Heaving Concrete and Asphalt – Subgrade


In Minnesota, there are some excellent references that can be used as a guide for the design and construction of residential driveways. However, what is needed is a code that addresses the key elements of a driveway structure. I find it fascinating that there is nothing in the code that addresses the design or the construction of a residential driveway.

In my professional opinion, the key elements to a properly designed residential driveway are as follows: the proper subgrade with the required compaction, a solid base upon which the driveway material will rest (this blog is based on an asphalt driveway), the right asphalt thickness, and once the driveway is constructed, preventing water from entering the subgrade (water management system).

Proper Subgrade

What is a proper subgrade? The answer – one that is NOT prone to frost heaving.  Damage from frost heaving can result in the buckling of garage door trim which can cause the misalignment of your garage door. This misalignment can make it difficult to open and close the door.

Buckling of garage door tripm. Clay soils push concrete soils upward in winter.

Buckling of garage door apron. Clay soils pushes concrete soils upward in winter.

The subgrade should either be granular or con-bit.  Most subgrades today consist of clays and silts, soils that are considered expansive – these materials are not the proper subgrade.  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.

Concrete and asphalt can move as much as 8 inches when the soil beneath it freezes.

Concrete and asphalt can move as much as 8 inches when the soil beneath it freezes.

Many driveway problems occur 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 often placed wet. This area is prone to becoming “wetter” over time because other elements feed moisture into it (poor water management systems). Disregarding this compaction and allowing the subgrade to become saturated are the key reasons why driveways settle and are subjected to frost heaving.


Moisture Intrusion Solutions: Solid Base & Asphalt Thickness

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 3-7 years, 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.


The building codes govern the applications of these materials and practices. 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.




WATER in my BASEMENT! What should I do?



water in basement

During the summer months, when heavy downpours from thunderstorms occur, you may experience water in your basement.  You ask yourself, “what can I do to prevent this from happening?”  The following are items you can review at your home to help determine the causes for this basement water.

  • Does the grade around your home’s foundation have a slope of 6 inches within the first ten feet of soil/landscape?
  • Do you have gutters on your house? Maybe you need gutters to collect the rain runoff from your roof?
  • If you have gutters are they:
    • Clean and not plugged? (They need to be checked several times during the year.)
    • Extensions on your downspouts of a minimum of five feet away from your foundation?
    • Gutters that are sized appropriately for the amount of water runoff created from the area of your roof?
    • Are there enough downspouts to empty the gutter fast enough?
  • Is the sump pump operational? Do you have a battery backup system if you lose power? Do you need a second pump in case of failure?
  • Do you have a wet basement often? You might need a collection system installed in the basement.

guttersmeasuring slope around foundation

These are some of the basic questions that may  point you in the right direction when solving your wet basement problem. Feel free to email, comment, message, or call Complete Building Solutions with your questions. We offer moisture intrusion solutions and help prevent flooded basements throughout Minnesota.

(612) 868-2922

LIKE US on Facebook 

To READ UP on WATER MANAGEMENT check out Complete Building Solution’s latest guest column with the Golden Valley Sunpost  HERE