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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
Integral-Abutment Bridges: Geotechnical Problems and
Solutions Using Geosynthetics and Ground Improvement
John S. Horvath, Ph.D., P.E.
Professor; Manhattan College; Civil and Environmental Engineering Department;
Bronx, NY 10471-4098; john.horvath@manhattan.edu
ABSTRACT
The integral-abutment bridge (IAB) concept was developed at least as far back as
the 1930s to solve long-term structural problems that can occur with conventional
bridge designs. Unfortunately, the IAB concept as executed historically turns out to
have its own inherent post-construction flaws. However they are fundamentally of a
geotechnical, not structural, nature. As a result, bridge engineers, who are more
familiar with dealing with structural issues, have been slow to recognize the true
source of IAB problems and develop appropriate permanent solutions for them. Thus
IABs represent an interesting case study in soil-structure interaction that requires the
coordinated attention of both structural/bridge and geotechnical engineers working as
a multidisciplinary team if the concept is to be improved for better long-term
performance. This paper is intended to be an contribution toward that goal and
illustrates the potential use of modern geotechnolgies for IAB problem solving.
THE EVOLUTION OF INTEGRAL-ABUTMENT BRIDGES
The conventional design concept used for most bridges with a short to medium
span length consists of a superstructure resting on abutments as shown in Figure 1.
There may be one or more intermediate piers but their absence or presence is not
relevant to the present discussion and does not affect any of the conclusions and
recommendations made in this paper.
Because of natural, seasonal variations in atmospheric air temperature, the bridge
superstructure will change in temperature and concomitantly change dimensions,
primarily in the longitudinal direction as also shown in Figure 1. Typical ranges of
longitudinal displacement for relatively modest span lengths are of the order of
several tens of millimetres (one inch). However, the abutments supporting the
superstructure are for all practical purposes insensitive to air temperature so remain
spatially fixed year 'round. The relative displacement between the moving
superstructure and fixed abutments is accommodated by a synergistic combination of
expansion joints and bearings as shown in Figure 1. Thus the key elements of
conventional bridge design can be summarized as follows:
• Seasonal thermal displacements of the superstructure are natural and
unavoidable. The magnitude of these displacements turns out to be relatively
insensitive to the specific materials (steel versus portland-cement concrete
(PCC)) used for the bridge components.
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
• The abutments are for all intents and purposes rigid structural elements that are
fixed in space and time (foundation settlements are not considered here as they
do not materially enter into the present discussion). This makes the ground
retained by the abutments also fixed in space and time.
• There are explicit measures taken to isolate the moving superstructure from the
fixed abutments + soil and vice versa, at least in terms of longitudinal
displacements.
Although the design concept shown in Figure 1 has been used for a long time and
works well enough in practice, the expansion joint/bearing detail is often a source of
significant post-construction structural maintenance and expense during the life of a
bridge. Therefore, the IAB concept was developed to eliminate the troublesome and
costly expansion joint/bearing detail. This is accomplished by physically and
structurally connecting the superstructure and abutments as shown conceptually in
Figure 2.
IABs have been used for road bridges since at least the early 1930s in the U.S.A.
[1]. Over the years and in different countries they have variously and synonymously
been called frame bridges, integral bridges, integral bridge abutments, jointless
Figure 1. Basic Elements of a Conventional Bridge
abutments
superstructure
expansion joints
primary direction of
thermally induced displacement
bearings
Figure 2. Basic Elements of an Integral-Abutment Bridge
approach slabs
(optional)
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
bridges, rigid-frame bridges and U-frame bridges. There is also a design variant
called the semi-integral-abutment bridge. Relevant to this paper is the fact that
collectively such bridges have seen extensive and growing use worldwide in recent
years because of their economy of construction in a wide range of conditions and
using a variety of structural materials. Thus they comprise an important aspect of
modern transportation-engineering practice as evidenced by the specialty conference
for which this paper was prepared.
