I have seen many a garden wall that has cracked or moved because of the builder cheating or miscalculating the materials for construction of the foundations.
Ask a house builder what the most important part of the house would be and he, I am certain will say, is the quality of the footings that support the 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.
Work on the basis that up to one third of your budget could be spent below ground - get this very important feature right and your structure, decorative or functional will last forever. Get it wrong and you could be faced with expensive reparations.
My general rule of thumb for garden walls is 50% extra width of concrete than the width of the wall you are intending to build.
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 a metre 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 your concrete footings you might come across an area of soft ground, pre-excavated ground, infill or a service such as drainpipe of cable.
Certainly with infill you need to ensure that no settlement will take place after construction because this soft spot is the likely area for the footing to fail. In the case of failure a crack will be transmitted through your wall and the severity will be mirrored by that of any movement in your concrete plate.
Re-enforcing steel is relatively inexpensive and should be considered 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 then the footing will act as a lintel and bridge the problem area.
Make sure that any metalwork that is inserted in 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 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 together properly.
If you do not tie the two areas 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 use some kind of method of levels. The easiest way is a peg and straight edge. By driving in a peg, laying a 3 metre straight edge on the top to another peg and tapping the second peg down until the bubble is in the middle will ensure you have a true reference for the top of your concrete.
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.
If your concrete is fluid then the material will find level naturally and as long as the walls of the footing are continuous then all you have to do is keep mixing until the material is at the right height. Use a rake to pull the material about and then a tamper to manipulate the surface.
Tamping is both a good way of creating the fine levels but also serves another very important functions. When concrete is poured it will have air trapped within it. This air can cause severe weakening of your concrete and must be removed. Tamping causes vibration and as the material vibrates the air will rise to 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 meat where the levels change because this could be a source of weakness.
Gauging your 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 cement) it is important that all of the mix is that gauge all the way through. Using a bucket as a measuring gauge is ideal because you eliminate any error that might be caused when judging by shovel loads.
I have witnessed on so many occasions a labourer adding 5 heaped shovels of ballast and one level shovel of Cement (O.P.C. - Ordinary Portland Cement).
Ready mix Concrete
Ready Mix is an easy way of getting the job done quickly and should be considered if you have a lot of work to do.
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.
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.
Damping down
Not always a priority but sometimes, if it is extremely hot or the soil very dry you will have to balance the water content after the concrete starts to go off (remember, concrete does not dry the same way as washing does. A chemical reaction expels the water but also needs water to complete the process. Id concrete goes off too rapidly is can crack so dampening down the surface will ensure that the process of curing is controlled.
Repairing poor foundations by underpinning - what to do when things go wrong
I would like to touch on under pinning in the hope of demonstrating what happens when a footing fails.
Clay soils are probably the worst with much movement from the contraction (dry) to expansion (wet) of the soil. Houses that are built on clay suffer with subsidence or cracking caused by movements in the soil and in 95% of the cases this would not have happened if a sufficiently re-enforced footing had been created before hand. A house that is built on soil that moves and cracks appear normally have to be underpinned.
Under Pinning is the excavation beneath and around a footing at a spot where a footing cracks or is likely to fail. New concrete is then poured into these areas to act like padstones. To avoid any further damage the 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.
I designed a garden in Whitmore Vale near Churt, Surrey a few years
back. The property was built into the side of the hill that forms one
part of the valley with a stream at the bottom.
The low ground would flood and because the soil was clay in this particular area (although there are areas of sand too) The house was protected from flooding but it suffered with subsidence. It was just after the recession had finally passed and a team of Welsh miners came down as subcontractors and excavated a series of deep cavernous holes under the property to under pin the foundations. It was a monumental mini civil engineering job and fascinating to see.
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