Balancing Urban Development and Water Conservation

Posts By: Tori

Basing the Driveway. Why its Important to Get it Right.

Posted March 16, 2021 @ 8:51am | by Tori

Basing the Driveway. Why it’s important to get it right.

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 catagories 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  


Images by Indiana DOT and Purdue University Here


Filed Under: Paving | Permalink

Greening the Driveway.

Posted March 5, 2021 @ 6:08am | by Tori

Greening the Driveway


As we have started to come to terms with the changing environment, many of us have become aware of the choices that we make. 


For the past 20 years, Earth Wizards has designed, installed and maintained various environmentally friendly driveways so we’ll share our take with you.


Just as a refresh, there are two terms that you’ll often hear. Permeable and impermeable. 


Permeable, or pervious, allows water to move through, whereas impermeable, or impervious, does not. 


As the built landscape has moved to a predominantly impervious nature, that means less water reaches our aquifers underground. Not only does that affect our highly-valued drinking water supply, but the damage of runoff has become detrimental to property, livelihoods, habitat, soil erosion and a host of other ills.


What type of driveway is right for me to lessen my impact?


It might seem like a simple answer but of course it never is. Let’s cover a few options and then we’ll delve into what really might make the most sense for you.


  • Permeable/Pervious

This can be done as pavers (both concrete and clay), concrete and/or asphalt. It’s important that the underlayment (gravel) is designed with a uniform-sized angular aggregate. 


Think of a tubful of angled marbles. The water collects in between the marbles until the drain can move the water through. 


  • Less surface

Concrete runners work great for a single car garage. Less pavement means less runoff. What to do with the non-pavement areas? Can be grass or a low ground cover that is suitable.


A driveway with a permeable paver pad. Concrete runners allow cars and SUV's alike to maneuver properly.


  • Rerouting

Often we consider rerouting runoff onto the property, ideally a raingarden or bioswale. 


A raingarden should be designed to handle the amount of runoff it receives and pond (6-18”) for no more than 72 hours. A bioswale is a graded vegetated channel that doesn’t pond with any significance (less than a few inches)


  • Underground Storage

This type of system allows for stormwater to be collected underground. Think of a bathtub underground. Similar to raingardens being designed to function properly, underground storage units also need to be designed appropriately to collect runoff and then allow it to infiltrate into the underlying subsoils.


By randomly putting a barrel with some rock (the old “french drain”) approach, the size of the system is limited and it’s important to know at what point it will fail and how water will move on/off your property when that happens.


What other considerations should be factored into my decision?


Subsoil type. Generally sandy soils are ideal for permeable, raingardens and underground systems. This isn’t to say that it can’t be done in less than appealing soils such as a more clayey type but it gets trickier in terms of designing appropriately.


One of the most often overlooked considerations is the carbon footprint. The amount of material coming off-site and being moved in. If there’s significant excavation, as with these options there’s more than a conventional approach, that means more equipment, more trucking and more disposal. 


Additionally looking at the location source of the new materials. The ideal aggregate to use in permeable pavements is an angular granite base rock as the rock is more igneous base than sedimentary so it will hold up better to the introduction of large amounts of water. For the Mpls/St.Paul metropolitan area this means St. Cloud. 


What else to consider? 

  • Budget. Permeable pavements and underground storage will be more expensive than rerouting into a raingarden or bioswale. 


  • Maintenance. Permeable pavements will require attention to areas that get clogged. Often overhanging trees or surrounding exposed soils will contribute to this issue. At some point, it may be necessary to remove the pavers, underlying choker course and replace that top layer of aggregate and pavers. So there’s some decent labor involve either that you hire out or do yourself.


With a raingarden or bioswale, the maintenance is similar to any garden area. If you choose to be a messy gardener because it’s better for pollinators less so than the Kew Gardens attendant. 


  • Available space/mixed use. What amount of green space might I have to install a raingarden or bioswale? Can my driveway also be used as an outdoor patio rather than two separate hardscape areas?


  • Pollinators. It’s the talk of late and for good reason! A native planted garden will do so much more for pollinating habitat than permeable pavements. 


Then there’s your aesthetic. Your preference with how you want your property to look and feel. 


The last important thing to mention is combining approaches. An entire driveway need not be permeable. Maybe it’s a portion of the driveway to be made permeable, some portion downsized and a spot for water runoff to flow into a native planting. 


So many choices and this is where it becomes fun! 


Start with how you want your property to function, flow and feel. All these things can be incorporated into that ideal green driveway. 


And then it’s become more than a driveway but a reflection of your style, your ethics and your home to be proud of.


I have already recommended you! Our consultant, Shelly, was extremely knowledgeable. She explained (the process) in technical detail. She is the reason I chose Earth Wizards over the other 5 bidders. Installation crew was terrific and exceeded my expectations.

- Don P.
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