PROBLEMS WITH THE IAB CONCEPT
Overview
Although the IAB concept has proven to be conceptually successful in eliminating
expansion joint/bearing problems as well as economical in initial construction for a
wide range of span lengths, it has not turned out to be problem- and maintenance-free
in actual service. This is because the IAB concept suffers from an inherent,
fundamental flaw. Specifically, the IAB concept fails to explicitly and proactively
address how the relative displacement between the moving superstructure and fixed
ground is to be accommodated. This derives from the fact that the IAB concept fails
to recognize that it does not, and cannot, fundamentally alter nature and the laws of
physics and the resulting tendency of a bridge superstructure to undergo seasonal
temperature and length changes in its longitudinal direction. All that has changed
between conventional versus IAB bridge designs (i.e. Figure 1 versus Figure 2) are
the details of how this thermally induced displacement occurs, and the nature of the
resulting problems and maintenance issues it generates. Thus IABs as currently
designed still have maintenance costs as did their jointed predecessors which inflates
the true life-cycle cost of an IAB.
Causes
The fundamental cause of in-service problems for IABs as they are currently
designed is illustrated in Figure 3. As the bridge superstructure goes through its
seasonal length changes, it causes the structurally connected abutments to move
inward and away from the soil they retain in the winter, and outward and into the
retained soil during the summer. The specific mode of abutment movement is
primarily rigid-body rotation about the bottoms of the abutments although there is a
component of rigid-body translation (pure horizontal displacement) of the abutments
as well. Because rotation is dominant, the magnitude of the range of horizontal
displacements is thus greatest at the top of each abutment.
At the end of each annual thermal cycle, there is often a net displacement of each
abutment inward toward each other and thus away from the retained soil as shown in
Figure 3. The primary reason for this is that the inward winter displacement is
typically of sufficient magnitude to cause an active earth pressure 'soil wedge' to
develop adjacent to each abutment and follow the abutment inward, with the soil
slumping downward somewhat in the process. Because of the fundamentally inelastic
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
nature of soil behavior, this inward/downward soil displacement is not fully recovered
during the outward summer cycle. It is relevant to note that this net inward/downward
soil displacement will occur no matter what type of soil is used and how well it was
compacted during original construction. It is also of interest to note that this tendency
to develop a net inward displacement of the abutments is exacerbated when the bridge
superstructure is composed primarily of PCC due to the inherent post-construction
shrinkage of PCC.
Consequences
There are two significant consequences of the annual thermal cycle of IABs. The
first was recognized at least as far back as the 1960s [2,3,4] and is the relatively large
lateral earth pressures that develop on the abutments during the annual summer
expansion of the superstructure. These pressures can approach the theoretical passive
state, especially along the upper portion of the abutments where horizontal
displacements are largest. Passive earth pressures are typically an order of magnitude
greater than the at-rest pressures for which a bridge abutment should typically be
designed. This tenfold increase in lateral earth pressures far exceeds any normal
margin of structural safety built into the design and thus can result in structural
distress and even failure of an abutment.
Recent research indicates that this long recognized seasonal increase in lateral
earth pressures may be a more significant and potentially problematic issue than
initially thought. This is because the summer-seasonal increase in pressures is not
necessarily constant over time but can increase over time. The reason is that not only
is one seasonal cycle of inward-outward-inward displacement nonlinear, but each
succeeding season is nonlinear with respect to the preceding one. This means each
winter the abutment moves inward slightly more than it did the preceding winter and
each summer it moves outward slightly less than it did the preceding summer. As a
result of this net soil displacement inward toward the abutments and the fact that the
Figure 3. Thermally Induced IAB Abutment Displacement
summer
position
winter
position
final position at end of
annual temperature cycle
initial position at start of
annual temperature cycle (shaded area)
superstructure
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
bridge superstructure still expands each summer the same amount as the preceding
year, the summer lateral earth pressures increase over time as the soil immediately
adjacent to each abutment becomes increasingly wedged in. This overall behavior is a
geo-phenomenon called ratcheting. The soil mechanics behavior causing ratcheting is
quite complex but is well- and thoroughly described in the literature [5].
