Mechanically Stabilized Earth (MSE) Retaining Walls (405160-15)

Crash Wall Design for Mechanically Stabilized Earth Retaining Wall (MSE Wall) (405160-15): Phase I: Engineering Analysis and Simulation

Problem Statement

Mechanically Stabilized Earth Retaining Walls (MSE) are used to make changes in elevation within restricted lateral distance.

Vehicular traffic may exist on either the high (fill) side of the MSE wall on the low side, or both sides. For traffic on the high side, a conventional traffic barrier might be placed on or near the top of the wall and mounted on a moment slab. For traffic on the low side, a conventional traffic barrier might be installed adjacent to the wall or the wall itself may serve as the traffic barrier. However, typical MSE wall panels are not designed to resist vehicle impacts without significant structural damage to the panels and the earth fill, and damage to those would require expensive repairs.

A proposed design for placing a crash wall on the face of MSE wall panels is shown in figure 1. It is intended to provide improved crashworthiness and reduce structural damage to the MSE wall system.

Figure 1: Typical Crash Wall Section from PennDOT drawing sheet BC799m_03.

Background

Mechanically Stabilized Earth (MSE) retaining walls typically consist of special backfill soil reinforced with either steel strips, steel bar mat, or polymeric materials. The reinforcement is attached to the retaining wall (panels) to provide stability of the MSE structure. On top of the retaining wall and the backfill soil, a barrier–moment slab subsystem is installed. A crash wall constructed of reinforced concrete can be cast against the MSE wall panels with steel anchors embedded between the crash wall and the MSE panels as shown in figure 1. If properly designed, such a treatment can reduce the need for costly repair arising from vehicle impacts with the MSE wall panels.

Objective

The overall objective of this study is to design and test a crash wall to protect an MSE structure from vehicular impact.

In this phase of the project, the research team will review, model, and analyze the proposed crash wall design to determine its expected performance under TL3 and TL4 impact conditions of NCHRP Report 350.

Benefits

To date, there is no known crash tested design of a crash wall that protects an MSE structure. This project would develop a practical design that meets the new crash test criteria. It would save cost of repair of MSE wall including traffic detour costs.

Implementation

TTI will provide all the supporting information and written discussion for submitting a request to FHWA for acceptance of the crash wall for use on the National Highway System.

Work Plan

Work on this project will be performed in two phases. Phase I will include the literature and engineering review, and computer simulation of an MSE crash wall to evaluate it’s ability to withstand a TL-3 or TL-4 design impact as detailed in tasks 1 and 2 below.

Upon the conclusion of Phase I, the pool fund member states will outline the work plan of Phase II of the project. Possible follow-up work may include full scale crash testing using 8000S test vehicle per NCHRP Report 350 TL-4 and/or engineering design, numerical simulation, and full scale crash testing of a crash wall for TL-5 impacts with an 80 kip truck. This proposal addresses the Phase I effort. Phase II will be funded in the next fiscal year.

Task 1 — Perform Engineering Analysis/Design/Drawings

The researchers will review the design proposed by Pennsylvania Department of Transportation (PennDOT), shown in figure 1, and any other information from the supporting states. TTI researchers will work closely with the supporting states to select the appropriate design parameters to be used to design the crash wall. As part of this task, TTI researchers will perform engineering calculations on the crash wall attached to the MSE wall. The crash wall will be designed with respect to AASHTO LRFD Test Level 4 requirements. TTI will develop suitable drawings and details that can be used for construction of the proposed crash wall. These calculations along with detailed drawings will be provided in the final report for this project. Once the crash wall is designed, the details of the crash wall and MSE system will be used for subsequent finite element modeling and simulation.

Task 2 — Perform Computer Simulations

The LS-DYNA computer program will be used to evaluate the performance of the proposed crash wall design resulting from task 1.
The simulation task will consist of the following subtasks:

1. Build a typical model of an MSE structure with the following entities:
   1. Soil (backfill and overburden)
   2. Soil reinforcement (steel strips)
   3. Wall panels and support pad including their reinforcement
   4. Reinforced crash wall.
2. Incorporate the C2500 truck model and Single Unit Truck model to simulate NCHRP Report 350 test levels 3 and 4 respectively.
3. Perform impact simulations of the proposed design.

Figure 2: A single-unit truck SUT test vehicle next to a typical MSE 10-ft tall wall.

Task 3 — Write a Research Report

The research team will provide a report detailing the engineering analysis, design, modeling and simulation work performed under Tasks 1 and 2. The report will include a plan for designing a crash wall for either TL-4 or TL-5 impacts with 80 kip trucks and for performing crash tests of the wall.

Final Report for MSE Crash Wall – 405160-15

Executive Summary