Spread footing or Helical pier for near surface organic soil 1

[NativeGator]

I am an Electrical and not a Geotechnical or Structural Engineer.
I am trying to get a single story concrete block house built in central Florida on a lake and have run into a problem.
The house will be built on a stem wall, be about 3 feet above finished ground level and have about 200 – 225 linear stem wall footage.
The problem is that the water table is ~ 3 feet below the surface and the Geotechnical report indicated that at or just below the water table I have a 2 foot layer of organic matter with a 7.5% organic composition.
Their recommendation is to de-water, remove the organic matter, fill and compact before beginning the home.
My problem is I can find no one in the area willing to take on the job much less give a price. They all say that since the water is probably the lake water level, keeping the excavation dry while removing, replacing the organic matter and compacting would be futile.
My builder has recommended the use of a wide spread footing to support the structure without removal of the organic matter. He has indicated Helical Piers as another possibility. Helical piers blow my budget way out of whack!
There are many houses built around and adjacent to me that do not seem to have settling problems. They have not had the organic layer removed or been build on piers. The houses are on slabs not stem walls.
The soil profile is as follows 0-3 feet –brown to tan fine sand (SP), <5 hammer blows.
3-5 feet (water table @ 3 feet) dark brown organic sand (SP-Pt) 7.5% organic content, ranging from 10-20 hammer blows.
5-7 feet green slightly clayey sand (SP-SC), 20-50 hammer blows.
7-14 feet green sandy clay (Cl) ,50-10 hammer blows.
14-20 feet green slightly clayey sand (SP-SC), 10-25 hammer blows.
20-30 feet grey fine sand (SP), 20-50 hammer blows.
My question is, with the profile I have on the property can I get away with spread footings or will I have to go with piers.
Another question is, why do all the foundation people seem so reluctant to excavate the organic matter.
Any help or incite will be greatly appreciated.

Thanks
NativeGator

 

[TDAA]

As far as why they do not want to price it out:
Like the contractors said, it will be tough to dig that out with the lake adjacent to it. Without adequate dewatering, it is likely that the sands below the organics will become loose, and excavation may become harder to perform as you go down. This can lead to a cycle that is hard to (know when to) stop. The good thing is that the sands will normally recompress once you get weight back on them.

Options:
You mentioned the adjacent houses are on slabs, not stem walls. Are they PT slabs? If the foundation is constructed as a raft, then the bearing pressure will be reduced compared to a wall footing. Also, you do not have a thick layer of organics.

This type of a design could impact the geotech’s decision to pull out the material. Keep in mind, or recommendations are based on proposed construction. Talk to them about your choices, and your acceptance of risk of damage to the structure. The option provided to you has less risk than building on the organic materials. The helical piers also have less risk. A raft built over the organics will carry risk in between the removal, and the wall footings constructed on the organic.

The best person to work out the details is the original geotechnical engineer. Too often, they are the person left out of the project once the report is published. Involve them in the discussions with your builder.

 

[NativeGator]

TDAA, thanks for the quick response.
What do you mean by PT. If it is pre-tensioned then the answer is no.
The homes have been built over a period ranging from the 70's to now.
I will talk to the G-Tech again.

NativeGator

 

[TDAA]

Post-tensioned. It should offer the home a chance to move as a whole, and should reduce distress on the building. This may not be feasible, or easy, based on the layout of the structure. Also, not sure of the cost in your area.

Just keep in mind that the geotech is the first to be named when the building moves, so your guy might be worried about getting sued, and going with the safest alternative. That is where you knowing the potential risk comes in.

 

[oldestguy]

Hello:
TDAA is giving good advice, get back to the geotech eng. and ask for them to think about it a while.

Neighboring practies mean a lot,since they usually have resulted from "what works".

If you go to a "floating slab", you need to involve a structural engineer. He (she) can design the slab for minimal deflection (not no deflection).

In our area we tell the structual guy to design the slab so that it can span over any square area 10 x 10 feet. While not a perfect solution, it usually results in a structure that has no noticeable differential settlement.

In addition, your block walls should have frequent control joints to allow for some movement, yet to prevent cracks in the wrong places. I call for them at every door (inside and out), every window and in all walls at spacing of no more than 15 feet.

In your weather situation, this crack control is needed also for temperature effects.

 

[DRC1]

Organics ae just bad news. They will decay and compess over time at rates that can be difficult to predict. They also are generally poor draining wich can cause seepage problems. The layer is thin and at the surface working a section at time with a few pumps it should be feasible to excavate and replace with 3/4 in stone. The reason for the stone is that it does not need to be compacted. Be sure the stone can drain.

