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These days the city is repaving the main road close to my office. Looks like the rainwater drainage system is being upgraded and curbs and drains are added to manage the flow of rain water. Few casual google searches helped understand some of basic cross section for a new asphalt road. An image of the search is attached below.

Cross Section

Below is a picture from the current repaving of the road, where an excavator is scoping out the dirt and trucked out. Then the dirt is replaced by different types of crushed stone.

Road Picture 1

Road Picture 2

Questions: What is the engineering explanation for all this additional rework? To begin with was the road sinking? To the casual user the road appeared to be perfectly in good shape, thus a waste of tax payer money. How can the additional cost be justified?

References:

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  • $\begingroup$ Would you say the road being repaved was in a similar condition to the one next to it (which does indeed seem to be in very good condition)? If they were in similar condition, the lack of cracks on the neighboring road does make "structural" reasons unlikely (assuming they carry a similar volume of vehicles a day). And the last time there was heavy rain, were there any drainage issues? That seems the most likely reason. $\endgroup$
    – Wasabi ♦
    Jul 7, 2015 at 2:14
  • $\begingroup$ The paved road is the old road. I have not seen any crack out of the ordinary. The is steady slope toward the font of the excavator. There is no know rain water drainage issues. $\endgroup$
    – Mahendra Gunawardena
    Jul 7, 2015 at 2:26
  • $\begingroup$ @MahendraGunawardena: Your question has taught me something. I didn't know crushed cement/concrete was being used as a form of aggregate for road foundations. I supposed it's a way of using a engineered material with uniform properties. Another reason may be the lack of suitable sized aggregate within a reasonable distance from the location, it's an issue that a lot urban areas worldwide are dealing with. One thing I became aware of years ago, was due to the lack of any hard rock aggregate in Bangladesh, clay is baked and then crushed to create an aggregate for road construction. $\endgroup$
    – Fred
    Jul 7, 2015 at 3:45
  • $\begingroup$ @fred, I did learn something new too. :-) $\endgroup$
    – Mahendra Gunawardena
    Jul 7, 2015 at 11:11
  • $\begingroup$ I do not know where "the city" is but comments made to me about similar situations in India when I was there in 2014 suggest that answers may lie somewhere between the engineering ones offered by others and that the tenderers uncle is in charge of the department that issues approvals and the aggregate supplier is .... . $\endgroup$
    – Russell McMahon
    Jul 8, 2015 at 12:25

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During my student days, ages ago, during one of my civil engineering lectures the lecturer told everyone that roads were designed and built for a 20 year life. After which the road could be dug up and new foundation materials laid.

After a prolonged period, and particularly with roads that are heavily utilized, foundation materials can move and need to be replaced and compacted anew.

The replacement of the foundation materials in your example may be an opportunistic upgrade brought about by upgrading the drainage system. Instead just upgrading the drainage now and ripping up the road "a few years later" do everything now. It also means the road and the new drainage system will blend in with each other and ensure the drainage system functions as designed.

Also, if work for the upgraded drainage system involves deep digging it may damage the edge of the foundation of the road which would require the road foundation to be relaid.

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When a road needs to be repaired there are a few typical procedures that are used depending on what is wrong with the road.

When the base (aggregate) of the road is replaced, it means that there was something structurally deficient with the road. The base is there to:

  1. Provide drainage so that water doesn't collect under the road surface.
  2. Replace weak soil.

If the aggregate layer isn't thick enough, too much water may be collecting under the road surface and causing weak spots. Also, if the layer isn't thick enough the load from vehicle wheels is not able to distribute over a large enough area. This may be over stressing the soil.

The direct answer to your question is that, yes, there was something wrong with the road. They aren't digging it up for the fun of it.

The signs of a weak subgrade may be easily over looked if you aren't specifically aware of them. The signs start out as cracks and progress to pot holes as things get worse.

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This is definitely not a simple repaving job. The second sentence gives it away: drainage system is being upgraded and curbs and drainage are being added. Converting a roadway from a flush shoulder with a swale (ditch) drainage system to one with curb and gutter is not a straightforward process. There are several considerations when installing new curbs on an existing roadway, and possible reasons for the reconstruction:

  1. A drainage system will be required. Because a straight section of road slopes from the center to both sides, curb inlets will be needed on each side. A drainage system is typically connected at one or more points between sides of the roadway, because the water has to be channeled into a pond in order to treat it before sending into natural waterways. Placing large pipes across the roadway will inevitably necessitate the sort of reconstruction you see in the pictures.

  2. Flush shoulder roadways have a different profile than those required for a curb and gutter roadway. With flush shoulder roads, water drains to the side anywhere and the swale has the proper slope to carry the water away. With curb, a gutter (or in your case the edge of the pavement adjacent to the curb) has to have a slope to carry the water to the nearest drainage inlet, which is placed at a low point. A gutter or edge of pavement has to match the slope of the rest of the roadway, so, especially in flat areas, the whole profile of the roadway needs to be adjusted to have what is referred to as a "sawtooth" where there are low points, high points, low points, and consistent, smooth slopes in between to direct the water to the drainage. This can sometimes be done by merely building up the pavement to create high points, but that may not be possible because it would reduce the clearance for vehicles under that pedestrian overpass. This could be the reason for the reconstruction.

  3. Future traffic considerations. Pavement designs take into account the predicted traffic volumes (especially the heavy vehicle loads) the road will experience in 20 years, the lifespan of the road. The current pavement design may be insufficient for this volume. Again, with that overpass, this may not be able to be resolved with merely making the pavement thicker.

  4. Existing deficiencies. If there was rutting or potholes forming in that area caused by some weak soil beneath the roadway, the right way to resolve that is to reconstruct the roadway foundation.

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