Foundaiton optioins for this building

[Robbiee]

Hello,
Doing a small one-storey building to be used as office and vehicles/equipment storage. Total floor area is about 8000sf.
The site is near a lake and the soil is soft. The soil report suggests to replace the top 10ft. under the footprint plus 10ft beyond the footprint with engineered fill and place the building on conventional strip/spread footings. see sketch. The soil engineer mentioned that this is the best option for the footings.
My concerns are: excavating 10 ft. near the lake where the groundwater is about 5 ft. below the surface would be challenging, and the cost of the excavation, dewatering, and the engineering fill.
Is this really the best option for this small building? I am thinking of 30in diameter piles spaced at about 24 ft connected with 4 ft. deep perimeter grade beam and 8in thick structural slab ( 2-way slab) on the grade level.

 

[DaveAtkins]

What about a mat under the entire building? Or would it settle too much?

DaveAtkins

 

[Robbiee]

Dave,
The top 10 ft of soil can't be founded on. A mat means we still need to excavate 10 ft. and then back fill on top of it. There is no basement.

 

[jayrod12]

I would be asking about driven piles as well.. Then you don't need to sleeve the holes. If water level is 5 ft down you'll have to sleeve every cast-in-place pile.

 

[SlideRuleEra]

Two concerns with your four-pile proposal:

1. No redundancy, if there is a "problem" with just one pile... the building is in trouble.
2. Mobilization of pile driving equipment is a major cost. The equipment to install 30" diameter piling will be large (expensive).

The loads must be very light (for piling), perhaps you consider treated timber piling, with a reasonable spacing (say, 5', or so). The pile would provide 100% of support for a mat foundation that is also the floor for the building.

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[Robbiee]

Jayfod12: In a similar site we did drilled piers (caissons) with temporary sleeves and a concrete mix designed to not wash away by ground water.
SlideRuleEra,
I am afraid you mis-read my post. My proposal will have 20 to 25 piers not 4. Any comments on the cost difference between the proposal of my soil engineer of replacing the top 10ft of soil with compacted engineered fill and the 30" pile proposal? Keep in mind that the soil is soft and the depth of these piles will not be more that 25ft. So, they can be drilled in a couple of days.

 

[cvg]

how does the geotech recommend placing engineered fill 5 feet below the water table in loose sand? how would you even maintain the excavation without shoring and dewatering? what type of fill material did he recommend? you might have to fill with concrete. sounds expensive.

 

[SteelPE]

I'm am not a pile expert here so evaluate my comments as such, but this seems like a huge expense to put in such a small building. Have you investigated to possibility of using helical piles? Have you also looked into geopiers? I know people's opinions of geopiers varies especially when dealing with water but they are a cost effective solution for smaller buildings.

 

[SlideRuleEra]

Robbiee - Sorry, I did misread your post, don't where I got the number 4 from. It was just before lunch.

Soft soil and 25' depth are ideal for treated timber piling. Construction of a pile supported mat should be much simpler and faster than grade beam / structural slab. I still suggest making an estimate of 320 -350 treated timber piling with mat to compare with 20 - 25 drilled piers, 30" dia., with grade beams including necessary excavation / backfill, plus structural slab.

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[Brad805]

The geotech is suggesting replacing approx 4,500 cu yd of material. If we assume something like $40/cu yd (a guess) to remove/replace the fill that would be around $180k. If the contractor wanted to complete the work quickly they could use lean mix concrete. In my area that would bring the cost to around $90/cu yd or roughly $400k. I assume either way you will have a grade beam (or footing), and a slab of some sort. The cost difference between the engineered fill option and the suspended slab design boils down to the difference between the suspended slab vs the grade supported slab and the additional interior piles (exterior piles somewhat comparable to a footing). I suspect the remove/replace option may be the more cost effective route; however, if there is the potential for ground water problems when you excavate the cost of the remove/replace could increase dramatically and slow down the project significantly.

That said, I still think your pile supported option should be investigated further. I suspect even with soil improvement the geotech has indicated a potential for movement. The pile supported option is much better in that regard, plus it will alleviate some of the unknowns with the soil stabilization option. Even if the pile supported option were 25% more expensive than soil stabilization I would likely still recommend that to my client. At the end of the day it is up to the owner to decide.

What size of vehicles do they want to store? In my area they are always very large, so pile supported slabs are rarely feasible.

 

[RFreund]

We did a similar job and rammed aggregate piers aka geopiers ended up being the way we went.

I wonder though -> if you have the space to excavate could you just pull out the material even if it turns into a pool. Then just fill 'the pool' with large 3" stone or crushed concrete then choke out the top portion with something that has fines? Doesn't seem right, but I'd be curious to know why/why not?

EIT

http://www.HowToEngineer.com

 

[hokie66]

Driven piling, whether concrete or timber, would be my solution. Which type of piling would depend on the location/availability.

 

[Robbiee]

Great input form everyone. Thank you all. The vehicles to be stored are the typical vehicles used to service public parks such as light trucks, snow removal and lawnmower equipment.
I have not worked with timber piles in my 20+ years of experience so find it uncomfortable to use them for that reason.
contacted a helical pile contractor to get their input and Will meet the geotechnical to discuss and will see what happens.