| BETTER BRIDGES
30 Years of Research Pays Off
Three bridges for the price of two,
without compromise. A new generation of bridges and box culverts last
longer and are quicker to build.
by Bob Barrett, Al Ruckman, and Mike Adams
"There is nothing new under the sun,” goes the old
proverb. This is certainly true with reinforced soil. Researchers in the
1970s at the U.S. Forest Service and Oregon State University resurrected a
technology used many thousands of years ago. Mesopotamians constructed
towers (ziggurats) hundreds of feet high using only alternating layers of
sand and palm fronds. The Chinese added organic tensile inclusions in
parts of their Great Wall. What our pioneering researchers did contribute,
however, was adapting polymeric materials as tensile reinforcement. These
materials have stable engineering properties and can last for thousands of
years in the benign environments of retaining walls and bridge abutments.
The USFS team used non-woven polypropylene fabrics to
stabilize roadways over soft ground and to build reinforced soil retaining
walls. They experimented with facing techniques and backfills, and used
sawdust as backfill in some cases. This early work had a huge payoff. The
use of geosynthetics to assist in crossing soft ground has become
universal. So has the use of geosynthetics in retaining wall construction.
The next generation
It takes time to assimilate new technologies into our
paradigms. It takes a while to learn all the applications, limitations,
and nuances. So it is with reinforced soil. We have found that this
simple, powerful technology can be adapted to many areas of transportation
activity — retaining walls, improved foundations, rockfall barriers,
bridge abutments, bridge piers, and more. Compared with traditional
methods and materials, adaptations of reinforced soil technologies deliver
superior longevity, superior earthquake tolerance, are easier and quicker
to construct, and, serendipitously, cost less.
Researchers at the Colorado Department of
Transportation, the Colorado Transportation Institute, the University of
Colorado/Denver, and the Federal Highway Administration expanded on the
pioneering work of the USFS to quantify engineering properties of
geosynthetically reinforced soil and to develop design protocols for the
various applications. This 30 years of research and demonstration is now
paying off with field constructions that are living up to early promise.
Critical research findings
The research team focused on defining behavioral and
performance properties of the magical composite that results from adding
tensile inclusions to soil. Early work included a comparison of clay soil
with granular soil as backfill. Full-scale tests to failure showed that
clay soil was stronger — but clay was ruled out as a backfill. Clay
performs well if it is kept at or below optimum moisture. Above optimum,
it will creep under loading and literally drag the polymeric
reinforcements with it. Clay presents greater challenges in construction,
resulting in uncertainties in the final product.
More importantly, this work revealed that geosynthetics
will not creep in granular soils at the loading levels measured in
laboratory and full-scale constructions. The load transfer to the
inclusion is over-predicted by current analytical models by factors of 20
and more. With close spacing (8 to 12 inches) in compacted, granular soil,
geosynthetic creep will not happen.
It was discovered that facing pressure, the horizontal
component of vertical loading, in these composites is limited to the bin
or wedge between reinforcements. The reinforcement is fully capable of
sustaining all the vertical load. Resistance to horizontal loading that
must be contributed by the facing element is minimal. For example, with
8-inch spacing, the small triangle at the face requires about 11 pounds
per square foot of resistance to contain. You could do this with one hand.
It was also determined that the load the facing element must sustain is
independent of height. It is the same at 10 or 100 feet. With this
important finding, connection strength requirements are much reduced,
allowing the use of more economical materials such as wood timbers and
ordinary concrete blocks.
Geosynthetically reinforced soil composites are
typically heavier at the back than the front. Using eccentric loading is
an artifact of design criteria for externally supported wall systems. They
cannot overturn. Bearing capacity analyses are based on supporting just
the simple gamma h or vertical weight of the structure. Still, bearing
capacity does not control the design. The critical analysis is external
failure (global, circular). Therefore, with spacing already determined to
be less than 12 inches, the backfill already determined to be granular,
the length of reinforcement is determined by simple circular failure
analyses. Having met this test, bearing, overturning, and sliding are
typically moot.
