Pad foundations

[PaulCrowther]

hi
I am building a timber framed conservatory and will have it sit on 9 pad foundations each measures 450 mm by 450 mm and a depth of 500 mm,my question is rather than fill the holes with concrete on which i will place concrete blocks to support the joists ,i thought i would fill up the holes with mot type 1 aggregate and compact it down................will this work?

 

[freddiemercurystwin]

No

 

[bernardgreen]

I agree with that No, it won't work, and 500 mm may not be deep enough to reach stable ground.

 

[KenGMac]

PaulCrowther Hi

As above NO!

Why? because, T1 no matter how hard you compact it could if there is a lot of rain? does that ring a bell recently? cause the ground at or near the surface level to become saturated, when that occurs, saturation, the top area of the column of T1 will spread outwards in all directions into he saturated top soil.

To try to elaborate? if the top [say 100mm] of top soil becomes saturated, then the T1 will spread in all directions into the saturated soil, if Saturation goes deeper than the 100mm then the volume of T1 moving in all directions, sideways will allow the load from your Conservatory to move, downwards? and in effect crack up

The main reason for Concrete as the pads is that the load from the Conservatory WILL be transferred DIRECT to the strata at the base of the Concrete pad, But, and this is a very large BUT? Will the strata at 500mm be capable of supporting the imposed load??? this is the Elephant in the room?

Now for the Elephant? OK get rid of the Top Soil average 300mm to 400mm or so, are you sure that the Strata at 500mm deep is capable of withstanding the imposed load?

This is a non-contentious question? how do you know that the strata at 500mm is OK to accept the proposed loading? albeit the load is low but if you get it wrong, the Conservatory WILL Subside and if you make an Insurance claim [if it were me handling it] would in all probability be declined, why
1/. No Soils investigation.
2/. No Structural Engineering involvement

Sorry if I appear shall I call it Harsh, but as I see it do not proceed with T1and please consider your next moves carefully?

Ken

 

[PaulCrowther]

Thanks so much for the replies,stupid idea, feel a pratt now ,the idea for this was from swift plinths(https://www.swiftfoundations.co.uk/shop/swift-plinth-plus/) ,they seem to think a concrete block straight onto a gravel grid on soil is good enough to support a log cabin so i thought i go one better and replace the soil with type 1 to make it more solid .My next idea is to mimic the foundations of my house,it has stood for over 80 years without any problems ,depth is 375mm,3 courses of brick onto a concrete strip of 150mm,if it supports a two storey solid brick house surely its fine for a few bits of timber and insulation and a sheet of polycarb.