House foundation on difficult soil 2

[IronDonut]

I'm attempting to build a 2 story house on property that I bought a few years back. The land is waterfront land in Florida.

Prior to building I had soil borings done. For the first 8 feet the soil is sand and clay with a blow count of 5.

From 8 feet to 16 feet I have a layer of loose to very loose organics with a blow count between 0 and 2.

From 16 to 22 feet I have dense sand with a blow count of 12.

Down to 100 feet I have a few 5 foot thick sections with high blow counts between 12 and 50.

Not until 100 feet do I get some decent bearing starting at 16 blows up to 73 blows at 110 feet.

My builder basically has left the foundation with me. He says he has no experience in complex foundation systems and basically I need to handle coming up with a solution. I've already talked to the Geopier folks and they say that because of the subsurface organics layer their system won't work. They suggested that I put a surcharge load on the foundation site. Basically a 5 foot thick block of concrete to settle the site and then build on top of that. Sounds kindof brute force to me.

These kinds of engineering problems are totally outside of my realm of knowledge (I'm in software) so I want to get some advice from you what I should do.

Thanks for your time.

 

[DRC1]

I would not use the surcharge method. first you would need a lot of block. Second, I think you would need a lot more than 5 feet. Third, surcharging is used to drive water out of clays and silts at a predictable rate. Organics are not predicable and have significant secondary settlement.
Once you get below the organics at 16 feet the material is not too bad. I would consider pile. Up here we would use timber. Down there precast may be the way to go. You should be able to get 25 - 40 tons out the piles. the number of piles will probably be driven by the practical size of the grade beams. If you talk to a good local structral engineer, he should be able to help you out.

 

[arnel123]

I would suggest to use an strip footing or grid type footing for more economical. for just two story building i think it would not be a problem since the load is not quite massive. so think about it and try to consult some structural engineers if it would be possible..but i think its possible since strip/grid foundation is good for loose soil..

 

[VAD]

I would lok into the use of helical screw piles for this site to bear on the 16-22 ft soil.

 

[jgailla]

IronDonut,

You can likely use augercast piles bearing at 30 to 40 feet and get 20 tons or so. Augercast piles are commonly used in coastal Florida and Georgia. The cost is about 60 grand. Berkel is a good contractor for these. With organics at 8 to 16 feet you will get minor settlement over time. This will not cause structural failure but you will get slab and wall finish cracking, trouble opening doors and windows, etc. If your builder doesn't know anything about deep foundations and soft organics he shouldn't be building in coastal regions. Preloading can be a more economical option, but you'll need to do testing on the organic layer. Depending on the organic content and consolidation characteristics, you might get by with 5 feet of extra soil and have a wait of 6 months for building. Get a geotechnical engineer and discuss your options.

 

[ChrisMcLean]

What about a highly reinforced raft type foundation? I realise it would be a large footprint but if the site was excavated and a geostextile mebrane (terram or similar) laid then gravel could be rolled in layers to provide a suitable bearing strata to form the raft without punching in. Any settlement, already described as being likely to be small, should be resisted adequately by the reinforced slab and edge beams.

I'm not sure whether or not this is a feasible option (my structures tend be rather lighter and probably smaller) but I suspect it may well be cheaper than deep piles. Your builder sounds like he may not be the right man for the job if has, as you say . no experince of complex foundations. A specialist groundworks company may be a better bet.

 

[dmoler]

Usually the augercast piles are used here (Florida) for those. Is this on the Intercoastal or near the ocean. If near the ocean, the costal construction control line stipulates piles. For homes next to waterfront, augercast would be the way to go, especially when the hurricanes come and the storm surge rips through.

Most of the houses I have done on the intercoastal uses 14" dia augercast piles for 35 tons installed to around 45 feet. Remember, in Florida, any pile over 40 tons must be load tested, and this would require much added cost. For houses, 35 tons is the standard here.

I have used the preloading sucessfully for many houses on the finger canals and intercoastal. You have to have a seawall in place before hand, and monitor adjacent structures (dont want them to be dragged down also).

 

[IronDonut]

Thank you everyone for your replies and input. This situation has me kindof frustrated. The builder basically said it's not worth the effort to build on this land. Meanwhile as you can imagine I paid a huge amount of money to buy the land (maybe mistakenly) so I have to find a way to make it work.

The thing about it is that there is hardly an inch of this riverfront that doesn't have construction on it so I find it really hard to believe that the situation with this land is so unique that it can't be done. Rather I think the builder just doesn't want to deal with it because it's going to require some extra effort. Truth be told he is a great builder I've seen a lot of his construction but has zero waterfront experience. And to his credit he admits this and says that an outside expert needs to be brought in to deal with the foundation.

The land river front (St. Johns River - Jacksonville) the water is sometimes brackish but mostly fresh and is tidal. There are a few feet of level change a day.

 

[cvg]

I would listen to what your builder says - consult an expert. You need a local engineer with experience in your area. When you got the borings done, did you have an engineer evaluate / recommend anything? If not, take your boring logs to a good geotechnical engineer and get an opinion.

 

[BigH]

As cvg says - get a geotech on the project. Site doesn't seem all that bad to me - I would go with piles to the 16 ft. layer. Short timber piles - but the others suggested would be okay too depending on the "typical" practice in your area. I once had a 10 ft compact sand stratum sandwiched between two soft clays - with the lower one being sensitive. We drove timber piles to the middle of the sand layer - spaced the piles out so that they wouldn't have overlapping pressure bulbs and it has held up a two storey school auditorium without any reported problems. It was a nice job. Saved the school district some $90k on the piles.

 

[IronDonut]

Everyone again I really appriciate your advice. I'm an engineer but computer systems are such a different animal than this stuff I'm out of my element.

On the timer pile solution a two story school auditorium that will weight a whole lot more than a typical 2 story house.

I will contact a geotechnical engineer today.

Thanks again.

 

[BigH]

Iron Donut - you're right ---- so you won't need so many piles! Actually our piling was something like 10ft apart with grade beams connecting everything together.

 

[dmoler]

A lot of houses here have been done with timber up to about 40 years ago. Unfortunately, I have had to underpin some, but I only see the ones that have problems.

With timber, the problem is the bending and structural considerations with the hurricane winds. The interface/connection of the wood into the concrete foundation (gotta use concrete due to the termites and bugs) tends to splinter/weaken/break apart over time due to bending. In Jacksonville, this might be less of a problem. By the way, I am not sure of the design, but if I were you, I would use a 2nd story masonry, and not wood frame as sometimes are used, due to hurricanes. I wish I was no the St. Johns River right now, great fishing up there, really big redfish.

 

[DRC1]

Why don't you ask the neighboring proerties what they used for foundations, who was the engineer and who was the contractor. Sounds like they may have developed an optimal solutions. Talk the latest buildings built- they probably were able to capitalize on the experiences of others.

 

[BigH]

Good Point DRC1

 

[MRM]

IronDonut,
Your observation about the development on either side of your property is generally valid. However, if problems related to settlement exist in those other structures, it would be very difficult to tell just by casual observation. It sounds like your builder may actually be very concerned about giving you a good final product and not getting a serious call-back. It can't hurt to ask others about their foundation too as DRC1 suggested.

Your site is buildable, but it probably will take some additional money you're not excited about spending to give you a foundation system that will work. The geotech engineer you retained should be able to answer your questions and give you good options that will work.

 

[jdmm]

Nobody seems to have suggested preloading which seems to be to be a suitable solution in view of the deep soft/loose conditions. This does require a geotechnical engineer and you are on the right track now.