The effect of environmental conditions on the selection of foundation types for bridges over water is discussed. The design of foundation for bridges over water is a challenging and tedious task and the designer would encounter extreme problems.
The most crucial factor that may control the design of bridges over water is environmental conditions rather than ground condition unlike the design of bridge foundation on land which govern by ground condition.
Therefore, it is considerably significant to understand the environmental conditions that govern over water bridge design and specify their influence on the selection of foundation types and their construction methods.
Fig.1: Bridge Foundation Over Water
Effect of Environmental Conditions on Foundations for Bridges
Environmental factors that affect the selection of over water foundation types and their construction methods are discussed below:
- Exposure conditions and water depths
- Sea or river current
- Ship collision
- Floating ice
Exposure Conditions and Water Depths of Bridge Foundations
When bridge structure is constructed in open waters of broad bay crossing, it is commonly exposed to strong currents and wind. This aggressive condition leads to undesired influences, such as, deteriorating uncompleted structure and restricting the operation period of floating construction plant.
This undesired condition encourages the utilization of sizable prefabricated structural members. For example, open well caissons and box caissons. The caisson units are commonly floated and transported to the bridge construction site. After that, they are submerged and placed on the prepared bed or piles.
Fig.2: Open Caisson Sunk on Prepared Bed
Box caisson can be employed for the case where sea water is sufficiently deep for box caisson floatation. Weather condition plays significant role during transportation of the caisson box to the bridge construction site, sinking and installation of caisson units. So, it is necessary to consider possible delay in construction due to poor weather condition into account.
As far as open well caisson is concerned, it is suitable for shallow water in which shallow draft bottom section is floated and moved to the construction site and submerged by taking out the soil from the open wells while the walls are lifted gradually.
It is worth mentioning that the construction process of open well caisson is more sensible to the effect of weather conditions compared to the installation process of box caisson units. The box caisson construction can be carried out in considerably short period of time unless poor weather condition halt installation operation.
Effect of Sea or River Current on Bridge Foundation
Piles or piers may be subjected to drag force which is generated by sea or river current. This force creates scour holes in locations where the soil is vulnerable to erosion at sea bed level.
The presence of these holes around cofferdams are undesired since it generates circular movement of water in this location because of temporary conditions such as partially driven sheet piles.
Current drag force is likely to create issues while sheet piles or bearing piles are installed. This is because such force at high velocities oscillates and consequently damages the pile prior to placement of pile caps or temporary grit provision which prevents the pile oscillations.
Effect of Ship Collision on Bridge Foundation
Collisions are likely to occur due to ships, which are out of control and possibly cause bridge pier failure. In specific broad estuaries, deep water channel may swing from one side to another in a considerably short time and almost all piers of the constructed bridge would be at risk.
The collapse of the old Sunshine Skyway Bridge in 1980 due to collision of a vessel with one of the piers of the bridge is a realistic example of ship collision risk.
Fig.3: Collapse of the Old Sunshine Skyway Bridge
Fig.4: Failure of old Sunshine Skyway Bridge in 1980 due to collision of a vessel with one of the piers of the bridge
So, it is necessary to take necessary measures to prevent the danger of ship collision to piers. These precautions would certainly increase the total cost of foundation construction.
Examples of precautions against ship collision include providing artificial island around the pier to stop vessel prior to collide with bridge pier. This protection measure should be considered for piers in shallow water.
Fig.5: Penang Island Bridge Piers are Protected from Ship Collision by Providing Artificial Island
Fig.6: Details of Artificial Island Surrounding Bridge Piers to Protect the Pier Against Ship Collision
If artificial islands are constructed in deep water, the amount of stones used for wave protection and mattresses employed against scouring would be inevitably large and it would create problems for water navigation.
Another protection measure is the provision of fender piles which are commonly connected by large ring beam. The ring beams should be distant from the pier to permit pier deflection while they swallow kinetic energy of moving vessel and make it to stop.
Fig.7: Provision of short piles around piers as a protection consideration against ship collision, Sunshine Skyway Bridge
Fig.8: Protecting Pier Bridge Against Ship Collision
Effects of Floating Ice on Bridge Foundations
There is a similarity between the effect of floating ice sheet impact and collision ship influence on the bridge pier. So, their design is also similar to each other. However, there is an extra danger in the former case which is the gradual increase of pressure due to the amassing of ice packs.
The direction of build up pressure is transvers to the bridge pier or vertical when pressure ridges start to form. It is recommended to use single pier bridge rather than group piles because single piles will alter the direction of ice floes and prevent ridge pressure formation.
For gravity base structure, the best form of bridge foundation in areas subjected to floating ice is the application of slender piers with a massive base. This is because the latter would provide resistance against sliding and overturning forces whereas the former does not create much resistance against floating ice sheet force.
Moreover, if the utilization of piles are needed due to ground conditions, then a ring of closely spaced skirt piles should be installed around the group pile.
Finally, for the case the movement of water is parallel to the river bank, the introduction of cut waters to the piles would provide satisfactory answer to the floating ice forces. This is because cutwaters would cut and break ice floes and consequently avoid the impact and pressure of floating ice sheets.
Effect of Earthquakes on Bridge Foundations
Earthquakes impose devastation force on the bridge piers especially in deep water because the force exerted on the bridge superstructure would be combined with the load on the pier and consequently substantial overturning moment at the base of the pier will be produced. The mass of water replaced by pier is specified to be added to the mass of the pier.
The eccentric load supported by the pier is crucially large in deep water which makes the selection of slender pier and massive base a desirable option. It is recommended to use circular column because seismic forces do not have particular direction and it can be in any direction.
Another problem is the liquefaction of loose to medium dense soil due to ground shaking. This issue may be tackled by using pile foundation to densify the soil to an extent that can support piers safely. The depth of liquefaction may be computed using particle size distribution and in situ density.
Finally, liquefaction may cause tsunami or develop submarine flow slides and subsequently a horizontal force at the base of the pier will be generated.