Settlement in backfill by column bases

[BauTomTom]

Hallo

I have the such an case (see attached sketch at best) where my column bases are around 2m deep. This needed of course be backfilled, in layers 150mm and each compacted to 98% MODASHTO
On top I requested a DCP test just to make sure, that all layers were properly compacted.
Now the DCP test results give me such irregular levels so that I am confuse what to do.
The layers differ from 50kPa up to 400kPa
And the curve is just a zick zack.

How critical must I see the compaction of this backfill?

I know the contractor compacted it but it seams like not really properly or maybe not even every layer, or the layers were to high

Bau Tom Tom

 

[BauTomTom]

here the results graphics

 

[ishvaaag]

Not a complete answer but remind that settlement is judged more than anything by the effects of compression on a
Depth of influence

for footings from 0.5B above base of footing to 2B below base of footing

for mats minimum to a depth of 6 to 15 m (10 m typically) or higher

where B is the maximum width of footing or mat

I would try to estimate settlement following this idea, then maximum width of footing or mat, then given a permissible settlement I would try to estimate an allowable pressure and see if it fits your intent.

 

[BauTomTom]

TO ISHVAAAG: thanks for the rule of thumbs, it is ok.

one other question, can a contractor use for such an backill compaction a rammer? Is this not to light? is there a rule what should be the weight of the compaction tool?

TomTom

 

[csd72]

Sory, for clarification is the compacted backfill above or below the footing?

 

[Ron]

The DCP is a poor method to determine ACTUAL compaction. It gives one an idea of the compaction, but must be correlated with actual in-place density tests.

Where are the compaction results during backfilling? Your specification was "98% MODASHTO". There should be in-place density tests to verify this. Why was that not done?

I wouldn't get too excited about variable DCP results...in fact, I would expect them without further correlation.

How consistent were the fill materials? We they all the same? Were there fines (either clay or silt) in the fill material?

 

[BauTomTom]

TO CSD72: Of course I mean int backfill above the footing

TO RON: You are right so why I got the TROXLER Test done on top to get the proper compaction test result in %. Of course the top layer (final level) gives compaction results up to 102%. I think it woudl be to much afford to request this test for each 150mm layer.

TomTom

 

[csd72]

Bau Tom Tom,

Sorry, just the way the posts were going seemed to imply otherwise.

Compaction tests should be carried out in-situ for each layer as it is compacted and the compaction redone if a given layer fails to meet the required criteria. This is not something that can be tested after the fact.

To avoid tthese issues, a lot of sesigners will specify a lean concrete mix under shallow footings rather than compacted fill over.

 

[BauTomTom]

TO CSD72:

each layer? so it means by 2m backfill that you will have like around 8 compaction tests just for one footing. Seam to be also expensive.

Lean concrete mix was was also something which I considered, or soilcrete at least

TomTom

 

[HeavyCivil]

Bau Tom:

I would expect the cost of testing every lift to pale in comparison with the cost of actually placing and compacting material in such small lifts. The tester is there anyway - it doesn't slow the operation down for him to run a proctor.

 

[csd72]

Bau Tom Tom,

Compaction IS expensive, this is why most places do their best to avoid unecessary volumes of compaction.

It is essential that each layer (usually 150 to 300mm depending on the spec) is tested as the tests only test the compaction of a certain layer of soil. Testing the top of a 3m lift will not prove anything about the compaction of the bottom 3/4 or so of the fill.

This is the reason why products like soilcrete were invented, though I have never used it underneath a footing so I cannot comment on that.

 

[BAretired]

For small volume fills such as this, a low strength concrete such as Fill-crete is probably the simplest solution. This avoids having to take numerous compaction tests.

Next best is a well graded gravel fill which needs very little compaction.

As a corrective measure for fill already placed, could you design the grade slab to span over the filled zone?

BA

 

[hokie66]

No matter how much you compact soil, there is always some settlement left. Maybe not much, but some. Therefore, like csd72 suggested, most of us would fill deep excavations with lean concrete or similar before building the footings. Also has safety benefits, as working in deep excavations without shoring kills a lot of people.

 

[BauTomTom]

Hi Guys

I see on your answers that lean concrete is there the best solution. this is fine because we have still some other bases to do so I will just tell them to backfill the footing with soilcrete or rather 5Mpa lean concrete?

Probably soilcrete with 10% cement is good enought or?

we need only discuss the rate but it seams that this is as well the cheaper solution

I mean here just the backfill ABOVE the footing not under the footing.

BauTomTom

 

[BauTomTom]

the other question was about the tool.

is a rammer not to light for such an compaction? when shall be a compactor plate used and with which weight?

are there some rule of thumbs?

BauTomTom

 

[csd72]

BauTomTom,

5% cement should be good enough. You are justreplacing the soil so the strength will not be an issue.

compaction equipment is a difficult balance in this situation the lighter the equipment the longer you need to compact it but you also do not want to use too large a piece of equipment above the new footing.

 

[BauTomTom]

TO CSD72: you right there is also a limit but such a rammer seams to me really beeing very unsufficient.

So you say 5% cement is enought? Are you using the same soil? Or do you bring better soil in?

do you actually also compact the soil crete?

BauTomTom

 

[csd72]

5% is sufficient for reasonably clean granular material if it is mixed well.

If the existing soil has a large clay content then I would not use it for this purpose.

Not sure about your part of the world but in most western countries you can get a sand/cement mix from concrete suppliers delivered wet to site.

Supply it wet and it will effectively be self compacting.

 

[BauTomTom]

TO CSD72:

I also thought rather to use a G5 material or even the concrete agregates. But maybe it would be better still to place it in layers and compact or? Can you just through the it into the 2m deep hole and leave it as it is?

TomTom

 

[csd72]

It would probably be worth doing a bit of compaction while it is still wet but generally the cement content is relied upon to fill the voids and stabilise the soil.

You really only need to achieve something like a 0.1MPa capacity which is nothing really.