Besides have a level, properly graded plot to build on, what is acceptable underneath. For example, large trees were taken down and some roots still exist in the ground. I also found debris (trash) buried in some areas a foot or so down. Can I use fill dirt on top of all this when I grade it or do I need to dig down and remove all this stuff first? Thanks
per Section 403 of the 2009 International Residential Code, the basis for most local and state building codes. Check with your building department to see if you code differs.
R403.1 General. All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, crushed stone footings, wood foundations, or other approved structural systems which shall be of sufficient design to accommodate all loads according to Section R301 and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil. Footings shall be supported on undisturbed natural soils or engineered fill. Concrete footing shall be designed and constructed in accordance with the provisions of Section R403 or in accordance with ACI 332.
You should dig down to undisturbed soil having a soil bearing capacity adequate for the load your building will impose on the width of footing for your building. This is how footings are sized, weight of the building and the soil bearing capacity of your soil.
Organic materials such as peat, tree roots, etc., are to be removed. In lieu of deep footing you may retain the services of a professional engineer to design an engineered fill as shown in Section 403 above.
A footing placed on disturbed material may settle causing damage to the footing, foundation and building as well.
Hope this helps.
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