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
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
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 objective of the research is to evaluate the MASH 2016 soil specifications and soild evaluation procedures as well as provide recommendations for revised soil procedures for the revision of MASH 2016.
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
· Variations to the system deflection and working width of
a barrier system due to drastically different soil strengths.
· Failed crash tests on an otherwise crashworthy system due
to artificially high soil strengths.
· 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:
· Clarify areas the MASH 2016 soil specifications that may
be ambiguous or unspecified.
· Improve and/or expand upon the soil strength requirements
to provide more consistency in testing procedures and soil strengths between
crash test labs.
· Simplify and streamline the soil evaluation procedure to
reduce time and costs spent on soil evaluations.
specifications and evaluation procedures found in MASH 2016 were developed
through a study in the mid 2000’s for inclusion in the first edition of MASH. Through
a survey of the force vs. displacement characteristics of posts-in-soil at
various crash testing labs throughout the US, 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 over 10 years now, there is likely many more dynamic and static
post-in-soil tests conducted per MASH evaluation criteria which 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.
comparison on soil strengths was conducted by the members of Task Force 13
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 over-compacting their
soil installations and utilizing artificially high soil strengths.
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 has 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.
Potential tasks include:
· Review the soil specifications and evaluation
procedures in MASH 2016.
· Review the types and strengths of soils as
well as the evaluation procedures used at various crash testing labs conducting
· Evaluate the need to tighten the MASH soil
specifications including the inclusion of a maximum soil strength.
·Identify any deficiencies, limitations, ambiguity,
and/or inadvertent effects of the current soil specifications.
·Provide recommendations for the revision of
the MASH 2016 soil specifications and evaluation procedures.
The results from
this research should be widely disseminated and presented throughout the
country. It is envisioned that AASHTO and the Technical Committee on Roadside
Safety (TCRS) would implement the recommendations from this research into the
next edition of MASH. Since MASH is the standard for the evaluation of roadside
safety devices, the research described herein would affect every state DOT,
especially the roadside designers, policy and standards engineers, and safety
engineers. Thus, relevant and interested organizations would include each state
DOT, AASHTO, TCRS, and FHWA.
Since MASH is the standard for the evaluation of roadside safety devices, the research described herein would affect every state DOT, especially the roadside designers, policy and standards engineers, and safety engineers. Thus, relevant and interested organizations would include each state DOT, AASHTO, TCRS, and FHWA.
|Sponsoring Committee:||AKD20, Roadside Safety Design
|Research Period:||24 - 36 months|
|RNS Developer:||Scott Rosenbaugh, Nathan Shulz, James Kovar, and Erik Emerson|
|Source Info:||TRB AFB20 Mid-Year Meeting 2019, Reno NV|
|Index Terms:||Soils, Strength of materials, Specifications, Crash tests, Barriers (Roads), |
Safety and Human Factors