We’re putting an addition on an old house and had to tear out 30′ or so of existing foundation and rebuild. The problem area was where the existing foundation ended and our new wall started, nearly 6′ deeper–in coarse sand.
How do you pour concrete up against 6′ of sand that wants to sluff off, while keeping the concrete in the forms and not sneaking around the less than secure sandy edge?
A week ago, we held back the sand with plywood, and supported the end of the existing foundation to prevent cracks while pouring the footer. Thus leaving a sluffed off gap under the existing foundation of one to two feet. Then we placed our ARXX forms up to the existing foundation, shoveled out any loose sand, formed the gap (ARXX to sand/existing foundation) with thick ply cut to match the profile of the sand wall.
We then placed another ply form 18″ outside of the concrete wall and filled this with roadmix and sand to create a plug outside of the concrete forms.
We poured the 8″ wall (12″ at transition to existing) yesterday with 95% success. Everything held together although we backed off the vibrator a bit when it looked as if the sand was beginning to give a bit.
It seems that there must be a better way to terminate a wall in these sandy conditions without damage to the existing foundation from sluffing sand giving out from underneath. Our inspector, two concrete contractors, and a few other handy guys couldn’t think of anything better than what we came up with.
Any ideas? We’ve had such interest in our project that a few more similar projects are in the works and our excavator sub says sand is everywhere in this part of town so we will run into the same problem again.
Thanks for any and all ideas!
Replies
Trout:
Only solution that jumps right to mind is to dig to base of existing foundation, drive corrugated steel at the interface between new and old to a depth of 6' plus whatever the engineer says would be necessary to hold the sand after you dig the rest, then dig the additional 6' depth. The steel will hold back the sand on one side and the new concrete on the other. You can pour what you want against it knowing that you won't be buried by sand, and the existing house won't fall in on you either.
If you can't tell from the above, I've never done that, just an idea. Hopefully you'll get some better ones.
PS: does anyone go into the trench or pit when you're setting the forms (or ever)? If so, Yikes!
Edit - after pondering this a bit more, drive the steel before digging anything. An extra 6' of the steel is cheap insurance. When you then dig to the base of the existing foundation, torch the steel at that level and go. I'm now thinking if you dig part of it first, then you'd be driving steel at the end of a sand trench, and have the driver operator curse at you when the driver had to be dug out after the sand wall collapse. Not good.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Edited 3/8/2006 9:49 pm ET by philarenewal
PS: does anyone go into the trench or pit when you're setting the forms (or ever)? If so, Yikes!
We try to always have either a lot of room or a support of some kind, but it's easy to underestimate the danger after working somewhere for a few weeks. Luckily we've had light rain lately which keeps the sand moist, but not washing out. Dry sand doesn't last long.
The great thing about ARXX ICF forms is that when the wall is first being put together there is little danger since we work from the inside. However, if there are any problems prior to, or during, the pour it might be necessary to get in the trench.
:-)
>>"The great thing about ARXX ICF forms is that when the wall is first being put together there is little danger since we work from the inside. However, if there are any problems prior to, or during, the pour it might be necessary to get in the trench."
Double Yikes! I'm just touchy about it 'cause we've had a bunch of collapses in phila. over the past few years. I happened to be working a couple of blocks from one the day it went down (no, it wasn't my fault, honest -- some good photos in the story, link below). Nobody hurt, luckily. I'm told the rocket scientists digging the foundation for a high rise condo next to it touched the 150+ yo basement party wall with an excavator shovel. Pop, pop, pop and then about a third of the width of the building fell in the hole. The guys working in the hole heard it pop and ran before it fell. Before the fire department demolished the rest of it (as in the pics), you could still see into the rooms, stairwells, furniture, everything. Looked like someone just cut it in two. Four story brick building.
http://www.temple-news.com/media/paper143/news/2005/09/27/News/Building.Collapse.Displaces.Students-999865.shtml?norewrite&sourcedomain=www.temple-news.com
Work safe.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Double Yikes!
I should have included that we brace the most dangerous areas and have the trench dug a few feet wider than normal to help minimize the risk. Still, risk management is not being in the trench any more than absolutely necessary.
