Strong foundations

the_matrix

Hi MSE

I am building a two storey extension. We have nine trees very close by. What's the best strength foundation mix to use to get a good strong foundation but without digging too deep.

I'm hoping not to use steel reinforcement as it is expensive.

If you could please give me the cement-sand-stones ratios please.

Thank you

Bye

mysk_girl

Strength of foundations is only partially related to the actual strength of the concrete. The properties of the ground it is resting on are very important, and moreso than the concrete grade. If you have 9 trees in close proximity and there is any chance that you have clay soil, you really need professional advice as the combination of the two (trees and clay) can cause heave and serious damage to the house. Building Control will also want to inspect the bottom of the foundation to confirm that the soil is capable of supporting the extension. Do you have an architect involved?

mysk_girl

And a bit more clarification - the strength of concrete is of lesser importance if you are building unreinforced foundations - e.g. strip footings (which I think is what you want from your post). If you want reinforced footings, such as a raft, or ground beams, then the strength of the concrete IS important, but I would certainly expect a Structural Engineer to be involved for that. In either case, the type and strength of the ground will dictate the form, size and depth of the footings.

Not all tree/clay combinations cause heave, it depends on the type of tree, proximity to the footings and susceptibility of the clay to volume change - not all clays are created equal when it comes to such things! I have seen buildings on shallow footings in clay with trees proximate with no issues, and one which had 2.5m deep strip footings that ended up with cracks you could put your hand through.

Your architect should be able to recommend a way forward, or you can ask Building Control in your area for advice.

iamcornholio

You don't need strong foundations, as roots rarely damage foundations

The problems occur due to moisture extraction by the tree, which shrinks the ground, causing foundations to move.

If the tree is within what is termed "influencing distance", then the foundations will need to be designed to deal with potential movement (deeper or certain design type) - and not made stronger

You really need to get some help on this as you don't seem clued up on building. The council will need to approve your proposals too

Avoriaz

the_matrix wrote: »

....I'm hoping not to use steel reinforcement as it is expensive....

I hope this doesn't come across as patronising or obvious but do remember that the most expensive option is the cheap one that fails in the future and needs expensive remedial work.

I think you need expert professional advice that will be worth it in the long run. It should only cost a small fraction of the overall cost of a two storey extension.

the_matrix

Thanks for your replies. I'll get a professional to work it out for me.