I started on this series of posts, reached this point months ago, and found myself writing way too much about the minute details. I erased it all the other day, and started over.
The bridge starts at the bottom. The bottom may be hundreds of feet below the surface, or right on top of rock. The design starts at this point, and expertise of geologists, and soil engineers, is necessary. At this point, the most important consideration is whether what's available can handle the load.
The foundation of a bridge has to handle the dynamic load. The dynamic load is the dead weight, and the forces that will act on the final structure. The dead weight is determined by the culmination of all materials that will be placed, but that weight can't be calculated until all the required components are known. The final weight can't be determined until the required components are known, which can't be calculated until what will support the bridge is known, and all the forces that will act on the bridge are known.
The final components are determined by theoretical design considerations, which require an understanding of what's available to support the bridge. That's first determined by knowing about what the bridge will rest on. Is their rock? If not, what type of soil is available? Is the available rock stable? Will the soil support the bridge with piling?
Bedrock can support a tremendous amount of weight, but the thickness of the rock, the amount of fissures, and the type determine the design. Geologists are consulted, and if necessary, samples are examined from drilled holes. After examination, the foundation may be secured by drilling dowels into the bedrock, or some rock may require removal to reach more stable rock underneath.
Without rock, the soil determines the type of support. Some soil only requires driving piling to the point the friction against the soil is enough to support the structure. Other soils may require drilling down to a subsurface strata that can support the load, and filling the hole with reinforcing steel with poured in place concrete. That, or the instability of sand may require "water jetting" concrete piling with a hollow tube in the middle to a depth that will allow the soil friction to support the load. In some situations, the soil will support the bridge and the only thing to do is to spread the load with a spread footing. A spread footing can last the life of the bridge, or when certain conditions exist, a hundred year flood can lead to scouring, which erodes under the footing, and the bridge is compromised.
Regardless of the method for the foundation, laying out the location can be an onerous task. Optical instruments have lessened the work, but arriving at control points may be as simple as walking with a trap on the end of pole, to spud barges, large tugs, and fighting a current to place sheet piling for a cofferdam around the future foundation, and the control points on the bottom of a body of water. The locations are critical, since all design loads are done on theoretical points on the final structure. Obstructions, locations errors, or broken piling can lead to construction stopping and new design parameters. Whether a single piling, or clusters of piling to handle larger structures, this is the point where the engineer representative calculates the bearing, if the piling is driven, and watches to ensure the contractor doesn't stop driving piling before it reaches bearing.
Bearing is the amount of blows by a piling hammer. The weight of the hammer is calculated before driving, and may have been determined by driving test piling. The hammer may be a large single cylinder diesel engine that uses the explosion of the diesel to blow the piston upward, which causes an equal amount of force pushing downward on the pile, or a hydraulic ram, which accomplishes the same purpose. Bearing is achieved when the number of blows to drive a foot of the piling below the surface match known quantities for blows, to what is known as bearing, or "refusal". Refusal is when a piling is driven for a minute, and it doesn't go down any further, or the amount is less than an inch. This can be a disaster for a pile driver. If the engineer representative doesn't agree with the contractor, demands to keep driving, and the piling breaks, engineering design changes, extra costs, and possible shut-downs can lead to angry words, with resentment. I've seen this happen, and the project rep was wrong.
Not achieving bearing requires adding to the piling, and driving again. On concrete piling, breaking the top of the concrete piling, adding an exact section with rebar, and concrete. On steel piling, welding a section is required. The welds are x-rayed for determining if the amount of porosity is within acceptable limits. Welds that fail are removed with arc-gouging, and/or grinding, then welded again.
Another aspect of piling is one I experienced on a project. The piling reached refusal about 7 feet above the final grade. The excess piling had to be removed. In that situation, the piling was 18 inches square pre-stressed concrete. The stressed cables prevented using a concrete saw, since releasing the tension can have disastrous results for the saw operator. We tried breaking the concrete with hand-held concrete breakers, since the pilings were above the water, and there was no access with larger breakers. That was taking to long. The final outcome was renting a machine that sheared the piles. It was expensive, but the time, with labor, and the cost of a piling crew shut down, was lower than the other method.
After the basic part of the foundations are designed, such as pilings, manipulated rock, or spread footings, and the lowest foundation is finished, the next step can be as simple as leaving piling above the surface to the final elevation of the caps, or placing reinforced concrete with protruding dowels for the columns. The concrete is usually placed around clusters of piling, and the amount can be tremendous. Whether at ground level, or dozens of feet below the surface, the next step is building up, whether it's poured in place concrete, or pre-cast sections. That requires another post.
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