Removing Organic materail 1

[lakeboy2004]

I am in the process of building a home and have had a soil boring test done which has come back telling us we have orgainic material or "muck" about 7 feet down and about two and a half feet thick. We have companies come in to quote us on removing the material and our bids are over 50,000 dollars. The area to be cleaned out would be about 2500 square feet. We have checked on pilings and they are running around 50K as well. Could we possibly pump concrete or do something else that would be a lot less expensive? Any help would be greatly appreciated.

 

[fndn]

If you intend to build a 9 ft basement home, your excavation would be around 6.5 ft deep. So you could remove and replace(undercut) the 3 ft of soil beneath the basement elevation and place crushed 3"-5" size rock. This can be compacted by the bucket of the backhoe doing the excavation. For a 2500 sq. ft area, you're looking at around 400 tons of rock which will be may be at $15 a ton or 6 grand. Then add the removal cost of the 3 ft soils, maybe another 2 grand. To keep costs down loose these spoils in the rear and side yards. Hauling it will cost extra.

Realize that you may need to add extra rebar in the foundation walls and may be spread the footings. This is typically 3 grand. So in total around 11 grand. Good luck. By the way, when they backfill the exterior basement walls, have them exclude the muck materials.

 

[BigH]

What is above the "muck"? Fill? - likely. What is the muck - peat? or . . . ? Any physcial property information for it? What kind of fill? How long has it been there? Was it compacted - or what did the soil borings tell you? Are you actually planning a basement? What is underlying the "muck"? What type of piling were you planning? Did a geotech supervise the boring or write a report or did you just hire a local drilling company to put down a hole? If you had a geotech - get him to give you some guidance.

A lot depends on these kind of questions. If the fill has been there a long time and is of good quality, you might be able to put your home on a rigid raft-like foundation. You could also dig out and put in a 3 ft crawl space or so - in effect matching the load of your house by the weight of the fill removed. This would be an alternative to the basement which, in a way is a good idea for it will give you a pretty good amount of "room" for later development (shop, in-laws suite, etc.)

It seems that your piles were to be pretty robust. Sadly you aren't in a country with cheap labour - you could do hand dug caissons at 10 or so suitable points, then use grade beams to support your house floor . . . I think that I would have a local geotech review your plans, review the soils data and give you some advice - his cost is much much less than the high figures you have had quoted.

 

[lakeboy2004]

Thank you both for your ideas. This is the first time I have used this forum and should have given more detail. First of all, my home is to be built in Florida and it will be on a lake. I had a geotech company named "Ardaman & Associates" conduct the boring. They have several locations in Florida and have a very good reputation. Their recomendation is to excavate but as I mentioned above it is really expensive. I think the biggest obstacle to this project is that we directly sustained three of the four hurricanes that hit Florida this year and this has significantly rasied the water table. The water table is about 6.5' down and the ground would have to be dewatered to ensure that all the muck is removed.
I will try to answer a few of the questions that were posed:
we will not be building a basement, the fill above the muck is standard good fill, the neighborhood was developed in the late 50's so the soil has been undisturbed up to this point in time, the soil beneath the muck is sand and will be fine for building on, muck (in my report from the engineers) is labeled as "organic material", the types of pilings or piers were to be made of some type of less corrosive material than galvanized steel (they wouldn't tell me the exact material). Again, thanks for any suggestions that you may have.

 

[BigH]

