Drainage Considerations
It is often said that the three most important things to consider in designing a pavement are drainage, drainage and drainage. It is absolutely critical that any pavement be well designed for proper drainage. Drainage problems are frequently a major cause of parking area pavement failures and should be given strong consideration during the design and construction phases.
Drainage provisions should be carefully designed and should be installed early in the construction process. Parking area surfaces should have a minimum slope of 1 percent to facilitate drainage. Pavement cross slopes of less than 2 percent are hard to construct without the formation of flat spots (bird baths) that tend to pond water. However, proper drainage depends just as much on the adequacy of the elevation data in the plans. There is a tendency among designers to overlook the need for grade information at key points in intersections, cross-overs and transitions between grade lines. Designers are encouraged to use valley gutters and interior drains to remove water quickly.
Automatic sprinkler systems have become very common in residential developments. The result is more and more water on pavement. While constant exposure to moisture can deteriorate the pavement and the subgrade, periodic sealing will extend the life of the pavement.
Except over well-drained soil, a granular base should include a subdrain system to remove intruding surface water. Ideally, the subgrade should be shaped and drained so that, if the pavement were taken away, there would be no spots where water would pond and saturate the soil.
Subgrade Issues
The designers are concerned about soil properties when determining the thickness of the base and pavement, in selecting the type of base and in deciding whether the subgrade should be improved. Weak soils require substantial quantities of pavement to obtain adequate bearing strength and may warrant subgrade stabilization. Weak soils are often difficult to prepare, and their sponginess may make it more difficult to compact the base and pavement.
When a soil is brought to the right moisture content, shaped to grade, compacted to the specified density and protected from moisture, the likelihood that it will perform satisfactorily is excellent. This is the function of the subgrade under a hot mix asphalt pavement.
Since the moisture content of a subgrade is subject to change, sensitive soils may undergo changes in volume and strength. No factor is more critical to pavement durability than the reaction of subgrade soils to changes in moisture over the life of a pavement. Within a pavement structure, the condition of the subgrade will vary from location to location and is often the most difficult parameter to control.
For these reasons, understanding the soil type under a pavement structure may help avoid common pitfalls that will slow down a construction project.
Subgrade Preparation
Because the subgrade must serve both as a working platform to support construction equipment and as the foundation for the pavement, it is very important to see that it is properly graded and compacted. The finished surface should be smooth, uniform and free of localized weak spots. One way to evaluate the adequacy of support is to proof roll using a loaded water or dump truck. A properly prepared subgrade will not deflect excessively under the weight of the loaded truck. The trucks tires should not leave deep tracks in the surface. However, appearances can be misleading, the degree of compaction should also be determined by testing. Note that most fine-grained soils are firm when dry, even if they are not well compacted. Therefore, in addition to proof rolling, it is important to monitor the in-place density with a nuclear density gauge.
All topsoil should be removed and low-quality soil must be improved or removed. Soil can be improved by blending aggregates with the soil or by chemical stabilization using cement, kiln dust or hydrated lime. In small private projects or in isolated areas, it is common to remove poor soils by undercutting. This procedure includes removing and replacing mud, muck or very weak soils and replacing them with compacted borrow soils, aggregate or asphalt.
The areas to be paved should have all rock, debris, roots and vegetation matter removed. Grading and compaction of the area should be completed in such a manner as to prevent yielding areas or pumping of the soil. The subgrade should be compacted to a uniform minimum density in accordance with the project specifications. This will ensure a stable working platform.
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