Do these foundations sound excessive?

[indus]

Sorry if this is not the right sub forum

Looking at my tech drawings for an outbuilding, it is 7.5m x 3.5m, cavity wall, double pitched tiled roof and single storey.

It will be at the back of my garden, there are a number of large trees (30-40ft high) within about 3m of the proposed building. My soil is clay.

The foundations suggested are

1) Six concrete 'padstones', four on the corners and then one each in the middle of the 7.5m walls. The ones on the corners will be 1.2m square and 2m deep. The other two will be 1m square and 2m deep.

2) There will be six bits of steel lying on these padstones with spec 203x203x46kg/m. Each one 3m long

3) It also says

' Beams to be encased with grade 35 concrete. Minimum cover to steel shall be 50mm. Allow for steel mesh at centre of surrounds. The end of the beam shall extend to the centre of the pad'


I know nobody likes to second guess another professional's opinion/calcs but does this sound about right or over the top for a single storey 'non habitable' room?

Thanks for looking

 

[noseall]

I suppose a nuclear bunker could be construed as a 'non-habitable room'.

 

[leew2]

Is this being built to building regs? outbuildings under 30m^2 are generally exempt unless you are using it for sleeping accomodation or something like that. 7.5m x 3.5m is under 30m^2. The footings you describe sound like those required to meet building regs which are generally IMHO excessive.

 

[noseall]

sound like those required to meet building regs which are generally IMHO excessive.

...yeah, but you think that anything above one foot deep is excessive...

 

[indus]

Thanks.

It is not being done under BRegs because as mentioned due to its size being under 30m2 it is not required.

The engineer does seem to have specced it as though meeting BR. Its difficult to second guess them though and then if the whole thing falls down I'll only have myself to blame.

It just seems as though its a lot for a single storey outbuilding...

 

[jeds]

It sounds over the top to me. Have you investigated mini piling? I would have thought that would be a fair but cheaper than you are looking at for that little lot.

 

[freddiemercurystwin]

Thanks.

It is not being done under BRegs because as mentioned due to its size being under 30m2 it is not required.

The engineer does seem to have specced it as though meeting BR. Its difficult to second guess them though and then if the whole thing falls down I'll only have myself to blame.

It just seems as though its a lot for a single storey outbuilding...

How would you like him to spec it then?
Just because a development does not need to meet Building regulations does not mean an SE will downgrade their design.

 

[Possel]

It will be at the back of my garden, there are a number of large trees (30-40ft high) within about 3m of the proposed building.

Is this really correct? "a number of large trees (30-40ft high) within about 3m"? How many exactly? If it is correct, I would suggest it is why the design seems to be excessive - the SE does not want to be accused when you get problems from the roots/clay shrinking etc.

 

[indus]

It will be at the back of my garden, there are a number of large trees (30-40ft high) within about 3m of the proposed building.

Is this really correct? "a number of large trees (30-40ft high) within about 3m"? How many exactly? If it is correct, I would suggest it is why the design seems to be excessive - the SE does not want to be accused when you get problems from the roots/clay shrinking etc.

Yes, at least two large mature trees, one 3metres away and one 4 metres away. Other smaller trees also close by.

This is the reason as you correctly say that he has probably specced the foundations like this.


I remember reading elsewhere that some people recommend so kind of expansion material underneath the floor should the ground buckle upwards.

Can anybody explain this to me?

Thanks

 

[jeds]

The expansion material absorbs the expansion and contraction of clay due to the wetting and drying cycle.

The trees are definitely significant and large trees that close will always need special foundations. The only question is what method and how it is specified. Going on price I would think mini piling would be the cheapest option but it does depend on a lot of factors.

 

[theoldun]

Indy, Was going to keep out of this, but as we wish to take advantage of our complimentary ticket for the grand opening, will give you our opinion.
We do not like the foundations as described. Reasons.
Pads are normally used for point loads such as columns. Pads are normally centralised under the point load, or in this instance central to the corners. In your instance pads will not be central to corners as 3metre long UCs would not be long enough. 75% of the four corner pads will be inside of the perimeter of building. We would have thought that as you have 300mm walling 2 number 203x102x23 USBs bolted together with spacer would have been better than 1 number 203x203x46 UC. With 203x203 UC half of the 100mm block and brick are corbelled over the beam and sitting on the casing concrete. However if you do go down this road in our opinion steels need bolting down on to pads with 100mm clayboard below beam concrete soffit to assist possible heave and steels cleated to-gether and 50mm clay shield to three sides of pads Re bar or mesh the slab to also sit on beams with 100mm clayboard below. DPM over slab turned up brickwork, 75mm insulation, depending on use 75mm screed with fibres, or if it is only to be used as passion wagon, 22mm floating chipboard floor.
If we were backed into a corner with this type of construction, that’s the way we would go. With the above method there is very little difference between a 1500x600 deep fill foundation in either amount of excavation and concrete fill. If anything we feel that it will be more costly.
Suggest you phone Abbey Pynford, Watford and have a chat with them and ask for quotation on short bored piles and ring beam. They are on the web and have some very small rigs.
Conclusion pads and UC are over the top, but we are not SE and this is only our opinion.
Regards, oldun

 

[RonnyRaygun]

If two metres is deep enough based on soil type and tree type, then why not use strip or trench fill? Other buildings nearby?

Suggest you phone Abbey Pynford, Watford and have a chat with them

Had an interview with them a not long after graduating...got the job but didn't take it...still don't know how to design a pile!

and ask for quotation on short bored piles and ring beam. They are on the web and have some very small rigs.
Conclusion pads and UC are over the top, but we are not SE and this is only our opinion.

Regards, oldun

If access is a problem there's also the Grundomat system. The rig can be left at the front of the house and just a large hose brought through. The piles are basically vibrated into the ground using a vibrohammer running on compressed air. We specified this system on a £3.5million terraced house in Westminster with no problems.

 

[indus]

Thanks for the input guys.

The work actually started 48 hrs ago! All six pads have been dug out and today the concrete has started to be poured. 100mm Clayboards have been placed on all four sides of each pad.

No idea why the engineer has designed it this way round and not trench or piling, but as long as it forms a solid foundation and the new building doesn't fall down I suppose its all good.


Thanks for all the input, as always its been an education and learning process for me