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Here's how a typical earthfill carrying a roadway joins a typical bridge

earthfill joins a bridge

Parts of the earthfill slopes are greenish - that's grass - and parts are whitish - that's concrete.

The earthfill slope is fully covered in concrete where it joins the bridge but the rest of the earthfill is only partially (lower part only) covered in concrete, the rest of the slope is grass-covered.

This photo is of a 20+ meters high earthfill and lower earthfills' slopes are usually not covered in concrete at all except where they join bridges - the join is usually fully covered regardless of how high the earthfill is.

What's so special in that part where the earthfill joins a bridge that this part is fully covered in concrete while the rest of the earthfill is either partially covered or not covered at all?

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  • $\begingroup$ I'm no structural engineer, but it seems obvious that you don't want the slope right at the bridge end to erode. $\endgroup$
    – Olin Lathrop
    Apr 2 '15 at 13:20
  • $\begingroup$ @OlinLathrop Why not just cover it with grass then? Grass prevents erosion rather well. The rest of the earthfill is important too and it works just fine with grass. $\endgroup$
    – sharptooth
    Apr 2 '15 at 13:22
  • $\begingroup$ No, grass doesn't prevent erosion as well as concrete. It also won't stop overall gradual settling of the slope. You can flip the question around and ask why not concrete everywhere. I think the answer is cost, which only is worth it in the areas that need to be especially stable. $\endgroup$
    – Olin Lathrop
    Apr 2 '15 at 13:29
  • $\begingroup$ FWIW I don't think I've seen a concrete strucuter like this with bridges in Germany. Could the area beneath the bridge be flooded on occasion? $\endgroup$
    – mart
    Apr 2 '15 at 17:40
  • $\begingroup$ @mart Yes, it could be flooded for up to several days in spring but that's not very likely. $\endgroup$
    – sharptooth
    Apr 3 '15 at 7:04
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This is called Slope Paving. (In the US at least.)

It is done for two reasons at bridge abutments. These reasons are related:

  1. Steeper slopes are possible.
  2. Protection from undermining abutments and piles.

Steep Slopes

The soil that is used to create the earthen fills will only stand up naturally to a certain steepness. By placing a layer (~4in) of concrete over the soil, the slope can be steeper than would naturally work. The concrete reinforces the soil.

Steeper slopes are beneficial because they reduce the footprint of the embankment fill. This reduces the length of the spans by allowing the bridge abutments to be closer to the roadway underneath.

Abutment Protection

The concrete slope paving also helps to protect the soil from eroding under the bridge abutments. The abutments (and piles) rely on having soil around them. This soil needs to be kept in place to ensure that the bridge performs as designed.

Some locations also put drains for the bridge deck near the abutments. It is simplest to let these drain pipes empty on the slope under the bridge. If the slope was not paved, the water would quickly erode the soil slope.

The area under the bridge is partially shaded, so it can be difficult for vegetation to grow there. Natural vegetation would help with erosion, but if no plants grow, the bare soil would erode quickly. This concern is eliminated by covering the slope with concrete.

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I remember a plenty of older bridges that have these places with soil as well. It's always barren earth with no vegetation whatsoever due to shortage of light and total lack of rainfall.

It's often harboring homeless, rowdy youth or kids, who seriously erode the slope with their feet. There's often trash, broken flasks, discarded refuse of homeless sifting through their "loot". Generally, unpleasant, dirty, barren spots.

Covering them with concrete, especially "grid" slabs with gravel filling the holes, making walking or sitting quite unpleasant (if much safer) - solves some of these problems. Considering there won't be any foliage, there are hardly any disadvantages.

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    $\begingroup$ Although I think your point of preventing human based erosion is a valid one, your answer has a few too many stereotypes about the people who might cause the erosion. $\endgroup$
    – Chris Mueller
    Apr 2 '15 at 14:57
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    $\begingroup$ @ChrisMueller: Political correctness may be all trendy recently but I'm a hobbyist geocacher, and the bridges are frequent cache hiding places. I've been to quite a few in person and I'm just reporting the first-hand observations - these places are really seedy locations, especially the river bridges (presence of a road or railway below seems to discourage the "guests"). $\endgroup$
    – SF.
    Apr 2 '15 at 16:27
  • $\begingroup$ The people I most often see making use of the area are motorcyclists waiting out a storm. $\endgroup$
    – Chris Mueller
    Apr 2 '15 at 19:10
  • $\begingroup$ @ChrisMueller: With river bridges? Unless they are on dirt bikes, that's unlikely; usually the approach from the road above is quite uncomfortable. Besides, storms happen only ever so often. Personally, I often find kids (still not the "difficult teens") playing there, they seem to find this kind of places fascinating. But there's more than a little "traces" of the less pleasant guests. $\endgroup$
    – SF.
    Apr 2 '15 at 21:34

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