Design and analysis of deeply buried polyethylene drainage pipes
The aim of this study is to improve the knowledge base for the design of nonpressure high density polyethylene (HDPE) pipes under high earth loads.
The properties of HDPE and behavior of HDPE pipe in compression are reviewed. Current design and analysis methods are also reviewed and their deficiencies are pointed out. Among the deficiencies considered in this research are the use of tensile strength for compressive strength and the treatment of time-dependent properties. Also, current design procedures do not consider such factors as residual stresses, physical and chemical aging, stress concentrations, and environmental stress cracking. These factors were not considered in the present study either.
Laboratory tests performed on high density polyethylene pipe sections of different sizes and lining are described. Deformation rate tests involved the diametrical compression of pipe sections to high vertical deflections at various deformation rates. The tests showed pipe stiffness increase with increasing deformation rates. Stress-relaxation tests involved compressing pipe sections diametrically to specified vertical deflections and monitoring the load decay with time. The tests showed continuous decrease of the pipe Young's modulus with time.
The performance over three years of an HDPE pipe under high fill is monitored. No wall crushing, structural buckling or excess deflection occurred. The pipe is in good physical condition of nearly round shape with most of the deflection occurring during the construction of the fill. Visual observation showed circumferential cracking of the unlined sections near the couplings, and buckling of the liner below springline level in the lined sections. Neither of these problems are addressed in current design practice.
Finite element analysis of the field installation is described. Time-dependency of the pipe material was incorporated by reducing the pipe modulus for the corresponding period of time under consideration. The predictions showed reasonable agreement with field measurements. The analysis also showed that the pipe can sustain circumferential stresses that are much higher than those proposed by current design procedures.