I would talk to the geotechs again and try to get them to think out of the box. Excavate is one way and a safe way given that you have room for open cut. Did they give you any guidance on safe excavation slopes? As you've indicated the fill is long term (and you should know precisely from the soils report what kind of fill - good fill is not an option - is it clayey silt fill? sandy silt fill? and what are the "N" values or other strength information. Given this, I still think that consideration be given to a "partial" basement (i.e., crawl space) that would unload the soil fill by a couple of feet (say remove 250 psf) so that you could place your house on a rigid raft-style foundation - the idea of a near rigid raft is that even if the house "tilts" it should be uniform without differential causing cracking of brick-work or misalignment of windows/doors etc. Your geotech should know the "house-loading equivalent" on the raft - if less than what you remove, then you should be fine without settlement of the "muck". Did they do a consolidation test in the 'muck' - likely not as it is expensive, but it would be nice to know the quasi-preconsolidation pressure - if, as you say, the water table rose significantly, then the muck may very well have been dried out to an extent - meaning that the preconsolidation pressure is a bit higher. Push the geotechs a bit - as I indicated earlier: What did the other houses in the area do? Also, you need to determine the amount of "risk" you are willing to take - a positive expensive solution or a more marginal solution that saves money but, well, . . .

 

[eric1037]

You may want to consider helical piers or anchors. They are usually cheaper than other deep foundation alternatives.

http://www.abchance.com/index.html

or

http://www.atlassys.com/helical.html

 

[RepairXprt]

I would like to add a possibile variation to your choices. You may wish to investigate the cost and validity of using a deep compaction grouting process to bridge that softer soil strata.

 

[lakeboy2004]

Again, Thanks to everyone who has posted. In response to BigH, three other homes within 400' have had their foundatins crack and the home that is next door has had piers or jacks put under it to lift it back up. I don't know how far it started to sink or settle but enough to warrant someone coming in and fixing it. These homes were all built in the late 50's or early 60's and I am sure they didn't run and kind of soil tests or prepare the homes for eventual settling.
I will give you a brief update. I received a quoted for helical piers (I have been waiting a couple of weeks for it) and they want to install 63 piers at a cost of $61,700 dollars. They are basically spreading them about every five feet around the perimeter of my home and under the few weight bearing walls. I had another excavating company suggest that I take my plans (and soil report) back to the engineer who stamped his seal of approval and see if he could modify the foundation to support the home. I have not received my plans back from the engineer but my draftsmen has told me that more than likely he will put in piers around the perimeter and under the weight bearing walls. I am looking at possibly doing this on my own with the help of some friends and someone to auger some holes. My plan is to have a local person come in and auger the holes down to say about twelve feet (based on engineers recomendations). I will then insert probably an eight inch diameter pvc pipe. Rebar will be added and then concrete. I am hoping that this is the route that I am going to go. Does anyone have any suggestions or comments?

 

[JedClampett]

Why use PVC pipe? If you use steel pipe and fill it with concrete, it will act as load carrying member as well as the concrete. It will take a hundred years for the steel to corrode and you will still have the concrete core.
I think you will have a problem trying to get concrete to consolidate in an eight inch pipe if there is reinforcing in it. The cage of reinforcing will also get all twisted and/or mis-aligned. Eight inches is kind of small to work with.

 

[lakeboy2004]

Thanks for your idea jedclampett. The main reason I was thinking of using pvc was for cost purposes. I can get the 8" pipe for about 3.00 a foot. I will check into the steel and see what kind of price range that will be. Are you thinking that I won't need the reinforcement in the cement if I use the steel piping?

 

[BigH]

WRT JedClampett's good suggestion - why not use precast reinforced concrete pipe - if you are going that way. Then just fill it up with concrete. I used to wonder if you could use, say a 1m RCP and "sink" it like they do in well foundations for bridges. Dig a bit of a hole, place in the pipe - dig out dig down inside the pipe and a bit to the side - somehow tap the pipe down, continue until you get it down the depth you want - never did say what was below the muck. Then concrete it. You wouldn't have to worry about support as you would be "in the pipe" when digging. (for my size, might need a 1.5m RCP. Put these in, use grade beam across them and build your house. As these would be "large" diameter, you might get away with fewer of them than you think. Also, you might be able to buy "used" pipes that are still sound that highway dept or city department don't want or will sell cheap.

 

[lakeboy2004]

BigH,
Below the muck is the good typical sandy soil you would expect to be there. When they did the soil test they went down to 20' and didn't run into anything unusual. I hope to hear back from the engineer this week and will suggest the steel and concrete piping to him.

