Raft foundation Bearing pressure

[DaveHolder]

Hi guys,

I was wondering if taking the weight of the soil off from the concrete in order to reduce the bearing pressure of the raft is correct? For instance concrete density is 24 kN/m3 and soil is 20kN/m3, so the total load = (24-20)x the thickness!

What do you think guys?

Thanks

 

[WARose]

Depends what the soil level was when you started digging. In a true raft foundation, the soil is excavated down to a level where the weight of the structure is balanced by the weight of the soil removed. (I.e. you are not significantly increasing the net bearing pressure.)

 

[JStephen]

In the past, I've seen discussions of designing based on "net" versus "gross" allowable bearing, which is essentially the difference you're describing (with the adjustment made only to concrete below grade). This idea is alien to some, as I recall- check with the geotechnical engineer and see how they intended recommended bearing to be interpreted if in doubt.

 

[GeoEnvGuy]

If you are dealing with a raft design it is usually because you have poor subsurface conditions. The unit weight of naturally deposited soil can varies typically between 16 to 22 kN/m3. The unit weight of poor soils is probably closer to the 16 than 20 kN/m3. Most of the recommendations I see in my area are for thickened edge slab on grade. The width of the perimeter thickened edge is considered for bearing pressure as a strip load.