Home FAQs General Concrete Knowledge We're going to pour a concrete slab foundation on a slope. Do I need to remove the topsoil?
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We're going to pour a concrete slab foundation on a slope. Do I need to remove the topsoil?
Q:In order to pour a 24' X 32' concrete slab foundation on a slope, which will be used to build a workshop, do I need to remove 6" to 12" of topsoil if I will be using 4 dump truck loads of rock with the edges rolled? I am having problems with city codes and with the concrete contractor. The four loads of rock have already been laid down without the topsoil having been removed. We want it done correctly.
A: Placing concrete directly onto grass or topsoil is never a good idea. I would recommend removal of the topsoil, as it will tend to compact over time, leading to settlement of the slab and cracking. Rolling the area with the gravel already installed will most likely not do an adequate job of compaction. Remember that some types of soil and geographic locations will require excavation beyond the topsoil to achieve adequate load-bearing and/or to provide protection from frost heave. You also referred to your project as a concrete slab foundation, leading me to believe that you intend for the slab to be the load-bearing member for the walls of the workshop. If this is so, you need to make sure that the edges of the slab are designed to handle this load - normally this is accomplished by using a turndown around the entire perimeter of the slab to thicken the concrete and increase the load-carrying capacity. This turndown area should also be wide enough to distribute the load from the walls and roof, and should be reinforced with the appropriate amount of rebar. I would recommend consulting a local engineer for construction details appropriate for your geographic area, soil conditions, and type of building.