If you happen to notice Spring’s heave and pothole haven around town you begin to understand the importance of a good foundation. Not only is a good foundation vital to a home’s structural soundness, it’s also vital to a pavement’s longevity.
Preparing a good foundation for a driveway involves a few things.
First, understanding what native subsoils exist. Yep, back into the soil type discussion. Certain types of soil can be more problematic than others. Soil type can be broken down into silt, sand and clay, and most likely your property is a blend of two or all three. Below is a fun visual to understand the 3 main categories and mixtures within.
Now how do soil particles play out with your driveway’s structural soundness?
Spring is one of the times that you’ll see a great deal of movement on the pavement surface. As the frozen ground is unfreezing, the moisture has to go somewhere. If the soils are not well-draining for the water molecule to move down (or the frost is still in the ground), then there will be movement of that moisture going up. Aye, there’s the heave.
In clay soils, this can be extremely noticeable particularly when the ground is thawing, still freezing or doing the thaw and refreeze action.
Sandy soils present less complications as this soil type drains quicker.
So if you had to choose the perfect soil type of your property, sandier soils would be the way to go. That is as long as the surface grades are working right for drainage on the property, but that’s an entirely different issue.
However, an important item to note are layers. Generally your property doesn’t have the same soil type 20’ down. So what might appear to be sandy for the first 12” may change below that and of course what’s below can play a role in things too.
So how does this discussion of soil types relate to installing a driveway?
Well, the pavement surface needs to be supported properly. And what does this mean exactly? Yeah, think of it as two things; 1. The shock absorber to all the movement that’s happening below ground with freeze/thaw conditions and 2. The structural stabilizer of what’s on top.
The base layer is composed of aggregate. In a driveway this generally means a ¾” minus aggregate meaning that it’s a rock of a ¾” diameter and smaller. By incorporating fines (the smaller) allows for compaction. Another important factor is for the aggregate ideally to be fractured or angular to allow for maximal density.
There are certain circumstances where it may be reasonable to install a layered aggregate base in really unstable soils. This might be a 1-3” aggregate below a ¾” for instance.
How deep should the base layer be?
Yeah. This isn’t set in stone. The standard in the Twin Cities Metropolitan area for driveway construction is 4-6”. Is that enough in clay? No. Enough in sand? Yes. But again, no property has the one soil type 20’ down so how exactly do you know how deep is deep enough?
Without involving a civil engineer to analyze and design - ‘cause that can add to the cost of things - one of the best ways is for a driveway contractor to proof roll. Why? Because it’s specific to your site and is the best approach to know enough is enough...and without doing too much (‘cause that can get costly too). Over-engineering, or over-compensating on designing, is a thing too and more trucking, labor and materials in and out means more expense.
How is proof rolling done?
A loaded truck drives on the surface to see how much displacement (aka “rutting”) occurs. If significant, that area is dug out and a new base aggregate layer, potentially geotextiles as well, installed to reinforce that “soft spot.” It could be limited to one location, several or the entire length of your driveway.
If you really want to geek out on proof rolling, Purdue has a great research publication. find it Here