Foundations – What You Need to Know

OK then, you have done your site investigation, the outline planning is through and you are ready to start designing your foundations. Here’s what you need to know.

There are five main types of foundation common to low-rise buildings within which category domestic housing falls. What type of ground you are building on will determine what type of foundation you will need to use.

Foundations and how they work.

· Trench Fill

· Strip Foundation

· Wide Strip Foundation

· Raft Foundation

· Piled Foundation

One Important note. When you expose your trench you need to get building control, NHBC or an independent structural engineer to inspect your ground prior to pouring your foundations. Forgetting to do this could mean digging up your foundations as a worst case, especially if the ground is not suitable for the type of foundation you intend to pour.

Stay safe get your inspections done.

Trench Fill.

Trench fill refers to a technique used to fill up deep foundations trenches. What happens is this, you go to site and dig out for your foundations, but you end up about 1600 or perhaps even 2000mm below ground level. Well obviously you don’t want to fill all of that depth with good strong foundation concrete, so you order a weak concrete mix from your ready mix company and pour that into the trench until you reach the depth at which you propose to lay your foundation concrete.The trench fill needs to be only as strong as a good supporting soil, to provide a suitable base for the strip foundation.

Strip Foundation.

A strip foundation is the simplest form of house foundation. It consists of a Gen 1 grade concrete laid to a width of 650m wide and usually to a depth of 350mm, with reinforced steel to either top or bottom of the foundation or in some cases both. It is preferable to have an engineer design the foundation for you, and provide you with dimensions prior to starting. Strip foundations are being replaced with wide strip foundations and are now relegated to supporting internal walls within the dwelling.

Wide Strip Foundations.

Wide strip foundations are similar to ordinary strip foundations except that they are wider. They are made wider in order to spread the load over more area of soil. The wide strip foundation is usually used in poorer bearing soil conditions.Wide strip foundations are usually 1 m wide x 450 mm deep, with reinforced steel to either top or bottom of the foundation or in some cases both, although an engineer can specify other dimensions depending on ground conditions and the load being imposed onto the foundations.

Raft Foundations.

Basically this is a slab of concrete laid over the base of the house with reinforced steel supporting the concrete. While the raft foundation is still used it is slowly going out of fashion, as there is a tendency for the raft to tilt in very poor soil conditions.

Piled Foundations.

As good quality sites become scarce, the poor weak soils that previously had been unexploited are now being built on. Weak or fluid soils or soils that contain a large proportion of compressible elements are prime candidates for piling.A pile is simply a square length of concrete with four reinforced steel bars running through it.

The pile is normally 350mm square depending on engineers specs and can be anything in length from 4m to 12m.The engineer specifies how many piles are required to support the house and where they need to be placed. This is normally at junctions and intersections within the house.Simple piles are driven into the ground using a ram. They are pounded down until they refuse. That is they won’t go down any further.

Once all the piles are driven in the tops are cut off leaving only enough to steel exposed to tie into the beams. Next a mud-mat or 50mm layer of concrete is laid over the site. Then the line of the house is marked out as well as all internal load-bearing walls. Then a small retaining wall is built to either side of the piles in order to form a trench within which the foundations can be poured.

Once the block work has been completed, reinforced steel is placed into the area where the concrete will be poured and as per engineers designs is laid and tied to form a steel cage that forms the basis for the foundations. These are tied into the top of the piles. Once the steel has been laid, fitted and tied the concrete is then poured in the same manner as you would pour a strip or wide strip foundation.The concrete is agitated using a poker in order to get all excess air bubbles out of the foundation.

All you have to do now is wait until the concrete hardens and you can start building your house. Normally with piled foundations you use pre-stressed slabbed floors. This is a very safe and fast method of building.