Wow, that's some mess in the story link.
Are you using any kind of shoring? Here (in CA) any trench over 4' deep must be shored - or dug with sloped walls so it can't collapse.
I hope your footing is on solid ground. - lol
duh.. i thought it was 6" not 6'..
damn.. i'd have steel sheet pileing driven for this.. and i'd have a soils engineer sign offMike Smith Rhode Island : Design / Build / Repair / Restore
Are you using any kind of shoring? Here (in CA) any trench over 4' deep must be shored - or dug with sloped walls so it can't collapse.
Some shoring. I'm sure what we have to do on occation doesn't meet osha specs, but we're actually more safe than most of the guys we've seen. [knock on foam]
:-)
trout .. i work around excavations a lot..
6' of sand can be just as liquid as the surf off a Rhode Island beach.. and it will come down on you without a sound , in the blink of an eye
just getting a foot caught in the flow will make a believer out of you
one guy in a trench in Rhode Island was dug out by the backhoe, but it cut his head off getting to him
unstable soils in a 6' excavation is asking for troubleMike Smith Rhode Island : Design / Build / Repair / Restore
"we're actually more safe than most of the guys we've seen"
That's what they all say - lol.
You're playing a version of Russian Roulette there. I hope it works for you.
You guys are killing me. Based on a question reguarding pouring concrete against "coarse sand/gravel" you've suddenly become very knowledgeable about the soil conditions on our site. You've also become quite aware of our pit wall-to-workspace clearances and practices--without even being there.
Yes there is a large amount of sand in general, compared to other places. Yes, sand can be very dangerous. Yes, excavations of any kind can be dangerous. Yes, there are also an awful lot of stable soils at our site. Yes we take the precautions necessary to work safe and come back for another day. Yes, we inspect the soil conditions daily. No, we aren't building on a beach and our soil is not like beach sand.
No, this is not our first rodeo.
hey trout, bite me...
<<<<<The problem area was where the existing foundation ended and our new wall started, nearly 6' deeper--in coarse sand.
How do you pour concrete up against 6' of sand that wants to sluff off, while keeping the concrete in the forms and not sneaking around the less than secure sandy edge?
A week ago, we held back the sand with plywood, and supported the end of the existing foundation to prevent cracks while pouring the footer. Thus leaving a sluffed off gap under the existing foundation of one to two feet. Then we placed our ARXX forms up to the existing foundation, shoveled out any loose sand, formed the gap (ARXX to sand/existing foundation) with thick ply cut to match the profile of the sand wall.
We then placed another ply form 18" outside of the concrete wall and filled this with roadmix and sand to create a plug outside of the concrete forms.
We poured the 8" wall (12" at transition to existing) yesterday with 95% success. Everything held together although we backed off the vibrator a bit when it looked as if the sand was beginning to give a bit.
It seems that there must be a better way to terminate a wall in these sandy conditions without damage to the existing foundation from sluffing sand giving out from underneath. Our inspector, two concrete contractors, and a few other handy guys couldn't think of anything better than what we came up with.
Any ideas? We've had such interest in our project that a few more similar projects are in the works and our excavator sub says sand is everywhere in this part of town so we will run into the same problem again.>>>
sure sounds like yur first rodeo
Mike Smith Rhode Island : Design / Build / Repair / Restore
Hey, Mike,
How's it goin'?
Bad day, hunh?SamT
no sam....good day
couldn't figure out what trout is trying to say.. either he's working in sand , or he's not..
apparently he's notMike Smith Rhode Island : Design / Build / Repair / Restore
Mike,
The way I understood his first post was that the site has patchy soil, some sandy, some not.
But, whatta-eyeno?
SamT
Steady there boss. I don't know the guy personally but I've dealt with him on other posts and I think he's a regular guy just asking for good advice.
You obviously have some so pass it on. Just criticizing the guy doesn't help.
PS: I have to tell you, we seem to agree that if someone told me to go in a 6' plus trench with walls that weren't at 45's I'd walk off. I wouldn't care what the soil conditions were. This is a hint to anyone that might think about risking their family's future by doing so.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
hey trout, bite me...
