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UNDERMINED FOUNDATION IN SAND

Dinosaur's picture

Going to see a client's farmhouse tomorrow with my excavation guy.


It's in a flat area near a river; ground is sand all the way down to the water table; and there have been issues in the past with water infiltration under and around the foundation wall. About 10 years ago, the HO had someone install a partial block wall on one side of the crawl space and a 3x3x3 block-walled sump and sump pump to keep the ground water under control. That has been working okay. However....


...it's not his principal residence and he hasn't been up much the last year due to some family health issues.  A couple of days ago I get an e-mail, 'please take a look at the hole in my backyard and tell me what you can do about it.'


Well, the hole in his backyard is the result of his gutter being so full of pine needles and craap that it actually bent outward into a nice spout shape right in the middle, and poured the whole roof onto one spot. The erosion dug a hole about 2 feet wide by 5 feet long by 3 feet deep...which got it below the bottom of the foundation wall. The foundation wall has no footings, and is undermined for a distance of about 5 feet.


This house is at least 125 years old. It's built on a stone + rubble + concrete foundation wall about 16" thick and from 24-48 inches high, depending on where. I seriously doubt there is any rebar in it given its age. The original house is of squared log-on-log construction, which means that to some limited extent the exterior walls can span a gap. There are two visible cracks in the foundation wall in the area that is undermined, but neither of these is new and neither appears to go through. The wall is not sagging yet.


In this area, the Canadian Shield bedrock is probably 100-200' down. Worse, I know if we dig more than eight or ten feet--say, to pour a footing pad for a concrete support pier--we're probably gonna hit the water table; his well is only 15-20 feet deep.


Don't tell me we need an engineer; I've already figured on that. He'll be brought in to crunch the numbers once we figure out what we want to do. What I'm looking for are ideas which I can use in conjunction with what my excavator has to suggest.


 



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #1 of 20)

they screw up by pumping the water, leave the water where it at, the wet sand at the beach is tight, sand needs water to remain tight. Now if they want a basement, it not the right location, it just not possible. The foundation was good before they started pumping water

(post #109087, reply #3 of 20)

It's on a 5-year flood plain. Neighbours' houses get 2 or so feet of spring melt water over the front yards at least once in 5 years. He's lucky, just a coupla feet higher. Hasn't been flooded in the 15 years I've known him.


So, yeah, basically, nobody shoulda been dumb enough to build a house there inna first place. But then there's N'awlins....


This is damage caused by specific problem which can be fixed. I'm worried about what I gotta do to fix the damage, not the cause. If I just slap backfill, it'll eventually wash out next time his gutters overflow.


My first impulse is to dig out about a foot or two down, all along the foundation wall, maybe six feet out from the wall, and put in a flashing of maybe epdm on a nice slope to carry roof run off out away from the foundation. Then bury it. That's the easy part, tho.


Have to figure out how to deal with the undermined area. If I just backfill, it'll run inside the crawl until it piles up high enough. If I stop the inside of the void with block or batterboards so the backfill won't all run in, what backfill runs under the foundation wall will settle eventually and won't support that wall worth a snot.


What do ya think?



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #4 of 20)

fill the hole up with white sand, put a garden hose on the sand for couple hours, trash the pump

(post #109087, reply #6 of 20)

Hmmm.... Never would have thought of that one.


Gonna have to think about how to present that to the HO, too....


You're saying if the ground had not been dried out the gutter spill wouldn't have been able to erode the sand out from under there?


Understand, the pump only empties what drains into the sump from a french drain inside the crawl. Supposed purpose was to keep the crawl dry enough to install the furnace.


In the spring, when the river breaks up and the neighbouring properties are under water, that sump fills up pretty quick and pump kicks in every 15 minutes or so. Rest of the year, it might take a day or two or more in dry weather before the pump kicks on.


This July, it friggin rained every single damned day. And I mean rained.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #8 of 20)

I think bb is right on about filling it with sand. He's off about the pump though...........

Some of the saddest, emptiest people I have known are those who were raised to believe that they were above others. Even when among their peer group, they're not really happy because the bond which they share is easily broken, due to it's illusionary nature.

