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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.

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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:

• Concrete (C)
  This is the stiffest material of the four comparisons. It’s stiffness often means that its thickness will be the least among the materials in supporting heavy loads. Under most loads, concrete offers long term reliability and resistance to settling. 
  
• Cement treated base (CTB)
  is a mixture of granular soil aggregates, portland cement and water. It differs from a typical cement slab in that a lower percentage of cement is used compared to concrete. 
  
• Asphalt treated base (ATB)
  Similar to a concrete treated base except that Asphalt is used instead of concrete. 
  
• Dense graded aggregate (AB)
  This material is typical of many bases used in ICP and is often used as a subbase under CTB, ATB and concrete slabs. It is the least expensive base material among the others and offers the most design flexibility. Drawbacks include the additional thickness and/or more excavation needed for certain loads and designs.

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) 

Test your Understanding...

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Other Lesson Components ::

ESALs & Loads

Soil Characteristics

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Additional Resources:

Subgrades
E.J. Breneman - Soil - Stabilization and Modification
AASHTO