 

[NativeGator]

Thanks everyone. I have a meeting set up with the GeoTech group to discuss my options wrt to the foundation.

 

[dgillette]

This lake in central FL isn't Scott Lake south of Lakeland, is it? ;-) I worked in Lakeland in the '80s, mostly for the phosphate mines or reclamation thereof. [For the rest of you:

http://www.foxnews.com/video2/playe..._sink&FNL&Disappearing Act&acc&Science&-1&exp

See also "Hurricane Proof."]

Just how much settlement is it physically possible to get from a 2' layer of saturated sand with 7.5% organics? The 10-20 "hammer blows" (SPT I assume) would suggest sand-to-sand contact, and not sand floating in a matrix of organics. More likely is interlayered moderately dense sand (the ubiquitous central FL fine-medium sand?) and peat, in which case the settlement could approach the combined thickness of the peat layers if the water table ever fell so the peat could oxidize. Even if the peat didn't oxidize, it could compress by a pretty large fraction of the peat's original thickness. NativeGator - Did you see the samples or pictures of them?

Regardless, the post-ten slab ought to be pretty safe. It should prevent local uneven settlement of the floor, and any side-to-side tilting of the floor should be small.

 

[NativeGator]

dgillette,
The house will be on Lake George outside of Salt Springs.
I have not seen the samples or pictures and probably would not know what I am looking at even if I did.
I have an appointment set up with the Geo-Tech people next Friday to talk over my options.
Does anyone have any suggestions or good pointers.
I really would like to build on a stem wall (my wife has back problem and a slab hurts here back) but if it is going to be too much trouble then we will consider other alternatives.

 

[Grouser]

You or the Geotek should be able to do a lot of hand auger retrievals to evaluate the upper soils in the proposed building pad location. Assuming no major setback requirements, you may be able to identify a more suitable location/orientation for the foundation. It shouldn't cost more than $300. The geotek can give you better advice if the subsurface conditions are better characterized. dgillette's sand and peat seams are a definite possibility.

Clean sand below the water table doesn't need much compaction. Based on the gradation of the upper 5-6' of soil, ask the geotek what is the permiability and what capacity pumps would be necessary to dewater the proposed replacement excavation or if a dewatering system is necessary.

You could consider cushioning the floor over the slab to ease your wife's pain. Let us know What the geotek says.

Good luck.

 

[NativeGator]

I will keep you all posted as things progress

 

[TDAA]

Although not the most common practice (that I am aware of), you could always frame up a floor above the PT slab. This would also give a nice location to run some of the machanical / electrical items you may have. Just be sure vetilation requirements of the space (or any other requirements) would meet code.

 

[NativeGator]

My wife and I had a meeting with our Geo-Tech Friday and this is what we came up with for possibly the most economical solution to the problem.
We discussed the use of a backhoe or equivalent to remove the soil down to and below the organic layer. We would back fill with # 57 rock as the organic layer was removed to a level greater than 2 feet above the water table. If needed we may add permeable fabric between the rock and the supporting soil to help keep the rock from creeping into the soil. Vibratory compaction/tamping may also be used if reuired to help remove voids if thay exist.
The rock trench would be >3'wide and 3' to 5' in depth depending on the organic layer.
If we decide to go with a spread footer of 3' to 4' the width of the rock trench would be widened to accomadate the wider footer. Width of the rock would alway be ~ 1' wider on each side than the proposed footer.
The footer would be poured on top of the rock and the stem wall on top of that.
We have a meeting with a new builder who has his PE and does both commerical and residental building week after next.
I would like to hear comments on the proposed solution and if we are barking up the wrong tree?
I await any and all comments and suggestions.
I think the meeting with the Geo-Tech was the right answer, thanks guys!

 

[Grouser]

It sounds workable to me. A trench box would hold the excavation open better for backfilling but would require a bigger machine to drag it. See if the backfill can be placed as an advancing slope that will push less dense material in front of it. This slow deposition will help eliminate voids. You may need to slightly extend the excavation to make a pocket for the undesireable material outside of the foundation footprint.

Have the gravel compacted per the geotek's recommendation or until no further depression of the equipment is noted. You could start compaction lower, I don't think #57 rock will wick much moisture. If it does, add more rock.

Remember the high water table will likely cause undesireable moisture conditions on/under your wood-framed floor. You need to be prepared to deal with it.

Good luck.

 

[NativeGator]

Thanks Grouser, we will keep your suggestions in mind as we talk to the builder.
Do we have any other comments from others out there?