excavated deeper than calculated depth 1

[mattdukes]

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?

 

[BAretired]

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

BA

 

[BAretired]

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

 

[mattdukes]

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?

 

[BAretired]

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

 

[mattdukes]

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.

 

[hokie66]

"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.

 

[BAretired]

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

 

[mattdukes]

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...

 

[BAretired]

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

 

[JacksPanic]

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.

 

[fcu45]

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

 

[msquared48]

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

 

[mattdukes]

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.

 

[BAretired]

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

 

[hokie66]

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.

 

[mattdukes]

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.

 

[hokie66]

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.

 

[mattdukes]

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?

 

[hokie66]

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.