Foundation on concrete slurry

[ZTGR]

Hello all,
an over-excavation been done for a footing designed for 10 kips/ft2. the footing suppose to be at 3 feet above the existing over-excavated bottom. a footing for such a high pressure should be on native rock, the bottom of the over-ex is native claystone. the contractor is planning to place 2000 psi concrete slurry to bring bottom of footing to initial level. is this acceptable geotechnically?
Thank you

 

[ashjun]

Yes, it is quite possible to place a concrete block below the footing to distribute the load deep under. The 3 feet block could probably be designed for bending moment since the superimposed loads may be large – on the whole, it would appear to be the part of the footing.

I have seen some of the foundation works in which there was an over excavation purposely done because the top thin layer was weak/compressible and replaced by well compacted earth or lean concrete 1:3:6 mix.

However, one needs to be cautious of not placing a layer of high stiffness in between the soil and the footing and should be compatible to the sandwiching layers; viz the modulus of the concrete filling the gap should be in between the modulus of the concrete and the constrained modulus of the soil if not greater.

 

[Grouser]

Geotechnically: the extension of the foundation downward is not a problem unless the additional weight thereof causes excessive settlement or shear failure in the bedrock. What are the rock's strength properties?
You will probably want to do this for all the footings of the building for their behavior to be similar. Otherwise you will need to analyze each footing's settlement probabilities and design to assure tolerable differential settlement between support conditions.

Other considerations: is the rock surface level or sloped? You will want to 'key' the concrete into the rock slightly to prevent sliding. You may need to 'bench' the rock to allow steps in the concrete if the rock's surface is steeply sloped.
Is there any ground water flow along the rock surface (aquaclude) that the concrete will interupt? If so, you will have to make allowances for it and divert or relieve it to prevent a buildup in or around the building.

 

[BigH]

I found Grousser's comments on the potential for sloping ground to be quite insightful for this could be a problem depending, of course, on the actual conditions.

Doubt very seriously if replacing soil with lean concrete would be detrimental to added settlement. Soil/weathered bedrock and concrete, for all intents and purposes, are of the same unit weight (give or take, ok?).

10ksf is not that high; most definitely not for sound rock - even if claystone; it is 5tsf - about 500kPa which for sound claystone (caveat - no weak or compressible clayey seams) should be okay - for glacial till in Toronto, we will use 400kPa or slightly more (clayey silt till, not silty sand till).

I am not sure what "added" moments you would get for 1m of lean concrete - surely, it can't be different, much, if the material was weathered rock - you didn't say what the "overexcavated" material would be but I'm presuming it might be weathered claystone; dig your excavation and muck out well to remove all loose/softened zones (on bottom and sides) and pour neat against the sides - you might consider the excavation to be 200 to 300 m wider each side than your footing. As for strength, 2000 psi concrete is about 12MPa - isn't this much stronger than the bearing pressure! - sure. Even 10MPa concrete should be okay as a concrete fill.

Hope some of these thought prove useful - best to you all.

 

[Focht3]

I agree with Grouser and BigH - in all their comments. And I wouldn't worry about bending in the lean concrete, either. So what if it cracks - where's it going? What could cause damaging settlement? Nothing. And I'll bet the footing is heavily reinforced, anyway.

 

[ashjun]

To clarify: Lean was provided only for excavations less than about 0.4 m. Reinforcement would not be an option then. My apologies

 

[jheidt2543]

Just a question, is it required that the overexcavation be corrected with a lean concrete backfill? Three feet is a lot of concrete. How about using an engineered backfill of crushed stone? The lean concrete is going to cost on the order of $100 - $125/CY, the engineered fill will cost around $20-$25/CY in place (Midwest USA costs). A well compacted, engineered fill should be able to support the load.

 

[sc]

Another problem with crushed stone backfill is that adequate and consistant compaction cannot be assured if the trench is narrow. There is the added problem that the vibration from the compaction equipment may cause the soil sides to collapse.

3 foot over excavation is a fair bit. What depth is the footing supposed to be?

The reason I ask is for health and safety, as some-one should not be in the trench without shoring if the trench is greater than 5 feet deep. The use of shoring would drive up the cost of backfill considerably, hence the contractor wanting to use concrete as it may be cheaper.

regards
sc

 

[BigH]

JHeidt: A thought I had "overlooked" since I responded to the "straight" question. Good point.

As for settlements, I think we are overplaying this aspect? 3 ft of well graded crushed granular stone, even compacted to 95% modified, will not be induced with that much settlement - especially if footing width in order of 5 ft or so - you worry about settlements if the structure has a special sensitivity, or there are other aspects playing about. But I would put the settlement of the 3 ft of well compacted granular as adding in the order of 1/4 inch to 1/3 inch of settlement - at the most. A lot, in this post, depends on the presence or not of "softer" clayey seams (even firm clay is "soft" compared to the claystone).

 

[jheidt2543]

BigH,

Thanks for the comments and the pancake flip.

One other consideration besides cost, the concrete will weigh about 145 pcf, while the crushed stone will weight about 120 or so. That means the concrete will add about 25 pcf to the footing subgrade. Not a lot, but something and depending on what is below the footing subgrade, could make the difference in a settlement calculation.

As for fitting a compactor into the trench, they come in all sizes, it is mainly a matter of how thick a lift you want to place and how much labor to compact it.

 

[BigH]

Could use a hoe-pack. Don't know about your crushed, but our well graded crush is something like 2.5g/cc - almost same if not same as concrete. In TO, our crushed is in order of 137pcf dry (MDD) - so might be more like 130 or so - still, I doubt that his soil or underlying material is that settlement sensitive - it is claystone. - but for other situations you have the point. (By the way - it ain't a pancake flip - it is a flapping "clap"!!)

 

[jheidt2543]

BigH,

Sorry about misinterpreting the "flapping clap". I must have been hungry when I wrote the post!

 

[BigH]

Me too - it is one "just one of those things" that can mean what you want it to, eh?? But, we can agree on the