pile or not 5

[gkhnhlk]

We are planning to built a compact wastewater treatment building with a wide spread foundation extended over a 100m x 40m rectangular area. All process tanks are concrete. The depth of building below grade will be 2.5m. The total weight of the building is approximately 20000 tons.

We first wanted to see what would be the pile carrying capacities are and tried to drive 400x400 square 12m long concrete piles... However the piles could not be driven more than 5 meters.

What would you be your thoughts?
Forget about piles and make a compacted bed and build the structure on top of it?

Attached is the borehole profile. We had 8 boreholes and all are the same.

 

[rowingengineer]

nothing attached

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[gkhnhlk]

the file is atached now (I hope..)

 

[BigH]

Your loads are quite heavy. 200,000 tonnes spread over 4000 m2 is approximately 500 kPa (490). My first question in reviewing your borehole log - Who logged it? A geotechnical engineer or a geologist? Other than the first layer where it is noted to be medium dense, we have no data on the consistency of the clayey layer - you seem to have "rock" above the marl? Geologically is this correct in your area?

We do not have the standard penetration test "N" values for the silty sand; we do not have the undrained shear strengths of the clayey materials - or RQD values, etc. for the gypsiferous rock (doesn't really sound like rock from the descriptions). We would want to know the overconsolidation history of the soil. If the layers below 6.8 m are all overconsolidated by more than 500 kPa, then a mat foundation would likely be the way to go (with that overconsolidation, the shear strength should be sufficient).
If not, and likely not, a detailed study would be needed - AND IT IS MOST DEFINITELY NEEDED NOW. Do yourself a service and hire an experienced geotechnical engineer in your location and have him do a proper geotechnical investigation, obtain the proper soil properties for modeling the site and propose/recommend the most suitable foundation system for your structure(s).

 

[ishvaaag]

20000 BigH, weary eyes sometimes at night; yet always good advice.

 

[Ron]

What BigH said!

 

[hokie66]

BigH's advice about getting a local geotechnical engineer to assist is appropriate. But 50 kPa (per ishvaaag's errata) is not much, so a mat should work. Be sure to allow for hydrostatic uplift.

 

[BigH]

- when they don't put in the commas, my eyes do go batty. 50 kPa is prety light - and I would use a mat. No need for piles - but on a job your size - you need the geotech to lead you right.