 

[RepairXprt]

Lakeboy2004! Hi again. I read the follow up posts and would like to add my nickel.

Piles - (I will call mech devices "pushed" or "driven" into the ground to develop skin friction and end bearing)
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The piles that you install whether steel, PVC, segmented concrete piles, etc. will require a large external drive weight of at least 2x the final expected load, if to be installed prior to construction to be of any use. Unless you have access to a pile driver or a vibratory driver, you may not be able to get your piles sufficiently set to develop your long term skin and end bearing requirements.

For a typical one-story house with (a) brick masonry veneer and (b) with a heavily reinforced grade beam structure to span the piles could easily be in excess of 1500 plf. you get to play a game with depth of beam to limit you span deflections to L/600 between supports versus the cost of the piles. Most commonly the economic break in the pricing will be between 8 and 14 feet, depending on local labor and material costs. thus if I did the math right, for a final load of around 7.5T you will need an available driving force of at least 15T if not more typically to 20T to provide the crowd on the pile to be driven hydraulically get it to set with a reduced risk of future settlement of the pile.

Spread Footings:
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Depending on depth to strata, you may have the option of excavating only the material required to install the spread footings. However in FL the labor rate may not make this an economically attractive. But by only excavating the footings, you limit the amount of excavations cost required. You will still have a type IV foundation system, except you will use the existing remaining soil as a form and your final loads will be transferred to the sand below.

Piers (mech products installed by means of drilling, auguring, etc)
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These are by far the most common pre-construction support systems. The drilled shaft C.I.P., the helical, ebo-rods, etc are controlled by other factors other than the total "push" load. Depending where in FL, you should have good success with either the Drilled shafts or the helical. The recent price increases in steel have made the use of steel products such as helical piers less desirable economically than at the beginning of the year due to several oversea demands for steel here in the states. The cost for truck mounted installation of these pier systems should be more economical than repair product because of the reduction in labor by using the truck mounted equipment.

===========================
Advice - not asked for but given anyway
A. I specialize in diagnostic and repair. I can tell you from years of repair experience that an improperly designed pile and/or pier under a foundation is no significant improvement over just having a slab-on-ground (SOG)design -- you just paid more for the foundation.

B. This is the one of those times in the construction of a house where having an experienced contractor install the “right” product will be a money saver downstream otherwise you may wish to locate your house (Investment) elsewhere.

C. The most expensive foundation repair costs are derived from fixing foundations which already have had piers/piles installed.

D. If your designing your own layout, try to balance your pier loads bewteen the interior and exterior piers to limit your settlement induced deflections.

E. Find out if your going to be designing that foundation for conventional "stick frame" or "modular"/"Truss" design. Makes a small difference on how you want your interior piles placed/spaced.


Homer Parker PE

 

[BigH]

Just a point in that the piles will not likely be 'taken' as friction - he doesn't want to go all that far below the "muck" - why a large dia hand dug caisson (using RCP for lining) might work okay - slow but . . . It will need to be big enough for end bearing. If he gets down there and the sides don't 'cave' below his liner too bad, he might even be able to bell it a bit for a bit more base.

I agree that the right contractor for the right system is best but it seems that the price of the right system the right contractor wants to install is going to cost 1/3 of the house! That's expensive. I know that I wouldn't want to pay that much. We drove 100 wood piles in Northern Ontario for only $50k US back a few years ago. But that was a school addition and there is more money in that.

 

[dirtsqueezer]

We have a local company called Geopier that installs stone columns that we've used for projects with smaller budgets. The process uses crushed rock instead of concrete, and doesn't involve specialized equipment for installation (a simple track-mounted auger and point compactor), both of which make the cost significantly cheaper. I should mention this is a different process than Hayward Baker uses, as equipment is more sophistocated, and requires more monitoring and inspection.

I don't know that you have the service available in Florida, but it might be worth a look.

http://www.geopiers.com/