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Pavement Subgrades

Pavement Subgrades.

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Pavement Subgrades

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  1. Pavement Subgrades The “subgrade” is the in situ material upon which the pavement structure is placed. Although there is a tendency to look at pavement performance in terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance ( source: Pavement Interactive) CEE 320Steve Muench

  2. Importance of Subgrade • The Performance of the Pavement is affected by the Characteristics of the Subgrade Materials. • Some of the important subgrade properties that affecting pavement design include: Strength, Stability, Drainage, Gradation, and Ease of Compaction.

  3. Preparation of Subgrades

  4. Subgrade Performance A subgrade’s performance generally depends on two interrelated characteristics: • Load bearing capacity. The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good. • Volume changes. Most soils undergo some amount of volume change when exposed to excessive moisture or freezing conditions. Some clay soils shrink and swell depending upon their moisture content, while soils with excessive fines may be susceptible to frost heave in freezing areas.

  5. Poor Subgrades • Poor Subgrades should be avoided if possible. There are some methods to improve subgrade performance. • Removal and replacement (over-excavation). Poor subgrade soil can simply be removed and replaced with higher quality fill (this 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 made acceptable by using additional base layers. These layers spread pavement loads over a larger subgrade area

  6. Subgrade Physical Properties • Stiffness (resistance to deformation under load) • Strength (bearing capacity)

  7. Basic Common Subgrade Stiffness/Strength Characterizations used in Design • California Bearing Ratio (CBR) • Resistance Value (R-Value) • Resilient Modulus (MR)

  8. Typical Values

  9. Typical Values MR = k1qk2 K1 =5600, k2 =0.47 q = s1 + 2s3

  10. Over-Excavation Recommendations

  11. Some Stabilization Recommendations

  12. Subgrade Reaction (K) p=kD

  13. Subgrade Reaction (K) Relation of Load, Deflection and Modulus of Subgrade Reaction (k)

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