Foundations

lostinfrance · June 14, 2008

[quote user="wiskersnatch"]

First off it is unwise to build anything without proper foundations. If you don’t know for sure what it there you must assume nothing.

The depth and type of foundation will depend mainly on the proposed loading and the ground conditions. A strip foundation for example for a garage may be a certain size but useless if not buried deep enough to avoid frost heave (in a clay area). The permutations are vast but some general principles apply always.

If the ground is very soft you must dig until you reach firm ground and if that is over a 2 m you will need to think about piles or a raft. As you will know there are various types of foundation.

The most common on domestic dwellings is the ‘Strip’ foundation. For a house, in firm ground with no clay, a convenient way to calculate the size is to take the wall thickness and measure out and down from each side at a 45 degree angle. This will automatically give you an approximate size. (As the loading bears down at 45 degrees). The top of the foundation should be a minimum of 600mm from GL (in firm ground). If you have clay and frost the distance must be NLT 900mm to avoid frost heave. The concrete should be C30 as a minimum (just ask for 30 Newton concrete). Lots of builders use C20 but frankly the cost difference is so small it is better to go with 30. By the way, I have used concrete in various places and have to say French concrete is as good as I have ever come across.

As a minimum your foundation should run 150mm each side of the wall (so a 250mm wall would have a foundation 550mm wide, round up to 600mm). This is enough to support a domestic dwelling of 2 stories. It will certainly be enough for a Garage.

A raft foundation is basically a big slab, most of the slab should be the thickness of about 150mm to 200mm (depends on loads) and the edges, which shall bear the major loads should be deeper (imagine a waffle inverted). The problem you may have is that the base, although solid is unlikely to be reinforced and a raft needs to be reinforced. It is not a good idea to treat any slab as a raft because a raft is designed to float and has specific areas designated for heavy loads (most often its perimeter but also other areas sometimes).

Your slab is likely to be a floor and not designed for huge point loads i.e. walls supporting a roof. It may well support cars and tractors but when you introduce walls and roofs and wind loads and snow loads etc you are in another world altogether. If you introduce new heavy loads to the edges you are likely to crack it, unless it is extremely thick or reinforced or both.

Typically Rafts are used in areas where ground conditions vary, so if there is any subsidence the whole structure moves, sinks etc and does not crack. Rafts more often than not do not have piles, they are meant to float (that is a relative term) when they do have piles they are not, strictly speaking, raft foundations. They are piled foundations.

Remember, concrete is most excellent in compression but less than useless in tension and varying ground conditions gives areas of tension and compression. Any point you have the ground pushing up and weight (walls) pushing down will be a point of tension. Reinforcing (metal) is great in tension, which is why it is used.

Of course a raft may be the same thickness all over but that is a waste of concrete.

Piles are another form of foundation. These can be used in extreme soil conditions such as marsh ground. Each pile or set of piles (they are often grouped) has a Reinforced concrete pile cap and these can support reinforced beams or a reinforced raft (Not really a raft as it can work without even touching the ground, a raft is deisgned to float).

Piles are costly and not often used on domestic dwellings. Piles can be driven (a big hammer) or poured. If poured a hole is bored with an auger, a steel reinforcing frame usually dropped in (not always) and concrete poured in and vibrated. Piles can be driven till they meet hard ground or can, where hard ground is not available at a reasonable depth, can rely on friction, even in very soft ground. After driving or pouring a pile should be loaded with a known load and its movement can be measured. There are tables for the movement allowed, you just need to know the imposed and superimposed loads, the safety factors are built into the tables. Needles to say piling is an expert job. I mention it here as you may find it of interest, I don't suggest you pile.

I hope that even if you don’t find this useful you find it interesting. It is a vast area and many people tend not to give it due consideration. My guess is that this is because foundations are hidden.

But back to your question. I have a suggestion which may be helpful. Instead of using the slab as a foundation why not dig around it and drop in a strip. I would recommend also installing about 4 x 8mm reinforcing bars. In a row, evenly spaced. These need to be covered in concrete so use spacers to support them before you pour the concrete. (Spacer you can buy at any builders' merchant). The cost is low and the added strength is huge. For real 'belt and braces' install bars in the bottom and the top (50mm cover). If you do this you will be sure to have a very strong base to build upon. Also, try digging down beside the slab to see its depth.

Did you know that concrete gains 50% of its strength in the first 7 days and continues to get stronger for about 50 years. There after it losses strength very, very slowly. When poured in huge mass concrete does ot go off quickly. Apparently inside the Hover Dam there are areas where the concrete is still soft!

Regards

CY

[/quote]

A good and informative post.

Take my word this poster has been there and done it, it is not from a book or web page,it is fact.

Keep up the good work.

lostinfrance · June 14, 2008