The first thing to consider when paving any area is the foundation or base, without a solid base, it doesn't matter what you do with your surface material, it will simply not hold up. The primary material used to create a solid base is "A" Gravel, which creates our solid foundation and is easy to grade.
"A" Gravel is approximately 3/4 inch stones, mixed with sand, and often a packing friendly material. Sometimes the material is a small amount of regular earth or clay, sometimes it is recycled asphalt or concrete. Generally at O'Brien Paving we use gravel with a mix with recycled concrete or with a natural ability to pack. Depending on the conditions of the area and the use of the finished surface the "A" Gravel base should be between 8 inches to 18 inches thick. For a standard driveway or light traffic parking lot a 10-12 inch "A" Gravel base is optimal. Often when re-paving an area the base will be perfectly sufficient and this is evident by the type of degradation on the surface, which our experts will take in to account when quoting.
Why "A" Gravel Works Best:
The "A" Gravel mixture is balanced to create a material which does 3 things very well while still being easy to grade:
1.) Stays put -> The stones in gravel act as barriers for the sand so that when pressure is applied from the top or water is flowing through the gravel, the smaller particles stay in place.
2.) Allows water to flow -> The sand's primary purpose is to ensure that water will always be able to flow under the surface and not pool as it would with clay or dense soil. Pooling water is a problem for two reasons; it can freeze and expand in the winter pushing up the asphalt then create a cavity when melted allowing the asphalt to sink, or pooled water it can create a soft spot in the base which can not withstand pressure from the surface. The sand also fills in the gaps between the stones and does not allow them to fill up with the non porous soil or clay around the area being paved.
3.) Packs tightly -> While conserving the ability to allow water to flow the base needs to be strong and packed tightly so that it can withstand heavy loads and not shift substantially. The packability, like the stones also help hold the gravel in place. Small amounts of naturally occurring soil (or very fine sand), mixed in recycled concrete, or mixed in recycled asphalt all do this job splendidly. The soil or very fine sand, and the recycled concrete have extremely small particles allowing them to get very close to each other with no gaps (packed), the recycled asphalt has bitumen which is a malleable material allowing the same to occur by conforming to the shape of the materials around it.
In most circumstances worrying about anything below the 12 inch mark is not an optimal way to spend your money. Often the sub-base is more than stable enough to do what it needs to in order hold up for as long as the surface can.
There are of course some cases where a little more attention should be paid to the sub-base. While not exhaustive the two main reasons are heavy traffic and clay deposits.
1.) When we say heavy traffic we literally mean weight, if a surface will be driven over by heavy equipment or transport trucks it is often good to have an engineered sub base of gravel fill or "B" gravel that goes deeper, this base is made of cheaper, harder to grade materials, that have the same properties as "A" Gravel.
2.) Clay deposits are areas with so much clay that they will hold too much water and do not act as a stable base. This can be over a whole driveway or parking lot, or just in a small section. It is the most common reason for us to go deeper than 10-12 inches. Again, our experts can detect these areas by how the surface moves or has moved and the solution is easy, dig out enough of the clay and fill the hole with gravel.