That's the level of your input? *chuckle*
trout... as you know from reading these threads... only a select few ever rate my term of endearment...
stay safe , buddy..... no job is worth the risk of your life or those of your fellow workers...
i get daily reminders from my guys.. the ones i paid to go to OSHA 10 classes..
they serve the same function as the slave riding in the chariot with caesar , whispering in his ear... " sic transit gloria"
it can all end in an instantMike Smith Rhode Island : Design / Build / Repair / Restore
Actually, I am pretty knowledgeable about trench work. I engineered all kinds of trenched telecommunications, sewer, and water systems for several years in all kinds of soil conditions. When you're deeper than 3'-4' there's always something of a "pucker factor" - even with some very serious shoring.
In your earlier posts, you sounded pretty concerned about the amount of time you (or your crew) spend in the hole. Nrevous people usually have reason to be nrevous.
In your earlier posts, you sounded pretty concerned about the amount of time you (or your crew) spend in the hole. Nrevous people usually have reason to be nrevous.
Well, yes, I am cautiously nervous. Construction as a whole is dangerous and the more dangerous aspects deserve respect--like working off ladders, roof work, scaffolding work, working around others with nailguns, working around heavy equipment, working with chemicals, working in high levels of dust, etc.
If we can cut our exposure to any dangerous condition, our chances of getting hurt are also reduced. OSHA wants to engineer a situation safe with no regard to time of exposure, while overall risk managment has to involve limiting exposure in ways that make sense, as well as reducing or eliminating risks as appropriate.
:-)
i'd try to build a dam with mortar...
Trout,
A: Drive Rebar as a retaining wall. Pour and pull. edit: JMHO, but not over 2' deep.
B: Drill 2" holes in the sand under existing and fill with thin cement/water/ few-drops-detergent slurry. Drill another and fill it. Repeat till you think you've injected enough. If you get 300-500PSI in 24hrs, that's good 'nuf. I think I'ld limit it to about a 4' drop in 24hrs.
More people will chime in with better ideas.
SamT
Edited 3/8/2006 9:52 pm by SamT
Has anyone answered your question yet?
The best politics is right action. -Mohandas K. Gandhi (1869-1948)
Has anyone answered your question yet?
No. Glad to see you though.
We're up against the same problem already with a simple 2' high retaining wall along a portion of the base of the existing foundation. This step makes the transition from the old to the new slab and needs to retain the intact soil under the rock/concrete wall so we're resigned to nothing closer than the retaining wall tall (2').
The catch is, the closer we make the retaining wall to the foundation the higher the risk of losing the wall, but the more useful the space becomes. Right now we've configured the area over the step as closet space, and a bench for a front loading washer and drier. Obviously we don't want to replace the wall, but we hope to come up with a solution allowing the step width less than 4' for the 2' drop (actually 3' drop to the bottom of the specified mono-poured footer).
Trout,
If you're not big on driving corrugated steel, how about drilling, pounding in a rebar or two and pumping grout?
As another option, I dug up the attached article which gives another idea of just driving pins into the soil at an angle as the trench is excavated (there's a hand drawn sketch that looks do-able). If the soil is sandy, you could probably pound the pins in with a demo hammer or a jackhammer.
http://tc17.poly.edu/sn.htm
Again, I wouldn't do any of this without an engineer, but I don't do excavation for a living so what do I know.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Thanks for the ideas. Keeping in mind that we're working under houses, often with limited access for heavy equipment, the more simple the process the better.
If it becomes too much of an ordeal with too many engineers then we're almost better off scrapping that section of foundation and rebuilding. One section of foundation wall wasn't as deep as the architect thought and he wanted the homeowners to hire a structural engineer to patch the problem at a cost of more than just having us tear out the old and rebuild.
We're going to try driving closely spaced rebar and see how that works.
