? on proper install of wall in area subject to freeze - base preparation?

[jim_wilson]

I'm wondering if anybody here might have some suggestions on how to do this properly.

First off - I live in MA, so the reason why I'm asking this question is because I want to make sure I do the install properly to avoid freezing damage.

Second: I understand the basic rules for installing stuff around here to avoid frost heaving : the base must go down below the frost line

but - I guess I'm sort of confused because I see some things around here built with no real base that goes below the frost line - but they do last for decades without being destroyed by the winters. Stone walls and granite curbing along the streets being two of the most common ones.

The project: I acquired a stock of what was supposed to be granite curbing (odds and ends) - from a local quarry that was going out of business. What I want to use them for is to build a short retaining wall (no higher than about 2ft high exposed at it's highest part) - along the back part of my property line so I can then raise the grade of my back yard and make it more level.

This wall will start at about 2ft high - run for about 75ft or so and end flush with grade. The blocks are about 3ft long x 12 to 18 inches high. Along the highest part of the wall I will stack them two high - and I've gotten a number of recommendations from stoneworkers and masons about how to bond the stacked blocks together, with the most common recommendation being to use some of the new epoxies that are out there.

Behind this I would then backfill the yard up to level of the top of the new wall.

To prevent the pressure from the backfill eventually toppling the wall - I was planning on building up some concrete on a taper (thinner at the top wider at the base) behind the granite blocks to keep the pressure from the ground being able to eventually topple the blocks. I was thinking some sort of waterproof barrier along the face of this concrete would help prevent water intrusion and the freeze/thaw cycle destroying the thing eventually.

My biggest question is: what should I be doing for a base under all of this? I ask about granite curbing installs - because every one I've seen around here basically just lays the curbing down in a concrete base maybe 6 inches thick - the curbing itself extends maybe 12" below grade if you're lucky - so there's no way the base under the curbing extends below the frost line. I see the same thing with stone walls built around here. They typically only go below ground a foot or maybe two if they guy doing the wall wants to spend a lot of money.

Do I just have to get down deep enough to get below the organic layers of soil and that should be good enough? I know there's a nice hard layer of packed hard sandy gravel/clay (no organics at all) - about a foot and half down on this property. The topsoil and the layer directly underneath (that has some organics in it) - is probably a foot to two feet deep.

Dig up the organics - put down a base of 3" trap rock or something like that - and just put the wall on top of that?

 

[dstig1]

Dig up top soil, put in a draining base of crushed rock - they call it "class 5" around here but it is something like <3/4" crushed limestone with fines (or local equivalent used for road bases/driveways). Compact and level as needed to grade, but make sure you compact in lifts of no more than 3-4". At least 6" total would be good (fill half, compact, fill the rest, compact again). I did that for my retaining wall 15+ years ago and it has been rock stable and dead level ever since.

 

[dave1949]

Rock walls with their base above the frost line probably do move with the frost heave in many cases. I think a lot depends on how exposed they are, good snow cover or not, grass, leaves, etc. My driveway is good deep stone, but in winter it heaves a good 6-8 inches. I know this because in summer it slopes down from the garage door, in winter it slopes down to the garage door. Running water doesn't lie. :laughing:

I'm pretty sure if a rock wall was laid on that driveway, which is compacted as good as you will get, it would be riding along up and down with the frost.

My understanding, from reading stone work books, is there is a rule of thumb to follow: Dry-stacked walls (no mortar) can be above the frost depth, they will move and shift a bit, but if well laid will not fall apart. Walls with mortar should be on a base that is below frost depth because any shifting will crack the mortar joints. Some people use a Portland cement, sand and lime mortar. The lime is supposed to be less rigid than cement alone.

I've built a few dry-stacked walls -- with dirt and stone bases -- and they seem to weather the frost heaves okay. Never tried a mortared wall. I'm not sure how the epoxy will react to heaving, it may have some flex to it. Probably a lot has to do with how much pressure the slope behind the wall exerts. For a two foot high wall, it seems like it wouldn't be much.

