Before any excavation work starts, you must plan the work carefully and give due consideration to the safety aspects, particularly any potential hazards such as services and cables beneath the ground.
Typically, deep strip foundations such as the ones shown here for a new extension will be a metre deep. The anticipated depth will be marked on the drawings for the work but site conditions as the work proceeds may require that they are deeper than originally specified. The District Surveyor will inspect the excavation and determine whether the base provides suitable load bearing capacity. Factors such as the proximity, height, and species of any trees will also be taken into account as well as the volume change potential of the soil. Trees can draw a lot of moisture from the surrounding ground and inadequate consideration of this may lead to subsidence of the new work.
Setting Out Trenches
The first job is to set out a base line for the work. This might be one of the main walls of the new work. All other setting out will be in relation to this so it is important to get it accurately set out. Fixed points such as the corner of an existing wall are used to establish this base line. Take measurements to the first point on the base line from two known points on your approved drawing and mark the first end of the base line. Repeat the process for the other end.
With the base line in place, begin setting out the rest of the lines denoting each of the walls. Lines at right angles to one another can easily be established by using a builders square like the one shown in the picture. When setting out the lines, always double check your measurements, and check that the diagonals of square and rectangular elements are equal. This ensures that the angles are indeed right angles. If the ground is on a slope, make sure that measurements are taken horizontally – not along the slope as this will be different. Remember, the layout of the building or extension must be in accordance with the plans approved in the planning application. Deviation from this could result in an order to take it down.
Once the lines have been established, it is common to make up profile boards. These are flat timber boards mounted on pegs driven into the ground and are set a metre or so away from each end of the new walls. This ensures that they won’t get in the way or get knocked. On the face of these boards, the line of the foundation edges and brickwork edges are marked so that a string line may be run between them to identify the outline of various elements on the ground.
Mark out the trenches for the new foundations with spray line marking paint. These aerosol spray paints make line marking very quick and they will be easily visible as work proceeds.
Excavating Foundation – Mini Digger
Determine a suitable order of working. Much will depend on the method you are using – hand digging or the use of a small digger. When using a mini digger like the one shown, you will want to work backwards away from the building if possible so that the machine is sat on solid ground at all times.
If the machine has to be used over an existing part of the excavation, steel plates will need to be laid over the trench spanning sufficiently to be fully supported by the ground on either side. Steel plates like the ones seen covering exposed road works, strong enough to support the digger, can be hired.
As the excavating work gets underway, keep a careful watch for buried services such as drains and cables. If any are spotted, stop work immediately and make the area safe before continuing. Failure to do so can result in damage like this
The bucket used on the mini digger in the pictures is 600mm wide – the appropriate width for the foundations to be excavated. Gradually excavate to the required depth, taking a modest layer of soil away with each pass. The excavated soil can be deposited from the bucket to a spoil heap or be taken straight to skip. On this project, two men with barrows worked back and forth between the digger and the skip on the front-drive. When digging by hand, the spoil can be loaded straight into a barrow and removed from the area.
As the depth of the foundations progresses pay attention to the sides of the trench. These should be vertical and straight. Firm soil will hold up well but areas of loose soil can result in the sides of the trench collapsing. If there is a risk that the sides will fall away, suitable support will need to be positioned to allow work to continue. Alternatively, the trench may be dug with angled sides so that the soil is raked back and is self-supporting. The big disadvantage with this is, of course, that much more material needs to be excavated and, consequently, backfilled
The depth of the excavations should be checked at regular intervals. This can be done with a steel tape or depth rod either from known levels or from string lines between the marked finish levels on-site pegs.
In the picture, you can just see the top of a clay pipe which has been exposed. This just goes to highlight the importance of excavating carefully and removing shallow layers at a time. This drain feeds to a soakaway in the garden. The area around this will need to be dug by hand to prevent any damage to it. The drain run itself is towards the end of one of the foundations. However, beyond this point, there is to be a new garden wall so the foundations are continued for this at the same depth. This will allow the drain to be bridged by steelwork set in the concrete foundations so that it is protected. The depth of the base of the footings will need to be below that of the clay pipe otherwise there is a risk that it will be damaged.
Excavating Foundation – By Hand
Once the bulk area has been dug out, remaining sections such as those around drain runs can be completed by hand. A fork is used to loosen compacted soil and a shovel to scoop it out. The base of the final trench should be level.
Additional care must be taken when excavating next to the walls of existing structures so that there is no risk of undermining these. The foundations in the pictures are coming away from the house wall at right angles so excavation next to the wall is limited. It’s surprising what you uncover sometimes – in the pictures you can see how there is very little in the way of footings under the main house. This was built some hundred or so years ago and yet has stood there without any problems.
Safety Around Excavations
Where excavations are to be left open, it is important to take steps to guard against accidents. Temporary fencing should be placed around the area if it is to be left unattended. Alternatively, suitable trench covering can be positioned over the excavations.