On Foundation

[geotechniqa]

-In bearing capacity calcualtions of a building foundation, say 4 floors, one has to assume a value for the foundation width (B)(We encountered dense to very dense glacial till at various depth ranging from 2 to 3 m, thus we are going with strip and/or square footings). How B is assumed in the bearing capacity (BC) relation (1 , 1.5 m ,0.9 m..)

- AT the time of drilling the water table is encountered in 2 BHs (out of 10) only at around 2.5 and 3 m. For BC calculations I will assume a water table depth of say 2 m.. ? what do you think.

-When talking about differential settelment. what are the 2 points we refer to commonly?. Is there any recommended relation (soil is dense sandy materials, thus only elastic settelement is expected).
BEST REGARDS AND THANKS IN ADVANCE

 

[civilperson]

Perched water, not found throughout the site. Not a water table.

Glacial till and dense sand are two distinct soils, which are you using?

 

[escrowe]

Local codes may stipulate a minimum 'B.'
Settlement generally controls foundation design.
Elastic settlement analysis applies to granular non-cohesive soils.
Estimate differential settlement as 1/2 total, apply as appropriate,
such as shortest column spacing or wall length.
Check angular distortion vs. building material type (many sources, start w. Terzaghi).

 

[DRC1]

The water table may or may not be pearched. in poorly dained soils it may take several hours for water level to stabilize, yet many drillers only check water level at end of drilling. Check logs carefully for other indications of water table.

Assume a reasonable bearing capacity, such as 2 tons/sf (100 Pa/m2) (I think) and compute a footing size, adjust to someting that makes sense such as 1m x 1m and compute your capacity. If, based on your results you need a smaller or larger footing, repeat.

Differential settlement is the diffence between 2 adjacent footigs. Although if everyting is uniform 1/2 total is used. However, footings that are heavily loaded or overlyng poor soils may be subject to higher settlements and the effect of these conditions should be investgated.

 

[BigH]

Where is your locale? You didn't indicate if your glacial till was "sandy" or "clayey" (like the Peel Till Plain in Ontario) although your use of "dense" suggests it is a "sandy" till. You also didn't say what depth of footing you need for frost protection or what the overlying material is. For the clayey till plains in Ontario we would normally use 300 kPa (N=30 - but this is a clayey till plain that is well understood) - we would also use a minimum footing width, say of 1 m minimum dimension. The minimum width might control. Bearing typically as DRC1 says seldom will control - in glacial tills - it is settlement and glacial tills are so overconsolidated that settlements are not generally a worry either. Geotechniqa - obvioiusly your company has experience in the area - what "rule of thumb" do they typically use for the dense to very dense glacial tills?

 

[geotechniqa]

Well but usually we calcualte the settelment using the allowed bearing capcity value (after calculating the bearing capacity). My foundation could rest on compacted fill to 100% Proctor of on the till which is gravelly sand , some silt/clay materials. The foundation depth is below 1.2 m (frost depth.
AS I said Water table was detected in 2 BHs at 3 and 5 m at the time of drilling. I will for conservation consider that the water will, due to seasonal fluctuation will rise 1 m. Thus the design water table is 1 m.
Alos any hint on how conservative will should be when assigning the water table will be appreciated.

I disagree with you guys,as in dense sands or overconsolidated soil (Failure is brittle and the stress-strain curve has a peack)imlying that the bearing capacity failure would happen before the allowed settelment is reached (with a little pit of settelment)

 

[oldestguy]

Interesting discussion. Glacial till can mean any of the materials left by the glacier, generally not affected by water washing.

Has any of us "experts" ever seen a brittle, shear failure in dense till? Have we ever seen an excessive settlement situation in same situation?

How do we know which one governs? Many factors here, right?

I'd opt for settlement, using a high GWT. Water can vary over the years.

 

[escrowe]

If the material is a 'brittle' or 'cemented' paralithic formation, why not core it and determine unconfined strength?