Some of the saddest, emptiest people I have known are those who were raised to believe that they were above others. Even when among their peer group, they're not really happy because the bond which they share is easily broken, due to it's illusionary nature.

(post #109087, reply #9 of 20)

Great project.

I think first I'd try to get as much weight as possible off that section of undermined foundation. Then try to install an adequate footing.

Can you somehow sandwich the existing foundation...maybe with steel plates and through bolts? It would have to have some integrity and you said "rubble"...but if you COULD do that to make the wall temporarily self supporting then excavate a few feet along the wall each side of the already undermined area...maybe few feet perpendicular to that spot also...like a big rectangular pad.

Then tie bunch of rebar in there and pour it full up underneath that wall? Kind of like a big flat grade beam? I think something along those lines might work out here in our soils.

But the other issue I'd try to resolve is those gutters. My first thought is remove as many as possible and come up with some other way of dealing with that water...maybe a series of French drains...but our ground doesn't freeze here...is there a way to deal with roof run off that isn't as maintainence(sp?) sensitive as gutters?

 

(post #109087, reply #10 of 20)

Generally, you put the 'gutters' on the ground, or even better, in the ground.


Frost line here is 5', but winter freeze isn't a problem for roof run off. When the ground is frozen any liquid water that comes off the roof--mid-winter rain, sun melt., etc.--hits the ground, melts a trench in the snowpack, then runs off on top of the ground following the slope. So the trick is to grade away from the house.


But when the ground thaws --usually between late May and mid June (snowpack is gone by then, along with surface water)--any roof run off will percolate straight down until it hits something relatively impermeable or already saturated. In this case, that would be the water table, which corresponds to the level of the river more or less. Rain enough, and that saturation level will rise both generally and locally under eaves.


Solution: Felt or rubber flashing, attached to the F-wall, buried in the ground and sloped to carry run off far enough away from the foundation that it doesn't do what it just did.


But like I said to Brownbagg, that's the easy part.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #11 of 20)

when ever you have a sand foundation, its usually in a high water table location, once the water leaves the sand the sand will loosen and erode. the problem is the crawl space. This house is hurting itself with the crawl space. This is a site that the french drain and pump is causing the problem. only other way you could build like this is on a pile foundation. So you want the water there so the sand will remain tight. what you have describe is a typical south alabama/Florida foundation, this is why we do slab on grades and no basements

(post #109087, reply #13 of 20)

I hear where you're coming from; only diff with this situation is the freeze-thaw cycle which I don't believe has anything to do with this.


House was there for over 100 years with this pissant 'almost' foundation; have to figure nobody used the crawl for anything. About 10-15 years ago, the pump and barricade wall on the east side of the crawl were installed along with slab and new furnace; partly in response to undermining of east F-wall caused by a really bad spring thaw--ice jammed under the bridge and river rose 30 feet in eight hours. 


Three seasons outta four, he's not low enough for the water table to keep the top 2-3 feet of sand tight. Only in spring, for about two months.


Also, French drain feeding that pump is all over on east side of crawl; this damage is 45 feet away on west wall. Got about a foot of daylight under the F-wall; about 3-5 feet long, 2-3 feet below grade (F-wall isnt full height). Right in the middle of the wall, too.


Ultimate solution would be to jack it up and pour a slab on grade, but we'd never get approval from AHJ 'cause they're more focused on protecting the river than they are on the houses. Won't give any new build permits on any 50-year or more frequent floodable site.


Will probably have to build a small block barricade wall inside the F-wall to hold the backfill in place, and will take your advice to soak it good so it settles and provides some sort of support for the F-wall. Then a buried flashing of epdm or xps, graded to keep run off away from the foundation.


Gotta go call my dirt guy; we're supposed to visit today if it ever stops pouring.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #14 of 20)

you could do an auger cast pile along side of footer and then encase a pile cap with existing footer on top of pile cap

(post #109087, reply #15 of 20)

You just went beyond me, bro. You're gonna have to explain that like I didn't understand any of it. Heh, heh....


BTW, no footers on this foundation. They dug a trench in the sand, lined the sides with planks, shoved in what rock and rubble they had on hand, and poured. Bottom is raw.


