Designing foundations for bridges over water presents unique challenges, particularly when compared to the design of foundations on land. While land-based foundations are primarily influenced by ground conditions, foundations for bridges over water are more heavily impacted by environmental factors. Understanding how environmental conditions affect the selection of foundation types and construction methods is crucial to ensuring the safety, durability, and efficiency of bridge projects. This article discusses various environmental factors, including exposure conditions, water depth, sea or river currents, ship collisions, floating ice, and earthquakes, and how they shape the design and construction of bridge foundations over water.
Environmental Factors Affecting Bridge Foundations Over Water
Exposure Conditions and Water Depths
One of the most critical environmental factors when constructing a bridge over water is exposure to harsh conditions, such as strong currents and severe weather, which can impact both the construction process and the long-term stability of the foundation. Bridges built in open waters, particularly those spanning broad bays or deep water channels, are often subjected to high winds and currents. These aggressive environmental conditions can hinder the completion of construction and restrict the operation of floating construction plants.
To combat these issues, large prefabricated structural members, such as open well and box caissons, are often employed. These caisson units are floated to the construction site and then submerged and placed onto prepared beds or piles. Box caissons are generally used in deep waters, where they can be floated to the site more easily. However, weather conditions must be carefully considered during transportation, sinking, and installation, as poor weather can cause delays.
Open well caissons, on the other hand, are more suitable for shallow waters. These caissons are floated to the site, where they are submerged by removing the soil inside the well. The installation of open well caissons is more sensitive to weather conditions compared to box caissons, making it a less favorable option when weather unpredictability is high.
Sea or River Currents
When constructing foundations for bridges over water, the impact of sea or river currents cannot be underestimated. Currents generate drag forces that can cause scour holes around the foundation’s location, particularly when the soil at the seabed is prone to erosion. This creates additional problems for construction, such as instability during the installation of cofferdams and piles. The scour holes can lead to circular water movement, further exacerbating the situation by destabilizing piles and cofferdams.
The drag force exerted by high-velocity currents is especially problematic during the installation of sheet piles or bearing piles. The oscillation caused by these forces can damage piles before they are even securely placed, thus hindering the progress of construction and affecting the integrity of the foundation.
Ship Collision
Ship collisions present another significant environmental risk for bridge foundations over water. In areas with busy waterways, ships that lose control can accidentally collide with bridge piers, causing severe damage or even structural failure. One of the most infamous examples of this is the collapse of the old Sunshine Skyway Bridge in 1980, when a vessel collided with one of the bridge’s piers.
To mitigate the risk of ship collisions, various protective measures must be implemented. Artificial islands, for example, can be built around piers to prevent ships from directly striking the bridge structure. While effective in shallow water, these islands pose challenges in deeper waters, where they may interfere with navigation and require large quantities of stone for protection against wave action and scour. Additionally, fender piles, often connected by large ring beams, can be installed around the pier. These fender piles absorb the kinetic energy of a moving vessel, stopping the ship before it reaches the pier.
These protective measures, while effective, significantly increase the cost of construction and can be challenging to implement in deeper water environments where space and resources are more limited.
Floating Ice
In regions where bridges are subject to freezing temperatures, the impact of floating ice must be considered. Floating ice can have a similar effect on bridge piers as ship collisions, as ice sheets can collide with and apply substantial pressure to the pier. The danger is exacerbated by the gradual build-up of pressure caused by the accumulation of ice floes. Pressure ridges may form when ice sheets begin to pile up, increasing the risk of damage.
To protect against the effects of floating ice, it is recommended to use single pier foundations instead of group piles. Single piers help redirect the direction of ice floes and prevent the formation of pressure ridges. Additionally, gravity base structures with massive bases are ideal for such conditions, as they provide resistance against the sliding and overturning forces caused by floating ice. If piles are necessary due to the ground conditions, a ring of closely spaced skirt piles should be installed around the group pile to help mitigate the impact of ice. In rivers where the water moves parallel to the bank, introducing cutwaters to the piles can break up the ice floes, preventing them from building up pressure against the pier.
Earthquakes
Finally, earthquakes present a significant threat to bridge foundations, particularly in areas prone to seismic activity. The combined forces of an earthquake and water movement can exert tremendous pressure on bridge piers, especially those located in deep water. In deep-water conditions, the eccentric load supported by the pier can be substantial, generating large overturning moments at the base of the pier.
To mitigate the effects of earthquakes, it is recommended to use slender piers with massive bases, such as circular columns. Circular columns are particularly effective because seismic forces are multidirectional and can act in any direction. Another major concern in seismic zones is soil liquefaction, which occurs when loose to medium-dense soil becomes unstable due to ground shaking. Liquefaction can lead to the generation of horizontal forces at the base of the pier, potentially causing failure. To address this issue, pile foundations can be used to densify the soil to a level that can safely support piers. The depth of liquefaction can be determined by assessing the particle size distribution and in situ density of the soil.
Conclusion
The selection of foundation types for bridges over water is heavily influenced by environmental factors, including exposure conditions, water depth, currents, ship collisions, floating ice, and seismic activity. Each of these factors presents unique challenges that require careful consideration during the design and construction phases. By understanding the impacts of these environmental conditions, engineers can select the most appropriate foundation types and construction methods to ensure the stability, safety, and longevity of bridges in these challenging environments.