Drilled shaft - regarding displaced soil weight

[Nayan67]

Hi everyone,

I'm currently in the process of estimating the axial capacity of a single drilled shaft. I've come across a situation where the bearing resistance of the drilled shaft is insufficient to counter the imposed load, regardless of the penetration depth. This was due to the contribution of the weight of the drilled shaft. I can potentially achieve the required resistance by reducing the weight of the displaced volume of soil, . However I have no idea whether reduction of the displaced soil weight is allowable or the equation used to estimate the axial resistance of drilled shaft implicitly takes into account of such displaced soil.
If any of you have insights regarding the inclusion or exclusion of the displaced volume of soil, I would greatly appreciate your input.

Thank you in advance.

 

[geotechguy1]

Can you show us your calculations or give us the soil profile / parameters you are using for design. It all sounds fishy to me.

 

[Nayan67]

Geotechguy, thank you for the response.
FYI, the pile capacity was assessed for the strength IV load combination (load at the top of the shaft is 9662 kN).
I have attacehd the snap shot of the design sheet along with the capacity demand ratio plot. In capacity demand plot, I have defined the capacity as the resistance and the demand as the factored load. Factored load is estimated for two cases: (1) including the weight of the shaft and reducing the displaced weight of soil, and (2) including the weight of the shaft without reducing the displaced weight of soil.

Kindly have a look.

 

[dik]

I usually write my SMath programs to include the option of the weight of the concrete, weight of the concrete less the weight of the soil, or the absence of concrete weight. The first is conservative, the second is defensible, and the third is not uncommon depending on the jurisdiction and/or the geotech report. Does it work with the second approach?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[geotechguy1]

What diameter is the shaft? The soil from 0 to 20m weighs 2,500 kN am I reading that correctly?

 

[Nayan67]

Dik, I used option 1 "Include soil weight of the soil - represented by the orange line in the plot", and option 2 "Include concrete weight less soil weight - represented by the blue line in the plot". I didn't go with option 3 as the diameter of drill shaft is 3.5 m and I can't simply ignore its weight. Yes, it did work with the option 2. However, I have no idea whether the effect of soil weight is implicitly incorporated in the axial resistance equation or not. Anyway thank you for sharing your view.

Geotechguy1, the diameter of shaft is 3.5m.

 

[EireChch]

Sorry have skimmed this thread but shouldnt you be taking the weight of soil displaced and subtracting it from concrete pile weight. The net increase is then added as part of your loading.

Often times, the increase due to concrete weight is ignored given weight of soil and weight of conrete are pretty close, in terms of geotech accuracy

 

[dik]

That's the approach I generally use. He should check with the geotech to see what the allowable bearing pressure is based on, I suspect.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Nayan67]

Thank you EireChch and dik for your input.