MSE Wall Leveling Pads: Concrete vs. Gravel

[djs23]

Here in Colorado we have a proposed MSE retaining wall (Allan Block) with heights varying from ~6'-4" to ~24'-9". The wall is surcharged with highway vehicle loads (250 psf) and it has a toe-slope of 2H:1V. I have the wall embedded to H/7, plus I have 4' of horizontal soil cover per AASHTO recommendations. In addtion, much of the toe is to be paved over with 4" of concrete for slope protection. My question is regarding the advantages of a concrete vs a gravel leveling pad. It appears that CDOT specifies only concrete leveling pads. Why would CDOT do that? Is a concrete leveling pad the way to go?

 

[concretemasonry]

A concrete pad is not in agreement with the development of the segmental interlocking block wall (SRW) and a concrete pad does not comply with the philosophy of the wall system that has been used internationally for over 20 years.

I suggest contacting Allan Block to get to the bottom of the situation since I know this has come up in many provincial areas and they constantly work with DOTs on numerous industry projects for similar products also.

Dick.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RFreund]

Concretemasonry - I wouldn't say that is not in agreement? Most manufactures allow a low strength concrete (500psi) 6" thick as a leveling pad. Granted I agree and would try to submit a granular leveling pad as concrete leveling pads are usually difficult to work with (may need leveling mortar or sand bed).

EIT

 

[emmgjld]

"It appears that CDOT specifies only concrete leveling pads. Why would CDOT do that?"

Because they are CDOT and all it takes is a single problem and a master solution is imagined. Cynical, I know. But fits what I have personally seen in over 40 years. The trully frightening result is that CDOT specs are adopted by most county and city govs and the foolishness is magnified.

 

[concretemasonry]

In MN, the MDOT uses SRWs according to the licensors requirements because of the support provided to engineers. The same follows in most countries that use far more SRW walls. Our municipalities have standard design plate for engineers and contractors regarding installation and none show concrete.

CDOT may have some unique and provincial situations due to the rock and lack of geo-grid length possible for development of the soil strength in some mountainous locations. Then, an application not recognizing the feature of a flexible wall may be used if engineered without the developers support.

In Spain, there is a mountainous stretch of road curving through rock cuts for several miles and the curved wall segment heights vary between 5' and 40' engineered without concrete "pads".

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dcarr82775]

What is the downside of using concrete for the leveling pad? I don't see how the concepts behind MSE walls fall apart because of the leveling pad.

 

[JML4TCM]

It is a leveling pad not a footing. They are flexible systems, why put them on a rigid "footing"?

Concrete causes a constructability issue too. An uneven concrete surface can cause leveling issues when setting the blocks.

NCMA allows the use of a concrete leveling pad, but not sure why you would want to use it. What is the benefit of concrete?

 

[Riggly]

I have used concrete leveling pad on a 22ft MSE wall, and there is no problem with the wall. As long as the concrete is level, it creates a good surface to lay the first set of units. Some of you might think that the friction between the concrete and leveling pad might be low, but remember most of the wt of the MSE wall will fall behind the leveling pad, and that's where most of the friction will develop. The face units theoretically acts principally to concrete the soil. In fact, I think if there is a good concrete leveling pad, it would be faster to lay down that first row of units.