Our company, Complete Building Solutions, investigates construction defects – including but not limited to the building envelope, water management issues, soil issues, settlement problems, concrete issues, and driveways. We also can provide engineering services and construction oversight.
As we conduct our investigations we have found that many of the defects observed do not comply with the building codes. And, as most people know, the building industry is governed by codes. We rely on the International Building Code (IBC), the International Residential Code (IRC), and the Minnesota State Building Code as our references when conducting our investigations as we look for construction defects. They all do a good job of addressing all facets of a building. But, there is nothing in the code that addresses the design or the construction of a residential driveway.
In Minnesota, there are some excellent references that can be used as a guide for the design and construction. However, what is needed is a code that addresses the key elements of a driveway structure. In my opinion the key elements are having the proper subgrade with the required compaction, a solid base upon which the driveway material will rest (incidentally this blog is based on an asphalt driveway), the right asphalt thickness, and once the driveway is constructed keep the water from entering the subgrade (another words – a good water management system).
What is the proper subgrade? The answer – one that is not prone to frost heaving. Damage from frost heaving can result in buckling your garage door trim to causing the misalignment of your garage door making it difficult to open and close.
The subgrade should be either granular or should be con-bit. I can tell you that most subgrades under driveways consist of clays and silts, soils that are considered expansive – those are not the proper subgrade. So if you have subgrade consisting of clays and silts, it should be removed and replaced with granular material or con–bit (this is normally referred to as a soil correction). Here at CBS we recommend using con-bit rather than granular material. Whether it is con-bit or granular it must be compacted and it should be compacted in lifts. Each lift should be about 8-10 inches. Once the subgrade is prepared then the base is constructed.
The area where we see most of the driveway problems is right in front of the garage door. This is an area where the native soils are used for backfilling and they are seldom compacted properly and are placed wet. It is the area where the contractor has to excavate in order to install the footing and construct the foundation. Typically the excavation extends out from the building’s foundation several feet and extends down to the footing.
That is also the area that is most prone to become wetter over time because other elements feed moisture into it (poor water management for one). Disregarding compaction and allowing the subgrade to become saturated are the key reasons why driveways settle and are subjected to frost heaving.
As previously noted, poor water management can affect driveway behavior. Since the presence of water reduces the strength of the pavement structure, it is important to not let water enter it. I can’t tell you how many times I have seen downspouts dump huge amounts of water right next to the driveway.
The next element of importance in a pavement structure is the base. The base is the layer directly below the driveway surface (either asphalt or concrete). AS previously stated, this article is based on a hot mixed asphalt driveway. The base material, in my opinion should be a class V material or con-bit (recycled concrete and asphalt). The thickness of the base will depend upon the thickness of the asphalt.
Let me explain – the design of the pavement structure should be based upon a term called “Granular Equivalent” (GE). The granular equivalent concept defines a pavement section by equating the thickness of the base and asphalt layer to an equivalent thickness of granular base material. Note: this is not my term, but a term used by the Minnesota Department of Transportation (MN/DOT) for designing pavement sections.
As an example, a class 5 aggregate base has a granular equivalent of 1 per inch of thickness and an asphalt course has a granular equivalent of 2.25 per inch of material. So, if I have an asphalt thickness of 3 inches, and a class 5 course of five inches, my granular equivalent would be 11.75 inches. If I had an asphalt thickness of 2 inches and a class 5 base of seven inches, my granular equivalent would be 11.50 inches.
So you see the granular equivalent can be any combination of asphalt and base. It goes without saying that the higher the GE number, the better the pavement structure (I have personally seen this to be true). Low GE’s might not show pavement distress in the first few years but within 3-7years, signs of distress will begin showing up.
The next element in the pavement structure and the ones everyone sees, is the surface course. To be frank, I am not sure what specification most residential paving contractors use for their design mix. I have seen HMA thicknesses from 2” up to 3” used for driveways. I think most paving contractors will use 2”-2 ½”. Especially in large residential developments. If you are an individual homeowner the contractor will probably use 2 ½” – 3”.
In my professional opinion, I would recommend 3” – 4”.
Now getting back to the GE. I would recommend a GE between 10.5”-12” for residential driveways. Similarly, for reference – the Minnesota Asphalt Pavement Association (MAPA) recommends a GE of 11.5” for driveways also.
Neither the IBC, nor the IRC include information for residential driveway designs. Nor is there a recognized national standard. Maybe it is far “fetched” of me to think that one could be established, but at the very least, I believe the Minnesota State Building Code should address the issue.
Incidentally, I have over 30 years of experience with the Hennepin County Department of Transportation dealing with bridge and roadway designs.
Bruce Polaczyk, P.E.