Asking for recommendation for foundation design

[edrichwell]

I have a warehouse design project. The geotechnical report produced the following result:

1. Net allowable bearing capacity = 40Kpa, 2.0m depth, foundation width is 3.0m or less
2. Idealized geotechnical parameters

Depth Description N-value Es(Mpa) C(kpa) unit weight(kn/m3)
0-2.0m Soft Elastic Silt N-value=4 Es(Mpa)=2 C(kpa)=6 12
2-3.0m Medium Stiff Elastic Silt N-value=9 Es(Mpa)=10 C(kpa)=25 12
3-13.50m Soft Elastic Silt N-value=3-4 Es(Mpa)=2 C(kpa)=6 12
13.5-22.0m Medium Stiff Elastic Silt N-value=7-9 Es(Mpa)=10 C(kpa)=25 16
22-24m Stiff Elastic Silt N-value=18-33 Es(Mpa)=30 C(kpa)=80 18
24-30m Hard Elastic Silt Refusal Es(Mpa)=40 C(kpa)=200 20

3. Ground water table at 6.25m

I've always read of silt being a problematic soil and belongs to the category of soils vulnerable to potential failure.

Thank you for your inputs.

 

[hokie66]

I would guess that driven piles would be your best solution. 40 kPa at 2 metre depth is very poor bearing material.

 

[Okiryu]

Appears that the Medium Stiff Elastic Silt from 3 m to 13.5 m is controlling the design. The shear strength is low 25 kPa but the modulus is not too bad (10 MPa). So, the recommended bearing capacity of 40 kPa is reasonable to me (actually, I would expect something like 50 kPa). Are these volcanic loam silts? I am asking this because the unit weights are low. Check if these silts are sensitive (lost of strength after remolding).

 

[dik]

With all that silt, frost heave may be an issue if you are in a cold climate.

Dik

 

[Ron]

Personally, I've never liked the term "elastic silt"....in my opinion they are anything but elastic! They are compressible and poor support for a foundation. As dik noted, if you are in a frost region, they can be problematic because of their ability to imbibe a lot of moisture.

Okiryu....the silt from 3 to 13.5m is soft, not medium stiff. That layer is below 13.5m.

Since your building is a warehouse, I assume the footprint is relatively large. I agree with hokie66 that piles would be the better option and due to the soft materials in the upper strata, driven piles would be better than augered concrete piles as you would likely have difficulty with controlling the pile diameter in the upper 15m of soil. The piles could support caps and grade beams or a mat.

 

[Okiryu]

Ron, yes, you are right. I misread the OP's post. To correct myself, most of the load will be taken by this layer:

3-13.50m Soft Elastic Silt N-value=3-4 Es(Mpa)=2 C(kpa)=6

With that low modulus, I expect that settlements are large (larger than the typically maximum allowable of 25 mm/1 inch). I agree with hokie and you about considering piles unless the geotech is very confident about settlements are okay with that soil conditions. Does the report talks about expected settlements? But even if settlements are okay, the shear strength is also very low, so if you have the footing at 2m, the stresses at say 3 m (top of the soft silt) for the size of your footing, may be about 35 kPa. This is larger than the qu of that layer (12 kPa). So, I would also be concern about a bearing capacity failure.

Also, based on the recommended bearing capacity and the size of the footing appears that the loads are not high (less than 500 kN??). I want to believe that the geotechnical report already considered settlements and bearing capacity associated with the recommended allowable bearing capacity/allowable bearing pressure.

 

[dik]

If you have a high water table... silty soils often collapse into the auger hole. Can you do spread bored piles @ 2m?

Dik

 

[Okiryu]

dik, good point. As Ron suggested, driven piles can help on this.

 

[dik]

a driven pile may be *really* long, judging from the soils... Did a 747 runway once where the modulus of subgrade reaction was 25... almost needed to fit the 747's with pontoons...

Dik

 

[dik]

How is your slab loaded? Racks? Just curious...

Dik

 

[L0k]

I prefer to install 14m long wickdrain or sanddrain then preloading.