Design Guide Offers New Look at Mitigating Highway Rockfall Hazards
Each year, rockfalls along highways cost States millions of dollars
in claims and litigation. Even worse, several States have reported injuries
and deaths as a direct result of rocks falling onto roads. According
to Oregon Department of Transportation (DOT) Geotechnical Designer Don
Turner, "In Oregon in the last 20 years, anywhere from 5 to 8 people
have been killed and 10 to 20 have been injured [due to highway rockfall].
Even more common is property damage to vehicles when a rock hits a windshield
or rolls in front of a car that swerves out of the way and hits a tree.
Many of the smaller incidents aren't even reported."
To address this hazard, highway agencies design "catchment areas,"
which are ditches along the side of the road that channel falling rocks
away from vehicles. To date, catchment areas have been used inconsistently
throughout the United States because not enough research had been done
to provide engineers with the data they need to make informed design
decisions. A new design guide available from the Oregon DOT is stepping
in to fill this gap.
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| This photo shows a rockfall catchment area. |
Oregon DOT recently completed a pooled-fund study with the Federal
Highway Administration and seven other State DOTs, the results of which
have been published in the new Rockfall Catchment Area Design Guide.
The Design Guide details the study's test methods, fieldwork
performed, means of analysis, and research results, and presents catchment
design charts for practitioners in a user-friendly form.
 |
| The photo above shows a 24-m (80-ft) high test site. |
To gather data for the report, a team of researchers rolled more than
11,000 rocks off 4 different rock cut slopes of 3 different heights.
The rocks were rolled into three different catchments. A large number
of rocks were needed because of the high number of testing variables;
the rocks ranged in size from .3 m (1 ft) to .9 m (3 ft) in diameter
and were rolled 12 m (40 ft), 18 m (60 ft), and 24 m (80 ft) down any
of four slopes (with angles ranging from 45° to almost 90°).
The rocks were rolled into one of three different catchments (with angles
ranging from 0° to about 14°). Researchers then plotted a series
of charts that depict the size of the catchment needed based on the
different combinations of factors. For instance, if an engineer needs
to create a catchment for a 24-m (80-ft) high slope at a nearly 90°
angle, a Design Guide chart will show that a 4.5-m (15-ft) wide
ditch is needed to retain up to 90 percent of falling rocks.
 |
| Rocks are dropped by an excavator from a 12-m (40-ft) high
slope. |
In addition to catchment charts and guidelines, the Design Guide
offers step-by-step guidance on applying the catchment design procedure
using sample problems and highway project case studies. The case studies
also demonstrate other design considerations such as constructibility
of the catchments and cost/benefit comparisons of alternate designs.
The Rockfall Catchment Area Design Guide can help transportation
agencies in several ways. Even with an engineering concept as simple
as a ditch, it takes significant effort to determine how wide it should
be to ensure maximum safety from falling rocks. The Guide takes much
of the guesswork out of designing a catchment, thus saving States and
localities time and money. Another advantage of using the Design
Guide is that it can help determine the potential effectiveness
of existing catchments without an agency having to conduct tests on
each one individually. And ultimately, the advice given in the Guide
can help make traveling through areas with rock slopes far safer for
drivers.
An electronic copy of the Rockfall Catchment Area Design Guide is available
on the Oregon DOT Web site at www.odot.state.or.us/tddresearch/reports.htm.
For more information on the Design Guide, contact Liz Hunt, Technology
Transfer Coordinator at Oregon DOT, 503-986-2854 (fax: 503-986-2844;
email: elizabeth.a.hunt@odot.state.or.us).
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Articles in this issue:
Prefabricated Bridge Elements and Systems: A Winning
Idea
Boosting Roadway Safety with Rumble Strips
Preserving the Future of Pennsylvania Bridges
Work Zone Research from A to Z
Design Guide Offers New Look at Mitigating Highway Rockfall Hazards
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