Pavement Design – From Intuition to Good Science

In talking with managers, property owners, and association boards, we are commonly asked why we’re recommending a specific depth of asphalt. Often, the age of the existing asphalt and its depth are all we need to know. Intuition tells us that an asphalt surface that is 3″ deep and has lasted 20 years is likely to last another 20 years or so if replaced at 3″, assuming conditions and traffic levels aren’t likely to change. Add an inch to make the depth 4″ and it’s likely to last even longer.

But the central problem is that contractors simply don’t know the depth of the existing asphalt. It costs both time and money to find out by taking core samples – investments many companies don’t want to make to price a project for a prospective customer. So most customers are left with getting 4″ recommendations on most repairs with 6″ recommendations for higher traffic areas. The end result is that customers often pay for far more than they need (or in some cases, far less) because they don’t know that there’s a better way to do some basic exploration and some simple design to “right size” a solution. Add to that fact that many pavement contractors don’t have basic knowledge about good pavement design and customers end up with solutions that are somehow lacking. This is one of the reasons we have made taking core samples a more common practice, particularly on larger projects where a 1″ difference in the asphalt thickness can mean big bucks.

The depth of asphalt required for the pavement to have an acceptable life expectancy varies greatly. We thought a quick crash course in pavement design methodology might be helpful to shed some light on how knowing the basics can both save money and result in a better solution.

In simplified terms, there are essentially 3 steps to designing an asphalt pavement section.

1. Determine how much strength is in the underlying native soil.

strength of soil

Drill deep enough through an asphalt surface and the underlying base rock (most pavements have at least some base underneath them) and you’ll hit native soil. In Northern California, the soil is typically some clay variety that has very little strength. In southern California it varies more – foothill areas may be very rocky, areas near beaches sandy, and other inland areas may be some type of clay. Soils engineers remove samples with a boring rig (a vertical drill) and take them back to a lab. There they determine the soil’s “R value” or its response to a specific load when it has a specific amount of water added to it (CalTrans outlines Test 301 in a 35 page document that can be found by clicking here ). A series of tests results in a number, the R-value itself, which may range from under 10 for a very soft clay soil to 50 for a strong, rocky soil.

2. Determine the type and quantity of traffic and the target life of the pavement surface.

pavement surface traffic type

Caltrans and cities might use actual traffic counters for this purpose on freeways and high volume roads, but for most parking lots and private roads, estimations are usually sufficient. A private road in a small condominium complex may get 100 cars per day, 1 miscellaneous delivery vehicle a day, and 2 garbage/recycling trucks per week. The drive lanes in a busy shopping center may have hundreds or thousands of cars, several mid-sized delivery trucks, and several large 18 wheelers per day. The designer or engineer adds up the respective vehicles, their ESAL’s (equivalent single axle loads – a car might be a .0007 and a heavy truck a 1.5) and determines the total number of ESAL’s for the entire pavement life. This number can then be converted to a traffic index (TI). Again, a small condominium complex may have a TI of 4.5 or 5 while the drive lanes in a busy shopping center may be a 6.5 or 7 (the TI is typically rounded to the nearest 0.5).

3. Determine how much strength will come from the base rock material or treated soil and how much strength will come from the asphalt (or concrete) pavement itself.

Asphalt base rock strength

The soil R value and traffic index, when entered into engineering software that makes a number of calculations, outputs several combinations of base rock thickness and asphalt thickness that will yield the desired strength and life for the pavement. Generally 1″ of asphalt has the equivalent strength of about 3″ of base rock. Sometimes cement, lime, or other similar hardeners can be mixed with the soil to add significant strength without excavating and exporting soil or importing and grading base rock.

The same methodology can be slightly modified and used for new pavements (where only the native soil is the starting point), replacement (where the asphalt is removed but the soil and some portion of the base rock remain), or overlays (where the soil, base rock, and some or all of the existing asphalt remain).

Knowledgeable contractors and resourceful engineers can “value engineer” a solution that minimizes costs by building strength in each layer with the most cost effective materials and processes. Such costs can vary greatly based on prices for materials, proximity to asphalt plants and recycling sites, trucking costs, equipment access, and numerous other factors and variables.

The bottom line is that a 2″ pavement section may be ideal if it’s placed in low traffic areas with a thick base rock foundation and stable soil. On the flipside, a 6″ asphalt section may be inadequate if it’s in a higher traffic area with little to no base rock and unstable soil. The key is to make investments in the design stage to, at a minimum, make very educated guesses about proper section thicknesses based on the age of the pavement, its original thickness, and its maintenance history. On larger projects where the cost of the design is justifiable and “pays for itself,” following proper methodology with the aforementioned 3 step design/engineering process will provide the best results.


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