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How to Prepare Soil for a Concrete Slab

Concrete is beneficial in the garden and landscape. Offering a solid surface that holds up to rain and traffic, a concrete slab can provide a patio for entertaining, a base for a garden shed or a spot to store anything off the ground. Before you can pour a concrete slab, you must prepare the soil beneath to ensure that your slab remains level and to reduce cracking from unstable soil.

Concrete requires a firm soil base.
  1. Remove the top layer of soil, including all the sod, with excavating equipment or with a shovel. While soil types vary in different geographical locations, the surface is usually unsuitable for pouring concrete. You may have to remove 5 inches or more of soft soil to reach the hard compacted soil.

  2. Replace the removed soil with sand. Sand compacts well and provides an optimal base on which to pour concrete. The amount of sand you will need depends upon how much soil you removed. A concrete slab may be 4 to 6 inches thick, so add sand until it reaches the proposed bottom of your slab.

  3. Pack the sand with a sand compactor, available from construction rental stores. You will run the compactor over the sand base and the machine will vibrate the sand, forcing the tiny particles to fill in tightly.

  4. Tip

    If your soil is very soft, you may have to dig deeper and make a gravel bed beneath the sand for extra support. Construct the frame for your concrete slab before filling in with sand.

    Warning

    Don’t pour concrete over disturbed soil. This situation may occur if you’ve recently replaced a buried water line. The soil, even after backfilling and compacting, will settle gradually, causing a concrete slab to crack. It may take a year or longer for the soil to settle sufficiently. Avoid pouring your slab over buried tree roots that may grow and cause the slab to heave or break.