
Subgrade Improvement Options
- Removal and replacement (over-excavation)
Poor subgrade soil can simply be removed and replaced with high quality fill. Although this is simple in concept, it can be expensive. - Stabilization with a cementitious or asphaltic binder
The addition of an appropriate binder (such as lime, portland cement or emulsified asphalt) can increase subgrade stiffness and/or reduce swelling tendencies. - Additional base layers
Marginally poor subgrade soils may be compensated for by using additional base layers. These layers (usually of crushed stone – either stabilized or unstabilized) serve to spread pavement loads over a larger subgrade area. This option is rather perilous; a thick pavement structure over a poor subgrade will not necessarily make a good pavement.
When stabilization is required, multiple base options are available:
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Cross section design alternatives are shown with 3.15 in. (80 mm) thick concrete pavers and a 1.0 in. (25 mm) thick bedding sand layer over several bases. These are a sample of the possible material type and thickness combinations which satisfy a particular set of design requirements. Cost analyses of these and other pavement cross section alternatives should be conducted in order to select the optimal design. | NOTE: Concrete = min. 8 in. (200 mm) CTB = min. 4 in. (100 mm) ATB = min. 4 in. (100 mm) AB = min. 6 in. (150 mm) |