Soil Settlement
Soil Settlement
(OP)
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...





RE: Soil Settlement
RE: Soil Settlement
RE: Soil Settlement
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.
RE: Soil Settlement
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!