Stepped excavation - forces on soldier pile

[milkshakelake]

I have a stepped excavation where I'm using a raker on an existing wall. About 3' inboard, I'll have a soldier pile to do the next step of the excavation.

I'm wondering how to do the forces on the lower step. Is my concept correct? I'm basically using the resultant at the bottom of the existing wall so that I don't double count the height of the soil behind the existing wall (which is mostly going into the raker laterally).

 

[MTSOE]

Applying the resultant toe reaction to the top of the lower soldier pile wall seems reasonable to me. However, I am concerned about what appears to be a very shallow toe embedment for the existing wall. You show lagging behind the back of your lower soldier pile wall. This means that for the lagging to be installed (and there is air between the retained soil and the back of the soldier pile), a temporary interim stage will exist when the toe of the upper existing wall is supported only by a very small passive wedge of soil. I.e. the soldier piles won't be offering a reaction for the emdedded portion of the existing wall. I worry that your existing wall will be unstable (only 1 brace point - the raker) in the temporary condition during installation of the lagging.

Is there more to the story that I am missing?

 

[milkshakelake]

As I got further along the calculations, I decided to add some soil to cover that embedment a bit more, possibly at 1:1 slope, to engage more passive pressure. Or I can leave more soil at the flat end and increase soldier pile embedment. Alternatively, I can use a second lower raker. So there are multiple ways to tackle that.

My concern at the moment is the 7Ka times soil height, which is the active earth pressure triangle. I think my assumption was incorrect, and the max pressure should be 17Ka times soil height. Basically using the entire depth of the excavation, not just the step. Because I don't think the step (about 3' long) is sufficient to "kill" the total height earth pressure.

 

[HTURKAK]

I would provide additional lateral struts at the top of soldier piles and transfer the load 0.46 klf shear to the common deadman.
Say my opinion.

 

[milkshakelake]

I would provide additional lateral struts at the top of soldier piles and transfer the load 0.46 klf shear to the common deadman.
Say my opinion.

Thank you for the idea. In my calculations, the soldier pile size and embedment is not too high, so it's not necessary at the moment. Your idea would add some steel. But I'll use it if the design gets rejected by the plan reviewer.