Because ratcheting causes each summer's lateral earth pressures to be somewhat
greater in magnitude than those from the preceding year, it means structural failure of
the abutments may take years, even decades, to develop, a happenstance observed in
practice for other types of earth-retaining structures where thermally induced
ratcheting occurs [5,6,7]. Given the relatively long design life of most IABs (typically
100 years or more), ratcheting represents a potentially serious long-term source of
problems, primarily structural distress and failure of the abutments.
The second significant consequence of the annual thermal cycle of IABs is also
related to the net inward displacement of the abutments and has become fully
appreciated only in recent years. This is the subsidence pattern that develops adjacent
to each abutment as shown in Figure 4. This is the result of the above-described
phenomenon of accumulated, irreversible soil-wedge slumping behind each abutment.
The consequences of this subsidence depend on whether or not an approach slab was
constructed as part of the bridge. If there is no slab, there will be a difference in road-
surface elevation occurring over a short distance creating the classical 'bump-at-the-
end-of-the-bridge' condition. If there is a slab, initially it will span over the void
created underneath it by the subsided soil. However, with time and traffic the slab can
fail structurally in flexure.
Subsidence behind IAB abutments has received much more interest in recent
years compared to the traditional concern over increased lateral earth pressures. This
is because experience indicates subsidence develops and becomes problematic
relatively soon (a few years at most) after an IAB is placed in service [8,9,10,11,12]
whereas the ratcheting buildup of lateral earth pressures might not create problems for
decades as noted above. For example, [11] noted that a survey of 140 IABs with
approach slabs in the State of South Dakota, U.S.A. found a void under virtually
every slab. The void depths ranged from 13 to 360 mm (0.5 to 14 in), and the voids
extended as much as 3 m (10 ft) behind the abutment.
Figure. 4. Ground-Surface Subsidence behind IAB Abutments
subsidence zone (and void development if approach slab used)
new ground surface
due to long-term
abutment displacement
long-term position
of abutment
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
PROBLEM SOLUTIONS
Overview of Past Efforts
Although most recent research into defining IAB problems has focused on the
newly identified issue of subsidence behind abutments, some recent work related to
developing actual solutions for IAB problems has focused on the traditional issue of
lateral earth pressures alone [1,11,13]. Specifically, various types of relatively
compressible materials (generically referred to herein as compressible inclusions)
such as either resilient or normal expanded polystyrene (EPS) geofoam and tire
shreds have been placed behind IAB abutments. Conceptually, a compressible
inclusion is intended to serve as a sacrificial cushion between a relatively rigid earth
retaining structure and the adjacent ground with the overall goal of reducing lateral
earth pressures. A recent overview of the compressible-inclusion concept with an
emphasis on earth retaining structures used for transportation facilities can be found
in [14].
While these research efforts are a step in the right direction, available information
suggests they are significantly incomplete. Research to date indicates that although
the use of a compressible inclusion can be highly effective in reducing the summer
increase in lateral earth pressures it is totally ineffective for controlling subsidence
behind the abutments. In fact, experience indicates the presence of a compressible
inclusion may even exacerbate the subsidence problem even as it addresses the
summer lateral earth pressure problem. This is because the highly compressible
nature of a compressible inclusion that is so desirable under summer expansion of an
IAB becomes a detriment when winter contraction occurs. As the superstructure
contracts and pulls each abutment away from the retained soil, the relatively weak
compressible inclusion between abutment and soil is unable to restrain the soil from
slumping and displacing inward toward the abutment. This actually results in
subsidence behind the abutment that is larger in magnitude than if no compressible
inclusion were present. This has been observed for at least one IAB that was studied
thoroughly where a compressible inclusion consisting of recycled tire fragments was
used with an approach slab [11]. It has also been observed in large-scale, 1-g
physical-model testing where a compressible inclusion composed of resilient-EPS
geofoam was used with a coarse-grain-soil backfill [12].
