Retaining wall's sliding 3

[Tstruct]

Please have have a look at the attached retaining wall sketch.
My question is, our usual practice is to provide 6" thick blinding/lean concrete under every type of foundation. But when it comes to retaining walls, we need sliding resistance specially when there is no passive resistance available as shown in my case. What frictional resistance factor should I use for sliding calculation as foundation is not directly resting on soil rather on blinding concrete? There are two interfaces working here, soil-blinding concrete and blinding concrete-foundation. My question seems very naïve but this is the first time I am dealing with sliding. Thanks

 

[XR250]

Curious how y'all get the lean concrete to follow the keyway trench?
Also, why is this your practice?

 

[DaveAtkins]

This should be checked for sliding as if the lean concrete were not there. The interface between bottom of footing and top of subgrade will control.

DaveAtkins

 

[BridgeSmith]

I would agree with the approach that DaveAtkins proposed, but XR250 makes a good point. There's no way lean concrete is going to form that keyway unless you block it out with formwork and load it down so it doesn't float out. At that point, you've wasted so much time and materials that it would have been cheaper to have just poured the entire footing and blinding concrete area as one pour with the retaining wall concrete. Lean concrete isn't that much (if any) cheaper.

If you do manage to get it to that shape, I think you could theoretically calculate the resistance based on the footing and key dimensions to the outside of the blinding concrete. If you don't form it, you'd have to look at the capacity of the key assuming the concrete strength of the lean concrete, and consider sliding at the interface of the footing and blinding concrete, assuming the bottom of the footing is flat.

 

[HTURKAK]

Nowadays,the use of membrane or bitumen layer under the footing is common practice . If there is no membrane , maximum resisting force between wall footing base and foundation soil will be calculated with friction angle δ between wall footing and foundation soil.
In your case , ( with the use of footing key ,) you can mobilize full base friction .
Pls look to the following doc.
..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48