When digging out a trench for foundations, always dig it deep enough to remove all large pieces of vegetable matter in the soil. If any tree or shrub roots, for instance, are left in the ground they will gradually rot away and the soil surrounding them will eventually fall into the cavities they have left, leaving the foundations unsupported. These may eventually sink and will almost certainly crack. Fill any additional depth created with an extra layer of hardcore; this is obtainable from builders merchants.
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