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Excavating the Foundations

Now that the old concrete slab and paved areas as well as the remaining side wall have been removed, the site is clear and ready for work to start on the foundations for the new extension. As you can see in the picture, the new extension is to extend a fair way out and span right across the back of the existing house

Site plan detailing extent of extension Line marking paint identifying excavation line

The position of the foundations are marked out on the ground with a spray line marking paint. This shows up well and makes it easy for the digger driver to see what’s needed. The foundations will be what are termed deep strip foundations – in other words a strip of concrete under the load bearing walls of the extension. These are around 600mm wide and will go down a metre – at least to start with. Dale from G L Smith and Sons, tells me that the final decision on the depth is dependent on a load of factors including the load bearing capacity of the soil at the base of the footings as well as the proximity, type, and height of any trees nearby. As he goes on to explain, nearby trees can have a major effect with their roots drawing moisture from the ground. Once the foundations are excavated, the District Surveyor will inspect before work continues.

The access to this particular job is quite restricted so getting a full size digger in is out of the question. A mini digger is brought in for the excavation work but even this is a bit of a squeeze. Fortunately, the tracks are retractable making it just narrow enough to fit down the side access path with an inch to spare on either side.

Mini digger on site Limited access for the mini digger

When you look at the controls on a mini digger, it can be a bit baffling with an array of 6 levers and joysticks immediately in front of the driver – let alone the other levers and pedals on either side. It’s clear from the outset that the guy operating it has plenty of experience. After a few deft manoeuvres he’s lined up at the end of the first trench run ready to start digging. The bucket skims a modest layer of soil at a time. A two man team with barrows is kept extremely busy ferrying the excavated material to a skip on the front driveway. It’s surprising how much soil a mini digger bucket picks up in one go – the guys on the barrows certainly have their work cut out.

mini digger controls mini digger excavating

mini digger excavating Mini digger bucket being emptied

Digging trenches requires a good deal of vigilance and it’s soon obvious why it’s done in thin layers, as a the top of a drain run is partly exposed. This is one that they expected to find as it feeds to a soak-away in the garden. But, as the driver tells me, you have to keep a good look out and take it steady. Yes, there will be services that you know are going to be there. There’ll be pipe runs and the like marked on the drawings, and manholes and gullies also telling you what to expect and where. But there’s always the possibility of one that nobody knew about.

Drain run almost invisible Exposed drain highlighted

Measurements are taken at regular intervals to check how work is progressing and we’re soon down to the right level. Final clearing of the base of the trenches is done by hand and the finished excavations are surprisingly neat. The drain runs that pass through the trenches are left undisturbed and will be bridged. One of G L Smith and Sons team describes how this will be done with steel work spanning between the foundations on either side. The base of the foundation on either side has to be lower than the bottom of the pipe. This ensures that no pressure is put on the pipe itself, or the soil next to it. Otherwise, there’d be a risk that the pipe might fracture at a later date.

Foundation excavated Foundation excavated

With the foundations excavated, the next job is concreting them. Site access is restricted and the road outside is pretty tight so it’s a tough call between mixing by hand and having a ready mix delivery

If you live in the Hertfordshire area and are looking for a professional building contractor, you can get in touch with G L Smith and Sons via their website: http://www.glsmithandsons.co.uk/

G L Smith and Sons