What to do about my gravel driveway

Factual Questions
21

This is sort of related to the topic at hand, and I am hoping perhaps one of the civil engineer types here can give me some guidance.

I am in the process of designing my own home and it has a long driveway. I really am hoping to avoid the gravel roadway (for one I believe locally the county isn’t too keen on them), and have been looking at alternatives.

One is Grasscrete

On the surface (hah! I kill me!) it appears to be a very good driving surface for a driveway that is used residentially. I would think a driveway that is used extensively would have some fallout from overuse, but for a residential driveway it seems like a good choice.

For one it seems to be great at being permeable which seems it would help reduce the gravel loss during water runoffs–it is easy to maintain as you just mow it and it seems to allow a more natural look rather then a black driveway such as blacktop or gray gravel.

thoughts?

22

I think that you’re getting a lot of information here. Some of it is general, some of it specific, so you’ll need to pick and choose what is applicable to you. I think that you’ve seen, however, that folks here who do this sort of design work everyday (raises hand) are more apt to ask you for a plan or a photo before getting too specific. That said, we let’s start in general concepts.

I know that terminology can be regional (and I’ll be using New England terms here), but in general “gravel” refers to a mixed gradation of mineral soils from smaller than sand up to cobbles. Typically the larger stuff is screened out. And you don’t want too many “fines” (silty particles that pass the 200 sieve). Ideally, a well graded gravel will give you the best structural base, because the finer gradations move in to fill the voids in the larger gradations. Think of a gravel road as your typical country “dirt” road.

What a lot of people call gravel (small, angular and relatively uniform sized stones) is called “crushed stone”. The railroad ballast mentioned earlier is a good example of that. You have to be a bit careful when working with all one sized stone, although the angularity usually ensures that the stones will lock together. You can get crushed stone in mixed gradations, just like gravel. Something we use around here is “dense graded crushed stone”. If you want a well constructed gravel road, it is hard to beat dense graded when placed on a gravel base. In lieu of dense graded, some people are turning to reclaimed pavement; the result of when pavements are milled down prior to being overlaid with new pavement.

This may help to address what your road should be constructed with, but there are other design factors to consider here that will keep you from rebuilding your driveway every time it rains.

Your road should pitch in a minimum of two directions. It should pitch from side to side, or if you prefer a crown, it should pitch from the centerline to either side. And your road should have a pitch along the length of the road. In general, 2% (1/4"/ft) is desired.

Stand in the middle of the driveway. Is the ground level about the same on each side? If so, consider a crown. If one side is decidedly higher than the other, consider just pitching it from high to low.

Now look down the length of the road. Is there a low point in the road? If so (silly as this sounds) make sure that’s where the culvert is. Otherwise, water will just pond at the low point.

Now you need ditches. I’d try to go 18 inches deep at a minumum. They serve a few purposes. If the ground on either side is higher than the driveway, you’ll need a ditch to intercept storm runoff, or it will run across your driveway. And even if the ground is lower on one side of the road, you’ll want to pick up that water if it looks like it will flow off your property and flood out your neighbor. Ditches served to channel that water parallel to your road and bring it to the low spot. And, if you’re in areas subject to frost, it helps to lower the water table under paved surfaces to avoid frost heaves.

So now you’ve got water flowing down either side of the road to the low spot, and into a culvert. 12" isn’t a bad size and will handle quite a bit of drainage; I like 18" as a minimum (it passes more debris that will clog a smaller pipe), but it’s real easy for me to spend your money. Obviously your ditches are going to have to deepen here, because your culvert should have some cover (depends on the pipe you use, could be from 6-24 inches) and you want your ditch to meet the bottom of the culvert. This also will increase the headwater depth which will help get the water through the pipe, provided it has somewhere to go once it gets through.

From your description, it sounds like the water just equalizes on either side of the culvert. All dressed up with no place to go. Unless you provide a positive relief for that water, all your hard work will be for nothing. You need to get that water flowing from the culvert away from your driveway and to the stream you mention. Again, without knowing how much water we’re talking about, 2% is probably a good rule of thumb.

The trouble with the generalities here is that I don’t know the pitch of your driveway, the grades on either side of your driveway, how much water shed is tributary to your driveway that will have to be handled by the culvert, and the amount of elevation change to get down to the stream. It could be that everything is damn flat there, and that always causes drainage problems.

I am not your civil engineer etc. etc.

Good luck.

23

Most places where I spec somthing like this are ditch banks or the bottoms of detention basins. However, it is a good alternative for driveways if you don’t mind mowing it. I personally would worry about differential settlement of the individual blocks - thereby creating peaks which would catch on mower blades - but if you don’t have expansive soils where you are, it could be fine.

Also, it will not like any big heavy vehicle (such as garbage truck or moving van).

24

I’ve used Grasscrete before (or one of their competitors, can’t remember what was finally accepted). I like it, with a few qualifications:

I think of it for areas where lighter duty traffic will be used. For example, I used it for overflow parking at a school that would only occasionally need that parking. I am more reluctant to see larger vehicles drive on it. Probably not an issue for a residential driveway, but you would know best.

The preparation for it is somewhat more intensive than for a paved road. Either roadway base needs to be thoroughly compacted, but when using these pavers it needs to be graded out - almost screeded - more like you would do for cement concrete than for bituminous concrete (asphalt). Asphalt is far more forgiving than concrete because it is flexible. In this case, you are working with a rigid paver. If they start to settle or to rise up, you’ll need to remove a section and recompact or regrade.

They’re being installed directly on gravel, and they don’t hold a lot of topsoil between the pavers, so the grass has to live in what is there. And the gravel will tend to wick away the water that lands on the driveway. I found that lawn grasses tended to not do so well, and drought tolerant weeds started to move in. That’s New England though; you may have some grasses where you are that do well in less than ideal conditions.

There are some alternatives now that use HDPE (Grasspave, I think?), and are unrolled on the gravel base. They sound like they are more flexible which might address some of my concerns, but I haven’t used them.