Lateral earth pressure on basement retaining wall from a narrow backfill? 3

My opinion is NO !!!.. Jansen's formula could be used for silo type structures havng square or circular cross-section.

The minimum horizontal design loading in this case, the active soil loading with triangular distribution..I would like to remind you that, ( since having two storey basement with floor support ), at rest pressure will develop in long term which is higher than active case..

Thanks, @HTURKAK,

I agree at-rest condition should be considered. I did say K₀ instead of K_a.

I think Janssen's formula can be used for long rectangular silos.

For a long rectangular section, the hydraulic radius will approach width /2 [ R= length x width / 2 (Length + width)], but conservatively I'm considering W instead of W/2.

Can you put a small slab across at the top... a couple of feet down to support the soil... make sure the area is drained and not use any backfill, or soil pressure?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

@dik, Thanks.

That is a good idea, you mean like an approach slab but suspended with no backfill under it?

It could be possible if I can convince the local authority; the back of the retaining wall is the property line.

The city is letting us dig beyond the property line for ease of construction, but I don't think they will accept a permanent void.

That is a good idea, you mean like an approach slab but suspended with no backfill under it?

Yup

It could be possible if I can convince the local authority; the back of the retaining wall is the property line.

Is it? else you may be adding fill material on somebody else's property.

The city is letting us dig beyond the property line for ease of construction, but I don't think they will accept a permanent void.

Other than the slab support being on their property, why would then object? You can provide a seal at that location to prevent water from entering and make sure the 'void' is drained... easy to do and with physical drains located 1' above the actual void drain to prevent water from building up.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Your situation is particularly sensitive to backfill placement. This is covered by Terzaghi "Influence of Compaction on Rigid Vertical Walls" as well as NAVFAC and USACE. You should be able to find this information at no cost on the internet. Terzaghi's older texts are free.
There is a 4-part curve relating lateral pressure with depth. I'm guessing this will govern your design, at least for part of the depth.

hetgen I think Janssen's formula can be used for long rectangular silos. For a long rectangular section said:

, but conservatively I'm considering W instead of W/2.]

Click to expand...

My thoughts are,

- You can use Janssen's formula for silos having substantial depth ... If you check the vertical pressure formula, you will observe that , the vertical pressure does not change with depth linearly..That is, the vertical pressure for depth 25 m , does not differ more than that for 20 m.

The following is copy and paste from the same book :( The grain (or other stored material) pressure for relatively shallow containment structures, say, under about 7 m in height, and with a height/width ratio < 2 (see also Fig. 1 l-22c), can be
computed using the Rankine or Coulomb earth-pressure equation with /3 = angle of repose of material (</> is also often taken as the angle of repose).


- You are right that , for a long rectangular section, the hydraulic radius (R) will approach width /2 (W/2).. You may be correct that,( if you want to analize )the lateral earth pressure which will develop for this case ( width may be 1.5 m or less) could be in the range of Janssen's limits..But if you are designing the basement wall, YOU MUST FOLLOW THE APLICABLE CODES IN YOUR REGION. That is, the minimum value ( in general ) corresponds to the active pressure.

Mr. DIK has a valid point.. you may provide a space with set back and get free from the lateral earth pressure. Just provide a retaining wall for H1 and some safety measures with railing etc.

There are insulation materials (Link) to fill voids such as this, but if the excavation is over the property line, that will not work. We have used that in warehouse applications where the building abuts a blasted rock face. I agree with HTURKAK as for the applicability of the use of Jansen's formula.

No guardrail... the space is filled at the top only... a small slab spanning from the building to the bedrock, with backfill and maybe sod. The void space is below. No earth pressure on the wall.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

I asked myself the same question the other day when trying to design a tall & narrow planter walls and found this You can check out references listed at the end of the paper.

@hotmailbox, thanks a million - the paper addresses the question directly.

Thank you all for your input!

I just scanned that paper. Does it include any consideration for how the backfill is placed? It doesn't appear so.
Like Prof Duncan frequently said, you can't analyze a wall as if the backfill soil was wished into place. For semi-rigid walls, how the backfill is compacted in place is a non-trivial factor on total lateral load.
You could easily increase your lateral loads by 2X with moderate compaction effort in a narrow zone.