Recently there has been road construction around on the main road leading into my office building. I observed the construction crew removing asphalt in the form of a trench as showed in pictures below. Later on they filled up with aggregate and poured a layer of black tar and filled with something. The trench has been filled and resurfaced in line with the existing road surface.

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Attach is the before picture (Satellite) captured from google maps.

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Attach is the before picture (Street) captured from google maps.

Google Maps Street Image

Question: The rain water is being directed to a near by manhole. What is the engineering explanation for digging the trench later on was poured with liquid tar (asphalt crack filler)?


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    $\begingroup$ What do you mean it was in the form of a drain, or were there actual drain features to it - a steady gradient, a soak-away, something like that? Or are you merely describing the fact that it's a trench that's rectangular in cross-section? Please add the photo of the road before the works too - it's not very meaningful without that. $\endgroup$
    – 410 gone
    Aug 22, 2015 at 19:07
  • $\begingroup$ I am sorry there are not before picture, but the it is an extension of the current existing road surface. $\endgroup$ Aug 23, 2015 at 11:14
  • $\begingroup$ @MahendraGunawardena, try using Google Street View to see if you can show the "before" picture. $\endgroup$
    – Wasabi
    Aug 23, 2015 at 13:43
  • $\begingroup$ @Wasabi, good idea, Though not crisp, I posted the picture from google maps $\endgroup$ Aug 23, 2015 at 13:57
  • $\begingroup$ @MahendraGunawardena, that seems like the satellite view. Can't you get the Street View, which would be a better quality? $\endgroup$
    – Wasabi
    Aug 23, 2015 at 13:59

1 Answer 1


Given the very little detail you've provided, it's not possible to be sure.

But I don't think it's any more complicated than a simple resurfacing. The top layer has been planed off, and a new top layer laid.

Planing is done when the underlying structure is fine, but there are superficial problems: a maintenance regime of frequent planing and resurfacing can help reduce the whole-life maintenance bill, by fixing problems early before they propagate to the underlying structural layers (layers which are much more expensive to replace).

Wikipedia currently says that this process is also called "pavement milling", "cold planing", "asphalt milling", and "profiling".

One possible reason (of several) why one might just plane the strip alongside the footway, is that traffic can cause indentations, bumps and grooves along the surface, and when that's along the edge of the road, that creates ponding of water after rainfall: instead of the water all draining off, some of it gets trapped in the dents and grooves. This creates a problem for pedestrians and any other kerbside activity getting splashed, and can decrease the lifetime of the surface.


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