Bored Pile or Pad Footing

[tngv752]

I've designed a car port. The Pad Footing is ended up 3m x 3m x 0.6m thick (10ft x 10ft x 24' thick). The geotechnical report show that the bored hole is stopped at 1m from natural ground level. If I use bored pile, I can end up with 600 diameter (24' diameter) and 1.5m long (5ft long). That mean I save a lot of concrete volume, but I believe that the cost to build the bored pile is more expensive.

Should I go for Bored Pile or Pad Footing ?

 

[hokie66]

That size bored pile sounds cheaper to me, but the size of the bored pile seems incongruous with the size of the pad footing. How did you determine the sizes?

 

[tngv752]

Pile Diameter is designed based on the load and soil friction capacity. Note that 450mm diameter is small, we may end up with much longer pile and longer socket.

 

[msquared48]

Soil friction seems unlikely to me with a bored pile unless there is vertical movement down of the pile with respect to the soil matrix. Could actually get downdrag load here due to soil settlement, reducing the allowable bearing capacity. Far more reaction attained from end bearing.

Driven pile definitely produce soil friction.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Zambo]

How big is this car port?

Presumably the "floor" of the car port is designed as a slab on grade or asphalt similar to a road. So the load is columns, roof and windload.

Point being the pad foundations seem big for such as structure (unless it's bigger than I imagine). If piles were being used I would have expected a say 150 x 150mm square driven pile about 3m long. Of course I have no information to back up my suggestion, just a general surprise about the foundation selected.

 

[tngv752]

It's just a small car port of 6m wide and 9m lomg. The big pad footing due to the wind uplift. The boredhole stop at 1m at the weather rock, therefore I dont want to go further. Note that the top 1m of the 1.5m pile is ignored. Therefore, the friction capacity is based on the rest 0.5m.
If I am not wrong, the pile normally is minimum 1.5m long.

 

[hokie66]

You certainly won't get the uplift capacity from the pile which you will get from the pad. The pad itself gives about 130 kN ballast. With 130 kN uplift, your 600 diam x 1500 deep pile is flying. I've seen small piles like this pulled out in a cyclone.

On the other hand, with a 6 x 9 roof area, assuming 4 posts, (130 kn x 4) / (9 x 6) equates to about 9.6 kPa (huge). Not even considering your load factors, something sounds screwy.

 

[msquared48]

Agreed Hokie...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[tngv752]

Sorry, the pad footing is 2m x 2m x 0.6m thick. The uplift force is approx. 67KN. Skin friction capacity is 50KPa (ultimate capacity factor 2.5) in sandstone below 1m natural ground level. Bearing capacity is 500KPa.

 

[BAretired]

In my area, drilled concrete friction piles are the usual choice of contractors. I would never go shallower than 16' deep (4.9m) for an unheated structure because of potential frost jacking. Such a pile would be much more economical than a 2m x 2m footing at a depth of 6' (1.8m).

If you do not have frost, the situation may be different for you.

BA

 

[apsix]

From what I've read here it's best to not drill into the sandstone for bored piers.
This leaves you with pad footings, or a slab with deep edge beam.

 

[tngv752]

The problem is that the boredholes are stopped at 1m at the sandstone. If we go further, we need to make sure that we have the same soil or better.

 

[apsix]

Augering through clay is relatively easy and cheap, up to about 600mm dia and 5 - 6m deep.
Drilling through sandstone isn't as easy or cheap.