Pavement Design Question 3

[MES2022]

I have a project where we engaged a geotechnical engineer and part of what they were asked to do was to give pavement design recommendations. The asphalt courses they recommended for the site were what I would expect for the type of use and loading that will be on site. The use is a customer parking lot with occasional truck deliveries. However, they are saying that because the soil subbase is hard and dry, no aggregate base course will be needed. This would be for areas paved in asphalt or concrete. I've not seen a recommendation before that did not include some type of aggregate base course. I have worked with this geotechnical engineering company before and have always been pleased with their work. I final design of the pavement is mine to make and I am still leaning toward a minimum of 6 inches of aggregate base course. Am I being to cautious?

 

[TigerGuy]

In arid climates, moisture migrates upward through the subsurface soils and becomes trapped beneath pavements. Once dry subgrades are quite moist when the pavement is removed years later.

I think the best step is to call and discuss with them to better understand their recommendation.

 

[Ron]

Put the base course in. That's a ridiculous recommendation and would deviate from the standard of care you are required to meet.
Even in light duty pavements, you have to consider constructibility. A competent base course allows you to get proper compaction in the asphalt course.

 

[MES2022]

Thank you TigerGuy and Ron for the responses. The moisture issue is the main reason I think having an aggregate course is necessary in the pavement design. It provides a drainable layer through the pavement. I talked to the city engineer about it today also and he agreed that the aggregate course should be put in. To me the idea of putting asphalt down on the subbase is asking for trouble. I am going to keep it in the detailed pavement section.

 

[EireChch]

I was of the opinion to put in a subbase too but also keen to know more about the pavement design. Surely it is in accordance with some local/national standard.

And surely there has to be an allowance to remove the subbase in said standard?

What was the subgrade out of interest?

 

[dik]

The base course is actually part of the flexible pavement system and contributes greatly to the durability.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[JedClampett]

I'd get the Geotechnical Engineer to revise his recommendation to provide a base course layer. You don't want some contractor getting hold of the Geotechnical Report and VEing out the AB based on the report. Contractor, "Owner, you get $.20 and I get $.80."

 

[SwinnyGG]

Full depth asphalt is extremely common. Much depends on the specifics of the subgrade - but if the subgrade is appropriate, full depth asphalt does not compromise durability and is significantly cheaper.

 

[MTNClimber]

I second SwinnyGG. There are areas where full depth asphalt constructed on a prepared subgrade is commonly used. This typically requires a thicker asphalt section compared to asphalt on aggregate base course, usually around 2" more.

 

[TigerGuy]

What kind of soil is the subbase?

 

[MTNClimber]

TigerGuy,

Base and subbase are clean granular material with good drainage properties.
Subgrade is whatever the existing material is on site underlying the pavement section.

 

[TigerGuy]

Subbase is typically such a material, but not always. I should have included subgrade. Clay and sand can behave quite different under pavement.

 

[KaBone]

make sure the subbase can handle the tack coat