TIME DEPENDENT RESPONSE OF PULLED-IN-PLACE HDPE
Abdul Ghafar Chehab
A thesis submitted to the Department of Civil Engineering
in conformity with the requirements for
the degree of Doctor of Philosophy
Kingston, Ontario, Canada
Horizontal directional drilling (HDD) is a trenchless technology used with increasing frequency to install polymer pipes without the costs and disruption associated with conventional ‘cut and cover’ installations. This technique, which was developed by industrial innovators, features complex soil and pipe responses which are not well understood. The success of this operation depends on knowledge of the pulling forces applied, the level of ground disturbance, expansion or ground fracture from mud pressure, and the effect of the pulling operations and axial tensions on the pipes. Tensile stresses in the pipe vary with time during and after installation, and along the pipe. This applies especially to polymer structures where the axial stresses during insertion and those that occur over the service life of the new pipe may influence the performance of the installed pipe.
The main objective of this study is to model the short term and long term response of pipes installed using horizontal directional drilling and to investigate the effect of the time dependent mechanical behaviour of polymer pipes, as well as other installation variables on the performance of the pipe during and after installation.
The mechanical behaviour of high density polyethylene, which is used to manufacture a significant portion of pipes installed using horizontal directional drilling is investigated, and two sophisticated constitutive models are developed to simulate the time-dependent mechanical behaviour of high density polyethylene. It is shown that one of the models is suitable for simulating HDPE response under nominal loading conditions while the other model is more suitable for high stress and strain conditions involving significant unloading or strain reversal.
The interaction between the pipe and the surrounding soil during horizontal directional drilling installations is also investigated and modelled. A FORTRAN algorithm is developed to calculate the short and long term response of elastic and polymeric pipes installed using horizontal directional drilling. The program implements the HDPE constitutive models as well as the pipesoil interaction model developed earlier in the study. The program is then used to conduct a parametric study of the sensitivity of short term and long term pipe response to different installation parameters. The effect of overstressing the pipe during installation is also addressed. The results of the study show that the performance of a pipe during and after a horizontal directional drilling installation depends on several factors including the installation length and geometry, the pipe, soil and drilling mud properties, and the construction quality. The stresses and strains in polymer pipes installed using horizontal directional drilling continue to change after installation throughout the service life of the pipe. A good constitutive model that can simulate the material’s long term behaviour is important for the prediction of these long term stresses and strains, and can help achieve better designs for horizontal directional drilling operations.
The uniaxial constitutive model developed in the study is generalized to form a multi-axial model that can simulate the response to biaxial stress and strain fields (plane stress or plane strain cases). The multi-axial model is implemented in a finite element code (AFENA) and its performance simulating multiaxial stress-strain fields is evaluated. It is shown that the generalized model performs well for multiaxial stress and strain fields and can therefore be utilized for analyzing applications involving multiaxial loading conditions such as the swagelining trenchless method.