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I am trying to estimate the volume of excavation needed for the foundations of a structure from the drawings provided.

The drawings clearly give the dimensions of the RC / cement footing but I suppose the actual excavation will need to be larger than this? How much larger should I plan for in the costing calculations.

Any rules of thumb?

Another issue that comes to mind is that although the RC / cement footing structures are rectangular or various dimensions, any practical excavation would have a slight slope in the walls that increases the excavation volume? Or is that not so significant?

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I don't know of any code which defines this, but a one meter "working perimeter" around the footing is probably more than enough.

Regarding the slope of the excavation walls, that's trickier to define with a blanket statement. The problem is that the slope is necessary to give the soil stability. Unfortunately, the soil's stability is highly dependent on its properties (cohesion, angle of repose, etc), so the requisite angle to stabilize the excavation is also a function of these properties. Therefore, a blanket statement of "a slope of X is adequate" can't be given without knowledge of the type of soil being discussed.

That being said, for non-problematic soils (i.e. non-expansive and non-collapsible soils, with a reasonable SPT, etc), a long-term excavation can usually be given a 2:3 slope (two vertical for three horizontal), so adopting such a slope for a temporary excavation would be very conservative. Depending on the soil, I believe even a 1:1 slope could be possible for a temporary excavation.

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