A concrete house pad, more typically referred to as a slab, or slab-on-grade foundation, must be properly planned for and poured to ensure that it will adequately support the structure. One of the first aspects of slab construction that must be taken into account is the dirt work. Dirt work, or sub-base and sub-grade preparation, is key to ensuring that the slab will be evenly supported and not fail. Top soil must be removed from the site before sub-grade fill can be compacted and base material imported.
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How to Do the Dirt Work for a House Pad
Step 1: Check Local Building Codes
Consult local building codes prior to beginning slab construction. Governing jurisdictions typically regulate several factors related to slab preparation and pour, including necessary setbacks and placement. Often, an inspection is required between base preparation and the concrete pour.
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Step 2: Check Soil Type and Depth
Scrape vegetation and topsoil off of the site. The depth to which the subsoil must be removed depends on the soil type and thickness and height of the planned slab. A well-drained, sandy site may require only enough soil be removed to accommodate the stone sub-base and the concrete slab. If the soil has a high loam or clay content, several inches of soil may need to be removed and replaced with a sand or gravel mixture. For shallow excavations, hand tools may be adequate but for sites where more than a few inches of soil must be removed, machinery may be warranted.
Step 3: Fill Low Spots
Compact the sub-grade material using a hand tamper or plate compactor. Note low spots and fill them in.
Step 4: Place Stakes in the Ground
Pound stakes into the ground in each corner and at intervals of several feet along the sides of the site perimeter.
Step 5: Use Strings Between Stakes
Stretch out and tie strings between the stakes at the height of the planned sub-grade. Ensure that the strings are level using a string level.
Step 6: Add Soil and Compact the Layer
Move soil around or import fill to fix any low or high spots. Compact the fill that is rearranged so that this sub-grade layer is uniformly dense and level.
Step 7: Lift Up the Strings
Raise the strings or temporarily move them. The new height indicated by the strings should reflect the planned sub-base layer thickness, about 4 to 6 inches or as dictated by local code or a professional engineer.
Step 8: Add Crushed Stone
Place the crushed stone for the sub-base on top of the sub-grade material. Only place enough at a time to create about a 3-inch layer of stone, spread it around with a garden rake or other tool so that it is roughly even and compact it before placing the remainder of the stone.
Step 9: Ensure Strings are Level
Replace or check the guide strings to ensure they are level and spread the top layer of the crushed stone. Bear in mind that most slabs must have thicker concrete around the edges to support the load. For a specific situation, this may be dictated by code. To accommodate thick slab edges, slope the gravel near the edges so it is thinner around the perimeter. Compact the crushed stone, check to make sure it is level, and make any corrections before continuing with slab construction.
Consider having a soil test performed. The results of a soil test will indicate whether or not imported fill will be required for the sub-grade and may influence multiple other aspects of construction.
It may be ideal to construct the wooden forms for the slab prior to placing and compacting fill for the sub-grade and crushed stone.
Before digging, contact local utilities or digger's hotline to mark the locations of any buried utilities.