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excavated deeper than calculated depth

excavated deeper than calculated depth

excavated deeper than calculated depth

(OP)

The actual digged or excavated depth is 2 meters to reach certain tuff rock layer, calculated depth of design is 1.5 meters.. what do you normally do in case like this?

1. Increase column size to control deflection
2. Put more bars to control deflection
3. Or nothing at all taking into account that the extra column length is below grade and the soil covering it can just hold it?

What do you usually do in your experience?

RE: excavated deeper than calculated depth

Fill 0.5 m with low strength concrete, then place your footing on top of that. Maintain column length as designed.

BA

RE: excavated deeper than calculated depth

If you are using an unreinforced pedestal between footing and column, increase pedestal height as required without changing design parameters.

The firm I started with in 1955 used an unreinforced pedestal atop each footing which had a dimension of one third of the footing size. That was large enough that it did not need to be reinforced. Its height could be varied to meet different site conditions without changing other design parameters.

Needless to say, it was not intended to handle columns with base moments required for stability.

BA

RE: excavated deeper than calculated depth

(OP)
BA.. but the footing needs to be put at 2 meters instead of the designed 1.5 meters. Can't put concrete below the footing. Do you redesign the column size and bars or let it remain as it is if it has to go down 0.5m more?

RE: excavated deeper than calculated depth

When a footing is over-excavated, you can fill the excavation with low strength concrete (fillcrete) to the 1.5 m level, then form and pour the footing on top of the fillcrete. What is the problem?

Alternatively, you can put the footing at the 2.0 m level and provide a stocky pedestal up to the underside of column.

You do not redesign the column for every minor site discrepancy.

BA

RE: excavated deeper than calculated depth

(OP)
It's not exactly overexcavated.. or more done on purpose I mean... it's only found out that 2 meters below are hard rock. so all the footings will be put 2 meters below instead of 1.5 meters. So you think all the columns have to be redesigned for 0.5 meter additional length? But I think this still falls within the margin provided by the load combinations.

RE: excavated deeper than calculated depth

"Can't put concrete below the footing." Why not?

BA didn't say anything about redesigning the columns. Just make the footing thicker, or as he suggested, place low strength concrete up to the intended footing base. Using the blinding layer to make up the overrun has the added advantage of giving you a nice level surface for supporting the footing cage and placing the concrete.

RE: excavated deeper than calculated depth

You can (a) add 0.5m fill concrete below the footing, (b) use a footing 0.5m thicker, (c) add a 0.5m high pedestal on top of the footing or (d) extend the columns 0.5m down to top of the footing. If you choose (d), you should check the column design for additional height. Do not rely on lateral support from the soil as it will not help.

You seem to be making a big issue out of a situation which arises frequently with foundations. I don't know what more I can tell you. Maybe someone else has something more pertinent to say.

BA

RE: excavated deeper than calculated depth

(OP)

I'm a new construction engineer. I guess I have to ask the structural designers about it. But my fellow construction engineers said for 0.5 meter difference.. they never bother ask the structural designers since 0.5 meter is just short and negligible or within allowance...

RE: excavated deeper than calculated depth

Normally, column steel is fabricated before construction starts. You cannot simply add 0.5m to the column length without modifying the column steel accordingly. Otherwise, you will be 0.5m short in the lap at the next story.

Whenever a change occurs which affects design, the structural engineer of record must be notified. It is his job to determine whether or not the additional 0.5m will affect his design, not yours. You must ask the structural engineer what method he wishes to use to extend the structure down to bedrock.

Your "fellow construction engineers" are completely off base with their comment and should be told so in no uncertain terms. They are a bad influence on you as a new construction engineer. Ask the designer what he wants to do and then do it.

BA

RE: excavated deeper than calculated depth

All discrepancies on site should be reported to the project structural engineer so he can make the call.

Quickest and easiest is to fill the 500mm with "blinding" concrete. But if this is over the entire site, it might be financially viable to have a look at redesigning the columns for the extra length.

RE: excavated deeper than calculated depth

In our issued for construction shop drawings
Our office puts a condition that the contractor is obligated to fill the areas over excavated in rock by concrete.

I believe strength of that concrete should not be less than the required bearing capacity of the rock.

Good luck

RE: excavated deeper than calculated depth

Personally, I see no problem whatsoever structurally with just making the footing thicker and lowering the bottom steel of the footing accordingly. Happens all the time.

You are making this problem too complicated by overthinking it.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: excavated deeper than calculated depth

(OP)

The total footings concrete already reach 60 cubic meter. If we add another 0.5m, it would add another 48 cubic meters for total of 108 cubic meter. The cost would be huge. Do you really implement this? Anyway. I'll just have the columns recalculated to see if additional 0.5 meter would still make it stable and redesigning it accordingly.

