Friday, June 6, 2008

Building a bridge... 7

Time to clear the air about these battered piles and why they are used in addition to the vertical piles. As I mentioned last time, if the piles were not installed properly, the bridge would sink. So, the piles hold up the bridge. But did you know that the piles are also holding up dirt? Before I get dirty, another of Mrs. Spit's favorite enginerd jokes:

Two engineering students were walking across campus when one said, "Where did you get such a great bike?"

The second engineer replied, "Well, I was walking along yesterday minding my own business when a beautiful woman rode up on this bike. She threw the bike to the ground, took off all her clothes and said, "Take what you want."

The second engineer nodded approvingly, "Good choice; the clothes probably wouldn't have fit."

From the top of the piles to the top of the road is a distance of approximately (I say approximately as I don't have the drawings in front of me) 4 metres / 12 feet. On one side of the piles will be the actual bridge girders, bridge deck etc. On the otherside we have the roadway. This is a large volume of compacted earth against a vertical wall. The wall keeps the dirt in place and functions much like a retaining wall.

My boss did his master's thesis on retaining walls, specifically mechanically stabilized earth walls. Thesis. So I'm not going there. What I will say is that the amount of lateral force exerted on a retaining wall increases as a linear function of the height of the wall. In non-math speak, a wall twice as tall as its neighbour will have to resist roughly twice as much earth pressure as the shorted wall. Remember the golden rule? Something has to resist this lateral force.

By driving the front row of piles as battered piles, they will function to carry load in two directions. Most of their strength is in the vertical direction. They will have a portion of their vertical strength reduced and directed at resisting lateral forces. In this way they will help hold up the bridge as well as the road fill next to the abutment.

A key to the photos along with this post. Top: Piles partially embedded with vibro hammer. Middle: view of the new abutment from the old bridge. Bottom: Waiting for the diesel hammer with all of the batter piles vibro'd into the ground.

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