Foundation options is swampy locations 1

[ataman]

Hi,

I am doing the structural design for a 2 storey house constructed in a very swampy location. The house is to be constructed out of 8" masonry blocks and 5" thick concrete floor slabs. I have done a few test pits and there is atleast 12 feet of "muck". Bedrock is nowhere to be seen. The site is going to be raised by about 4 feet for flood protection and the ground floor is going to be an additional 3 feet above the fill.

I am currently exploring foundation options and the raft foundation seems like a reasonable solution as opposed to more expensive options such as piling. My question however is whether the 4 feet of fill will be sufficient to distribute the loads so that I don't have any settlement issues considering that the house is heavier than a timber framed structure. If not, what would be a recommended fill layer and how far beyond the building footprint should I extend the fill layer.

Are there other economical foundation options that I can explore?

Thanks

 

[COEngineeer]

I want to know also. It sounds like pile is the only option?

 

[Donrolla]

If you put 4 ft of fill on top of swampy soils you are going to start consolidating the soils and settlement will occur....since you have to bring fill in maybe the best option is to preload the site.

 

[cvg]

which takes time and money, hence piles may be more commonly used...

 

[COEngineeer]

driven piles I assume right?

 

[SlideRuleEra]

Suggest that you get soil borings to determine if the piles can be point-bearing or have to be friction piling. If the the 4' of fill is placed for flood protection, fill settlement may drag friction piling (and the house) down with it.

Of course if the piling are point-bearing, the piling and house probably will "stay-put" (at a constant elevation) while the yard, access stairs, driveway, etc. "go down".

Sometimes sites like this are a loose-loose situation. A few months ago I had to talk a potential client out of building on a similar site.

http://www.SlideRuleEra.net

 

[VAD]

Now that you have found out that you have 12 ft of muck, What is the soil like below the muck.

 

[Dinosaur]

I concur with the advice to preload the site. Even if you elect to drive piles later, preloading will reduce the amount of downdrag experienced by the piles later. However, putting aside the house built of masonry block and concrete slabs, you should be able to pile up a huge amount of dirt to preconsolidate the muck so when you strike off the pile the soil is already overconsolidated WRT the load of the residential structure. Then you sell the dirt and recover that cost. All you have to do is wait longer to begin construction. This entire process can be accellerated by using wick-drains that give the moisture a path to escape when the surcharge is placed. A well(s) to draw down the watertable may also help speed things up. I caution you to look into the possible need for a permit to raise the prevailing ground elevation in this area. There are many regulatory issues that may need to be addressed before you can substantially begin construction. Good Luck.

 

[eric1037]

How deep is the groundwater? You could dig it out if it's only 12 feet deep. If more than 12 feet, then you could do a partial removal and displacement. For either of these situations, you would probably have to do some amount of dewatering.

I am not a big fan of surcharging organic soils for support of foundations, especially if they are thick. Sure, you can accommodate for primary and secondary consolidation, but the "tertiary" consolidation of organic soils can be significant, uneven and relatively difficult to predict.

Personally, I would either use deep foundations or get the organics out. As noted before, if deep foundations are selected, you need to be concerned about walkways, drives and especially utility connections.

 

[civilperson]

I agree with eric1037, 12 feet of removal and replacement in a controlled fill will give long term assurance that the organics vaporizing will not cause settlement. If the imported fill is placed in level lifts with compactive effort, the fill can be 6-8 feet beyond the limits of the foundation, (or the the width of the footings if greater than 8 feet).

 

[VAD]

Preloading, partial, full excavation, piling are all good options. generally, the cost factor could be a concern with some . However, you have to look at the risk factor as well. One type of piling that may be useful is helical piles, which can be readily installed. A group can be used at corners and intermediate locations with pile caps. I presume that you will be using a grade beam to raise the house the extra 3 ft.

With the raft foundation were you contemplating supporting the 5 ft thick concrete floor on columns transferring the load to the raft or a box created by grade beams. This could be expensive as well.

I am guessing here about the actual construction details. However, this is an important ingredient to your design and what approach you may want to take.

As SlideRuleEra has said sometimes you have to walk away from a site.

Problem with a house is that if cracks occur you will be readily taken to court. Litigation is high in this area and one needs to be careful and advise the owner accordingly.

 

[ataman]

Thanks everyone for your comments.

Time is also an issue so likely preloading and raft foundation is out. I will probably advise piling for the building foundations and removal of as much muck as practically possible along the road, sidewalk, areas

I will also be advising on a Geotech Eng to assist in the foundation design.

Thanks Again.

 

[dicksewerrat]

Drag in a very large house boat. Swamps are there for a reason. It is a low spot. Can you get a permit to fill the wetland? another way to build here might be to use 'geo-web fabric out past the footprint about 20 feet in your fill layers.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Gimbli]

Today we have some alternatives to driven piles such as soil reinforcement. I did not have enough time to check in all the thread answers and/or suggestions, but one alternative is the use of some type of stone columns. Geopiers seem to me a viable alternative based on limited information provided. With the Geopiers you will control settlement and the subsoil will be reinforced. Then, structure may be cast over conventional footings or a slab mat.

 

[fattdad]

If this is a wetland, you may have bigger (i.e., permitting) problems. For what it's worth, this sounds like a pile job to me also (i.e., if you don't have the time to build and preload a soil raft).

f-d

¡papá gordo ain’t no madre flaca!

 

[boo1]

Pile, with a raised wood floor, forget about the fill and concrete slab.

 

[dik]

Get a geotekkie and forward a set of finished plans to him for review. If litigation is common, you should advise, in writing, that they consider a different site. You should also advise them to have a home with lots of joints... control joints on ea side of door and window openings... either 'J' molding or different materials.

If any problems with the building, you should share the guilt!

Dik

 

[boo1]

Exterior siding dont use traditional stucco or brick. Interior floors dont use tile.
Design to consider the flood forces.