how to find soil upward pressure,if SBC 250KN

Sir,

We can simply define upward soil pressure. The addition of all overburden pressure from soil weight which is present above the footing as well as self-weight of the structure.

One more interesting thing is that it is also called as the ‘Allowable Bearing Capacity’ of soil.

Allowable Bearing Capacity=(Density of soil x Depth of foundation) + Self weight of foundation.

One more important note is that the distribution of allowable bearing capacity for non-cohesive soil is greater than in the middle portion.

For upward pressure calculation, you need the depth of the foundation. You see, upward soil pressure goes in increasing with increasing depth of the foundation because it takes into account the overburden soil. The method is already given in your attachment.

Upward pressure = ρD + W

Where: ρ: soil density, D: depth until the top face of the foundation, W: the weight of the foundation

It doesn’t require the knowledge of Safe bearing capacity. The upward pressure can be lesser than the SBC depending on the situation. If it is more than the foundation is unsafe.