Also as I have mentioned 2 meters below is hard rock. The calculations used assumed value of 150kpa... now it's more like 500kpa. But the contracts and payments already made so I think there is no problem with the footings actually larger than needed, is there? Because if they were changed smallers, the contract would still be the same and no refund or deduction will be issued.

RE: excavated deeper than calculated depth

That seems unusual. Is there no provision in the contract for credits or extras to accommodate design changes?

Perhaps the footings could be reduced in area and increased in thickness such that the volume remains about the same. Footing reinforcement could be reduced both in length and number of bars so there should be a net saving in materials.

BA

RE: excavated deeper than calculated depth

Very strange indeed. If the footings were designed for 150 kPa, they were not designed to bear on tuff. And if the level of the tuff was known, why were the footings shown to be founded 0.5 metre above that level? This should have all been allowed for in the contract documents, including unit prices for payment in the case of different conditions being found during construction.

RE: excavated deeper than calculated depth

(OP)

I don't know your definition of Tuff. You assume it is softer hence less than 150 kpa? I'm talking of tuff rock that is 400 kpa or so. We didn't know the depth of the tuff initially. I'm concerned of your statement that if footing is designed for 150 kpa.. it is not designed for let say 400 kpa?? But the load (live and dead ) will be the same. Hence I assume that it will even be better putting a 150 kpa designed footing on 400 kpa soil because the latter is simply harder hence the footing is like kinda oversized larger but no harm at all because the live and dead load won't change.

RE: excavated deeper than calculated depth

You took my statement to mean the opposite of what I intended. If the rock is indeed tuff as I know it, I would expect it to be good for 1 MPa (1000 kPa) or better, but the design pressure should be determined by your geotechnical engineer. If the footings are indeed designed for only 150 kPa, I would think there is a good chance you can found them on the material existing above the rock, but again, that should be determined by the geotechnical engineer.

RE: excavated deeper than calculated depth

(OP)
The geotechnical engineer wants to be on the safe side and juse put 150 kpa on paper which is what the designers use (meaning they didn't expect us to dig deeper). Now that we have tuff rock as bottom. Let's say it is really 1000kpa. What is wrong with the footing that is designed for 150kpa when the live and dead load is still the same. We are not talking of the soil bearing actually impinging on the footing. I mean, we can't take the analogy of the 1000kpa as the uniform live load on normal slabs or beams. It is on soil, upside down. Any arguments why 150 kpa designed footing can't be stable on 1000kpa soil when the actual live and dead load and footing size and bar sizes, etc. don't change?

RE: excavated deeper than calculated depth

You are talking in circles. I have already said that the tuff is almost certainly much better than 150 kPa material. The option I suggested is that rather than extend the footing to the rock, perhaps you can found them higher and save the added concrete, but it sounds as if your geotechnical engineer doesn't want to be involved.

RE: excavated deeper than calculated depth

(OP)

What we intend to do is to put the 150kpa designed footing putting right on the rock and just let the designers change the column parameter for the extra 0.5 meter length. We don't want to change the design of the footing anymore. So there is absolutely no problem in putting the 150 kpa footing - as is- right on the say 1000kpa rock, correct?

RE: excavated deeper than calculated depth

mattdukes,
I agree with your last statement. One other (pedantic) thing. Mr Pascal's name should alway be respected by capitalizing...kPa.

RE: excavated deeper than calculated depth

Quote (mattdukes)

What we intend to do is to put the 150kpa designed footing putting right on the rock and just let the designers change the column parameter for the extra 0.5 meter length. We don't want to change the design of the footing anymore. So there is absolutely no problem in putting the 150 kpa footing - as is- right on the say 1000kpa rock, correct?

There is no problem doing that, but you are wasting material. Why don't you want to change the design of the footing anymore? Why not stop and think about it and come up with a sensible design? Why do you insist on making decisions without reviewing all possibilities?

I suggest you sit down with all concerned and see if you can come up with something a little more intelligent than what you have proposed.

BA

RE: excavated deeper than calculated depth

(OP)

Because the designers said they no longer have time to redesign the whole footing just to accomodate the increased bearing capacity.. and they also said 0.5 to 1 meter length increase of the column can be made without changing any of the design... because they have large margin of safety already. So we'll use as is. And base on my own calculations, the bonus of having increase bearing capacity than designed is one more floor can be added in the future... which of course will be coordinatated with the designers. Thanks to your assistance anyway.

RE: excavated deeper than calculated depth

mattdukes,
By chance, would you be related or work with Pattontom?

RE: excavated deeper than calculated depth

Quote (mattdukes)

So we'll use as is. And base on my own calculations, the bonus of having increase bearing capacity than designed is one more floor can be added in the future... which of course will be coordinatated with the designers.

