 | Parkways and park roads |  |
THICKNESS:
The subgrade is critical in the design of pavements. The Soil
Manual which Rob McLeod gave to you in Terrain Analysis contains
a description of tests for soil properties; you'll see the California
Bearing Ratio and other testing procedures. Check those out.
The Asphalt Institute has developed a method of estimating the
necessary thickness of pavements based on traffic volumes equated
to truck axle loads of 18,000 pounds; you can find the detailed
explanations in the Institute's publication, Thickness Design, MS-1.
They classify traffic according to a system which they call IDT
(Initial Daily Traffic). That is the volume of traffic expected
the first year following completion:
Light traffic is up to 1,000 vehicles/day on two-lanes;
equates to 10 trucks.Medium traffic is 1,000 to 100,000 vehicles/day on two-lanes;
equates to 100 trucks.Heavy traffic is over 100,000 vehicles/day on two-lane or multilane
roads.
Four examples are given in the text; each is calculated on a
20-year service life, and an annual increase in traffic volume of 3%.
Example 1: A residential street carries 400 vehicles per day, CBR is 4;
requires an Asphalt Concrete Surface of 1.0 inches
over an Asphalt Concrete Base of 5.0 inches.
Example 2: An urban Interstate carries 10,000 vehicles per day;
CBR is 10; requires an Asphalt Concrete Surface of 2.0
inches over an Asphalt Concrete Base of 6.5 inches.
Example 3: A suburban street carries 4,000 vehicles per day; CBR is 3;
requires an Asphalt Concrete Surface of 1.5 inches over
an Asphalt Concrete Base of 4.5 inches over a Subbase
of 8.0 inches.
Example 4: A Parkway carries 8,000 vehicles per day; the road is
restricted to cars, buses and service trucks; requires
an Asphalt Concrete Surface of 1.0 inches over an Asphalt
Concrete Base of 6.0 inches.
An alternate is given for the Interstate where an equivalent
pavement is 2.0 inches, on 4.0 inches, on a 5.0 inch subbase; an
alternate for the Parkway shows 1.0 inches, on 4.0 inches, on a
5.5 inch subbase.
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