Retaining wall with footing cut into dense Chuckanut Sandstone bedrock

[Prestressed Guy]

I need to design a 10' tall concrete retaining wall. The soils report is allowing 400 psf for passive pressure but leaves me with a sliding failure.
The existing slope is a thin layer of soil and weathered / fractured sandstone, on top of competent sandstone bedrock. The footing of the retaining wall will at least 18" - 24" below the surface of the competent bedrock. I intend to place the footings in a trench cut into the undisturbed sandstone bedrock by sawcutting the edge of the footing into the bedrock and then chipping out the rock within the footing edges. This condition seems to me like it should be more like full bearing pressure. Any thoughts or suggestions on how to control sliding?

 

[DenverStruct]

Just by visual inspection, that wall will overturn and slide. Are you sure this is a retaining wall? The heel is too short. I expect the heel would be 4 to 5 ft long. The rebars also do not look right.

After seeing the drawing a little bit longer, it looks like it is a basement wall? It is not really a retaining wall?

 

[Prestressed Guy]

It is a restrained retaining wall. The top of wall will be tied with reinforcement to the concrete slab at the retained height. The wall will be braced to the bedrock with erection bracing to support the top of wall during backfill and casting of the slab. The analysis is more like a basement wall that is restrained by the basement slab and the first floor diaphragm but the basement slab will be the sandstone bedrock and the floor diaphragm will be the reinforced slab on grade over the heal of the wall.

 

[Prestressed Guy]

A basement wall is a retaining wall. It is just a restrained wall with "at rest" pressure rather than a cantilevered wall with "active" pressure.

 

[Prestressed Guy]

Here are the test pit log and wildcat dynamic cone log for the pit closest to my wall. It sure seams like being cut a couple of feet into that rock ought to keep my wall from sliding.

 

[Prestressed Guy]

Here is a prelim run on this wall. The lateral restraint needed by the footing bearing on the edge of the bedrock socket is 2406 plf. Given a 12" deep footing this also comes out to 2406 psf on the rock.

 

[EireChch]

Is the elevation of that test pit log located at the elevation of the restraint?

How have you calculated the earth pressure from the 2ft soil at the top and remaining intact sandstone beneath?

 

[Prestressed Guy]

Most of the fill behind the wall will be imported. The site is sloping downward from the street and this retaining wall is to create a level parking slab. The existing rock profile is fairly uniform slope of less than 2:12 across the site and the bottom of footing will be a minimum of 18" cut into the bedrock.

 

[Prestressed Guy]

Here is a wall section of the retaining wall I am working on. I have shown the typical depth of the cut into the competent homogenous sandstone.
Passive Earth Pressure

To the extent that this poor old structural engineer can understand it, Passive earth pressure is defined as the highest limiting lateral pressure developed at the onset of shear failure by wall moving (penetrating) in the direction opposite to the direction of acting earth pressure. Given that, it seems that 400 psf Passive pressure is appropriate weathered / fractured layer on top but i am well down into the solid competent bedrock.

Shouldn't I be able to use full bearing pressure. The values I have for this is 3000 psf bearing and 400 psf passive. Given wildcat jump from 31 blows to 222 per 10cm these numbers seem conservative in the extreme.