Newbie: In some soils, can holes be permanently self supporting if protected from ground water and.. 5

[NotEngineer]

Thanks for looking at my thread. I'm not an engineer, some general advice would be greatly appreciated. In some cohesive soils, can holes (90° walls) be permanently self supporting if protected (from above and sides) from ground water and weather?

I understand if you were asked to confirm that a large hole (11x11 meters) is self supporting you'd need a large amount of data about the site geology which I don't have currently, so this is just theoretical for now.

The plan is to protect the hole from weather by covering it with a rubber membrane which is supported to be high in the middle, so to drain to the sides. Protecting the sides from running ground water is more tricky. Currently my best idea is a trench around the hole (2' away from the hole) filled with clean rubble and gravel to below the floor level of the hole. The 2' thick wall of soil between the trench and hole would act as a (not 100% effective) vapour barrier to reduce the humidity inside the hole (which has some protected ventilation).

Thanks in advance for your help!

 

[jayrod12]

although my answer will inevitably be no, how deep is the hole

 

[NotEngineer]

Good point! I didn't mention the depth 11'. If you could explain why it would fall in (even if it was a circular hole?), if only to sate my curiosity I'd really appreciate it!

 

[jayrod12]

Without going into too much detail, the individual soil particles interlock with each other to resist moving into the hole. The deeper the hole, the higher the force of the soil trying to slide into the hole. But the interlock strength between particles remains fairly constant, there gets to be a point where the force of the soil wanting to move it into the hole overcomes this interlock strength.

For the record, I'm not a soils guy and that is a really crude explanation leaving out a ton of information, but it gives you an idea.

 

[NotEngineer]

Thanks Jayrod12! Perhaps if I had the circular hole walls battered outward, so instead of being vertical walls they were 10° or so they may be permanently self supporting? With an ideal soil composition.

 

[hokie66]

With regard to cohesion, you are looking at it backwards. When the soil dries, the cohesion reduces, leading to cracking. The only soil I know of which naturally stands vertical is loess, and you can't always depend on that.

 

[NotEngineer]

Thanks Hokie! Do you think there's a chance a very stable soil, for example 20% granitic clay, 20% silt, 60% well graded sands and gravel (reasonably compacted) be permanently self supporting if the walls were battered a % outward?

I don't think I'd dry out the soil, as it's not a 100% vapour barrier from the gravel trench, if it only held 10% moisture I think that'd be ok. As I'm sure you know, if it gets too wet it also loses cohesion.

Cheers!
Mike

 

[hokie66]

Yes, most soils will remain stable at a given batter. But establishing that batter is complex.

 

[NotEngineer]

Great! Yeah I'll definitely involve a Geo-engineer. Should he be able to calculate the batter once the soil samples have been taken and analysed by a lab? For the soil I described above, would you be surprised if it only needed a 10% batter or does that sound about right?

I'm only using this info to finish the preliminary CAD sketches as far as I can get them myself, in order to choose between different options.

Cheers!
Mike

 

[NotEngineer]

I forgot to ask, regarding the walls of the hole collapsing into the inside, it would make a big positive difference if the battered hole was round right? Thanks again!

 

[oldestguy]

Indicated where this is might help, since local soils info is useful here.

Placing a trench around to supposedly keep water away likely will cause infiltration right where you don't need it. Better waterproof the surface and slope it away 11 ft. Numerous ways for waterproofing are available.

With all this fuss, why not consider some "wall treatment", such as a sprayed on protective layer. Remember some swimming pools are made this way.

Look up critical height for clay soils and that may help some.

Regardless of all the info saying it is stable, I'd suspect in time it will fail in some respects. Question then, what is the useful life needed? Finally round shape and sloped back will help.

An explanation for the purpose may get some ideas for a different treatment from this forum.

 

[darthsoilsguy2]

is there an application to this? tank?

 

[hokie66]

Battering any soil only 10% (from the vertical) will do little good in slope stabilisation. In summary, If you don't want to batter to a stable slope, you need walls.

 

[NotEngineer]

Thanks again for the great answers guys! Sorry for the slow reply, I'm in the UK, it was pretty late when I posted. For some context I've inserted an image below of the mostly self build underground dwelling project. Any input is greatly appreciated! From the image it looks like humidity would build up between the walls of the hole and the straw bales, but there will be active ventilation.

 

[NotEngineer]

I should mention also, the plan to waterproof from above is a massive rubber (epdm) sheet which is supported at it's centre by a bamboo supported sky light, to be the highest point, so water drains off into the gravel trench. Domes finish prematurely before reaching the full dome shape to support 8' circular sky lights.

 

[NotEngineer]

Oldestguy, thanks, here's some of the info you said may be useful:

Lat & Long: 50.155853, -5.158280

Soil type (only topsoil info available right now) can be seen using the lat & lon at at

http://www.landis.org.uk/soilscapes/

Soilscape 13:
Freely draining acid loamy soils over rock
Loamy
Freely draining
Drains to: Local groundwater and the river network

I originally designed in waterproofing to the sides of the building design, but thought (with my lack of geotech experience) it may be better to attempt to keep the sides dry using ground works.

 

[jayrod12]

I'd be looking at some form of stabilization of the excavation walls if this is a permanent application. As oldest guy stated, some form of wall treatment, even shotcrete with a wire mesh would probably do the trick, I'd be leery about anything less.

 

[NotEngineer]

Thanks Jayrod,

As this is an Eco project it's design is to see what can be done without cement, can you suggest any eco variations?

Cheers!
Mike

 

[jayrod12]

Not likely, anything else would be less organic than a cementitious product. You could find a bunch of giant boulders and make a stone retaining wall using mud as the mortar, but i doubt that's a realistic possibility.

 

[NotEngineer]

Hmm, what about gabions and clean rubble?