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Unfortunately it’s very rare that a gravel driveway construction will require no excavation. At the very least, the topsoil should be removed as its load-bearing ability is poor and the heavy concentration of decomposed organic material makes topsoil particularly prone to retaining water. The depth of topsoil that needs to be removed depends entirely on your area and can vary anything from 50-300mm in depth. Once the topsoil is removed you should have an exposed layer of firm subsoil. Any further digging required will be governed by the desired finished level and will vary from site to site. Any soft areas that remain after topsoil is removed need to be excavated and then filled with compacted sub-base material before the membrane is laid.
It is a common misconception that geotextile membranes are placed between the sub-base layer (Type 1 or Type 3) and the finished surface (gravel) which is not the case. The membrane should be placed between the excavated ground and the sub-base.
The presence of a membrane ensures that the sub-base material doesn’t mix with ground below while still allowing the passage of water, and is essential where ground conditions are poor. However, it is good practice to put a second layer between the top layer and sub-base. This prevents the two types of stone merging – especially with the movement of vehicles.
Laying The Sub-Base
The sub-base is the most important part of any driveway construction. It provides the load bearing ability of the driveway by helping to spread the weight of traffic from above, whilst also creating a solid layer resistant to rutting and channels. The depth of the sub-base will depend on the size of vehicles using the drive with a typical domestic construction requiring a minimum depth of 100mm for foot traffic or 150mm to be able to take cars.
The most common sub-base material is Type 1 MOT which consists of crushed rock graded from 40mm down to dust. The size range ensures that the material interlocks when compacted whilst still remaining permeable to water. Increasingly, a product known as Type 3 MOT is being used which is very similar to Type 1 but contains a lower fines content and is more permeable to water. The use of Type 3 complies with Sustainable Urban Drainage Scheme (SUDS) protocol which is increasingly becoming the norm in the UK as flooding issues become more prevalent. All sub-bases should be installed in thin layers and compacted with a vibrating roller or wacker plate to ensure full compaction throughout the sub-base layer.
Laying The Gravel
Now the easy bit – the finished surface. The biggest mistake made by DIYers is laying the gravel too deep. The gravel performs no other purpose than to provide a decorative finish to the driveway. Laying the gravel too deep not only costs more and takes longer but has no benefit to the performance of the finished driveway. If you want to go deep make sure it’s with the sub-base! It is recommended that a maximum depth of 50mm gravel be applied as this provides good coverage of the sub-base whilst remaining shallow enough to prevent rutting.
The gravel/shingle used on the driveway should be 20mm in size and ideally be angular in shape, not like a smooth pebble. Smaller sizes such as 10mm and 6mm should not be used for driveways as the smaller particles get stuck in car tyres and also provide a handy toilet for neighbourhood moggies! With the exception of Welsh Slate which is very flat, larger gravels should be avoided as they can be difficult to walk on. The gravel is simply laid directly on top of the top membrane and levelled with rakes and shovels.
Geo grids for extra load bearing or securing the top dressing
A gravel driveway can require levelling and raking to keep it functional as the gravel is easily displaced laterally. Often, gravel driveways look aesthetically pleasing when first laid but over time the gravel migrates. This means that the earth below may become rutted and non-load-bearing, leading to voids that require regular maintenance. A geo grid system retains the gravel, by holding it firm in cells to provide a stable surface. This not only prevents rutting or movement of the top dressing layer, but when installed on the correct sub base thickness it can achieve a weight loading of 350 ton m².
The interconnected lattice structure of geo grids also makes it ideal for slope reinforcement that would otherwise be subject to deterioration due to gravity or wear-and-tear.