If load is assumed to be spread evenly over the footing area, you will have no additional capacity to carry another floor. Footing capacity will be controlled by the reinforcement which has presumably been designed to carry the present design load.

If you want to obtain additional capacity, use a square concrete pedestal 0.5m high on top of each footing. If the side dimension of the pedestal is 1/3 the dimension of the footing, you will increase the footing capacity substantially because you will reduce the cantilever length of the footing to L/3.

BA

RE: excavated deeper than calculated depth

(OP)

Thanks BAretired for the statement. Bending moments of footings are indeed controlled by reinforcement in the form of qu= factored column load / bearing area. So even if the soil capacity increases 5 times fold (from say 150 kpa to 750 kpa). If the shear, moment reinforcement is the same, increasing the column load would only strain the moment and shear bars even though actual soil bearing is higher. So from the beginning the reinforcements must be designed already. This is what you are emphasizing, agree?

Also from the formula. Increasing footing area would make designed qu or net upward soil pressure less requiring lesser bars. I'm wondering. Do you in practice use larger footing and less bars but more concrete for more overturning stability (instead of smaller footing and strong bars to obey the qu=factored column load/bearing area formula?

RE: excavated deeper than calculated depth

Consider a footing 2.7m x 2.7m with column 0.4m x 0.4m bearing directly on it. Assume that the factored uniform soil pressure is qu. Cantilever length = (2.7-0.4)/2 = 1.15m. Mu= 2.7*qu*1.152/2 = 1.785qu.

Now, suppose we add a pedestal 0.9 x 0.9 x 0.5 high to the top of footing. Factored soil pressure qudoes not change but the cantilever length becomes (2.7-0.9)/2 = 0.9m and Mu = 2.7qu*0.92/2 = 1.0935qu, a mere 61% of the moment without the pedestal. If the soil bearing is sufficient, this means a potential increase of 63% in permissible column load using the original footing reinforcement. The cost of the additional 0.4 cubic meters of concrete is trivial compared to the added benefit in additional load carrying capability of the footing.

Quote (mattdukes)

Do you in practice use larger footing and less bars but more concrete for more overturning stability (instead of smaller footing and strong bars to obey the qu=factored column load/bearing area formula?

In the above, I was considering a centrally loaded square footing with no consideration for overturning. In most buildings, overturning is not carried by individual pad footings but rather by stiff elements such as elevator shafts.

BA

RE: excavated deeper than calculated depth

It seems to me that if you have increased the soil bearing capacity by a factor of 5 (150 to 750kPa), by going .5m lower to bear on solid rock; you might be able to reduce the bearing area (size) of the of the footings enough, so that the original 60 cubic meters of concrete would allow the original .625m depths to be extended by about .5m to the rock surface. Thus the top elevation of the footings and the column designs would not have to change. Another possible solution would be just to lower the whole building by .5m, then neither the columns or the footing would have to change. smile

BA and Hokie.... I wonder if this OP’er. isn’t from the same neck of the woods, or at least the same school of engineering thought, knowledge and logic, as the guy we dealt with a few weeks ago on the eccentric footings, etc. etc. They don’t have the time to redesign footings for rock, but they can find the time to redesign the frames and columns, and now are even planning on adding another floor?

What the heck kind of engineering really goes on in their neck of the woods? He claims there was some GeoTech involvement in the foundation design, but then they didn’t know there was solid rock .5m below the intended bottom of footing elevation during their original design? I can’t believe this. This shouldn’t even be called engineering, because it certainly isn’t by any stretch of the imagination. One wonders if some of the good advice being given here might not actually be twisted around, and be mis-applied or mis-used. This kind of so called engineering is down right dangerous, and probably shouldn’t be aided and abetted.

RE: excavated deeper than calculated depth

dhengr,
I was wondering the same thing. See my latest post. I am not expecting an answer, but if not, there are other ways of finding out.

RE: excavated deeper than calculated depth

I'm not sure why the soil report did not reveal the location of a "tuff rock layer" at 2.0m depth. I wonder whether it remains the same 2.0m depth across the site or if its depth is variable.

I'm not sure whether the footings at design depth (1.5m) can safely sustain the loads using design pressures provided by the geotechnical engineer without extending them down by an additional 0.5m.

I'm not sure why it would be a huge cost to increase the thickness of the footings by 0.5m but there would be no financial consideration for reducing material quantities where possible to achieve a better design.

I'm not sure why the Structural Engineer of Record does not have time to get properly involved in the construction phase of the project.

In the absence of more information, I don't see how I can contribute to this thread in a meaningful way.

BA

RE: excavated deeper than calculated depth

(OP)
Thank you.

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