It is important that the ground beneath pavers and flatwork be properly prepared to provide a stable base. Otherwise it may shift after the paving installation is complete, which can lead to cracking, movement, and uneven paving.
For any base it is important to remove all vegetation, rocks, and debris to provide uniform support without soft or hard spots.
The recommended base for pavers consists of 4 to 6 inches of compacted crushed stone. This is topped with a 1-inch layer of bedding sand. A 4-inch stone layer is considered adequate for paving that will carry pedestrian traffic, such as a sidewalk, in temperate areas with good drainage soil. A thicker base layer is recommended if one or more of the following conditions applies: the paving is for a vehicular traffic application—such as a driveway, the soil is poor, the local climate is wet, or the area is subject to severe freezing. In addition, when the paving will go over silt or clay soils, a layer of geotextile fabric is recommended on top of the soil subgrade.
The base should extend 8 inches beyond the edges of the paving. In most installations, it is important to slope the paving so that it drains effectively. The slope should always be away from buildings or other areas that need to remain dry. To achieve proper drainage, the base should slope a minimum of 1.5%. Slopes more than 12% are not recommended because it can make the paving unstable.
Preparation of the ground should not take place in times of heavy rain, snowfall, or when the base material or sand is frozen. Work begins with excavation down to the appropriate depth. The existing soil then is compacted to form the subgrade. A walk-behind roller compactor is usually adequate over silt or clay soils. Sandy soils should be compacted with a vibratory plate compactor. Next the layer of geotextile fabric, if needed, goes over the subgrade.
The crew then places the stone base and compacts and levels it with a plate compactor. It installs the edge restraints for the pavers. It then places and screeds the bedding sand.
The base beneath concrete flatwork traditionally consists of much less material. If the existing soil is stable and drains well, it may simply be cut to desired depth and compacted. If the soil is poor, the weather especially wet or cold, or the traffic that the paving must support will be heavy, it is advisable to follow a procedure similar to the one recommended for pavers. However, it is not necessary to install a layer of bedding sand. In addition, the depth of the stone base may be less for flatwork than it would be for pavers in a similar situation. The base for a driveway in a good climate and good soil, for example, typically consists of less than 4 inches of crushed stone.
— Pieter VanderWerf is president of Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products.