is my structural engineer right on this? stepped foundation

[Moonshine]

My structural engineer has specified that there needs to be a stepped foundation from the concrete slab to the strip foundations as shown in blue below (only my rough mark up)

 

https://www.labc.co.uk/news/when-and-how-to-cut-stepped-foundations-on-a-sloping-site

 

is this really the case as i don't really know how they are going to build this on site, given the massive level differences.

 

can't the strip footings just be tied into the retaining wall? what is proposed seems like overkill, is there another way to tie the traditional footings to the retaining wall?

Edited May 26, 2021 by Moonshine

[mvincentd]

I had a similar initial detail and thought it too complex along with being problematic to actually construct on site, so instead we ditched traditional footings and beam/block floor in favour of another structural slab tying into top of retaining wall.

[Ronan 1]

What his reason for tying the strip to the basement slab ??

If we know what he's trying to achieve /mitigate might be able to suggest a easier to axhieve/ construct solution

[Moonshine]

17 minutes ago, Ronan 1 said:

What his reason for tying the strip to the basement slab ??

 

when i spoke to him previously he said that is what it says in the building regs, i presume that the loading on the retaining wall.

 

This is his drawing, note that the mass concrete base isn't connected to the retaining wall, though i would have thought that you would build blockwork / pour concrete onto the mass base, so i presume you have to shutter it to make sure it doesn't touch the retaining wall.

 

it seems crazy to me that this is needed as its a 400mm thick cavity filled wall and in some places has masonry cavity walls on top of it

 

 

44 minutes ago, mvincentd said:

we ditched traditional footings and beam/block floor in favor of another structural slab tying into top of retaining wall.

 

its too late for us, as we are committed to traditional footings at the top.

 

 

 

[Gus Potter]

It's a detail with many facets.

 

Hard to make constructive comment without more info, particularly on the soil, how far you are from the boundary and so on.

 

In some ways this can be quite clever. Here the SE may be using the steps to create a safe excavation. The SE may be backfilling with a material that loads the basement wall less so you make a saving here. You have beam and block floor so that affords a good stiff point to connect the basement wall into at the top so you can design it as a propped cantilever... again potential savings.

 

I have done a remedial job where you backfill with EPS blocks on the outside so you load the basement wall with very little lateral earth loads. The reason for this is that the lateral earth pressure loads need to go somewhere else after they are transferred to the basement wall... long storey.. best left.

 

I think there is more to this than meets the eye.

 

Interested to see how this develops.

 

 

 

[Gus Potter]

Moonshine. Our posts overlapped.

 

moonshine "its too late for us, as we are committed to traditional footings at the top." 

 

How far on are you..? maybe a design review could be in order.

 

"when i spoke to him previously he said that is what it says in the building regs, i presume that the loading on the retaining wall"

 

0 out of 10 to the SE for that response... very poor if that is all you got. If the SE had taken the time to explain to you over the phone (would take less than 5 - 10 minutes) the ins and outs then you would not feel the need to post!

 

 

 

 

 

 

[Ronan 1]

If it's solely for load transfer to the basement wall then you could simply dig you strip footing down vertically to a depth where the 45 degree line misses the basement wall. You should then have alot less of a dog and massively simpler also you should be able to trench fill the strip with leanmix up to underside of require footing depth which would further save money.

 

*Caveat* this is only if it's is due to load transfer from the loads on the strip to the basement wall. But he should be able to give you a proper explanation the generic  crap about regs sound well like crap 

[Moonshine]

1 minute ago, Ronan 1 said:

If it's solely for load transfer to the basement wall then you could simply dig you strip footing down vertically to a depth where the 45 degree line misses the basement wall. You should then have alot less of a dog and massively simpler also you should be able to trench fill the strip with leanmix up to underside of require footing depth which would further save money.

 

*Caveat* this is only if it's is due to load transfer from the loads on the strip to the basement wall. But he should be able to give you a proper explanation the generic  crap about regs sound well like crap 

 

I think that it is, as this is a construction note, 

 

"Underside of mass concrete foundations to be adequate depth so that retaining wall is not surcharged with within 37 degrees of underside of foundation to rear retaining wall face."

 

this the 37 degree line, so the footings are going to have to go deeper than what are shown on the drawings so it seems that the issue is that area in red when the wall returns

 

[Mr Punter]

Does not look like you will need to dig down too far - maybe a metre?