 

[Rustyiron]

Good drainage (3/4"+ stone with no dust) under & behind the wall. 12" under & behind, but the more the better. This does 2 things, obviously the stone will not hold water & heave, and the stone is sort of an insulator not allowing the cold exposed granite to act as a "heat sink" type of thing removing the warmth from the ground & sending the cold from the granite down into the earth. It sounds whaky, but if you want to be sure, but 2" of ridgid foam under the wall and a little under finished grade, extending out to the exposed side of the wall a foot or more. This will keep the frost out for sure. I'd spend the money on the foam instead of some type of epoxy glue.

 

[davedj1]

Part of the first row should be underground. I would then stagger the second course back an inch or two. Maybe use some fabric in between the two courses to tie back into existing grade.
Something like this: Google Image Result for http://extremehowto.com/wp-content/uploads/2004/08/892004120531_retwlgraph.jpg
Google Image Result for http://www.allanblockblog.com/wp-content/uploads/2009/03/ab-01-geogrid-typical.jpg

 

[jim_wilson]

Thanks for all the responses guys.

Davedj1: I was actually looking at some Allan Block products just last week for another wall I want to put up - and I plan on using some geogrid on that - so I should probably think of using the geogrid to help tie this wall in also.

I was also planning on sinking the first course into the ground about 4-6 inches.

Rustyiron:

I'm attaching an image I put together to see if I understand what you're saying correctly.

I've heard of using the pink rigid foam along "frost slabs" to keep them from freezing underneath - I guess I never thought of using it for a wall also.

Legend for the image:

grey blocks are the granite curbs I'll be using for the wall.

green is the grass layer

brown is the layer of topsoil

red is the geogrid tying the wall back to the soil behind it

the grey/yellow dots are 3/4 stone

pink is the 2" foam

the yellowish color is the lower soil layer(s) (under the topsoil)

 

[dstig1]

You don't need the insulation. With a proper compacted base and free draining backfill (better with a path to daylight or drain tile, but probably not needed for a wall this small), you will be fine. Besides - the face of the wall is uninsulated, so what will it actually do? Heat will travel the path of least resistance...

 

[jim_wilson]

You don't need the insulation. With a proper compacted base and free draining backfill (better with a path to daylight or drain tile, but probably not needed for a wall this small), you will be fine. Besides - the face of the wall is uninsulated, so what will it actually do? Heat will travel the path of least resistance...

I wasn't sure how insulation would make too much of a difference in this case - it's been my experience that without something that's a higher temp on one side of that insulation - that sooner or later everthing it is going to normalize - and freeze up.

I've seen some setups where - instead of using a full foundation in an area where the ground freezes - they use a "frost slab" ( I believe that is the correct term) - where the slab has insulation alongside it - and going out away from it under the ground. This is supposed to keep the ground underneath the slab from freezing and therefore avoids the necessity for a full foundation in ground that freezes down to a certain depth.

I honestly don't see what kind of a difference this makes - unless - you have a heated building on that slap that keeps the cold from migrating thru the building into the slab and then into the ground below it.

Which would be the case with a wall like the one I'm looking to build. The cold is going to freeze the ground sooner or later anyway around that insulation.

I think I can have a path to drain to daylight on this - so here's a revised (crude) - picture of how the wall would look in cross section. The blue is the drain pipe. The base would be compacted with a compactor - wrapped in filter fabric to avoid silt intrusion - as would the drain pipe.

 

[dstig1]

Yep, that would be the normal way to do it properly, at least around here. Here is the reference on what you are thinking about for insulated slabs. Frost-protected shallow footings is the technical name... If you read through it, you will see how and why it works, and for both heated and unheated slabs.

View attachment Frost protected shallow footing guide.pdf

 

[Carl_NH]

Hi Jim,

Interesting project - I have done similar here - got 300' of granite curbing and built a single high simple retaining wall for raised beds on a slope up to a stone wall at the front of our place. I used 3" or so 3/4 stone and tamped as the base - as I usually do build dry stack walls.

The concern I have about your project is stacking the curbing, adhesive degrading over time, and how to hold it together with the freeze/thaw cycles we have.

I would slope the blocks 10* or so back into the banking and also offset the second row maybe an 1" or so from the bottom row.

How to hold the top from pushing out may be a challenge too. Possibly a lag and an eye in each block connected to the geo grid material? That's a bit of hammer drilling in 20-25 blocks.

Just my thoughts and keep us posted with pics of course!

Carl