Dynamic response of guardrail systems encased in pavement mow strips

Abstract

Strong post guardrail systems have long been employed to keep misguided vehicles on the roadway. In order to combat vegetation growth around the posts, many new guardrail installations are being encased in pavement mow strips. By increasing material stiffness of the confining layers around the posts, this practice changes the dynamic response of the guardrail system to the impact of a vehicle. This research aims to analyze existing mow strip configurations by using experimental testing and numerical simulation and to develop predictive, full-scale simulations that enable assessment of the performance of guardrail systems encased in mow strips. To develop a test matrix of mow strip designs in preparation for possible testing, a state of practice survey of Texas Department of Transportation districts is performed. Current mow strip construction methods and designs are gathered. Mow strip dimensions, materials, and depths are considered in addition to the presence of asphalt and grout "leave-out" layers around posts. Seventeen configurations using wood and steel posts embedded in asphalt, concrete, soil, and grout are subjected to dynamic impact testing with a bogie vehicle. Dynamic impact tests from the test matrix are numerically simulated using the nonlinear, dynamic, explicit code LS-DYNA. Modeling of soil, steel, concrete, and grout components is investigated. Results from numerical simulation are validated using experimental test results. Four full-scale mow strip system simulations are assembled using subcomponent models. Impact of a vehicle with a G4(1S) guardrail system is simulated. Results are compared to a full-scale crash test for validation. Enhancements to the simulation are made to increase numerical stability of the vehicle model and improve interaction between the vehicle and guardrail system. In addition, three mow strip guardrail systems are modeled using the G4(1S) system as a baseline case. Concrete mow strip and grout leave-out encasements are also examined. Through numerical simulation of a full-scale crash test it is shown that a concrete mow strip with 457x457 mm (18x18 in.) grout leave-outs performs at a similar level in a full-scale crash test to an equivalent system with posts embedded only in soil. This system is recommended for a full-scale crash test.