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gravel driveway



So what’s the secret to getting a good gravel driveway into your new country property? Let’s start from the beginning.


You pull up to your friend’s place in the country and marvel yet again at that long gravel driveway that winds its way up to their home, because of how it’s so impressively smooth, hard and great to drive on. Then you think of another friend who’s driveway is a shocker – full of pot holes and like the waves of the ocean.


Must be the way they maintain it, you think to yourself.


But then again, maybe there’s more to it than that. Maybe it was how the driveway was originally built. ‘Built’?? you say! Yes, a gravel driveway is built, it’s constructed. And that’s what this blog is about – gravel driveways on country properties.


People don’t just grab some gravel from the nearby river bed and spread it out on the ground and ‘hey presto’ you have a driveway. There’s a lot more to it than that.


Think about the roads we drive on every day, the bitumen roads. The top layer of bitumen is only a skin, or facade over the top. Over the top of what? Gravel! The gravel underneath the roads you drive on creates a foundation or base for the bitumen to go on top later on, and in principle, the state of the bitumen on top reflects the condition of the base underneath. A poor base will eventually mean a horrible road to drive on.


So let’s start with the basics.


1. Determine the existing ground condition.

Step 1 is to determine the quality of the soil, the ground that the gravel is going to sit on. Is it moveable clay? Is it saturated with water? Is it rocky or sandy? A soil test will identify this for you, and every new home needs a soil test, so you’ll be able to use it for this as well. Rocky or sandy soil is ok, but a highly reactive clay or ground that’s really waterlogged can be a recipe for disaster if it’s not properly attended to.


2. Preparing the existing ground.

So the next step is what to do about the soil. If it’s that reactive clay or really wet, then you’ll need to look at whether you remove some of it to make way for the gravel, or whether you’re going to build the driveway up by piling the gravel on top. Whatever happens, you need to remove the top layer of vegetation (grasses, etc). This is to get rid of what could rot later on and cause instability underneath. You’ll also need to look at drainage solutions – will you need to build spoon (swale) drains beside the driveway to get the water away? Or will you need to dig trenches to put Agi pipes in or even concrete pipes under the driveway to let water through.


This step of what to do about the soil is a key step. Highly reactive clay will always stay highly reactive clay, so covering it up with gravel won’t necessarily solve the problem. Removing the top layers that have been saturated all their life can make a long-term difference. More on this later in the blog.


Another thing is the time of the year. If it’s in the middle of the wet season, you’re going to have to take more soil out, or be more drastic in your approach. And vice versa in the peak of summer, you can take advantage of its dryness to get the driveway down with less fanfare.


Just before we move into the next step, we want to talk about the gravel. We actually prefer the term crushed rock, because it’s a more accurate definition of what’s used in the industry. Gravel is a natural product, but typically has rounded edges and smoother surfaces. Crushed rock is made from exactly that – breaking up big rocks by crushing them with a huge machine. It results in sharp edges and jagged shapes which are great as a base, because they lock together.


The other thing about gravel is the ability to get ‘fines’ or ‘smalls’ (literally gravel dust) with the larger or bigger rocks. The industry jargon for this is called ‘minus’. So a 20mm minus basically means a combination of crushed rock pieces with an average diameter of 20mm, along with very very small crushed rocks that could be as small as quarter of a millimetre – the ‘minus’ part of it. This combination allows it to compact down hard, as the small fine stuff fills up the gaps between the bigger stuff and it sort of all locks together – brilliant!


And one last comment about crushed rock – it’s made from various types of rock such as bluestone, but whatever is generally available in your area. Obviously the harder the rock the better. And in this day and age there are a number of recycled options using concrete, brick and other similar materials.


3. Applying the final gravel.

So step 3 is to put the gravel down. This is a 4-sided approach – size of gravel; quantity of gravel; how many layers, and level of compaction.


Unfortunately there’s no one size fits all rule for this. But here are some pointers. If it’s a really wet site, start with 70-100mm minus crushed rock with layers of about 100 – 200mm thick. This really big stuff is tops for the way it wedges itself down into the moveable stuff, and with the fines between it and a whole layer of it compacted down, it forms almost like a flexible ‘raft’ of crushed rock over the top of the clay or mud.


If you’ve got a big excavator or loader putting the gravel down, then it can just be ‘track-rolled’ – this means you’re just using the machine to compact the gravel with its own weight. But if it’s only a small machine, or you want to be doubly sure about the long-term of the driveway, then we recommend getting in a 2 or 3 tonne vibrating roller to give it some oomph.


But if the ground is good, then you’ll probably only need some 40mm minus, and maybe only a layer of about 80 to 100mm thick, track rolled. This stage can be a bit of trial and error. You might find that after you’ve put some down there’s still a section that’s like putty and you need to put some more big stuff down.


Then there’s the next layer. This is generally where you use 20mm minus crushed rock, sometimes known as scalps, which is the most common size of crushed rock around. Put this down at about 50 to 100mm thick, but once again it depends on the soil quality underneath – the wetter the soil is the thicker your layers should be. And although this 20mm minus is great for the top layer (it can actually be screeded to quite a smooth surface) you can put another layer on of what’s called ‘dust’ or ‘fines’. This can be graded to a very fine tolerance and also compacts to an extremely hard surface, although can be a bit more dusty in dry windy weather.


2 other essential ingredients to the layers of gravel going down are water, and shaping. Just like baby formula – just add water. Crazy as it may seem, if you’ve got a wet moveable soil, you need to get plenty of water onto the gravel as it’s being spread out and compacted. As it sets it goes very hard – if there’s no water then it stays loose- the fines or smalls don’t solidify.


And shaping. Make sure there’s a camber in the road – or, the centre or crown of the driveway is higher than the sides so the rain runs off it. The main cause of pot holes is water pooling in one spot and vehicles then driving over that wet patch – it slowly softens the gravel and then the vehicles push out the loose wet gravel and a pot hole forms. You don’t want water to pond on your new driveway when it rains!


So there you have it – the ins and outs (or the ups and downs) of a good gravel driveway. Of course, any decent excavation contractor will know their stuff and won’t need to be told any of this – in fact, an earthworks person that’s worth their salt will be able to give you expert advice based on your specific circumstance. But sometimes a bit of knowledge will go a long way to make sure you get a decent lead-in to your beautiful new country home.

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