huangyhg

concrete pad for gabion wall on rock
i have a ten foot high gabion wall that sits on a rock shelf. the base of the wall is nine feet wide. we are providing a concrete levelling pad that varies from six to eighteen inches thick to provide a slope to the wall.
do we need to provide reinforcement in the pad? the wall resists sliding forces by a steel dowel into the rock at 36 inch spacing. the top of the dowel is encased in a pvc tube in the gabions
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i would only suggest minimal reinforcing in the longitudinal direction if the concrete is exposed.
how about fiber reinforcement? seems like an alternate solution
i consider steel & polypropylene fiber to be good and useful products, but they are not normally considered substitutes for reinforcing steel. for many applications they are substitutes for welded wire fabric.
however, i agree with minorchord2000, reinforcing steel would be a good idea. over 25 years ago i learned that if you cannot accurately compute if, or how much reinforcing steel is needed, put in a mat of #4 @ 12", each way. unless you have a very large pour the cost of adding the steel is minimal and it let's you sleep at night when things don't go as expected.
i would put at least 0.0018 shrinkage steel in one or perhaps two mats with bar spacings no larger than 18". it goes against every fiber in my body not to put rebars in wet concrete, even though they may not be needed structurally. concrete is a beautiful material in that it will crack and find your reinforcement, even if your analysis was different than the reality of the situation.
for mse walls and "redi-rock" gravity walls i have done in the past, a concrete levelling pad is nice, but may not be necessary. often times, a screened gravel pad wrapped in a geotextile fabric ("burrito drain") is sufficient for site walls. who cares if there is differental settlement in a gabion wall? with as much as 18" of fill bearing on rock, there is not much potential for any settlement anyway.
if your rock dowel is sufficient to resist the bending forces generated by shear couple separated by the distance between the top of the rock shelf and the bottom of the first gabion basket, then you might be able to justify omitting the concrete levelling pad. another solution might be to cast a concrete curb at the toe of the gabions to take the sliding, but still level the first gabion course with gravel. just a thought.
no reinforcing is required for
sundale,
a concrete levelling or work pad over rock, while not technically necessaty for the design of the structure, makes it much easier to install the gabions than a bare rock surface.
i would suspect that if the base is rock, then the base was probably blasted out. if this is the case, then the base will likely have numerous joints and fracture planes in it, making a level base a near impossibility. this will make it very difficult to construct the base gabions. while it is possible to due, the increase time and labour cost can be considerable.
in my experience, a concrete pad is very nice in this situation.
gabionguy is right - you pad on the rock is nothing more than a leveling pad (dental concrete)- mud mat - blinding concrete - it adds no structural strength. i've not heard of anyone putting steel in a leveling pad before - so what if it cracks? gabions on crushed stone have "lots of cracks" (e.g., individual particles).
i've used hundreds of gabion installations to control erosion along pipelines. never used a bit of rebar in any pad. in fact, very seldom even use paddings of any kind, other than a sheet of geotech fabric.
going the big inch!

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