No visible settling/Additional Load/Peat/Retrofit?

[lexk]

Hello, and thank you for your help in advance.

I have an existing ranch home (approx 24'x 40' with an addition measuring 16' x 24' basically it is an "L" shape with each leg 24' wide and 40' long - overlapping a square 24'x24') that I have lived in for 7 years. It has an unreinforced concrete trench footing measuring 10" x 42" that basically is sitting on 2' of peat with the water table at 2.5' (rises/falls ~1') and 20' from a lake. I would like to build up one floor over one of the 24'x 40' legs. There is no observable active settling under the proposed addition, but there is some under the other "leg". The main reason I'm looking at helicals is I want to span *some* of the addition resulting in ~13 kip unfactored point load. I'm mostly concerned with the remaining portion.

I have got elevations (approx 2 yrs ago) and tied into a telephone pole, and intend to check for settlement soon. If we assume there is none...

The question is - if there is no active settling under the proposed addition, can I simply support the second story with columns (focusing the load to point locations) and support these with piles? or should I worry about possible future differential settlement? Is this whole concept crazy? I'm obviously trying to save money on piles.

If it is settling (non-differential - which I find hard to believe) and I support a portion, I will get differential - but I just can't afford to put in helicals every 5 ft on the whole house. Any thoughts are greatly appreciated.

Thank you, again.
Lex

 

[fattdad]

You'd be best served by engaging a geotechnical engineering firm to do a field exploration to properly characterize the thickness and engineering properties of the peat and give you recommendations for your new structural loads. Helical piles may be just the right thing, then again. . . . . How can we help over hear in cyberland?

You're asking the right questions. We just don't have all the information.

f-d

¡papá gordo ain’t no madre flaca!

 

[Patgeotech]

Lexk, why not look at straddling your existing house to enable the second floor construction. Would limit the amount of differential settlement significantly, but would perhaps require flexible jointing to enable a small amount of movement (check with structural engineer). One would need separate foundations for the second level, perhaps a reinforced ring beam could assist with the soil-structure interaction (help distribute the pressure in the soil). We have undertaken similiar straddling projects successfully but often requires a bit of the architectural approach/touch to blend the straddling columns in with the existing house. Also, contact a local geotechnical engineer and do the necessary to characterise the soils beneath your house. Hope this is of some help.

 

[lexk]

Thank you for your responses. I wasn't aware that any support capacity was attributed to peat (even though it is obviously supporting it now). I guess that is something I should look into.

With respect to Patgeotech's comments:
In simplistic terms, I believe that if the peat is currently supporting the structure, and no additional load is applied, then it should support it into the future. Therefore, (assuming I can take the 2nd story loads through the existing foundation to helical piles) I can simply support the 2nd story loads with these piles.

It is not entirely separate as it is passed through the foundation - this is, in essence the crux of my question. I guess what I should do is come up with some reasonable figure for the capacity of the peat, and try to model the foundation as a continuous beam with my proposed helicals and columns, in addition

to the support capacity of the peat and any existing and proposed loads. Part of my foundation is virtually unloaded (gable end roof trusses and floor beams span the other way) and part is partially loaded (hipped roof, and floor loads go the other way).

With respect to fattdad's comments:
I have two soil borings (from each side of the house ~50' away from eachother) as follows:
Depth soil desc N Bearing Cap
Boring #1
1.5'-3', Organic Soil with Gravel, Not Given, Not Given
3.5'-5.5', Fine Gray Sand, 4, Not Given
6.0'-7.5', Fine Brown Sand, 7, Not Given
7.5'-9', Silty Gray clay, Not Given, 500 psf
9'-10', Fine Gray Sand, Not Given, Not Given
10.5'-14.5', Medium-Coarse Gray Sand with Trace of Silt, 18, Not Given
14.5'-18.5', Medium-Coarse Gray Sand, 28, Not Given
EOB

Boring #2
0'-1', Organic Soil with Coarse Brown Sand, Not Given, Not Given
1.5'-3', Coarse Brown Sand, 6, Not Given
3.5'-5.5', Peat, 2, Not Given
5.5'-8.5', Medium Gray Sand, 17, Not Given
8.5'-13.5', Coarse Gray Sand, 11, Not Given
13.5'-18.5', Silty Gray Clay, 11, 1500
18.5'-20', Medium Gray Sand, 17, Not Given
EOB

The peat had %water ~50%, and everything else varied from ~12%-20%.

The engineering report with the above stated "It is anticipated that suitable bearing helical pier (8"x10"x12" helical configuration) sub-grade soils will be encountered at a depth of between 25' and 30' below the bottom of footing elevation. The granular soils at the specified elevations should be capable of supporting fifty kips."

As sort of an aside - My neighbor had (3) helicals placed (using my borings and ~25 away from one of them), each about 15-18 kips at about a depth of 15-20' - good test piles.

I don't know if this was clear, but the house is mine, and I plan on living there a long time. Again, I am trying to minimize piles, and don't particularly like the idea of having exterior columns (part of it has a brick veneer).

Thank you again.
Lex