The free diy home improvement guide with answers to your questions on a wide range of do it yourself projects.

Excavation for a Concrete Slab or Base

The area of the base will need to be marked out on site. For a simple garden shed base, this will be relatively straightforward. But, for the base of a new building or extension, measurements will need to be taken from the architects drawings and set out on site. Marking out building lines on the ground is usually done with line marking spray paint as seen in the image.

Square used to set out lines Spray line marking paint

Excavation Levels

Before laying a new concrete slab or base, the ground will need to be excavated to a suitable depth. Pegs are driven into the ground at the corners of the area so that their tops are at the required finished level as detailed on the architect’s drawings. By measuring down from the top of the pegs, you can establish how much soil to dig away - enough for the combined total thickness of the various layers. For an extension this might typically include 150mm consolidated hardcore, 50mm sand blinding, a damp proof course and 100mm concrete slab reinforced with steel mesh. So, in this example the total base thickness will be 300mm. Measuring down from string lines connecting the pegs will assist with checking the excavation levels as work proceeds.

Excavating

Breaker used to break up concrete slab shovelling up broken concrete

The method of excavating the ground will depend on a number of factors including existing ground and access for machinery. In the accompanying pictures you can see that an old concrete base had to be broken up first with a breaker before digging could begin. With this out of the way, it would have been ideal to use a full size digger to quickly clear the remaining soil. However, access to the rear of the property was restricted to a pedestrian pathway at the side. A mini digger was therefore used. Digging by hand for anything larger than a simple shed base can prove very labour intensive. Also see - Breaking up a Concrete Slab.

Mini Digger

Mini diggers are readily available from plant and tool hire companies. Before hiring one, make sure there is sufficient access to get the machine to the area concerned. Some mini diggers have retractable tracks allowing them to be moved along fairly narrow access routes.

As with all building works, safety is very important and the hire company will explain or provide literature covering all aspects in relation to their mechanical diggers. There are some fundamentals which are worth highlighting. The operator must be familiar with the controls and the digger should only work on a firm, level base. Any services such as power cables, water mains, and drains should be clearly marked and the digger kept clear of these. Before using the digger make sure you have read and understood all the operational and safety literature.

Digger bucket removed from boom Digger bucket changed

Various buckets are available for diggers and they are relatively simple to change. The bucket is mounted on the end of the boom by means of steel pins. With the bucket resting on the ground, these pins can be removed and the bucket swapped. Slide the pins out and place them to one side. Remove the existing bucket from the boom and place that out of the way. Lift the new bucket into position so that it sits with the fixing holes aligned with those on the end of the boom. Carefully insert the steel fixing pins and secure them. Take care when handling the buckets – they are surprisingly heavy.

Mini digger on site Mini digger controls

Before starting work, familiarise yourself with the machine by practicing with the controls in a clear area. The boom and bucket are usually controlled by two joystick mechanisms. The right joystick usually controls raising and lowering of the boom, and curling and uncurling the bucket. The left joystick usually controls extending and contracting the boom, and swinging round to left or right. Getting the hang of these controls in an unrestricted firm level area first will pay dividends as you need to focus on the area being excavated and the area around you rather than looking at the controls.

The digger can be moved back and forward on its tracks. These can be operated independently allowing the machine to be turned. With much the same principal as painting a floor in a room with only one door, excavation should start at the farthest point. You can then move the machine back from there, excavating each section as you go. Remember to ensure that the machine is on firm level ground before starting to excavate. Failure to do this can result in the digger tipping.

Extended boom of digger commencing excavation Emptying digger bucket into barrow

To commence excavating, extend the arm of the machine toward the farthest point with the bucket in the curled position. Then, uncurl the bucket and lower the boom. As the bucket scoops into the soil, begin to curl it and draw the boom in towards the machine removing a reasonable depth of soil as you go. With the bucket curled and filled, lift the boom and turn the cab to deposit the soil where required. Uncurl the bucket to empty

In the pictures, you will see how, with such restricted access, the soil was emptied into barrows which where then taken straight to the skip allowing the site to be kept as clear as possible.

Site cleared Spoil emptied into skip

Safety While Excavating

While excavating be vigilant - if you see any unexpected service pipes or cables etc stop immediately. Do not continue until these have been dealt with and it is safe. You also need to be aware of the area around you as you move the boom and bucket, and swing the machine round. Check for obstacles and other people in the vicinity.

Working your way in a grid like fashion across the site, excavate to the required depth by regularly stopping and checking levels against the pegs mentioned earlier.

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