Thickness varies from about 10-12"; one part is almost 16" thick. Total height ranges between 3 and 5 feet.


Yeah, I know, but it's been there a while so....



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #2 of 20)

JLC had an article on foundations installed on the end of Cape Cod.  Might look it up.

(post #109087, reply #5 of 20)

You've seen the set up in Sutton?


If you don't remember the sump pump is set up so that there is always about 6 to 8 inches of water in the pit. This covers about half of the drain pipes from the parimeter of the new foundation. The pump is only there to keep the basement dry not the ground around the house.


And yes the whole area is all sand as deep as we have ever dug.


When I get home I can dig up some of the excavation pics taken during the reno.


BB's idea is probably your best bet. Just keep filling with sand as it settles.

(post #109087, reply #7 of 20)

I don't remember the specifics in Sutton; only that I whacked my head on that doorway real good!


This crawl is all sand floor; the only concrete in there is the pad they poured to put the furnace on. Sump is just a well of block, about 3x3x3 IIRC. 4" feeder from a grid of buried perforated BNQ pokes into the well, sump pump sits in the bottom with a float switch. All pretty standard.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #109087, reply #12 of 20)

I'd probably use concrete to plug the hole while backfilling, but BB's idea of using sand is an interesting option.


Your idea of using EPDM is good too.  If you're digging anyway though, why not install a 4' apron of XPS insulation instead?  It will do the same thing, and keep the basement warmer to boot.


I helped build quite a few houses and additions on Nantucket, which is one big sand dune, and the southern coast of Maine, which is nothing but sand.  Most have regular foundations with water tables about like you describe.  Nothing tricky or unusual about them, except that on Nantucket a lot of the foundations are block instead of concrete.  You might consider running a drip course of crushed stone around the building to alleviate erosion.  You might also take the gutters (I thought you called them "eaves troughs"? ;-)  )  off and installing perforated pipe "ground gutters" around the foundation.  

(post #109087, reply #16 of 20)

How about a couple of helical piers? Take 3 or 4 hours, cost about $3,000.00 and you're home for dinner.

Florida Licensed Building Contractor, 40 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

(post #109087, reply #17 of 20)

I worked on a sea wall for a beach house.


The sea wall was there to keep waves out so they didn't carry off the sand.


The house sat on piers in the sand. As long as enough sand was around them they were fine. They went pretty deep and worked together to support the load.


A wall to contain the sand is a good idea imo.


Some drainage in the containment area would be a good idea in case you get a lot of moisture.


 


"There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves."
Will Rogers
______________________________________________________________ ______________________________________________________________ There are three kinds of men: The one that learns by reading, the few who learn by observation and the rest of them have to pee on the electric fence for themselves. Will Rogers

(post #109087, reply #18 of 20)

We took a look at it this morning.


My excavation guy thinks he can dig deep enough to form a whonking big footing under the exisiting wall, without hitting the water table. But before we can do that, we'd have to dig out about 600 cubic feet of sand and crud, by hand, inside the 3-foot-high crawl space, to build a temporary supporting wall outta 2x12s and Lally columns. That's gonna be a backbreaker; no easy way to do it. Access is via an exterior stairwell under the deck, through a trap door, so the spoils would have to be shifted elsewhere under the house or hauled out by bucket.


If he wants it done before this winter, I'm gonna sub the whole thing. I start back at work on the mountain in 8 days.


If not, I'll patch it with some form boards and sand fill, watered to pack it, the way BB suggested. Then as soon as the river falls to normal summer levels next year, I'll be looking for some young, strong, short labourers....



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

Successful on undermine? (post #109087, reply #19 of 20)

Were you successful on the backfill? I have the same problem an addition was added to slab on grade house and they put a basement under just the addition. Water runoff from gutter problems undermined 4-5ft under the slab where they connect no way to pack the fill. I  work industrial construction and we use flowable fill  but its 20 to 30 yards at a time so getting it to flow is easy could you let me know what you ended up doing?  And by the way were does the dirt go that erodes? down through the water table? Thank you

Sounds like a job for a mud (post #109087, reply #20 of 20)

Sounds like a job for a mud jacker.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville