Reduced loads on retaining wall in front of rock 1

[Greenalleycat]

Howdy,

Designing a big ol wall for a client that is ~3.4m high + footing (3.8m overall)
The face is supposedly mostly rock, so the wall is mostly retaining its own backfill
As you'd expect, using typical retaining wall theory I'm getting pretty big loads that make a realistic wall unachieavable
I've been looking into research on such walls to reduce my loads, and found a handy paper here that is applicable

https://www.academia.edu/26726769/Limited_reinforced_space_in_segmental_retaining_walls

The problem is, I'm only working out at about a ~15% reduction in load
As you can see from my sketch, I'm probably retaining 1/3 of the soil in the active wedge that I'd normally expect - so this isn't quite computing in my head
Thoughts, facts, feelings from the experienced crowd here?
(please don't just tell me to talk to the geotech. A man can try multiple pathways in his learning)

 

[Retrograde]

Are you sure that the rock face is stable in the long term? If not, you may be stuck with typical retaining wall theory.

 

[Greenalleycat]

Let's go with yes for now
The geotech report talks about using percussion breakers on the rock and I've spoken to other engineers who designed the house next door

I'm going to go to site and inspect it myself and will also talk to the geotech eventually
They just gave a bunch of retaining wall guidance that seems to be bonkers conservative so I'm upskilling myself ahead of that conversation...

 

[Retrograde]

You could look at active wedge theory.

https://www.researchgate.net/figure...m-1-and-b-forces-acting-on-the_fig1_346138861

 

[Trippelsewe]

You need to get a geotech to advise once excavation has taken place. Rock is very dependent on defects, it could be self-supporting, it could have a wedge shaped failure plane. Also the geotech can advise on the effect of stress relief on the rock which can have a big impact.

 

[MTSOE]

If (and this is a big if) you can satisfy your self that the rock will not apply lateral load, here are a few text sources for reduced lateral pressure from smaller active soil wedges. If you can't get ahold of hardcopies, try Internet Archive.

Chapter on Fascia Walls in Geotechnical Engineering by Handy and Spangler

Chapter on Lateral Earth Pressure in Foundation Engineering Handbook by Winterkorn and Fang

Walls, Bins, and Grain Elevators by Ketchum

Chapter on Pressures in Silos, Grain Elevatos, and Coal Bunkers in Foundation Analysis and Design by Bowles

 

[Tomfh]

You're right that it's better, as you're holding back less soil.

This has some interesting discussions, which suggests much better than your 15% reduction, simply via wedge analysis:

https://www.jeaconf.org/UploadedFiles/Document/96dbda29-b1b7-4bc7-9834-ea00e99fac63.pdf

If the rock has defects, it's a different story. You need to be confident there are no dip planes in the rock.

 

[Greenalleycat]

Thank you all for the responses/references
The client is a friend of mine so I have told him to pack a couple of beers and we will head up one evening to take a look at the rock

 

[XR250]

Can you not use the floor diaphragm to brace it?

 

[TLHS]

Yeah, is this not effectively a conventional residential basement wall?

Also, watch your drainage here. It's potentially got no other way to escape than what you provide for it.

 

[Greenalleycat]

@XR250 I forgot to say - the concrete floor on the left side is a possible 'Stage 2' that is shown in the drawings for context but may not be built for 5+ years (if ever)
I don't like relying on timber floor diaphragms for retaining loads either, particularly not as I'm already pushing the bracing available
My design is relying on the retaining wall to be 'self bracing' - if I pump the loads into the floor then I'll write off the rest of the house's bracing

@TLHS luckily drainage is the architect's problem!
That is why I've allowed a nominal 400mm space in the design though - to get a pipe etc into