A pavement’s total strength and performance depend on both its design (including mix composition and structural design) and the subgrade soil’s ability to support loads. Therefore, any action that may be taken to raise the subgrade soil’s load-bearing capacity (or structural support) would probably also increase the pavement’s load-bearing capacity, increasing the pavement’s strength and performance. Additionally, increased subgrade structural capability can lead to more cost-effective pavement constructions that are thinner (but not too thin). The completed subgrade must also conform to the heights, grades, and slopes indicated in the contract designs. This section addresses:
- Compaction increases subgrade support
- Increasing subgrade support using different strategies
- Lower-level elevation
- paving with HMA prime coats
- Other methods of preparing for lower grades
Compaction increases subgrade support
A subgrade soil needs to be compacted to the required density in order to offer the greatest amount of structural support. If not, after construction, the subgrade will continue to compress, distort, or erode, leading to pavement cracking and deformation. In most cases, sufficient density is defined as having a relative density for the top 6 inches of subgrade that is at least 95% of the highest density found in a lab. Subgrade below the top 6 inches in fill regions is sometimes regarded as acceptable if it is compacted to a relative density of 90%. The subgrade must be at or close to the optimal moisture content at which maximum density may be reached in order to reach these densities. In most cases, in situ or fill subgrade compaction will produce appropriate structural support.
Increasing subgrade support using different strategies
There are three solutions (any one or a combination of the three may be utilized) if the structural support provided by the in situ compacted subgrade is or is believed to be insufficient:
Stabilization. These materials’ binding properties often boost the subgrade’s capability for load bearing. Typically, cement is used with less plastic soils (plasticity index less than 10), emulsified asphalt can be used with sandy soils, and lime is utilized with highly plastic soils (plasticity index more than 10). Because the material cannot be absorbed into such a fine soil, a primecoat over silty clay or clay soils is ineffective for flexible pavements.
Over-excavation. The overall idea is to add superior load-bearing fill in place of inferior load-bearing in situ subgrade. Poor soil may typically be dug and replaced with superior load-bearing fill, such gravel borrow, at intervals of 0.3 to 0.6 m (1 to 2 ft).
Over the subgrade, add a base course and maybe a subbase course. An extra load-bearing capability is provided by a base course. Unless the subgrade structural support is very strong and the anticipated loads are very low, granular base courses are frequently used in new pavement structural designs. The same compaction and elevation criteria that apply to subgrade soils also apply to base courses.
The subgrade elevation should typically roughly match the subgrade elevation of the construction design after final grading, also known as fine grading. Because HMA, PCC, and aggregate are more expensive than subgrade and because HMA pavements compact differentially, with thicker areas compacting more than thinner areas, large elevation discrepancies should not be compensated for by varying pavement or base thickness. This will cause the subgrade elevation discrepancies to affect the final pavement smoothness.
Paving with HMA prime coats
If necessary, a primecoat may be used to prepare the graded subgrade or the top granular base layer for HMA pavements. A primecoat is a cutback or emulsion asphalt coating that is sprayed over the untreated subgrade or base layers. Primer coatings have three functions.
- To shield the subbase from the elements, fill the surface spaces.
- Maintain the subbase material while stabilizing the fines.
- encourages adhesion to the following pavement layers.
A primecoat is typically advised if a HMA pavement will be less than 100 mm (4 inches) thick and installed over an unbound material.
Other methods of preparing for lower grades
- Make sure construction traffic can sustain the compacted subgrade. Before being paved over, the subgrade should be corrected if construction traffic causes significant rutting.
- Subgrade ruts may reflectively result in early rutting of the pavement if they are not rectified.The area that will be paved must be cleared of any trash, big rocks, plants, and dirt. These things either don’t compress properly or result in uneven mat thickness and compaction.
- Apply an authorized herbicide to the subgrade underneath the area that will be paved. Future vegetation growth will be stopped or at least slowed down as a result, which might influence the stability of the subgrade or directly cause pavement failure.
To conclude, subgrade preparation for new pavement ought to provide a material that is (1) able to withstand loads without suffering significant deformation and (2) graded to certain elevations and slopes.
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