This leads to the option of truncated bases. Where
global stability permits, the lower layers of reinforcement can be as
short as a couple of feet. The authors designed a wall that was built by
Grand County, Colorado forces that is 55-feet high and has a 12-foot-wide
base. This wall was awarded the 1998 International Design Award by the
Industrial Fabrics International Association. CDOT maintenance forces
built a 45-foot-high wall at Silt, Colorado that is 8-feet wide at the
base. CDOT and Yenter Companies also built a 26-foot-high wall near Grand
Junction that is 3-feet wide at the base. These walls use ordinary
concrete blocks as forming/facing elements. Yenter’s Al Ruckman also
designed and constructed a 93-foot-high wall that is 12-feet wide and with
woven wire as the tensile inclusion.
Negative batter is also permissible up to about 60
degrees. Talk about counterintuitive! Cantilevering dirt!
Perhaps the most exciting new development is
demonstrated resistance to seismic forces. It is becoming accepted that
the geosynthetically reinforced soil composite will be the last structure
standing in an earthquake. The blocks will not go flying off in space.
Montana bridge
The Montana Bridge gives us a window to the future of
geosynthetically reinforced soil technologies. This bridge is a 191-foot
clear span that uses four 8-foot-deep I steel girders. It is founded on a
landslide in a high seismic zone. Micropiles and soil nails, extensions of
reinforced soil technology, were used to increase local stability and the
bearing sill rests on a geosynthetically reinforced soil improved
foundation. The back wall is the first application of a wrap-around
geosynthetically reinforced soil structure that will contain the
superstructure laterally in a seismic event. This appears to be the most
effective type of abutment for seismic resistance at any cost, yet the
cost is less that traditional abutment constructions. Yenter Companies
provided value engineering on this project, changing from traditional
piling and tiebacks to reinforced soil and saved the owners about
$500,000.
Other bridges
Just as concrete blocks can be dry stacked as
forming/facing elements on geosynthetically reinforced soil structures,
native stone can also be used in this application. The Ski Tip Bridge at
Colorado’s Keystone ski area illustrates this adaptation of facing
materials.
Yenter offered a money-saving value engineering
alternative to a traditional box culvert for a Breckenridge box culvert.
Using geosynthetically reinforced soil walls for the sides and precast,
prestressed concrete panels for the roof, cost was much reduced over the
precast concrete box option. The owners used the savings to add native
stone facing to the entrances of this ski-under tunnel. Negative batter
was used in the side walls to shorten the length of the lids.
Perhaps the most dramatic and conclusive demonstration
of geosynthetically reinforced soil in bridge foundation design is at the
Colony Development in Park City, Utah. This ambitious private project
offers ski-in, ski-out to every lot, which requires many ski-under roadway
bridges. The first of these bridges was designed by structural engineers
using traditional, mainstream methods and materials. Geotechnical
engineers offered geosynthetically reinforced soil abutments and the
resulting savings was about 33%. This is consistent with the projects
where value engineering has been applied.
What’s next?
Geosynthetically reinforced soil has a bright future in
the transportation industry. This evolving science is just now making an
impact on time and costs for building walls, boxes, and bridges. Not only
are we seeing savings, we are seeing better, stronger, and longer lasting
constructions. What sorts of constructions? Three bridges for the price of
two. Bridges that can be erected in 24 hours, avoiding detours and
excessive delays. Bridges without bumps. Bridges that can survive major
earthquakes. This translates to billions of dollars annually that can be
spent for infrastructure expansion and improvement.
The National Cooperative Highway Research Program
recognizes the potential for geosynthetically reinforced soil bridge
abutments and recently selected Dr. J.T.H. Wu of Reinforced Soil Research
Center of the University of Colorado/Denver to develop design and
construction guidelines for geosynthetically reinforced soil abutments.
This document will be the cornerstone for full implementation of
geosynthetically reinforced soil technologies in supporting bridge
superstructures.
Another transition is afoot. Reinforced soil is a
geotechnical discipline and will require stronger roles from these folks
in wall, bridge, and box design. The secret to efficiency and savings is
all in the foundation design and requires a willingness to adapt
geosynthetically reinforced soil and other reinforced soil technologies to
fit specific requirements. We will see this handing over of project
responsibility on an increasing basis, and this will eventually be
reflected at our universities.
Bob Barrett is manager of bridge design and
construction and Al Ruckman is president, both with Yenter Companies.
Mike Adams is senior researcher, Turner Fairbank Highway Research
Center, Federal Highway Administration.
Reprinted from Better Roads Magazine
February 2002 |
