Evaluation of MASH 2016 Soil Specifications and Procedures

AASHTO’s Manual for Assessing Safety Hardware (MASH) contains the testing and evaluation criteria used to evaluate various roadside safety features. As written in the preface of MASH’s 2nd edition (MASH 2016), “this document’s purpose is to encourage consistency in crash testing and evaluation.” One of the components to establishing consistency between crash tests is the use of a standard soil, as soil strength can affect the performance of safety barriers embedded within roadside soils. Although a specific soil is not specified, there are requirements for the strength of the soil. Strong soils are typically considered more critical that weaker soils because stiff soil conditions result in higher impact/rail forces, increased propensity for rail tearing and rupture, and increased rail pocketing. Accordingly, MASH 2016 lists a minimum force resistance for soils used in crash testing. The methods and specifications for the standardization of test soil conditions were developed in the mid 2000’s during the formulation of the first edition of MASH. MASH 2016 specifies that a dynamic impact test is conducted on a W6x16 post embedded in the soil, and the post-in-soil must maintain a resistance over 7,500 lb between 5 and 20 inches of deflection. The soil must be verified prior to conducting the crash test. To prevent having to conduct an elaborate dynamic test on test day, MASH describes a methodology in which a soil satisfying the dynamic testing strength is then evaluated in a quasi-static test by pulling on a similar W6x16 post installed in the same soil. The lateral forces recorded at displacements of 5, 10, and 15 inches then become the baseline values to which future static tests (conducted on crash testing days) are compared. Unfortunately, the establishment of only a minimum force specification has resulted in greatly differing soils among crash test labs and between tests. Recently, the members of Task Force 13’s subcommittee 7 conducted an inter-lab comparison on the soils used for crash testing. This comparison showed a wide variety of soil strengths being recording on test days, some of which could be three times stronger than other soils that had passed the minimum requirements. The inconsistent soil strengths were observed in comparisons between crash testing labs and in comparisons of tests conducted within a specific lab. Additionally, the roadside safety community has started to observe some questionable crash test results. The results may be as small as system deflections and working widths differing from the expected values to system failures of what had been thought to be crashworthy systems. It has been suggested that soil strengths greatly exceeding the MASH specifications may be causing these unexpected test results. Subsequently, questions have risen pertaining to the need for an upper bound, or force maximum, for the soil strengths. Additionally, the inter-lab comparison also revealed significant differences in soil installation procedures, soil gradation analysis methods, and soil testing equipment. Many of these differences are likely the result of ambiguous language or the omission of details within MASH 2016. Therefore, there is a need to review and revise the current MASH 2016 soil specifications and evaluation procedures to provide more consistency between crash tests and prevent unnecessary alterations to crashworthy safety devices. The soil specifications and evaluation procedures found in MASH 2016 were developed through a study in the mid-2000s for inclusion in the first edition of MASH. Through a survey of the force versus displacement characteristics of posts-in-soil at various crash testing labs throughout the United States, a minimum dynamic force was established to ensure strong soil conditions. The static baseline testing procedure was recommended as a means to avoid having to conduct dynamic tests immediately prior to the crash test. With the MASH soil evaluation procedures being around for more than 10 years now, most likely there are many more dynamic and static post-in-soil tests conducted per MASH evaluation criteria that can be utilized to better define strong soil specifications. Between 2012 and 2014, Task Force Subcommittee 7 had many discussions pertaining to the MASH soil specifications, testing procedures, and areas for clarification and improvement. Numerous recommendations were made, including using absorbed energy instead of force to evaluate soil strength, use of accelerometers instead of load cells to obtain force values, ambiguity and errors in the MASH 2009 test procedures, etc. These recommendations have yet to be included in MASH. An inter-lab comparison on soil strengths was conducted by the members of Task Force 13’s Subcommittee 7. This comparison of soil types, installation methods, and soil strengths revealed significant variability in the soil conditions between labs and between tests. A significant lack of consistency between soil installations was noted. Additionally, some labs had soil strengths approximately three times higher than other soils that had passed the MASH specifications. Thus, Subcommittee 7 questioned whether MASH only specifying a minimum force had resulted in some labs overcompacting their soil installations and utilizing artificially high soil strengths. Strong-post guardrail systems rely on the posts to rotate through the surrounding soil and absorb impact energy in order for the guardrail system to perform in a safe and reliable manner. Crash tests of guardrail posts installed within in rock, concrete, and asphalt have repeatedly resulted in excessive rail forces, rail ruptures, and test failures. Thus, there have long been recommendations to provide leave-outs around the base of a post installed in rock or pavements. Recent crash testing of guardrail systems that were expected to perform adequately have resulted in similar rail ruptures and failures. These failures have drawn concern that artificially high soil strengths may be causing unexpected failures during the testing of otherwise crashworthy guardrail systems. The objective of this research is to (1) Review the soil specifications and evaluation procedures in MASH 2016; (2) Review the types and strengths of soils as well as the evaluation procedures used at various crash testing labs conducting MASH testing; (3) Evaluate the need to tighten the MASH soil specifications including the inclusion of a maximum soil strength; (4) Identify any deficiencies, limitations, ambiguity, and/or inadvertent effects of the current soil specifications; and (5) Provide recommendations for the revision of the MASH 2016 soil specifications and evaluation procedures. MASH testing guidelines were intended to provide consistent and reproducible tests. However, as observed in the in the recent Inter-lab Comparison between the crash testing laboratories, soil strengths can greatly differ between labs and between tests. This can and likely has resulted in inconsistent crash test results, which include (a) Variations to the system deflection and working width of a barrier system due to drastically different soil strengths; (b) Failed crash tests on an otherwise crashworthy system due to artificially high soil strengths; and (c) Increased costs to roadway agencies and barrier manufactures who are trying to evaluate their barriers in unintentionally inconsistent soil conditions. The completion of this research project would result in the formulation of recommended revisions to the soil specifications and evaluation procedures in MASH 2016. These revisions are intended to (a) Clarify areas the MASH 2016 soil specifications that may be ambiguous or unspecified; (b) Improve and/or expand upon the soil strength requirements to provide more consistency in testing procedures and soil strengths between crash test labs; and (c) Simplify and streamline the soil evaluation procedure to reduce time and costs spent on soil evaluations.

Language

  • English

Project

  • Status: Proposed
  • Funding: $250000
  • Contract Numbers:

    Project 22-51

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    McKenney, Christopher

  • Start Date: 20200520
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01739669
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 22-51
  • Files: TRB, RiP
  • Created Date: May 18 2020 3:05PM