Monopole foundation in swampy area 2

[aalexm]

All-

I have a request to design a foundation in Ireland in what can only be described as a swamp. The client is insisting on a pad foundation, as piles (caissons) aren't typically used here. Coming from the US, I have rarely seen 'just' a pad used for a monopole, and never in bad soil conditions. The client has mentioned pile reinforcement under the pad to combat overturning.

My quandary is this:
There is 2m of fill (clay, brick and concrete) covering peat. The water table begins where the peat begins. 'Presumed rock or boulder' begins at about 4.5m. Rainfall varies heavily in this area, so shrinkage will occur. Even if a mat is designed with pile reinforcement, won't the shrinkage cause the mat to crack, possibly to the detriment of the stability of the monople? Wouldn't a large pile resting on or even drilled into the 'presumed rock or boulder' be a (structurally) better and presumably more cost effective solution?

I should mention that the client has no interest in replacing soil or the like. They just want to plop the monopole down and be done with it.

Many thanks-
Alex

 

[GeoPaveTraffic]

I suggest talking to a local geotechnical or structural firm to see what the local practice is. You need someone familar with the local conditions to know the best course of action.

 

[TDAA]

You may consider a waffle slab (perhaps with the ribs up to ensure a more uniform bearing). You may be able to offset the wight of the structure by lowering the bearing elevation (i.e. a compensated foundation).

 

[civilperson]

The design of the monopole is a vertical beam, small axial loads, (vertical) and large bending moments,(from horizontal forces of wind and seismic). The pile footing will have uplift on the upwind side outside rows. This is the limiting case and the reason a pad foundation is inefficient. Consider a cylindrical ring footing with a base and soil fill or a drilled shaft of generous dimensions,(4-6' diameter).

 

[DRC1]

A mat is either soil supported or pile supported. Once you put piles under it, the slab should be designed to be supported by the piles alone, so soil shrikage should not effect the mat. I would consider mini piles as they can be drilled into the presumed rock layer, such that they are effective in tension or compression. It is doubtful that the pole will provide sufficent weight to keep the back piles from going into tension.
I would also do a couple borings to determine if the layer is rock or if it is buolders. This will make a big difference in which ever deep foundation system you use.

 

[aalexm]

Thanks everyone for the posts. We're trying to get the geotech co. to go out and drill into the rock, but we're hitting a wall. They were supposed to drill the first time, but just stopped when they hit resistance.

Again, I appreciate the responses.

Alex