Ask a house builder what the most important part of the house structure would be and he will say, I am certain, is the quality of the footings that support the whole structure.
Just because the footings are out of sight does not mean that they are less important than the aesthetic features that are apparent to the eye.
If you are considering building a garden wall then work on the basis that up to one third of your budget could be spent below ground. Get this very important part of your construction element right and your structure, decorative or functional, will last forever; get it wrong and you could be faced with unsightly cracking and subsidence and potentially, expensive reparations.
The depth of the concrete is always something you need to interpret based on the height and width of the wall.
If your wall is to a be a single brick wide and only 500mm high then an 8 inch (conversion = 8 x 2.54cm = 20.3cm) wide and 6 inch deep (click to convert) is more than adequate but adequate provision is needed depending on requirements.
Bridging soft ground
Sometimes, when excavating for a concrete footing you might encounter an area of soft, pre-excavated ground, infill, or a service such as a drainpipe or cable.
Certainly where there's been infill you need to ensure that no settlement will occur after construction: a soft spot is the likely area for the footing to fail. In the case of any failure, a crack may be transmitted up through your wall with the severity of this crack increasing depending on the movement in the concrete footing plate. In extreme circumstances the (now separated) parts of the concrete footing may move apart as the soil around it heaves to and fro during expansion and contraction
Re-enforcing steel is relatively inexpensive and should be considered (a no-brainer for any professional builder) where the integrity of the sub soil is brought into question. As long as the footing rests on a substantial area of solid, previously un-excavated, ground -either side of any anomaly - then the footing will act as a lintel and bridge over the potential problem area.
Make sure that any metalwork inserted into the concrete is completely encased - a common fault of the DIY builder/landscaper is to lay the metal on the floor of the trench and pour concrete over it.
Allow good time
Always allow enough time to complete the job in one go and employ the help of a friend or two if the task dictates. Mixing and wheeling and then achieving good levels is hard work if you are on your own, especially if you have more than 1 cubic metre of material to mix.
If for any reason you are forced to complete the pouring of the concrete at a later date you need to make provision for the insertion of metal bars where the join will be. Ensure that the bars are bent at the ends so that the concrete can wrap around and secure the two sections of footing together properly.
If you do not tie the two areas of footing together there is a high probability that the two sections of concrete will move independently of each other, causing a shift - just like tectonic plates that cause earthquakes (although not as dramatic). To an extent, concrete is floating in the sub soil and as long as your raft is strong then the construction that is built upon it will move with it and stay intact.
Setting your levels
Once your footing trench has been excavated you will need to ensure that the concrete will settle to the correct levels. The simplest way is by using a metal peg and long straight edge and spirit level. Drive in a metal peg, lay a 3 metre straight edge on the top between that and another peg and tap the second peg down until the bubble on the spirit level is in the spot in the middle.
Always use metal pegs. Wood, whilst easy and cheap swells and can cause a crack across the plate. Metal is both easier to drive in and will not move or cause weakness in the footing.
If your concrete is fluid enough then the material will find its level naturally and as long as the walls of the footing are continuous then all you have to do is keep mixing and pouring until the material is at the right height. Use a rake to pull the wet concrete about (being sure not to disturb or damage the metal pegs) then a tamp the fluid mixture to manipulate and encourage the surface until you are happy...the tops of the metal pegs should just be visible.
Tamping is both a good method of achieving the correct levels but also serves another very important function. When concrete is mixed, air becomes trapped within it. Too much air can cause severe weakening of the concrete and must be removed. Tamping vibrates the fluid mixture, causing the air to rise to the surface. You will see bubbles popping as the air reaches the surface.
- Bumping up - if you are working on uneven ground you may need to step the footing. Bear in mind that the step should be equal to the depth of the material you intend to use. Make sure that there is sufficient overlap (see drawing) where the levels change because this could be a source of weakness. Whatever you do, make sure that each level of concrete footing is sufficiently bonded to the next. Never use wooden shuttering to cause a barrier between levels as wood will now allow bonding. Wood will absorb ground water, swell and cause a failure of the footing and transmit a crack up through the wall above.
Correctly gauging concrete
One of the most important considerations when constructing a footing is the consistency of the mix. If you are working to 5:1 (five parts all in ballast to one part Ordinary Portland Cement O.P.C) it is important that the entire mix is accurately gauged throughout the entire process. Use a bucket or similar receptacle as a measuring gauge. Using a shovel to measure can so easily be misinterpreted
I have witnessed on so many occasions a labourer adding five heaped shovels of ballast but only one level shovel of Cement. The consequences are that the mixture is weakened because the ratio isn't right.
Using ready-mix concrete ensures that gauging is accurate and it is also a way of getting the job done quickly and should be considered if you have a lot of work to do: there's no mixing required.
I recently flooded the floor of an old barn (70 square metres) with ready mix and it took me and two friends one and a half hours to do. The job was accurately gauged and consistently mixed which made it possible to get an exact job without the physical fatigue that would have been experienced if we had mixed the equivalent in ballast and cement.
I estimate that the job would have taken a very long day and a lot of energy if mixed by hand, in reality, using ready-mix, the whole floor was flooded and levelled in a short space of time.
You will also be surprised at the costs similarities. Ready-mix on the face of it costs more per cubic metre but the labour rate (if you are paying someone else) is much reduced.
Not always a priority but sometimes, if it is extremely hot or the soil very dry you may have to introduce extra water as the concrete starts to cure - concrete does not dry the same way as washing does. A chemical reaction expels the water but also, during the curing process, may need extra water to aid curing. If concrete cures too rapidly is may crack, so dampening down the surface will ensure that the process of curing is controlled.
Use a fine mist setting on a garden hose and do not spray the water directly onto the curing concrete as you may wash out the cement fat and weaken the mixture.
Repairing poor foundations by underpinning - what to do when things go wrong
I would like to touch on underpinning in the hope of demonstrating what happens when a footing fails.
Clay soils can move excessively as the soil contracts (dry) and expands (wet).
Underpinning is a method of excavating beneath and around an existing footing at a point where a footing has cracked or is likely to fail. New concrete is then poured into the voids and act like padstones; strengthening and supporting the original footing. To ensure stability during the underpinning process, excavations are usually staggered by excavating one metre and missing a metre and so on. Once the concrete had hardened enough the un-excavated areas are then completed in the same manner.
I designed a garden in Whitmore Vale near Churt, Surrey a few years back; an area renowned for its heavy clay and notorious for failed footings. The property was built into the side of the hill. Below the house was a stream.
The lower ground would flood often and due to the clay in this particular area. Although the house itself was safe from flooding, it suffered from subsidence. A team of Welsh miners were hired to excavate a series of deep cavernous holes under the property to underpin the foundations. It was a monumental mini civil engineering job and fascinating to see.