Digging the foundation trench
Before the concrete footing can be poured, the builder must dig a trench for it. The depth of the trench depends on characteristics of the site and building that should have been determined in the basic planning stage (see pages 82-88),
However, the width of the trench, and the care with which it must be dug depend on whether the concrete footing will be poured directly in the trench or into wooden forms.
TRENCHES FOR FOOTINGS POURED WITHOUT FORMS
When wooden forms will not be used, the trenches must be dug very carefully:
• the walls of the trench must be plumb (straight up and down);
• the earth forming the trench walls and bottom must be very firm so that there is no danger of the trench crumbling when the concrete is poured;
• the earth must be removed carefully so that the earth at the bottom of the trench is smooth, clear of all trash or debris, and undisturbed (undisturbed means that the earth has not been moved, loosened, or dislodged by the digging);
• the bottom of the trench must be absolutely level: 6-8 measurements with a straight board and level should be made along the length of the trench to be certain of the level;
• the trench must be exactly as wide as the concrete footing;
• the trench must be free of standing water.
TRENCHES FOR FOOTINGS POURED WITH FORMS
It is much more common to use wooden forms when pouring the concrete footing because the ground at building sites is rarely hard enough to stay in place when the concrete is poured directly in the trench.
Like the trenches for non-form footings, trenches in which forms will be used must be level at the bottom, and hard. The earth at the bottom must also be left smooth, clear of debris, and undisturbed.
However, there is no need to make the side walls of the trench plumb. The only requirement is that the trench be 30-45 centimeters wider than the footing will be, so that there will be room in the trench for the wooden forms and work space for the builders.
A FINAL NOTE ON DIGGING TRENCHES
When digging the trenches, any loose soil, or soil with organic material should be shovelled outside the foundation lines and discarded.
Hard earth that is dug up should be spread evenly inside the foundation lines: it will be very useful as part of the fill under the floor.
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