Was just wondering about this.
It is pretty well accepted, apparently, that a new asphalt driveway for
a residence should be laid over about 4" to 6" of gravel.
Never over soil directly.
The hot, liquid, asphalt I would pretty much think makes the gravel into
one, solid, "clump" underneath (the asphalt).
So, it's hard to see that any improved drainage results.
Increased "stability" perhaps ? If so, how ?
So what does the gravel add as a benefit over just laying the asphalt
on top of well compacted soil ?
> > Hello,
> > Was just wondering about this.
> > It is pretty well accepted, apparently, that a new asphalt driveway for
> > a residence should be laid over about 4" to 6" of gravel.
> > Never over soil directly.
> > Why ?
> > The hot, liquid, asphalt I would pretty much think makes the gravel int= o
> > one, solid, "clump" underneath (the asphalt).
> > So, it's hard to see that any improved drainage results.
> > Increased "stability" perhaps ? If so, how ?
> > So what does the gravel add as a benefit over just laying the asphalt
> > on top of well compacted soil ?
> > Thanks,
> > Bob
> A: asphalt pavement is not liquid
> B: the gravel is for drainagek and stability.
The folk responding about asphalt are right, so I won't duplicate their
effort. However, you might also be recalling the old Macadam road technique
with the modern version.
In my home town, many of the streets were Macadam and they did very well.
Occasionally, the street department would employ a tractor with wheel disk
to turn up the streets, and then they'd be rolled smooth, with a thick layer
of asphalt then applied. The final touch was a new coating of pea gravel to
reduce tar pick-up by car tires. The streets held up beautifully and the
technique most likely would be considered, "Green," in today's world.
Joe wrote in
Then there's "hardtop", which is common in my area on lightly-loaded roads:
You lay down the usual packed-aggregate roadbed, then spray really runny
tar on the surface. Then you spread pea-gravel on top of that, and just let
passing traffic roll it in the rest of the way. Excess gravel gets pushed
off to the shoulder over time. It's fairly durable if you keep heavy trucks
off of it.
Where is your hometown? I noticed you used the word "Macadam", that is
a rare word in the US. Named for a man named John McAdam. The first to
use stone and roll it to lock the stones together to make better roads.
Later he added oil/tar on top to help it last through heavy rains.
That is were the word "Tarmac" became popular and is still used at
airports. I come from a part of PA, USA where the outdated word
"Macadam" was still used to describe asphalt/blacktop. If I use the
word today not many people will know what it means.
The hometown is a small farming community in central MO, but dates back well
before the War of Northern Aggression. The streets there are nowadays a
typical mixed bag of asphalt, concrete and Macadam, but as far as I know,
the Macadam ones are still maintained by the occasional wheel disking,
rolling, new tar and a coat of pea gravel. FWIW, most of the older curbs
are cut granite.
I'm in upstate NY & grew up in Schenectady, Schoharie & Greene
counties in the 50s-60s. I used the term last year and was amazed
that none of the folks I talked to knew what it was. My wife grew
up in Saratoga county- and the folks I was talking to were all from
the NY city area. [and a bit younger]
I didn't think to look it up back then, but when Micajah used it I
Wiki'ed it. I knew it was named for Mr. McAdam- but didn't know
the specifics of *his* roads.
I've always used the term[improperly, it appears] to refer to any
The asphalt material is slightly pliable. The surface underneath needs to
be a hard compacted surface -- I think it is compacted modified stone (or
something like that). That creates a solid surface for the asphalt
material. The first layer of asphalt material needs to also be compacted
with at least a 5,000 pound roller and is called a stabilizing layer. Then
a second coat of asphalt goes on top of that and is compacted.
Huh. I'm late 50s in upstate NY and that's what EVERYone around here calls
it, both younger and my age. Or, at least, I've never had any strange looks
or questions. Probably because I'm in the boonies. I'll admit, though, "Red
Asphalt" is a sexier title for a drivers' ed video than "Red Macadam".
The enemy of a road, driveway or parking lot is water under the surface.
The inevitable vertical movement of the surface, when a vehicle passes over
it, creates what's called "Pumping" of the dirt under the pavement. It
liquefies dirt, clay or other fine material, removing even more support for
the surface layer with the inevitable cascading result of alligatoring and
pot holes. Compacted and drained stone doesn't lose support for the surface
when moved about by traffic and if the water can be turned away at the
surface level, then the underlying dirt doesn't liquefy. Just as important
as the water-turning ability of the road surface is the ability for the
water that makes it through to escape. That's why there are ditches and
stone underlayment. They're just as important in draining out water under
the roadway as they are to carry off rainwater to prevent roadway flooding.
I know of at least one very long stretch of an Interstate highway where the
designers failed to understand this. The cure was to trench deeply
alongside the concrete roadway of both lanes, on both sides, install drain
tile and then backfill with coarse stone to carry the undersurface water to
By the way, expansive soils are also an enemy of roads, just as they are an
enemy of basements. One lady, who was an NCSU graduate engineer, didn't
understand what expansive soils were and approved an entire highway
interchange to be constructed using expansive soil for the ramps. It never
even opened before the concrete began to crack and fail.
"Micajah" wrote in
I didn't know that. We just call it "hardtop". That's to differentiate it
from the softer gravel/sand mix which goes on roads that are lesser to
hardtopped roads. The big advantage of hardtop is that it's really cheap to
install compared to asphalt, but is a lot more durable (and cleaner and
washboard-resistant) than gravel/sand.