading on liner design; example calculations show significant reductions in the required liner thickness when large drops in water level occur during the dry season or when the dry season is longer than the wet season.

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