Development of a Comprehensive Analysis Approach for Evaluating Superheavy Load Movement on Flexible Pavements
AuthorNabizadeh Shahri, Hadi
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The movement of superheavy load (SHL) has become more common over years since it is a vital necessity for many important industries. Superheavy load (SHL) hauling units are much larger in size and weight compared to the standard trucks. SHL vehicles may involve gross vehicle weights in excess of a few million pounds often requiring specialized trailers and components with non-standard spacing between tires and axles. Such moves require the determination of whether the pavement is structurally adequate to sustain the SHL movement and involves the analysis of the likelihood of instantaneous or rapid load-induced shear failure.In this study which is part of a Federal Highway Administration (FHWA) project on Analysis Procedures for Evaluating Superheavy Load Movement on Flexible Pavements, a comprehensive mechanistic-based methodology was developed which consisted of the following analysis procedures: (1) segmentation of SHL analysis vehicle, (2) subgrade bearing failure analysis, (3) sloped shoulder failure analysis, (4) buried utility risk analysis, (5) localized shear failure analysis, (6) deflection-based service limit analysis, and (7) cost allocation analysis.The segmentation of SHL analysis vehicle is a procedure to identify a segment (or element) of the SHL configuration that can be regarded as representative of the entire SHL vehicle, this element is referred to as Load Nucleus. The vertical stress distribution (or any other pavement response) under the entire SHL configuration can then be estimated by superimposing the stresses calculated under the Nucleus, hence eliminating the need to model the entire SHL vehicle.Subgrade bearing failure analysis is an ultimate shear failure investigation that reveals the adequacy of pavement structure to withstand the shear failure. To this end, Meyerhof’s general bearing capacity equation is adopted to investigate the possibility of ultimate shear failure.Sloped shoulder failure analysis which falls under the ultimate failure investigation. A method to investigate the stability of a sloped pavement shoulder under a SHL vehicle move was developed by modifying the well-accept Wedge Method for slope stability. This method in conjunction with the use of 3D-Move Analysis software are capable of considering layered medium with distinct layer stiffnesses along with the unconventional SHL vehicle loading configuration. In order to account for existence of sloped pavement shoulder in 3D-Move Analysis software, which assumes pavement layers extending laterally to infinity, computed SHL vehicle-induced stresses are modified using a Stress Adjustment Factor (SAFShoulder). The SAFShoulder was determined based on results from large-scale pavement experiments conducted in this study.In order to conduct buried utility risk analysis, procedures to investigate the risk against the failure of existing buried utilities due to SHL movement on flexible pavements was developed. The available and widely-accepted state-of-practice procedures to examine the structural integrity of flexible and rigid buried utilities subjected to standard traffic live load were adopted in this study. However, significant shortfalls in the existing methodologies, which are the impact of the layered nature of the existing flexible pavement, role of unconventional surface loading from SHL vehicle, and the effect of vehicle speed, were addressed by the use of 3D-Move Analysis software. In order to account for the discontinuity due to existence of buried utility as well as soil-structure interaction, the results of existing 3D-Move Analysis software which assumes continuous pavement layers extending laterally to infinity, needs to be modified. This is accomplished using a Stress Adjustment Factor (SAFUtility). The SAFUtility was determined based on results from large-scale pavement experiments conducted on full-scale pavement structures.In localized shear failure analysis, likelihood of localized failure (yield) in pavement subgrade layer is examined using Drucker-Prager failure criterion. Such analysis is conducted by computing the load-induced stress level on top of subgrade layer. Stress level higher than subgrade failure criterion indicates likelihood of localized failure (yield) and need for mitigation strategies.In addition to the shear failure analyses, deflection-based service limit analysis is conducted since excessive surface deflections resulting from SHL vehicle move may give rise to the rapid deterioration of pavement structure and development of premature surface distresses, (e.g., permanent deformation). To avoid rapid deterioration, the SHL vehicle-induced surface deflection is limited to an allowable surface deflection.Complementary verification and calibration processes of a number of important theoretical-based aspects that were incorporated in the analysis approach were conducted. To this end, a comprehensive experimental program that included five full-scale pavement/soil testing performed at UNR Large-Scale Box facility was designed and carried out. Supplementary numerical modeling as well as measured data from Accelerated Pavement Testing (APT) facilities provided additional justifications to the procedures adopted in this study. The developed analysis procedures were then implemented into a user-friendly software package called SuperPACK (Superheavy Load Pavement Analysis PACKage) to evaluate SHL movements on flexible pavements.
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