DESIGN CONSIDERATIONS - 3.4

CONSTRUCTION PRACTICES

Preparation of Subgrade

Remove all large rock, debris, and topsoil from the area to be paved. All vegetation, including root systems, should be removed. To prevent future growth, the subgrade should be treated with an approved herbicide. Install all drainage and utility facilities and then properly backfill and compact.  

The subgrade must be properly shaped to meet true lines and elevations and compacted to not less than 95 percent of maximum laboratory density. The surface of the compacted subgrade should not vary more than ¾ inch from the established grade.

Areas showing pronounced deflection under construction traffic indicate instability in the subgrade. If the situation is not corrected by reworking and additional rolling, the areas must be removed and replaced with suitable material and compacted or stabilized using a geotextile. The use of Asphalt Concrete base or course granular material is recommended.

Untreated Aggregate Base

The crushed aggregate base course may consist of one or more layers placed directly on the prepared subgrade. The material must be spread and compacted to the required thickness, grades, and dimensions indicated in the plans or as specified. The minimum compacted thickness of each lift should be no less than two times the size of the largest aggregate particle, or 4 inches, whichever is greater. The maximum compacted lift thickness should be 6 inches.

Binder and Surface Courses

The upper lifts of the pavement may consist of one or more courses of Asphalt Concrete placed on the previously constructed Asphalt Concrete base. In general, the top or wearing course must not be constructed to a depth greater than 3 inches. Where a thickness greater than 3 inches is indicated, it should be placed in two courses consisting of a binder and a surface or wearing course.

The minimum lift thickness must be 1 inch, but this thickness should never be less than two times the maximum particle size.

Tack Coat

A tack or bond coat of CSS-1, SS-1, MC-70 or an approved alternate should be applied between each course at an undiluted rate of 0.02 to 0.05 gallons per square yard. The surface must be cleaned of all dust, dirt, or other loose material before the bond coat is applied. If emulsion is used, it should be diluted with equal parts of water or as specified in the proposal.

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DESIGN CONSIDERATIONS - 3.3

SUBGRADE CLASSES

soils have been divided into three classes: good (G), Moderate (M), and poor (P), CBR design values are assigned to these different subgrade classes.

Good

Good subgrade soils retain a substantial amount of their load-supporting capacity when wet. Included are the clean sands, sand gravels, and those free of detrimental amounts of plastic materials. Excellent subgrade soils are relatively unaffected by moisture or frost and contain less than 15 percent passing a No. 200 mesh sieve. A soil classified as good will have a CBR value of 9 or greater.

Moderate

Moderate subgrade soils are those that retain a moderate degree of firmness under adverse moisture conditions. Included are such soils as loams, silty sands, and sand gravels containing moderate amounts of clays and fine silts. When this soil becomes a cohesive material, it should have a minimum proctor density of 110 pounds per square inch. A soil classified as moderate will have a CBR value of 6 to 8.

Poor

Poor subgrade soils are those that become quite soft and plastic when wet. Included are those soils having appreciable amounts of clay and fine silt (50 percent or more) passing a No. 200 sieve. The coarse silts and sandy loams may also exhibit poor bearing properties in areas where deep-frost penetration into the subgrade is encountered for any appreciable periods of time. This also is true where the water table rises close to the surface during certain periods of the year. A soil classified as poor will have a CBR value of 3 to 5.

Very poor soils (those with a CBR of 3 or lower) often perform poorly as pavement subgrades. However, to improve their performance, these soils can be stabilized with granular material or a geotextile. Lime, fly-ash, asphalt cement, Portland cement, and combinations of cement stabilizers also can be added to improve the subgrade support.

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DESIGN CONSIDERATIONS - 3.2

 SOIL SUPPORT CAPABILITY 

The ability of the sub-grade to support loads transmitted from the pavement is one of the most important factors in determining pavement thickness. The sub-grade must serve as a working platform to support construction equipment and as a foundation for the pavement structure that supports and distributes traffic loads. 

Thus, it is essential to evaluate the strength of the sub-grade before beginning the structural design of the pavement. Figure 3-1 shows the spread of wheel load through the pavement structure and on to the sub-grade. 

If sufficient pavement thickness is not provided, the applied loads could cause greater stresses on the subgrade than it can resist. This may result in deflection of the pavement and ultimately in its failure. 

In street and highway construction, the subgrade provides the foundation for the pavement. Different types of soils have different abilities to provide support. A sandy soil, for example, will support greater loads without deformation than a silty clay soil. Thus, for any given traffic volume and weight of vehicles using the roadway, a greater pavement thickness must be provided on clay soils than on sandy soils.

SUBGRADE STRENGTH

Because thickness calculations depend on the strength of the finished subgrade, the soil must be tested for this information. Tests are based on bearing capacity related to the moisture and density of the soil. The California Bearing Ratio (CBR) is one of the most widely used methods of designing pavement structure. Once the CBR value is determined, the soil classification can be identified. Or, when the soil classification is known, a relative CBR value can also be identified.

The lower the CBR value of a particular soil, the less strength it has to support the pavement. This means that a thicker pavement structure is needed on a soil with a low CBR rating than on a soil with a high CBR rating. Generally, clays have a CBR classification of 6. Silty loam and sandy loam soils are next with CBR values of 6 to 8. The best soils for road building purposes are sands and gravels whose CBR ratings normally exceed 10.

The change in pavement thickness needed to carry a given traffic load is not directly proportional to the change in CBR value of the subgrade soil. For example, a one-unit change in CBR from 5 to 4 requires a greater increase in pavement thickness than does a one-unit CBR change from 10 to 9. 

A number of soil classification-strength systems are currently in use for roads and airports. A correlation chart follows for a general soil overview.

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