Plain concrete as a leveling block over sloped rock

[bonbon83]

Hi All,

we are engaged in a project that we need to design pad and pier foundation over sloped rock. Client asked not to chip or blast the rock (to generate even surface) due to residential area nearby. I came up with a solution that to put the pad on a concrete poured wedges (as a leveling pad). What type of reinforcement do I need? I am thinking of putting bottom bending reinforcement, shear dowel to the rock and some temperature reinforcement at the top. the rock has 2-way slope and putting reinforcement at the bottom is a bit hard. The load on the foundation is in form of huge moment 1200kN.m ,100 kN downward and 65 kN shear. what are your thoughts please?

 

[jayrod12]

How are you planning on transferring the moment to the rock? Is it just a giant pad?

They need to be able to do some form of mechanical levelling.

What does your geotechnical engineer say regarding the slopes?

 

[bonbon83]

How are you planning on transferring the moment to the rock? Is it just a giant pad? (there is no moment transfer to the pad, only trapezoidal pressure from main foundation reaction)

They need to be able to do some form of mechanical levelling. (for bottom reinforcement? I agree)

What does your geotechnical engineer say regarding the slopes? (He is ok, he asked us to use shear dowel)

please see attached image.

 

[jayrod12]

Then I don't really see a need for other reinforcing per se. Although I'd likely be tempted to have a top mat going in both directions with the ends hooked down extended to the top of rock (minus a bit of clearance between bar and rock). And I'd probably have a single horizontal with corner bars located just below the top surface, maybe 6" down, to keep the blinding concrete confined where the compression will be the highest.

How thick at the thinnest? Thickest?

 

[bonbon83]

I was wondering what would happen if I delete the bottom reinforcement? it cracks due to bending1 but is it detrimental?

 

[bonbon83]

Thanks Jayrod12, I like your thought.
How thick at the thinnest? Thickest? thinnest around 100mm and thickest 850mm for one way and for the other way, thinnest around 100mm and thickest 1400mm

 

[jayrod12]

Over a metre thick? I'd be looking at a couple of horizontals on the deeper areas. The intent would be to keep it confined.

There should be minimal bending stresses, more just compressive stress, and therefore the associated concrete tension.

 

[bonbon83]

Thanks again Jayrod12, wondering why you are worried about the confinement, concrete is pretty good at compression and I don't really have vertical rebar to worry about the buckling. can you shed light on this? My concern is bending of bottom and being difficult to place the bottom rebars due to rock slopes . for better understanding , the pad is 6X6X0.610m and per my model it exerts average pressure of 130kPa on the blinding concrete. this pressure is triangular and happens almost over 4m of pad width,

 

[phamENG]

bonbon - think about a typical concrete tension test. It's accomplished by turning a cylinder on its side and compressing it. As you say, concrete is great in compression. But as you compress it, you get a simultaneous, transformed tensile stress orthogonal to the compression (think Poisson). So jayrod is concerned with this tension that will induce a failure prior to a crushing it in compression. If it is properly confined and/or reinforced for that tension, you won't have much to worry about.

 

[STrctPono]

This is not uncommon for me, however our slopes are generally gradual. It many times is unpractical to level rock to the profile you want and we many time pour lean concrete from the rock layer to the invert elevation of our footing. I don't see any problem with this. If your slope is steep enough such that the sliding force along the faying surface between the concrete and the rock is greater than the resisting friction that will develop then I fully agree with the shear dowels. Corrosion of the dowels is something to consider and I would specify a duplex grade stainless steel shear dowel.

 

[bonbon83]

Thank you All for your inputs, what about bending reinforcement? Do I need that? Why I don’t ? Isn’t it identical to axially loaded pad which we consider bending reinforcement at the bottom?

 

[jayrod12]

It is, and at the same time isn't. Your concrete pad is designed to apply a uniform(ish) pressure to this levelling slab. So really the levelling slab is mostly just transferring load straight downwards.

I'm a big fan of confining the levelling slab more from a sliding resistance mechanism. The shear dowels will work, but on the jobs I've been to, quality control on the dowel installation, and the projection of the dowels into the levelling slab, has been a bit suspect. So the confining reinforcing also acts a bit like the suspenders to my belt dowels.

And also to resist the concrete tension that pham indicated. Often these levelling slabs are lower strength concrete and therefore the higher bearing stress from the moment may cause some issues. Especially if the moment is overturning in the downslope direction.