Quote:
Originally Posted by denizen467
^ So maybe two monster caissons for each viaduct, to eventually hold up the spanning of 2-to-3 tracks? They still seem awfully large for that ... if something lesser would be inadequate, then why is the rest of the UP North's railbed (where it's elevated on earth/rock in between retaining walls) adequate without continuous hardcore supports like that?
I didn't realize that the viaduct project was going to result in a higher railbed --- if they're elevating several miles of UP North by even a foot or so, jeez, that is a huge undertaking.
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Sorry to dredge up the stale post, but I stumbled across some helpful images while searching for something else:
http://www.facebook.com/media/set/?s...6907722&type=3
This is a similar rail underpass in Lima, OH. There are two rows of caissons that support the pair of abutment walls. You need a deep foundation here for several reasons.
First, the bridge is quite heavy, and its load is concentrated on the abutment walls at each end. Unless you're building on solid rock, you're gonna need to sink the foundation down to a more solid layer than the typical Midwestern surface clay. That means caissons, sank in a line maybe 10' apart.
Second, the vibration of moving trains and the movement of water through the soil conspire to push the walls outward, a "moment" force that would eventually topple the walls towards the roadway if left unchecked. Sinking a deep foundation resists these horizontal forces, just like a spade gets harder and harder to tip the deeper you sink it into the ground.
Currently on Metra's embankment, the sides are sloped, which helps distribute the weight of the trains over a greater area and keeps the soil in place. If that slope is below a given angle (the "angle of repose"), gravity and friction work together to keep the soil from pressing outwards and removes those moment forces. Any steeper than that angle, up to and including 90 degrees, you need some form of artificial retention.
Metra's planning to raise the tracks, but now in their new plan they need to provide enough flat space at the top to allow for three tracks. That poses a problem with geometry, because with 45' of flat space at the top and only a 66'-wide ROW, there isn't enough space on the sides to slope back to grade without exceeding that angle of repose. That means they need some form of retaining wall along the entire length of the line. But since they still have a small amount of width in which to slope the dirt, the retaining walls don't need to be full-height - the soil can slope down as far as is safe to build, and the wall can be partial-height and built out of cheaper materials like landscaping blocks.
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