Heavy Duty Pavement on Rockfill 1

[TTomaz]

Dear all,

We're currently supporting the design of a flexible pavement on a rockfill (1-1000kg), with expected high/very high subgrade reaction modulus. The C8/10 layer is defined according to Material Equivalence Factor (MEF). However, there is no/very little reference to high strength subgrade in "Heavy Duty Pavement Manual" from Interpave (2007) or other codes (at least those which I found), or how this would impact the subbase layer thickness in general.

Is it mandatory to use low-strength subbase over the high-strength subgrade? It doesn't seem to be coherent. The Heavy Duty Manual from Interpave allows to remove the reinfoced subgrade layer when CBR>5%, but doesn't say much about when the subgrade layer is CBR>30%.

Greetings.

 

[Ron]

The pavement layers should get progressively stronger as you go toward the surface. The subgrade should have strength adequate to reduce vertical deflection as this controls rutting. The base material should be stronger than the subgrade so as to mitigate load transfer to the lower levels. The surface should be significantly stronger so as to mitigate high stresses at the bottom of the surface layer which controls cracking.

You seem to be confusing terminology between a rigid pavement section and a flexible pavement section. A rigid pavement consists of portland cement concrete, a subbase and a subgrade. A flexible pavement consists of one or more layers of asphaltic concrete, a strong base of compacted, graded aggregate and a subgrade that may or may not be stabilized, depending on the loading.

Load and thickness analysis of the two types of pavement are approached very differently. Rigid pavements are analyzed by Boussinesq or Westergaard theory. Flexible pavements are analyzed by a mechanistic empirical approach such as the Structural Number concept or by Elastic Layer Analysis.

 

[TTomaz]

Thank you Ron for your support.

I agree with you that pavement layers should get progressively stronger. However, in our case, the subgrade is "naturally" a high-quality material (rockfill).

In our case, my understanding is that we're designing a flexible pavement, since the upper layer is composed of several separate high-strength concrete block paving h=8 cm (not asphalt or continuous slab) on a sand bed.

Several heavy duty pavements using concrete blocks are done using Elastic Layer Analysis (example here).

However, in our case, the subgrade is already "strong" due to the location we're placing the pavement (on a rockfill). Shouldn't we take advantage of it? Why placing a "lower" class unbound sub-base material on a high-strength subgrade material? That's my main point.

 

[Mad Mike]

A quick note - just make sure you have interpreted the properties of your rock-fill correctly. What invariably happens, regardless of the packing and preparation of the rockfill layers, is that you get pockets of fines, at least to the extent that the fines can govern the strength of the subgrade.

So there may be patches here and there, where your "rockfill" does not produce the strength at subgrade level which you had assumed it would...you can then get significant pavement failures (rutting) where the rockfill packing is subpar, and contains more earth than it should.

Maybe you've already quantified and are perfectly confident of the rockfill performance at high stress, in which case ignore this post.

Best,
Mike