Unique Substructure Solutions to Consider in Complex Bridge Projects
Bridge projects don’t always come with simple solutions. Several factors can complicate the design and construction of bridge projects, such as limited accessibility and poor foundation soils. Depending on the situation, an alternative substructure may be the answer to overcoming obstacles without sacrificing the project schedule or budget.
Soldier piles are vertical members driven into the ground or placed in pre-bored holes and spaced at equal intervals to support horizontal lagging. In addition to providing lateral support, soldier piles can be placed along an abutment wall to provide adequate vertical support for load bearing beams.
In Jefferson County, Ohio, a bridge replacement complicated by a combination of factors including bedrock depth, boulders, and the need to maintain traffic led to the decision to use a drilled shaft soldier pile wall that supports the vertical loading from the bridge. By using this technique, the need for difficult and expensive temporary shoring was eliminated and the amount of rock excavation required was minimized. In addition, low-strength mortar was used as backfill behind the abutment to minimize lateral pressure on the wall and the resulting deflection of the soldier piles.
Anchored Retaining Walls
Anchored retaining walls provide lateral support for excavations. They utilize sheet piles that are driven into the soil or soldier piles that are either driven into the soil or placed in drilled shafts. Ground anchors are then installed through the piling or walers into the soil or rock behind it. The anchors provide lateral support when the lateral loads are too high for the piling to resist on its own and when other methods of bracing cannot be used.
The location of the Jones Road Bridge in Lorain County, Ohio is subject to frequent flooding, as was experienced during construction of a replacement bridge. Traditional abutments with foundations placed below the scour depth would have been extremely difficult to construct, requiring extensive dewatering, and most likely would have experienced delays and damages due to the inundation of flood waters. Anchored retaining walls were used instead, minimizing the required amount of excavation, temporary shoring, and span length of the structure.
Micropiles are relatively small-diameter deep foundations that are installed by drilling steel casing down to the design depth, filling the casing with grout, and then removing the casing as necessary to allow the grout to bond to the desired formation. Micropiles can be installed with small, low-head equipment and are beneficial in situations with limited site access, complex ground conditions, or when ground disturbances need to be minimized.
A bridge rehabilitation in Cleveland, Ohio incorporated micropiles when the design was restricted by complex site conditions including limited overhead clearance, existing railroad tracks both above and below the bridge, and aging utility infrastructure underneath the abutment that required disturbances to be limited.
Light Weight Fill
Light weight fills can consist of foamed concrete fill or lightweight aggregate that can be used to reduce the net vertical load on a consolidating or weak soil layer without sacrificing strength. Used in lieu of regular fill, a light weight fill can be beneficial when there is a need to mitigate slope stability problems, limit the additional vertical loads applied to the ground, and prevent extensive settling.
The Unionport Road bridge in Jefferson County used light weight fill in the bridge superstructure replacement. When the design raised the grading at one abutment by a few feet, light weight fill was used to replace some of the soil, preventing the net load on the abutment from increasing.
Creative Solutions for Complex Bridges
Some bridge rehabilitations and replacements require creative, outside-the-box solutions. If a simple design just won’t cut it, consider applying any combination of these substructures for a unique solution that keeps the project on schedule and under budget.