1

I have a question. I live in Scotland and I asked builders to do my driveway, about 60 sq meters. It is a driveway where I'll be parking the 2 jeeps I have, with a total weight of about 6 tonnes. These builders told me they are going to excavate about 40-50 cm, put 20 cm hardcore type 1, then 10 cm hard sand, then 10 cm soft sand and finally lay the blocks or slabs. According to them the driveway will be very solid and it won't sink. They said this is the way they all do up here in Scotland.

So I had a look around in the little village where I live and I noticed that almost all the driveways where people drive and leave their cars, are sinking, you can fairly see "waves" made by the weight of the cars.

So I asked a few friends of mine in Italy ( I'm Italian ) and they said that the driveways have to be done in this way : excavate 20-22 cm lay a heavy duty mesh like the one you use for ceilings, then pour 12-15 cm concrete and after a couple of days lay the blocks/slabs using a mix of cement and lime. The driveway will never move for 100 years!

They said I could do like in the first example if I wouldn't be going back and forth with cars every day. Please give me suggestions cause the whole thing is driving me nuts...thanks a lot

  • 3
    Hello, and welcome to Stack Exchange. It's difficult for us to say precisely what will and won't work in this situation, and there's a number of ways to build a "good" driveway. I'd be careful, though, to avoid drawing parallels to countries with different climates; e.g., some parts of Italy never deeply freeze, which would make a big difference in the building techniques used. – Daniel Griscom Mar 5 '17 at 12:54
  • In Dallas TX driveways are poured concrete with either steel mesh or rebar. I cannot say what the underlayment is, but from what I have seen it is minimal sand layer on compacted clay soil. These driveways don't sink, but they do crack. Some fully detached houses with semi-circular front driveways are surfaced in small pavers and some of these have totally unacceptable waves in them. I suspect that the supporting layers were not engineered or installed to a good standard. – Jim Stewart Mar 5 '17 at 13:52
  • One deficiency of ordinary poured concrete is that it is impervious to water and so contributes to rapid runoff of rain water and contributes to flash flooding in heavy rains. I have seen one installation in Dallas of so called pervious concrete (water soaks right through) and it seems this has a history of use in Scotland en.wikipedia.org/wiki/Pervious_concrete. – Jim Stewart Mar 5 '17 at 13:59
  • Marco, from experience it could be a matter of expectation and assumptions. Some contractors just expect those waves and divots you describe and would consider this normal. Sometimes it's difficult to express exactly what you expect. I would ask the contractor and explain or show an example of what you want to avoid. Good luck. – BrownRedHawk Mar 6 '17 at 15:23
  • Thank you verymuch guys, you have been very helpful!! I think you are right saying that the weather plays an important role in all this, and here the weather is quite crap, I wouldn't say too cold, cause very rarely the temperature goes below zero, so it doesn't freeze often, maybe 10 days a year, -2 or -3 °C. But the problem is the rain, I think it rains 300 days a year, that's why maybe the concrete is not the right solution. Thanks a lot!! – Marco Mirani Mar 8 '17 at 11:03
2

The most important step is the "base". Here, we excavate down about 12", then compact the existing ground (your finished product is only as good as your base), then install 10" of crushed gravel (not round rock) that is evenly graded from 2"" down to fines (sand), then install a leveling course of 3/4" crushed rock that is evenly graded down to fines and then place the "wear" course on top. (Of course each layer is compacted as it is installed.) The wear course can be asphaltic paving, concrete slab, reinforced concrete slab, stone slabs, brick pavers, etc.

Compaction can be by manual tamping, but it's better with a mechanical tamper.

We use crushed rock for the base because it "locks" together and is cheaper than making the wear course extra thick.

All driving surfaces will settle. The key is to break it up so the cracks are not noticed. We do this by using slabs with "control joints". so when each slab settles, the cracks are small and "designed" into the surface.

