Minimum embedment of Pedestal Support 2

[engrAlan]

Can anybody share any ideas as to what is the minimum embedment of a pedestal support.

We have fabricated a pre-cast concrete 300mm x 300mm x 600mm depth designed to support a 50kg instrument junction box. i did a calculation to determine the required embedment to the ground considering the lateral resistance of soil and is constrained on top with asphalt pavement.i used a calculation guide from IBC and it only requires 450mm embedment.

However,the site inspector insisted to use 600mm embedment based on their similar project. in this case we are 150mm less than his requirements. What will be the most suitable solution to our case?

Thank you.

 

[TXStructural]

I'm not sure what parameters are considered by the method you used for design. Since we don't know enough about your location (soils, weather) or the nature of the project, we can probably only guess. Frequently, I have seen problems with shallow depth pedestals simply due to soil movement caused by moisture cycling or frost, so the inspector could be considering such things.

 

[engrAlan]

Attached is how i determined the required depth. Our Area is not subjected to seismic and snow. And usually Geo-technical report will have 0.15-0.20 ksf lateral soil bearing capacity with no water table within 2m below natural grade.

 

[TXStructural]

Your computation seems to be based on 0.2 ksf capacity. You mention that usual is "0.15-0.20 ksf" (assuming the unit conversion are all correct, since these are mixed units): is the official going on 0.15 ksf capacity (the previous project), which would probably give you a 1/3 deeper embedment?

And is the extra 6 inches worth even arguing about? Even paying retail, the added concrete costs just $2, and either of us could have dug the extra 6 inches for a couple of dozen holes in the time this conversation took.

 

[engrAlan]

Thanks for sharing your spare time TXStructural.

Cost for the additional 6" may not be an issue, but the issue is, how can we add this 6" to our precast pedestals? We already fabricated about 300 pcs, 300mm x 300mm x 600mm (450mm embedment and 150mm above the ground). These are intended not just for the junction boxes but also for cable tray supports along the entire cable route. Cable tray supports will not give the critical support requirement since it is close to the ground and supports were spaced close enough.

Can we just add the 150mm at the bottom and provide anchorage to our pre-cast? Or how can i convince the inspector that 150mm is not worth arguing in terms of structural stability by presenting the facts through calculation?

 

[TXStructural]

I'd ask the geotech for guidance on arguing the stability of the current configuration and embedment depth. The bearing values are very subjective, and subject to interpretation. Ask for a confirmation of the requirement. They may have "ballparked" a conservative value and might be able to suggest a higher value. That said, in many places, values involving the top 2 feet of soil are just a guess. You mention that this location is paved with asphalt, so the actual conditions in the top two feet may not be native soil, and may be well-packed select fill with better values.

As low as the forces are, adding an additional length of concrete would probably suffice, but obviously doing this to 300 pieces would not be trivial. If you have the ability to drop fresh concrete into each hole before placing the precast pedestal, you would probably get satisfactory results, particularly as low as the stresses would be. One possibility would be to place a block in the bottom of each hole and adhere it to the precast with mortar or other adhesive.

 

[engrAlan]

Thank you TXStructural,i will try convincing our inspector to place fresh concrete it would easier for us. The inspector's idea of using 600mm embednent is to reach a more stable/stronger foundation base, and adding fresh concrete would suffice. thanks again.

 

[RareBugTX]

I am with TxStruct, it is not worth the wasted time discussing something that will make him happy AND that will make your design a little more conservative and in this case, safer. I would have roughen the lower part of the pedestals or even more, make a larger diameter C.I.P. base pedestal of say 400 mm. We are talking very small amount of extra concrete. Thats my view.