Dupont and Allen (2002) outlined the causes of differential settlement of bridge
approaches as the following: compression of embankment fill, settlement of foundation soil
beneath the embankment, poor design or construction practices, and poor drainage practices.
A look at current practice began with a survey of all 50 states in which it was found only 21
states used special procedures when backfilling around integral abutments and end bents. The
authors noted that some states appeared to view the approach slab as the overall solution to
bridge approach problems, instead of design feature in need of additional improvement.
Many conclusions were reached in order to alleviate future approach problems including
approach settlement periods/using surcharge loading, lowered approach slabs with asphalt
overlays, and designing maintenance plans simultaneously with construction plans. Other
more viable or effective methods include improving drainage on/ around approaches,
reducing embankment side slopes, and longer/stronger approach slabs. Robison and Luna
(2004) accurately modeled the deformation and settlement of bridge approach embankments
in Missouri using PLAXIS software. By accurately modeling the staged construction process
the structurally important deflection, which occurs after the completion of the approach slab,
can be determined and minimized on future projects. Other recommendations for MoDOT
include enhanced soil exploration for high embankments (10 to 20 feet), the use of
geosynthetics, and the use of select drainage material underneath the entire slab and sleeper
beam. Some additional recommendations for better geotechnical performance of the
embankment are provided by Luna et al. (2008). A recommendation was made to include
exploratory boreholes 30-50 feet away from the abutment in the location of the approach
embankment. Embankment slopes should be limited to 2.5H:1V to increase stability, and
Horvath (2005) examined the geotechnical issues that accompany integral-abutment
bridges (IAB). The author states that the problems with IABs are geotechnical in nature, so it
would follow that solutions should be geotechnical as well. Many solutions do not address
the discontinuity between the moving structure and stationary soil. As the structure contracts
in winter, a soil wedge moves inward and downward into the opening gap behind the
abutment. This creates a long-term problem in addition to the passive pressure on the
abutment during the summer. The “ratcheting” increases the passive pressure seen by the
backwall over time. The movement of soil means that a loss of support will occur under the
approach slab no matter what kind of soil is used or how well it is compacted. Compressible
inclusions were found to be unable to hold back the active pressure of the slumping soil since
they were elastic enough to accommodate the expansion of the bridge in the first place. Two
different details were proposed in Figure 2.22, with the first more promising and cost-
effective than the second. The first uses a compressible inclusion in combination with a
mechanically stabilized earth (MSE) embankment which gives the soil enough strength to
avoid falling into the void. The inclusion acts as a joint, while also insulating the soil against
temperature changes, and possibly aiding in drainage. Passive pressures are reduced to
increase cost savings in design. The second detail is intended for soft soil under the approach
embankment and utilizes a wedge of expanded polystyrene (EPS) with a compressible layer.
In conclusion, any successful solution must support the soil on a year-round basis and be able
Figure 2.22. Proposed New IAB Design Alternatives (Horvath 2005)
Backfill is extremely important since it interacts with both the abutment and the
approach slab placed on top of it. Abu-Hejleh et al. (2008) evaluated the Colorado DOT
bridge approach design methods to determine their effectiveness and to provide additional
recommendations moving forward. The practice at the time included three different methods
for backfill: flowfill concrete, MSE Class 1 backfill, and MSE Class B free-draining backfill.
Five different bridges were inspected and forensic investigation was done to determine the
source of bridge bump problems. Performance of approaches using these methods improved
over the previous methods but some settlement issues persisted. Flowfill was still
recommended for unique scenarios where compaction is extremely difficult, but MSE Class
B fill had the lowest unit cost over design life since there was no necessary repair reported.
Final recommendations include the compaction of fill done wet of optimum and the use of
surcharge preloading if possible. In order to better support the sleeper slab, two different
methods were proposed in Figure 2.23. One included more MSE fill under the sleeper than
the standard 4’ at the time of the study. The other detail used piles for supporting the sleeper slab. The sleeper slab and expansion joint above it may be installed up to 1 inch higher in
elevation than the design in order to account for post construction settlement if approved by
the hydraulic, structural, and roadway engineers.
Figure 2.23. Recommended Supporting Systems and Drainage Details for Sleeper Slab: (a) Placement of MSE Wall under Sleeper Slab, (b) Use of Class 2 Backfill and Driven Piles to Support Sleeper Slab, and (c) Placement of Gutter and Half-Circle PVC Pipe to Drain Water
(Abu-Hejleh et al. 2008)
Nebraska DOT has a rather unique practice that includes the use of helical piles at the
end of an approach slab per correspondence with Mark Traynowicz of Nebraska DOT on
9/13/2018. Grade beams are used similarly to an approach slab and are almost always
supported on piling. Nebraska allows HP, pipe, or concrete piles, but in the case of an
approach slab replacement helical piles may be used as an alternate if they are the only piling