Is my concept for basement raft realistic 3

[oloksy]

I have a building with a total load to the foundation soil as 105kN/m2. If I decide to excavate 4m deep soil and I remove 72kN/m2 of soil. Then I replace the weight of soil removed with some part of the building. It means I’m left with about 33kN/m2 acting on the soil at 4m.

So since the soil bearing capacity @4m is 99kN/m2.
Since 33kpa < 99kpa can I safely use a raft?

 

[dik]

You need a good geotekkie...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EireChch]

Yes, you have a factor of safety of at least 3.

But that is only shear failure. You need to check settlement.

For raft foundations and any small foundation that is deep,shear failure bearing capacity is generally not an issue. Settlement calculations control your design

 

[HTURKAK]

- Common practice in my zone ,( if not otherwise mentioned ) the allowable soil bearing capacity is defined below 1.0 m of ground surface. If so, the soil bearing capacity @4m could be 99kN/m2+ 18*3 ≅150 kN/m2.

- The local contact stress could be much more than 105kN/m2 due to the deflection of the raft fnd.

- However you can use 33kN/m2 for settlement analysis.



According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

 

[oloksy]

So @HTURKAK are you saying my bearing capacity @4m below the natural ground level is now 150kpa?

If so then 105kpa<150kpa so the raft is okay right

 

[EireChch]

Olosky, you need a geotech to assess this for you.

HTURKAK, I think its very unusual to work out bearing capacity at 1m and then add on overburden. The formula easily accounts for embedment depth so why not just use it. Seems like does somewhat lazy blanket bearing capacity statements. Those general statements are fine for routine foundations but for something that is very deep, like 4m, i think it should be a specific assessment.

 

[HTURKAK]

Dear EireChch , The OP has provided very limited info .In this case ,just bearing stress 105kN/m2, depth of exc. 4m and allowable bearing capacity @4m is 99kN/m2.( why not 100kN/m2 ??)
I guessed the bldg is 10 storey bldg with one basement . If this is correct (Mr oloksy ;pls. correct me if i am wrong and please provide info for the proposed allowable bearing capacity ) i suggested a treshold value 150 kN/m2. considering the removed soil wt.

I do know how to calculate using bearing capacity equations ( Terzaghi, Meyerhof, Hansen..)




According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

 

[oloksy]

HTURKAK, yes it is a 13 storey building with basement. I just attached the soil report below.
My 105kpa was total axial load divided by area.

 

[HTURKAK]

Mr OLOKSY,
The attached soil report gives info only for CPT. You should have a soil profile and lab test results for better prediction of allowable bearing capacity.



According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10