Soil Settlement

[hiluxxx03]

I've got an instruction from our consultant that i don't understand the logic behind... based on the geotechnical report, we can excavate up to 1.5m from the natural grade level to achieve the needed soil bearing capacity but we must excavate 50cm more and backfill in two layers, 25cm per layer and compact up to 95% to prevent settlement. I really don't get why we have to backfill when we can have the 1.5m depth of excavation compacted up to 95% without backfilling or have the 2.0m depth excavation and have it compacted up to 95% and rest my footings there. as per our consultant, he said that soil bearing has nothing to do with settlement but what affects the settlement is the soil properties that's why we need to backfill a good material to serve like a spring that would receive the settlement... can anybody enlighten me regarding this matter? because as my experience, footings or foundations are rested on the natural ground level after excavation and no more backfilling only compaction...

 

[VAD]

Interesting question. The reason for overexcavating 0.5m and backfilling presumably with gravel is for this layer to act amongst other aspects as a cushion similar to a mudslab often used if the ground is a bit soft and mushy. However, it is hardly likely that this thickness is required for weak concrete or gravel. On the other hand, if the water table is high and chances are that water will migrate upward due to capillary action then the gravel layer if used for this purpose would act as a capillary cut off and allow the zone of significant stress below the footing (assuming the width of the footing is equivalent to the depth of significant stress) to exist within the gravel. This would influence the bearing and settlement characteristics of the gravel. It should be noted that if gravel is used for this purpose then this gravel should be drained which may be the case if the report addresses a residential or low rise building. However, I am not sure if this concept was considered or it was based on so called good practice which is often rooted with the experience of the geotechnical engineer. I guess one has to understand the subsurface conditions, loads to be applied to the substructure etc etc. Note that the zone of significant stress can be also taken as 1.5 B or 2B where B for rectangular or square footings where B is the width of the footing or for strip footings 4B, although this large value for strip footings is often not desired. Is the absence of this gravel layer a cause for foundation settlements in the long term? Do we need to examine this some more?. Note I have made assumptions which may not be valid.

 

[BigH]

Why not be a bit more specific about the problem. What soils are being excavated? What soils are lying beneath. Perhaps, there is a "soft layer" extending to 2 m and as only 1.5 m is needed for frost protection, he wants to found at that shallower depth so is replacing "bad" soil with good. But no real suggestions can be made unless there is more details - as VAD rightly pointed out - now it is just assumptions.

 

[molerat2210]

hiluxxx,

Your soils consultant sounds like they know what they are talking about. If you are structural engineer then you do not need to fully understand the issue. Leave that to the soils experts. If you post the full geo report then we could attempt to decipher the issue for you. Otherwise, it's a lot of second guessing.

 

[fattdad]

I'd hope that the 1.5 m is for frost protection and to some extent to enhance the ultimate bearing capacity.

I'd think the engineered subbase of recompacted soil would be to stiffen the elastic properties of the bearing materials to limit elastic settlement from the highest-loaded portion of the soil column below the bearing grade. Is 0.5 m enough? Don't know.

At this point, we don't know what the foundation is supporting, the areal dimentions of the footing or the elastic nature of the native soils. It's hard to know whether the geotechnical engineer is being rational or not.

Let's say the footing imparts (sorry, I have to use English units now) 2,000 psf and is 3-ft square. Let's say that the average stress in the upper 20 inches (0.5 m) is 1,500 psf (you know 2,000 psf immediately below the footing and like 1,000 psf at 20 inches (1.6 ft). If the soil modulus of the native soil is 100 tsf and you replace that with an engineered subbase of 300 tsf, you'd limit settlement from that layer. By how much? Here's the answer:

1500 psf = 0.75 tsf
0.75 tsf * 1.6 ft = 1.2 t/ft
1.2 t/ft / 100 tsf = 0.012 ft = 0.144 in

contrast that number with:

1.2 t/f / 300 tsf = 0.048 in (not showing a conversion step)

Then again, you may have a different stress distribution, footing size, attenuation of stress, etc.

At least that's how I see it. . .

f-d

¡papá gordo ain’t no madre flaca!