You mentioned installing "slabs". So, if you use concrete slab (with or without reinforcing) you need to divide it into "slabs" with control joints every 15' on center each way for non-reinforced concrete slabs and 25' on center for reinforced slabs. (Be sure to stop the reinforcing from going through the joints too.) The reinforced slab is good because it will "span" soft spots and not have differential settlement, but the reinforcing and location of the control joints need to be designed by a professional.

If you use smaller slabs, maybe something that could be picked up by 1-2 people, then each slab will move, twist, etc., but not be held in line and show more differential settlement.

If you're just building a couple parking stalls, I'd have my contractor install the gravel base and compact it and then stop and let you park on it for a couple weeks. This will take most of the settlement out. Then, the contractor could spread a thin layer of gravel to level it all up and then install the wear course.

What we use is not necessarily what you should use, because it may not be available without great cost. However, the principles are the same: 1) compacted soil, 2) good rock base, and 3) well placed sound wear course.

Make sure you have good drainage and your drive/slab will be there longer than you...maybe as long as The Appian Way.

0

I know you can avoid cracks in concrete by running deep joints where it changes in elevation as well as every few feet on even ground. Concrete still cracks but does so in the joints where it's not noticeable. This should help as well with any kind of movement of the slab. The strength of the slab poured or hand mixed is with the amount of rebar and mesh. I have always seen a large amount of both used in California where we build for earthquakes.It's also important to have the right mix of sand,rock,and concrete.You can call the plant that makes it and they should be able to advise as to what you need for where you are and what you are doing.The only other thing will be for it to be leveled out to begin with.This is done with a rod stick {2 by 4} preferably one that reaches end to end.

  • Thanks a lot for your help!! I decided to do how the contractor is telling me to do!!! The weather here is wet and super wet, so basically better to avoid the concrete and use sharp and soft sand so that there is abetter drainage!! It freezes maybe less than 10 days a year so probably it'd be better like the contractor suggested...thanks a lot – Marco Mirani Mar 8 '17 at 11:12
0

Agreed that different climate makes a big difference. In Alberta we will get frost 6-8 feet deep on areas that don't have snow left on them.

Rural roads here are largely gravel. There are paved roads between towns. When I visited the U.K. I noticed that many roads were pave there where here they would have been gravel. It may be that gravel traffic surfaces don't work rainy climates. Gravel roads here are regraded at regular intervals (4-8 times a year) to remove potholes, and re-establish the crown.

Here there are various ways to do drives.

A: Concrete.
Excavate to undisturbed subsoil. Fill with road crush (Crushed limestone -- whatever passes through a 3/4" screen -- sometimes called 3/4 on down. Pack mechancally every 4 inches. Top with 4 inches of concrete, usually with both rebar and mesh in it. Crack lines are about every 8-10 feet. This is by far the most common method used in the cities.

B: Gravel As before, take off the topsoil down to undisturbed subsoil. If you're cheap, you start with a foot of "river run" This is round rock from the nearest gravel pit. Not crushed, but usually screened to 3 or 4 inch. It does not require packing. This is topped with 6" of road crush and packed.

This type of surface requires occasional maintenance. I usually have to buy and spread 20 cubic meters of road crush every 6-10 years. (400 feet of lane spreading out to 30 x 70 foot parking pad.

C: Asphalt Sometimes you see this done when roads are paved in the neighborhood. Uncommon.

D: Pavers These are most often zigzag interlocking stones. Prep is similar as for concrete, but the last layer of road crush is topped with builders sand, which is then packed flat. Pavers are set this. The paver mold leaves bumps on the bottom edge that sets them about 1-2 mm apart. Finishing sand is brushed to fill the spaces.

The downside of both pavers and gravel is vegetation. Neither is weed proof. Chemical agents or a torch are most effective to keep a neat appearance.

E: A method I've only seen a few times: Use breeze blocks instead of pavers. A breeze block is 4" thick by 12x12 and has a pattern of holes in it. The block is 50-60% holes. It's made of the same sort of mix as concrete construction block, sometimes with a finer finish.

