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# Frictional resistance and 50% of Passive Pressure

## Frictional resistance and 50% of Passive Pressure

(OP)
Hi,

I have a Spread footing where the ratio of Horizontal Load Fa to the Frictional resistance, = Fa/Fr=1.1 <1.5

To increase the Factor of Safety(F.O.S) of sliding, 50% of the passive pressure with Frictional was considered.
However I am not confident with 50% passive pressure together with Frictional resistance of the Foundation.
Because passive pressure would act on the foundation only when the foundation moves, it means that when passive pressure is there no friction is available.
Please suggest me if I am wrong.

### RE: Frictional resistance and 50% of Passive Pressure

Friction and passive can act together. Passive resistance requires more horiz displacement to engage. Friction is a more reliable component and not as variable, particularly for shallow depths.
When sliding is a problem, shifting the position of the footing relative to the wall face helps (if that works for your site geometry).
However, the way your sketch is show, there doesn't appear to be much driving force.

### RE: Frictional resistance and 50% of Passive Pressure

As you note to engage the passive friction significant movement of the footing will be required. After this movement there will be still be friction between the bottom of the footing and the soil, however depending on the soil type the quantity of friction could be less. For example in a highly plastic clay, the amount of movement required to develop passive pressure could put you on the residual portion of the strength curve for the clay. You also need to consider if the structure can handle that amount of movement.

Mike Lambert

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