cantilevered retaining wall - sliding resisted by asphalt?

[susanwc]

Hi all,
I am working on a 6 foot cantilevered retaining wall that will have 4" of asphalt cast over the toe of the footing right up against the wall. Will the asphalt resist sliding forces?
thanks for the help

 

[msquared48]

I would not rely on asphalt to resist any lateral kick, particulaarly if it gets warm.

If you need further resistance, either use concrete or a concrete key.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[weab]

Also, I don't think I'd count on the bond between the concrete and asphalt. And if you are puting asphalt on top of concrete, is the footing even fully buried? Is there a chance that you'd have an erosion issue? I wouldn't count on the top 6 or 12 inches of soil always being in place during the life of the wall.

 

[beton1]

Also, what happens if the asphalt is removed for re-paving?!

 

[apsix]

They will need to re-pave quickly, before the wall realises.

As above, asphalt will creep, especially in warm weather.

 

[BigH]

I'm sorry - but who's kidding? 4 inches of asphalt? - warm, cold, hot . . . even if it did, it wouldn't make a dent in the driving forces.

 

[apsix]

OK, I admit it, half of my response was said in jest.

 

[TDAA]

Which half? ;p

 

[GPapazafeiropoulos]

@ susanwc:

Typical modulus of elasticity of asphalt is 1.5GPa and Poisson's ratio ?=0.35, found from the attachment.

Assuming plane strain conditions for the asphalt and the wall, the one-dimensional compression modulus is:

D=E*(1-?)/((1+?)*(1-2*?))= 2.4GPa

In addition, it is well-known that the active limiting conditions in the backfill is fully mobilized when the horizontal displacement of the wall divided by its height is approximately 1%, so horizontal displacement equal to 1%*2ft=0.02ft=0.06m=6cm is sufficient. The actual displacement will be below this value.

If you know the extent of the asphalt from the toe of the wall (the distance between the wall and the point where the asphalt ends), be it y, then you can check if the resultant force at the toe of the wall F, is lower than the quantity D*(4inch)*(6cm/y).
Here I assume that y=6m, so F<2446 KN/m.
The resultant force from the earth pressures at rest are equal to:
F=(1/2)*K0*?*h^2=(1/2)*(1/2)*(20 KN/m^3)*(6ft)^2=20KN/m < 2446KN/m then it is ok.
There were not considered active earth pressures in order to be more conservative in the calculations. The above calculations are not rigorous but rather simplistic. They are intended to show only that the asphalt layer can resist the sliding forces of the wall, in terms of static stability.

But attention must be paid in cases that the asphalt layer is removed and re-paved, or if it gets warm (you have to check the variation of modulus of elasticity with temperature). Finally, possible erosion problems may arise uner the wall's foundation, and the result of this will be the reduction of the rotational resistance of the wall. Check the value of the equivalent soil rotational spring under the wall's footing.

Best regards,

George Papazafeiropoulos
________________________
Second Lieutenant, Hellenic Air Force
Civil Engineer (M.Sc.), Ph.D. Candidate

 

[apsix]

They are intended to show only that the asphalt layer can resist the sliding forces of the wall,
IF YOU IGNORE CREEP.

Simple observation of asphalt on roads shows that it can suffer horizontal displacement, especially near traffic lights. Those squiggly white lines weren't painted that way.

 

[GPapazafeiropoulos]

Yes, apsix, there are a lot of factors which affect the problem, including creep. I stated that my calculations are simplistic. If somebody wants to consider creep (as well as other factors, such as the variation of E with temperature, the bonding of the asphalt with the underlying soil or subgrade, the friction under the footing of the wall, the potential separation of the retained soil from the wall, and so on...) one can make use of appropriate numerical models and extract results of the desired accuracy.