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From my related question, I'm trying to understand the decision-making that went into metro/subway design.

Why did Chicago choose an elevated metro system instead of an underground subway? Most other cities chose the latter.

Apparently Los Angeles, St Louis, and Detroit also had elevated metros before they were torn down in the 1950's. Did this have anything to do with Chicago's decision?

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I don't know about other cities, but Chicago appears to have had many troubles with its underground terrain, to the point that, to build their sewer system (once these things became "fashionable" :-)), they had to raise the entire city instead of building it underground.

From the link:

During the 19th century, the elevation of the Chicago area was not much higher than the shorelines of Lake Michigan, so for many years there was little or no naturally occurring drainage from the city surface. The lack of drainage caused unpleasant living conditions, and standing water harbored pathogens that caused numerous epidemics

So, any attempt to build an underground would have probably ended with just a flooded holes in the ground.

UPDATE: Searching for more info I found this geological map of Chicago. It seems to point that most of the area underground is formed by "beach ridges, sands and gravel"; maybe someone with knowledge about geology could inform us if this confirms the fears of water filtration for underground structures.

  • Oh yes, maybe I should ask another question about why Chicago was raised. – DrZ214 Oct 11 '16 at 22:52
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Based on my knowledge of geology, adding to @sjuan76's answer:

  • The geological map linked by Sjuan76 shows that most of the area underneath Chicago city is, as he says, beach ridges, sands, and gravel. These are extremely porous materials.
  • The quote SJuan76 references indicates a very low elevation and constant drainage problems.

These two factors mean that the water table is at or very slightly below the land surface - which in turn means that building a subway would require constant pumping during construction to prevent tunnels flooding, and watertight construction techniques.

This combination, especially at the time the elevated lines were built, would have made an underground system prohibitively expensive compared to an elevated system.

Also, according to the Chicago L history site, the early lines were built by private organizations, where the provision of the sewer system and raising of city buildings was handled by the city authorities.

Even though the raising project occurred prior to the first elevated line, SJuan76's link makes it clear that buildings were typically raised between 3 and 6 feet - not nearly enough to support a rail tunnel. That link also states that the infill material was soil, so the water table would have remained at or near its prior level, ensuring that any underground system would be plagued with drainage problems.

All the other cities referenced are low-lying, adjacent to major bodies of water (in the case of St Louis, the Mississippi), and rest primarily on soil, sand, and gravel. These cities would have the same issues with an underground system vs. an elevated system.

  • beach ridges, sands, and gravel. These are extremely porous materials. Does this mean they are hard to drill through? I thought being porous (more air) would make it easier to drill through. – DrZ214 Oct 11 '16 at 22:56
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    What happens when you dig through gravel? The digging is easy, but as soon as you stop, the tunnel collapses. This means you have to heavily reinforce the tunnels - the gravel, unlike rock, doesn't help you structurally. I'm not sure how much of an issue this is in practice. The big one is that porous materials allow water to flow through them, into the tunnel (especially when you're near and below a major body of water). – Max Oct 12 '16 at 6:42
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    As @Max says, easy to drill through, but collapse as soon as you take out the supports and let water through, so you have to waterproof everything and stay on the maintenance constantly. You also have a lot more issues trying to go under existing buildings because gravel moves more than rock. – Kate Paulk Oct 12 '16 at 11:37

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