Numerical Finite-Element Investigation of the Parameters Influencing the Behavior of Flexible Pipes for Culverts and Storm Sewers under Truck Load Omar Chaallal, M.ASCE 1 ; Madasamy Arockiasamy, M.ASCE 2 ; and Ahmed Godat, M.ASCE 3 Abstract: The objectives of this study are to evaluate the parameters that affect the behavior of flexible pipes for culverts and storm sewers and to examine the strength-limit states of such pipes proposed by the AASHTO bridge-design specifications. The flexible pipes investigated in the study are high-density polyethylene (HDPE), PVC, and metal pipes. The pipe diameters considered are 914 mm (36 in.) and 1,219 mm (48 in.). A three-dimensional finite-element (FE) model has been developed to simulate the behavior of buried pipes under truck load. Results from field tests conducted by the authors for large-diameter pipes under 2D, 1D, and 0.5D burial depths, where D is the pipe diameter, were used to evaluate the accuracy of the numerical model. Extensive parametric studies were then carried out to examine the effect of burial depth, backfill soil quality, and compaction level on the buried pipes, and their results are reported in this paper. The effect of these parameters has been evaluated in terms of thrust values at the pipe shoulder and springline, vertical diameter changes, and longitudinal strains. Regression analysis was carried out based on the FE model predictions for the first load pass and on results from a research study available in literature to predict the behavior of buried pipes with multiple load passes. The parametric study has shown that the performance of buried pipes varies with the soil type, and the increase of compaction level and soil cover reduces the stresses on buried pipes. The numerical predictions indicate that the provisions of AASHTO bridge-design specifications for HDPE pipes need to be reassessed for very shallow burial depths. DOI: 10 .1061/(ASCE)PS.1949-1204.0000186. © 2014 American Society of Civil Engineers. Introduction Thermoplastic pipes are being used increasingly in buried-pipe applications such as water supply lines, storm-water drains, and sanitary sewers. A massive renewal of infrastructure is necessary in developed countries, as well as extensions to developing areas. New applications for thermoplastic pipes are also being found, such as for leachate collection in solid-waste landfills and as liners for rigid pipe rehabilitation. Factors such as light weight, ease of fab- rication, resistance to chemical corrosion, toughness, and low cost have been the main motivations for increasing use of these materi- als in buried pipelines. Although these polymer materials do have a number of desirable characteristics, current design guidelines for buried thermoplastic pipes are still largely based on conservative- limit states. Therefore, additional work is required to improve the understanding of the behavior of buried thermoplastic pipes. Pipe vertical deflections are dependent to a large extent on the pipe diameter and the contact area of the load application (Klaiber et al. 1996; Lohnes et al. 1997; Jayawickrama et al. 2002; Sargand et al. 1998; Reddy 1999). The pipe-wall thickness and its surround- ing soil constitute an interactive system, in which the stiffness of the surroundings depends on the compaction level and is affected by the constraints afforded by the trench walls and the stiffness of the pipe itself. AASHTO M145 (AASHTO 2000a) recommends soil groups A-1 and A-3 as backfill for flexible pipes under shallow burial depths. In addition, it proposes a minimum compaction level of 95% for backfill material [AASHTO T99-97 (AASHTO 2000b)]. However, these soil types and degrees of compaction might not be affordable at some sites. Conard et al. (1998) reported that most pipes subjected to concentrated live loads and under 609.6 mm (2 ft) of soil cover failed at a vertical diameter strain between 1.9 and 2.9%. Phares et al. (1998) and Reddy (1999) ob- tained a longitudinal failure as opposed to a circumferential failure in their high-density polyethylene (HDPE) pipes. Therefore, it remains difficult for structural engineers to decide on an appro- priate design procedure when designing buried pipes with soil types and compaction levels other than those recommended by the AASHTO. Finite-element (FE) analysis, which is a more economical ap- proach than elaborate field tests, can be used to investigate the behavior of buried pipes. However, field-test results are required to validate the numerical predictions. FE analysis is clearly an extremely powerful tool for simulating complex structural behav- ior. In addition, it can provide a better understanding of various failure mechanisms and capture the numerous parameters that in- fluence the behavior of buried pipes. Literature review showed that elastoplastic FE analysis using a multilinear constitutive approxi- mation has been used to predict large-strain deformations and to investigate crazing in notched thermoplastic components (Trantina and Nimmer 1995). However, this approach would have difficulty in accurately predicting stresses for structures with odd geometries, such as holes and notches, where the strain rate is much higher than elsewhere. Linear viscoelastic FE analysis was used to predict time- dependent behavior and structural performance of HDPE pipes (Moore and Zhang 1995; Moore and Hu 1995). It was found that 1 Professor, Dept. of Construction Engineering, Univ. de Québec, École de Technologie Supérieure, Montreal, QC, Canada H3C 1K3. E-mail: Omar.Chaallal@etsmtl.ca 2 Professor and Director, Center for Infrastructure and Constructed Facilities, Dept. of Civil Engineering, Florida Atlantic Univ., Boca Raton, FL 33431. E-mail: arockias@fau.edu 3 Research Associate, Dept. of Construction Engineering, Univ. de Québec, École de Technologie Supérieure, Montreal, QC, Canada H3C 1K3 (corresponding author). E-mail: Ahmed.Godat@etsmtl.ca Note. This manuscript was submitted on August 13, 2013; approved on August 11, 2014; published online on September 30, 2014. Discussion period open until February 28, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Pipeline Systems En- gineering and Practice, © ASCE, ISSN 1949-1190/04014015(13)/$25.00. © ASCE 04014015-1 J. Pipeline Syst. Eng. Pract.