Proposed Improved Solutions: Basic Concepts
Because of the current extensive use of IABs, there is a critical need to develop
solutions to correct the behavioral deficiencies inherent in all IABs as they are
typically designed and constructed at the present time. As noted previously, past
efforts to develop improved designs based on the use of a compressible inclusion
alone have not been a total success because they did not address both problems, i.e.
the seasonal buildup of lateral earth pressures on the abutments and ground-surface
subsidence adjacent to the abutments.
The key to developing improved solutions that address both problems is a
thorough understanding and appreciation of the fundamental physical processes that
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
affect both conventional bridges as well as IABs. The following key concepts and
considerations were identified by the author and used to develop the improved
solutions presented subsequently:
• Expansion and contraction of a bridge superstructure due to seasonal temperature
changes is inevitable and unavoidable as it is a natural phenomenon that simply
cannot be changed in any significant way. This displacement will occur regardless
of the specific structural concept used for the bridge design. This means the
tendency for differential horizontal displacement between an IAB and the ground
surface adjacent to its abutments is unavoidable and must be addressed explicitly.
• The ground adjacent to IAB abutments must be made inherently self-stable on a
permanent, year-'round basis to prevent development of subsidence during the
seasonal winter contraction of the IAB. In essence, the ground itself must provide
the non-yielding, seasonally constant retention function formerly provided by the
abutments of conventional bridge design (Figure 1).
• There must be a design detail involving a structural element or material between
the self-stable ground and moving IAB abutments to reliably and predictably
accommodate the relative movement between them. This detail conceptually
replaces the expansion joint/bearing detail of conventional bridge design (Figure
1). Simply leaving a void between ground and abutment as has been done
occasionally in the past is not considered an acceptable design detail. Experience
indicates that a void is difficult to construct routinely and reliably in practice [15],
and it cannot be depended on to remain for the long service life of a bridge.
Proposed Improved Solutions: Concept Details
A detailed numerical study was conducted by the author to both define the key
behavioral aspects of IABs as well as investigate potential solutions using
geosynthetics [16]. That study drew heavily on the knowledge gained during the
1990s about geofoams in general and EPS geofoam in particular [17]. Although the
revised designs developed and presented in [16] will increase the construction cost of
IABs, the anticipated superior post-construction, in-service performance of such IABs
should more than make up for the increase by reducing future maintenance and repair
costs. Similarly, implementing these revised designs retroactively on existing IABs
should be cost effective by reducing their future maintenance and repair costs.
Two different design concepts were developed to accommodate different site
conditions. Both are shown schematically in Figure 5. The one likely to be more cost
effective in most applications is shown in Figure 5(a) and is appropriate for sites
where compression and/or stability of the native soils underlying the approach
embankment to the bridge is not an issue. The concept utilizes geosynthetic tensile
reinforcement (likely geogrids or geotextiles) to create a mechanically stabilized
earth (MSE) mass within the retained soil adjacent to each abutment. This reinforced
soil mass would be inherently self-stable for the design life of the bridge. In addition,
a relatively thin (typically of the order of 150 mm (6 in) thick) layer of resilient-EPS-
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
geofoam would be used as a compressible inclusion in a 'chimney' orientation
between the abutment and MSE mass. This durable inclusion is highly compressible
and thus functions as the desired expansion joint between the abutment and MSE
mass. Note that the compressible inclusion also thermally insulates the retained soil
(against winter freezing) and the geosynthetic tensile reinforcement (from summer
heat which can increase geosynthetic creep), and can be designed to also serve as a
drain for ground water. Functionally, this compressible inclusion allows the
reinforcement within the soil to strain in tension (which prevents the soil from
displacing inward and downward toward the abutment each winter) as well as allows
the abutments to move seasonally in either direction with minimal restraint. Thus
summer increases in lateral earth pressures are reduced to relatively small
magnitudes. Overall, lateral earth pressures acting on the abutments are significantly
reduced from current design levels which would achieve a cost savings in the
structural design of the abutment.
