Thickness of the subgrade

[Okiryu]

Hi ! In the US, what is the thickness of the subgrade? Here in Japan, we consider subgrade soils to extent 1 meter below the subbase course... just curious about what are the considerations for subgrade thickness in the US...

For example, if design guidelines indicate that the subgrade should have a permeability of "X", the depth of soils with that permeability will be 1 meter below the subbase course in Japan...

 

[oldestguy]

In my experience USA, "thickness of subgrade" usually is not even a factor considered in pavement design. Subgrade physical characteristics, such as shear strength, are more important. Permeability also enters into the overall design, but generally assumed as very low. Each state usually has its own methods, sometimes using the national highway association guidelines.

 

[Okiryu]

As always, thanks much for the input OG...

We have some guidelines which require to use a drainage layer unless the subgrade has a very high hydraulic conductivity. Our subgrade has not high hydraulic conductivities (fine-grained soils) so we are thinking on replace the subgrade with granular (high hydraulic conductivity soils) material to avoid installing the drainage layer (due to cost constraints)... The question is: what is the depth of this replacement? What is the depth of the subgrade?

 

[BridgeSmith]

The depth that will need to be replaced will be what is required to allow the subgrade to drain, so it will be based on 1) the rate that water is getting into the subgrade and 2) the rate at which the water can get out of the subgrade into the surrounding soil. "2)" has to be greater than "1)", so that you don't end up with a swimming pool under your footing. You'll probably need to involve a geotech familiar with the local soil conditions.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Okiryu]

Thanks BridgeSmith... this is for pavement design...and we do not have lack of drainage affecting our pavements so far but, the local guidelines require a drainage layer under the AC or PCC layer...which can be deleted if the subgrade is permeable enough.... So, our goal is to remove the drainage layer making the subgrade more permeable. In order to do that, we have to replace the current subgrade (fine-grained soils) with granular material....

 

[BridgeSmith]

Regardless of what's on the surface, the requirement for a drainage layer, and that it's dependent on the permeability of the subgrade, would indicate the concern is with saturation of the subgrade. Therefore, to meet the requirement, the subgrade needs to be able to drain to somewhere, either into the adjacent soil, or to daylight at a lower elevation. If your replacement material can be exposed at the edge of the roadway, so that it can drain into a roadside ditch or something, that would probably work. If not, then I would again suggest you consult a geotech familiar with the local soil conditions.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Ron]

I usually consider 3 layers for a flexible pavement system...

12" of stabilized subgrade (CBR 30)
6-8" of base material (CBR 80)
1.5 to 3" of surfacing asphalt (AASHTO layer coefficient 0.40-0.44)

This is for general pavement applications. For specific traffic counts, I consider other variations of thickness.

 

[Mad Mike]

I would consider the subgrade to fall within the defined Materials Depth for each of the road categories.

I also use 1m in general- our local pavement engineering manual prescribes a Materials Depth of 0,7 to 1,2m for the various road categories...these depths are measured from the final pavement level, so by using 1m, you are covering all your bases.

All the best,
Mike

 

[Okiryu]

Thanks all for the input...