Technical Notes On Paving: The Base

I have been battling a bug and not up to my posting standards of unremitting and sensational posts.  Oh well, eh?  Here, then, is a recycled post from an earlier time, yet more relevant now, by far, owing to my recent posts regarding driveways and large surfaces.  Thanks, guys, I’ll be back in fighting trim in no time flat……….enjoy:

It is a truism in landscaping, road-building and all things related to the installation of any hard surface that the base is perhaps the single most important strictly functional element in the entire deal. It is also true that any surface will reflect in terms of longevity that preparation. So what am I talking about?

A “base” or “Sub base” is that material beneath the surface. If we lay in an asphalt driveway, we use this weird gravel material, spread out by graders and then rolled over a bunch of times by those big double-drum rollers, or “steamrollers” as we used to call them as kids in America. We watch this occur as if it were nearly natural. “Hey, it’s those rocks they put in under roads,” is our usual statement, as if it meant something. Well, yes, it IS rocks, but it is also “fines”, almost dusty material crushed along with the rocks in these gigantic machines used to crush rock which then gets taken by conveyer belt to a system of “sieves”……meshes where progressively smaller materials can get separated from larger from the processing.

In the end, one of the “sieves” only allows a certain limited size to filter through – in this case, let’s call it “3/4″ minus” material. Naturally, the “minus” deals with the powdery residue which is as necessary to binding compact able material as it is to cement itself. What happens with this material is that, when applied at a proper depth and thickness to the roadwork, a combination of water and compacting with these huge machines occurs. Those “fines” serve to nestle in, carried by either water or force, or both, to combine to make an amazing durable, hard layer. It is nearly, but not quite, cement – just at the flexible end of “monolithic”.

The real trick is assessing the need for what depth we need to achieve. If soils under this layer are moist soils with lots of organic material in them, there will be a need to add a very thick layer of base material to stabilize and maintain a permanently hard subsurface. Once the upper, or finishing, layer is applied, it will deal with issues like water and erosive factors which would affect the substrate. The problems of durability then become how the sub base was applied. If it is a thin layer, insufficient for compensating for loose soil under it, it will sink as the organics break down, or will become misshapen as pressure is exerted downwards, into vulnerable areas. If however, it is of sufficient depth to compensate for the always-looser soil underneath, one can expect a never-changing level for a sidewalk with far fewer eventual problems, such as concrete breakage or grade changing events.

I have seen some classic failures in driveways and even walkways and patios when someone tried to pinch a few pennies of the sub base and failed to achieve a degree of compaction that assured the surface of permanence. The sub base needs to be pounded and then pounded down again to achieve the degree of compaction necessary. Once this is achieved, we’re half way there. But this is, by far, the most important task of all that proceed from it. Ignore this one at your peril.

My advice to those interested in getting a new driveway or introducing some hard surface onto their place is to make sure full attention is paid to this seeming detail. Ask how much base material the contractor is expecting to use and at what depth. Do some math. Make sure your contractor realizes you understand how important a well-compacted sub base can be. Here we have just that: in the foreground, the compacted gravel comes out from the concrete blocks at the right. Notice we are adding an inch or less of sand above it in order to lay in the brick pavers.

If you want to see this in all its stages, click on the right of the main page of this blog for October 2006 and September 2006. It’s reversed, I’m afraid, but every step is dealt with there. Check it out.

4 thoughts on “Technical Notes On Paving: The Base

  1. I learned a little about fines when I had the ‘terrace’ and the path to it created. Actually, I learned a lot, because I read the American Rock Garden Society’s book about rockeries and rock gardens and it covered the subject of paths, too. You are right when you wrote that it will set up almost like concrete. We couldn’t use a roller, and it is only a pea gravel path, so I guess it will all sink in over time.

    I would love to have our old asphalt driveway replaced with pavers. If we do, I hope I can remember what you wrote and will be able to ask the right questions. Thank you for the info.

  2. Barbee, I’ll always be glad to help from here. I could help plan some, if you were interested.

  3. Great technical expertise shown here. Great info on the subbase. Although not everyone will want to do this themselves, you have shown how this should be properely done.
    A great resource.

  4. I think some knowledge is proper and most usable, even if one is hiring this work out. I wish I had a nickel for every failed driveway I have seen over the past 20 years owing to an improper or insufficient road base. This sort of information so vital, it just bears repeating.

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