This week we began excavating the footings for a single story extension in stanley to the rear of our clients property. In contrast to how our footings are usually installed, this time we are building off a concrete raft; Now, as we said,this is different to what we conventionally do. The normal procedure usually is to dig the footings as a strip, 600 mm wide and 900 mm deep (or until you hit ‘good ground’.) We haven’t been able to do this however this time around.
Once the mini digger was on site, our excavator started to dig into the ground. We reached the usual 600mm but we were hitting nothing but soil. No clay, no bedrock, nothing but loamy soil. We continued to 900mm but still no hard ground, so we then had to stop digging and discuss with the client if they wanted us to continue further, or try a different approach. We advised them that, at this point, it would be best to get a second opinion from a building inspector.
When he arrived on site, he checked it over and agreed that we should build the single story extension on a raft. The next step was for the structural engineer to design the lay out of the raft, which he did the next day. When you change from a footing to a raft you have to excavate the whole area instead of just the strip footings, but you don’t have to go as deep. Once this was done, we had the steel rebar meshes delivered to site. These metal grids must be laid in the plot in a specific pattern so as to create the best strength and integrity when the concrete is poured on top of it. Normally, we push wheelbarrows full of concrete around to the dig site, but seeing as this was a raft, it would have been impossible to push the barrows over the top of the metal without disturbing the layout and potentially pressing down on the ‘tables’ between the layers. We instead had to use a concrete pump to get the job done.
All the concrete has since cured and set, and we have begun laying the first bricks and blocks. When we do footings, it’s always the same routine, so this time, constructing a concrete raft was a nice change indeed.
Pt 2 coming soon.