The design of the foundations will need to be approved by the Local Authority. The plans submitted for approval will detail the type and depth of the foundations. However, prior to concreting the foundations, the excavation works will need to be inspected and approved by the District Surveyor. Many factors have to be taken into account but, during his site visit, he will be establishing that the base of the trench is of good load bearing capacity and that there are no adverse factors such as nearby trees which may affect the foundations. Trees can draw a large amount of moisture from the surrounding ground and the height and species of nearby trees will be taken into consideration so that there is no risk of ground movement.
Also included in the plans approved by the Local Authority will be the specification for the concrete mix to be used for the foundations. Mix ratios are commonly specified as a proportion of cement : sand : aggregate. Typically this will be 1:3:6 for foundations such as those shown in the accompanying pictures. This mix is 1 part cement : 3 parts sharp sand : 6 parts aggregate. In practice, it is often easier to order ballast where the sand and aggregate is pre mixed. When using Readymix, mixes are specified by grade. You must ensure that the mix you use is as specified in the approved documents.
Shallow strip foundations are sometimes used. These normally comprise a 600mm wide strip of concrete set a metre or so into the ground. Two skins of brickwork are built up from this to ground level. The depth of the concrete strip is a minimum of 150mm but must be at least equal to the projection beyond the brickwork. The cavity between the two skins of brick should be filled with a weak concrete mix to resist any lateral pressure from the surrounding ground.
Building the brickwork below ground level is labour intensive and a wider trench will be needed. Completely filling a trench with concrete is normally much cheaper and easier, which is why deep strip foundations are normally used. These will usually be a metre or so deep (depending on factors mentioned earlier) and 600mm wide. The excavated trench is then backfilled with concrete to a couple of brick courses below ground level.
The base of the trench should be solid and level and the sides of the excavation vertical and straight. Where this is not possible due to soil falling away at the sides when the trench is excavated, suitable shuttering or formwork may need to be built. Formwork is simply a temporary strong wooden structure used to control the shape of the concrete work. You can see in the picture how a simple shuttering arrangement has been used where the sides of the trench were not straight.
Formwork and Shuttering
Formwork may also be needed in other areas. In the project shown in the pictures, an existing drain run passes through the trench and this needs to be protected. The foundation is cut short either side so that there will be no load bearing down on the pipe. You can see how some simple shuttering was used to hold the concrete back on either side. This drain run will later be bridged with steelwork.
Mixing Concrete Versus Ready Mix
Mixing the concrete by hand would not be practical for this job – even mixing concrete with a mixer is a labour-intensive process. Having concrete delivered by a ready-mix lorry is the most practical solution for anything more than a small volume. When ordering the concrete you will need to specify what mix you require as well as agree a time slot for delivery. The amount of concrete you are ordering will affect the price and remember that you may have to pay a surcharge for any cubic metres less than a full load. There are also other preparations you will need to make aside from ensuring that the work is absolutely ready before the lorry arrives. You need to be sure that there will be room for the lorry to park up as near to the work as possible – the less borrowing the better. Talk to neighbours and if needs be to mark out an area of the street with cones in advance. The last thing you want is a load of concrete turning up and not being able to offload it. The access route between the lorry and the foundation trenches should be completely clear so that a steady flow of barrow loads can go back and forth.
Borrowing and Pouring Concrete
When the lorry arrives, make sure that you have enough people ready to barrow the concrete to the excavations. You cannot expect the lorry to hang about so, if needs be, rope in a few helpers. As a rough guide, a cubic metre of concrete is about 25 barrow loads and weighs over 2 tonnes. The driver will set up the chute so that the concrete can be discharged into the barrows. Hold the barrow steady as it is loaded – concrete is heavy and can easily tip the barrow as it drops in.
Remember that concrete can cause alkali burns so wear protective gear and avoid skin and eye contact. The easiest way to work is to have one person working the concrete as it is tipped into the trench and two or three people borrowing it from the lorry. Work the concrete so that it fills all the space without leaving any pockets, voids or trapped air. Use a shovel to work it into place. The level for the top of the concrete should be clearly marked on-site so you know when you’ve reached it. Once the trench has been filled to this level, tamp the surface with a batten so that it is flat and level ready to receive the brickwork when it has set.
As soon as all the concrete has been poured, clean up the area. There will inevitably be a few slops and spills along the way and this will set hard if it’s not cleared up straight away. Use a shovel to pick up any spilled concrete then thoroughly hose the area down. Make sure you include the street where the lorry was loading the barrows.