Bearing Capacity - Unusual Case

[EireChch]

Hi All,

I have had a request land on my desk where a structural engineer needs a bearing capacity assessment of a foundation where the ground surface profile changes over the width of the bearing capacity failure wedge.

I have indicated that the exercise is pointless as a 6m wide foundation will be governed by bearing pressure based on settlement and not shear. This may not be true for foundations where there is an extreme change in ground profile over the width of bearing capacity failure wedge, but I am yet to confirm that.

I have been reading text books, online etc and I can not find a method to assess my specific case.

Das shows methods from Meyerhof for foundations which are set back from a slope, and where the toe of the slope is deeper than the embedment depth. Or where the foundation is sitting on an infinitely long slope. These cases are different than mine so I dont think they are applicable.

In my case the foundation depth is 2.7m, however on one side of the foundation the ground is flat for 2.3m then sloping at 1:3, over a height of 1.5m.

I indicated that we could just assume an embedment depth of 2.7-1.5m so 1.2m. This will be conservative. Apparently this is not acceptable. I think the structural has designed everything on a knife edge.

Interested to see if anyone has encountered such a problem

 

[Ron]

Under Terzaghi's theory, if your "q" is greater than passive pressure in the side wedge, should be ok.

 

[EireChch]

Thanks Ron,

Can you provide a reference for this? I have had a quick look and cant see anything.

Assessing the passive pressure in the failure wedge will be difficult. The best way would be to do a log spiral method, which is too complicated for what im doing to be honest.

I can conservatively assume a vertical face of the failures wedge beneath the corner of the footing and work out passive earth pressure assuming Rankine earth pressure. Rankine is conservative compared to log spiral but I dont mind that.

In the end I could have cases where q will be less than passive pressure so I am still left trying to assess the bearing capacity for such a case.