
Aggregate bases stabilized with asphalt or cement are recommended under very heavy loads, and over weak or saturated soil subgrades. These are sometimes used when adequate aggregates are not available or when a stabilized base is more economical than unstabilized aggregate. Poor subgrade should be avoided if possible, but when it is necessary to build over weak soils there are several methods available to improve subgrade performance.
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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Aggregate subbase is used in most applications due to its availability and cost. Thickness varies with ESALs, soils and drainage with ranges from 4 in. (100 mm) to 46 in. (1150 mm). Choosing a different base option will reduce the required material depth (See ASCE 58-10 design standard for thicknesses).
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) |