The other design alternative is shown in Figure 5(b). A self-stable wedge of some
kind of geofoam (most likely EPS blocks [18] but alternatively foamed PCC) or
geocomb blocks would be used as a solid lightweight-fill material in lieu of the MSE
mass. A relatively thin layer of highly compressible resilient-EPS geofoam is again
used multifunctionally as a compressible inclusion/thermal insulation/chimney drain.
This alternative is expected to be the one of choice for sites where the soils
underlying the approach embankment are soft and compressible. Use of a lightweight
fill material would minimize settlements and enhance stability of the ground adjacent
to the bridge as well as greatly reduce the loads acting on the abutment and the deep
foundations that would likely be supporting it in such soil conditions. Solid
lightweight-fill materials such as various types of geofoam are particularly attractive
Figure 5. Proposed New IAB Design Alternatives
resilient-EPS geofoam
compressible inclusion
(a)
(b)
geosynthetic
tensile
reinforcement
EPS-block
geofoam
lightweight fill
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
here as they are inherently self stable even when constructed with vertical side slopes.
Although geofoam materials are inherently more expensive than soil on a strictly
volumetric comparison the resulting overall savings would likely more than
compensate for the use of a geofoam material in lieu of soil. The benefits of
accelerated construction by using geofoam materials should also be considered.
Proposed Improved Solutions: Additional Comments
For the sake of completeness, it should be noted that in cases where the ground
underlying the approach embankment is weak and compressible there are other
potential alternatives to using a solid lightweight-fill material as shown in Figure 5(b)
that might be cost-effective on a project-specific basis [19]. This includes using a
granular type of lightweight fill material (expanded-shale aggregate, tire shreds, etc.)
in combination with geosynthetic tensile reinforcement to stiffen and retain the
material as an equivalent MSE mass as shown in Figure 5(a). Alternatively, some
type of ground improvement might be performed to strengthen and stiffen the native
soils in situ prior to constructing the approach embankment. After performing the
ground improvement, the approach embankment could be constructed using normal
soil and an MSE mass adjacent to the abutments as shown in Figure 5(a).
SUMMARY AND CONCLUSIONS
IABs are an interesting example of how new problems were inadvertently created
in the process of solving old problems. IAB problems are fundamentally geotechnical
in nature and can manifest themselves both structurally and geotechnically any time
in the life of an IAB. The primary cause of both short- and long-term IAB problems is
the irreversible net inward and downward displacement of the soil retained by IAB
abutments. This will occur regardless of the type of soil used and how well it was
compacted during construction. The resulting problems consist of irreversible
subsidence behind the abutments and the ratcheting buildup of 'summer' lateral earth
pressures on abutments. Either or both of these outcomes can result in serviceability
or collapse failures of the bridge components and thus are serious.
Research also indicates that relatively simple and cost-effective design solutions
to eliminate these problems can be achieved using a variety of modern geosynthetics
and/or ground-improvement technologies in an innovative, synergistic fashion.
Because the problems encountered with IABs as currently designed turn out to be a
complex problem in soil-structure interaction, any successful solution must address
the need to both:
• support the ground adjacent to the abutments on a permanent, year-'round basis
and
• provide for a compressible inclusion (essentially an expansion joint) between
abutment and adjacent ground to serve as an engineered, in-ground replacement to
the expansion joint/bearing detail of conventional bridges.
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IAJB 2005 - The 2005 FHwA Conference on Integral Abutment and Jointless Bridges
Baltimore, Maryland, U.S.A. ⋅ 16-18 March 2005
Several specific suggestions for improved IAB designs were presented in this
paper. An important benefit is that these solutions can be implemented as part of the
rehabilitation of existing IABs in addition to being applicable for new construction.
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