The surface is prepared as for pavers. These block are laid flat, with about 1/2" to 3/4" space separating the blocks. Spaces and holes are filled with top soil and seeded with short turf grass. This will withstand light traffic. The blocks keep traffic from compressing the soil to the point where grass doesn't grow.

  • Thanks a lot, you have been very helpful!! Here the weather is quite shit, never so cold like in Alberta, but it is very very wet, infact the problem here is the dampness!! So I'd say it would be better to avoid concrete and use like the contractor told me, the first 20 cm hardcore type 1, then 20 cm of sharp and soft sand and then finally lay the blocks..In a couple of years I'll tell you if the driveway sinks!! I am leaving my 2 jeeps, about 5 tonnes, on the driveway!! – Marco Mirani Mar 8 '17 at 11:08
0

the right way to do your driveway is dependent on a few things, but primarily, its water and cold. in scotland, areas down by the coast will have more rain, but less cold. inland or in the highlands, you will have less rain (and snow) but more cold, and thus more frost penetration.

i have seen this problem here in southern ontario for decades. its almost always entirely based on inexperienced contractors putting in things that they don't really understand. remember, for many contractors, their warranty is only as long as it takes for your cheque to clear the bank.

when you construct a driveway like you are describing, you have three issues that create the ruts you don't want.

first is the soil fluidity below the driveway. water, freeze/thaw cycling and vertical load all cause the soil to subside in similar ways such that areas of high load (where the tires are) to sink more than other areas. this is why the whole thing is dug out and replaced with coarse gravel. it helps to allow drainage (no water means less fluidity to the soil), it helps to spread the load out (the tires push down on a small contact area, but the gravel spreads the load horizontally through the base proportionally to the depth of the gravel bed)

second is the amount of precipitation you receive and how much it freezes. the more rain you get, the more "lubricant" there is in the soil, gravel and between the paving stones. this all contributes to things moving, which is what is makes the ruts. if you set the pavers into sand only, it maximizes drainage, but lets water get under the driveway. if its not properly sloped or drained, that water will lubricate and under cold conditions, freeze. freezing is the biggest destroyer of driveways as the mechanical pressures from ice formation are huge and can move things large distances. if you put the driveway on concrete (as your italian friend suggests), it has to be well drained and well designed/reinforced. it will keep the water out better, but over time as small fractures and lack of sealing let water in more and more, you will get more water permeation. but now it has nowhere to drain to (as the concrete isn't porous the same way gravel is) and it will cause more damage if it freezes than it would with just gravel. this approach is fine in warm areas like Mediterranean italy or southern california, but its very expensive to do it properly in cold or cold and wet areas like scotland.

thirdly is mechanical reinforcement. this is an often omitted step that lots of less qualified contractors omit. by adding metal, or preferably fibre-reinforced polymeric binding fabrics, in the right ways and in multiple layers, you can horizontally bind the base aggregates. this may seem counterintuitive, but binding the base horizontally almost entirely eliminates the rutting problem you want to avoid. since it prevents the base from moving on a microscopic scale, it also prevents vertical movement of particulates. which means no ruts. however, it has to be done well and with the right geotextiles. its not just weed fabric from the local gardening store. someone with experience with these systems in your specific area will be the best choice as they will know the available fabrics and how they interact with locally sourced aggregates.

a really simple solution is just to hire an independent engineer to design the parking area. they will specify exactly what to build and how to do it. then any knucklehead landscaper can install the parts. you'll get a better slab, they will learn the right way to do it, and everyone is happy. might cost you a couple thousand bucks extra, but its worth it if the slab stays flat for 30 years.

  • Thank you for your help, it's been very helpful, at the end I decided to do how the contractor is telling me to do!!! The weather here is wet, it rains 300 days a year and it freezes maybe less than 10 days during the year so the important thing is a very good drainage which probably I wouldn't have with concrete!!! Thanks a lot and I'll let u know in a couple of years if my driveway sinks!!!! – Marco Mirani Mar 8 '17 at 11:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.