Cheers
I don't think I'm visualizing your situation all that clearly, but here is a retaining technique that was signed off for me by a PE.The actual specs vary with the site conditions, do not use this as a reccomndation, just something to run by your PE.You can drive drill stem or well casing (steel pipe 3/16" wall) in the 3" to 5" diameter range into the ground twice as deep as you plan to excavate. Depending upon soil and site conditions, you may need to space these as close as 2' to 4' apart.I have done this in more cohesive soils than you describe on 6' spacing.It's easy to drive the posts. There is a tool that you can rent called a PostMaster that goes on a skid-steer. It's just a big drop-hammer. Works great.Then depending upon the soil and what you are doing you can either slide pressure-treated timbers in behind the posts as you go or weld 8 x8.5 C-channel to the front of the posts as you dig down.The one time I used this we used PT timber behind the posts. We've got one pending right now that may end up being the C-channel welded to the face.In any case, it is important to recognize that in the circumstances you describe a 6' deep hole is much more dangerous than a 4' deep hole, even with shoring.And in any case, I would not do anything without a PE approval.Alternately, you might check this website http://www.Shoring.comThere might be a dealer near you.I'd be interested to know what you come up with.The best politics is right action. -Mohandas K. Gandhi (1869-1948)
Thanks for the idea.
Apparently I'm not very good at describing the situation since it's caused so much confussion.
To put it in other terms, we've demoed a portion of a basement wall that is 7' high and need to form a simple concrete end cap, where the demo stops, that is 2' deeper than the exsisting wall. On one side of the wall is open living space. The other (exterior) side was originally backfilled with sandy soil for 4-5' from the foundation.
What we are trying to do is minimize the amount of backfill removed from the exterior side in order to form and pour the simple concrete cap on the foundation. The more backfill removed the more exposed the exsisting foundation becomes, requiring additional concrete to be poured under the exposed wall.
We've formed and poured the two wall-height "caps" but ended up with nearly 4' of the foundation unsupported from below because so much of the backfill was removed for safety reasons. To prevent cracking of this exposed wall section we lifted the house to eliminate weight from above and added supports on the sides so the freehanging wall can't separate and fall on someone. The gaps under these sections of wall are then filled with concrete as a separate pour.
Replacing settled, undisturbed soil with concrete under the foundation walls won't provide the same support since there will be a small amount of settling, although this appears to be our only cost-effective option, and the way this is usually handled locally according to the contractors and building inspectors that have looked at it.
A good sounding option appears to be having a grout pumping service drill and fill the area in question before excavation begins eliminating the problem with sand altogether. The way it's described, a grout slurry is pumped into the hole as it's being drilled to eliminate voids or air pockets, and a series of closely spaced holes essentially produces an in-ground wall of whatever thickness you need. This sounds like a good way to go.
A few degrees removed, we would have benefited a great deal by simply augering a few good size holes on each side of the wall and filling with concrete up to the footer prior to excavation. Not quite as good as the pumped grout above, but quick and much better than what we did last week.
We're going to auger a series of holes and fill with concrete prior to excavation for the 2' high retaining wall we're pouring in a few weeks.
Cheers
Sounds like you've got a handle on it.FWIW, helical piers can be a quick and easy solution to foundation support in the circumstances you have described. We've got a similar one going right now, the fill will fall out from under the foundation as fast as you can dig. We had two helical piers put in, these will easily support the end of the house.Also, you might look into Controlled Low Strength Material AKA Flowable Fill or Lean Fill. Your local batch plant will supply it. It is self-levelling, flows into anywhere, and has plenty of bearing competence for this application. This is what it is made for is replacing substandard soils or backfilling in otherwise impossible areas.The best politics is right action. -Mohandas K. Gandhi (1869-1948)
I have seen concrete blocks being placed with a crane. The blocksize is 4'wide x 2'deepx2'high or larger. after completion of the wall the crane removed the blocks for other uses. In any case excavate much larger than usual and have an emergency exit
The pumped grout is sometimes called mudjacking or slabjacking. Great way to deal with settling slabs & foundations. Our next door neighbor used it to level his foundation & chinmey. Two story house, stucco siding & new windows; jacked the low corner 4" and no cracked stucco, no stuck windows. Very cool technology.