The overall strength and performance of a pavement is dependent not only upon its design (including both mix design and structural design) but also on the load-bearing capacity of the subgrade soil. Thus, anything that can be done to increase the load-bearing capacity (or structural support) of the subgrade soil will most likely improve the pavement load-bearing capacity and thus, pavement strength and performance. Additionally, greater subgrade structural capacity can result in thinner (but not excessively thin) and more economical pavement structures. Finally, the finished subgrade should meet elevations, grades and slopes specified in the contract plans. This subsection covers:
- Increasing subgrade support by compaction
- Increasing subgrade support by alternative means
- Subgrade elevation
- Primecoats for HMA pavements
- Other subgrade preparation practices
Increasing Subgrade Support – Compaction
In order to provide maximum structural support (as measured by MR, CBR or R-value), a subgrade soil must be compacted to an adequate density (see Figure 1). If it is not, the subgrade will continue to compress, deform or erode after construction, causing pavement cracks and deformation. Generally, adequate density is specified as a relative density for the top 150 mm (6 inches) of subgrade of not less than 95 percent of maximum density determined in the laboratory. In fill areas, subgrade below the top 150 mm (6 inches) is often considered adequate if it is compacted to 90 percent relative density. In order to achieve these densities the subgrade must be at or near its optimum moisture content (the moisture content at which maximum density can be achieved). Usually compaction of in situ or fill subgrade will result in adequate structural support.
Increasing Subgrade Support – Alternative Means
If the structural support offered by the in situ compacted subgrade is or is estimated to be inadequate, there are three options (any one or combination of the three can be used):
- Stabilization. The binding characteristics of these materials generally increase subgrade load-bearing capacity. Typically, lime is used with highly plastic soils (plasticity index greater than 10), cement is used with less plastic soils (plasticity index less than 10) and emulsified asphalt can be used with sandy soils. For flexible pavements, a primecoat is not effective on silty clay or clay soils because the material cannot be absorbed into such a fine soil (TRB, 2000).
- Over-excavation. The general principle is to replace poor load-bearing in situ subgrade with better load-bearing fill. Typically, 0.3 – 0.6 m (1 – 2 ft.) of poor soil may be excavated and replaced with better load-bearing fill such as gravel borrow.
- Add a base course and perhaps a subbase course over the subgrade. A base course offers additional load-bearing capacity. New pavement structural designs often use some sort of granular base course unless subgrade structural support is extremely good and expected loads are extremely low. Base courses are subjected to the same compaction and elevation requirements as subgrade soils.
After final grading (often called fine-grading), the subgrade elevation should generally conform closely to the construction plan subgrade elevation (Figure 2). Large elevation discrepancies should not be compensated for by varying pavement or base thickness because (1) HMA, PCC and aggregate are more expensive than subgrade and (2) in the case of HMA pavements, HMA compacts differentially – thicker areas compact more than thinner areas, which will result in the subgrade elevation discrepancies affecting final pavement smoothness.
Primecoat – HMA Pavements
For HMA pavements, the graded subgrade or the top granular base layer may be prepared with a primecoat if necessary. A primecoat is a sprayed application of a cutback or emulsion asphalt applied to the surface of untreated subgrade or base layers (Asphalt Institute, 2001). Primecoats have three purposes (Asphalt Institute, 2001):
- Fill the surface voids and protect the subbase from weather.
- Stabilize the fines and preserve the subbase material.
- Promotes bonding to the subsequent pavement layers.
Generally, if a HMA pavement is to be less than 100 mm (4 inches) thick and placed over an unbound material, a primecoat is recommended (Asphalt Institute, 2001).
Other Subgrade Preparation Practices
- Ensure the compacted subgrade is able to support construction traffic. If the subgrade ruts excessively under construction traffic it should be repaired before being paved over. Left unrepaired, subgrade ruts may reflectively cause premature pavement rutting.
- Remove all debris, large rocks, vegetation and topsoil from the area to be paved. These items either do not compact well or cause non-uniform compaction and mat thickness.
- Treat the subgrade under the area to be paved with an approved herbicide. This will prevent or at least retard future vegetation growth, which could affect subgrade support or lead directly to pavement failure.
In summary, subgrade preparation should result in a material (1) capable of supporting loads without excessive deformation and (2) graded to specified elevations and slopes.
AASHTO Listserv Survey on Soil Cement
- Req. Density
- How Determined
- Asphalt Institute. (1997). Soils Manual. Manual Series No. 10 (MS-10). Asphalt Institute. Lexington, KY.
- Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.
- American Association of State Highway and Transportation Officials (AASHTO). (1993). AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials. Washington, D.C.
- Colorado Asphalt Pavement Association (CAPA). (2000). Guideline for the Design and Use of Asphalt Pavements for Colorado Roadways. CAPA. Englewood, CO.
- Handy, R.L. (1995). The Day the House Fell. ASCE Press. New York.
- Heukelom, W. and Klomp, A.J.G. (1962). Dynamic Testing as a Means of Controlling Pavement During and After Construction. Proceedings. First International Conference on Structural Design of Asphalt Pavements, University of Michigan. Ann Arbor, MI.
- Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook 2000. Transportation Research Board, National Research Council. Washington, D.C.↵
- Asphalt Institute. (2001). HMA Construction. Manual Series No. 22 (MS-22). Asphalt Institute. Lexington, KY.↵
- Colorado Asphalt Pavement Association (CAPA). (2000). Guideline for the Design and Use of Asphalt Pavements for Colorado Roadways. CAPA. Englewood, CO.↵
- Washington Asphalt Pavement Association, www.asphaltwa.com↵