Chapter 7 Pavement Design

Chapter 7 Pavement Design

According to the Federal Aviation Administration, airport pavements are primarily constructed to provide adequate support for the loads imposed by aircraft using an airport. The Federal Aviation Administration states that an adequate pavement design produces a firm and stable, all-year, all-weather surface. In order to effectively fulfill these requirements, the pavement must be of such quality and thickness that it will not fail under the loads imposed by the various aircrafts using the airport. It is also vital for the pavement throughout the airport to be durable and long-lasting in order to control operating costs. The pavement chosen must possess sufficient inherent stability to withstand the constant traffic, adverse weather conditions, and other deteriorating influences to a degree that it does not constantly have to be attended to.

Although aircraft landing gears are involved in the design of airport pavement, the Federal Aviation Administration does not specifically prescribe any component of landing gear design. In 1958, the Federal Aviation Administration adopted a policy of limiting maximum Federal participation in airport pavements to a pavement section designed to serve a 350,000 pound aircraft with a DC-S-50 series landing gear configuration. The main intent of this policy was to insure that future aircraft were equipped with landing gears that would not stress the pavements more than the referenced 350,000 pound aircraft.

Since, aircraft manufacturers have accepted and followed the 1958 policy. Even though aircraft gross weights have substantially exceeded 350,000 pounds they still have designed all aircraft landing gear to conform to the policy. This feat has been accomplished by increasing the number and spacing of landing gear wheels. Therefore, even though the policy of 1958 is not exactly the same as it was the year it was adopted, the landing gear stills falls to the responsibility of the aircraft designers and manufactures.

7.1 Existing Soil Investigation and Evaluation

The first step in the pavement design process is to inspect the existing soil present throughout the site. According to the Federal Aviation Administration, the Unified Soil Classification system should be used in all matters concerning civil airport pavements. Soil conditions include factors such as the elevation of the water table, the presence of water bearing strata, and the field properties of the soil. Some examples of field properties of the soil include the soils density, moisture content, and frost penetration.

The standard method for classifying soils for engineering purposes is ASTM D 2487, or more commonly known as the Unified System. One of the primary purposes in determining the soil classifications is to have some idea of how the soil will behave under different scenarios. The Unified System classifies soil first by grain size and then further subgroups that particular soil based upon its plasticity index.

For the Lake Mathews Airport proposed site, the soil characteristics for the preliminary pavement design were obtain from the United States Department of Agriculture. The Department of Agriculture provides a free software program called Web Soil Survey. This software provides soil data and information produced by the National Cooperative Soil Survey. It is operated by the United States Department of Agriculture Natural Resources Conservation Service and provides access to the largest natural resource information system in the world. Soil maps and data are available for free online at websoilsurvey.nrcs.usda.gov/app/ for more than 96 percent of the nations counties. Figure 7.1 illustrates the software that was used to obtain the soil characteristics for the Lake Mathews Airport site.

Figure 7.1. Web Soil Survey, a computer software program which provides free soil characteristic information for more than 96 percent of the nations counties, was primarily used to obtain the soil information for the Lake Mathews Airport site.

In addition to the Web Soil Survey software program, the United States Geodetic Survey geologic maps proved to be valuable aids in investigating the soils around the proposed airport. Figure 7.2 illustrates a portion of the map available from the United States Geodetic Survey that aids in illustrating the various soil types located around the Lake Mathews site. Once the different soil types are known throughout the area, the soil classifications can be found through data provided once again by the United States Geodetic Survey. An example of the soil classification table can be seen in Table 7.1.

Figure 7.2. Map provided by the United States Geodetic Survey which assisted in determining the soil type for the area surrounding the proposed Lake Mathews Airport.

Table 7.1. A portion of the engineering properties table provided by the United States Geodetic Survey which guided in determining the soil classification for the area surrounding the proposed Lake Mathews Airport.

From the engineering properties obtained from the United Stated Geodetic Survey, it was found that the lowest quality of soil is classified as LaC. This has a Unified Classification of CL and CL-ML for the first 12 inches. The classification of CL consists of all inorganic clays of low to medium plasticity, gravelly clays, silty clays, and lean clays. In addition, CL-ML was within the first 12 inches, this includes all inorganic clays that were previously mentioned as well as inorganic silts, very fine sands, rock flour, and silty or clayey fine sands. Between 12 inches and 32 inches, the soil has a classification of CH and CL. Once again this region holds inorganic clays, as well high plasticity, and fat clays. Weathered bedrock is at a depth from 32 to 54 inches.

7.2 Overall Pavement Thickness

Now that the engineering properties of the soil are known, the overall thickness of the pavement needed throughout the airport can be determined.

The strength of the subgrade is the main factor in determining the thickness of the pavement needed for a particular application. The value of the stiffness of the subgrade is required if the stresses and strains in the pavement and the subgrade are to be calculated. Subgrade strength is expressed in terms of its California bearing ratio (CBR) value. The CBR value is measured by an empirical test devised by the California State Highway Association and is simply the resistance to a penetration of 2.45nn of a standard cylindrical plunger to various penetrations in crushed aggregate, notably 13.24kN at 2.5mm penetration and 19.96kN at 5.0mm penetration.

The CBR value can be determined by conducting various tests on collected soil samples, or it can be obtained from a standard table provided by the Federal Aviation Administration. In the case of the Lake Mathews Airport, the CBR value and the soil characteristics for the site was found through a provided table. Table 7.2 illustrates how the Field CBR value was found for the soil around the Lake Mathews Airport site.

If the worst case scenario were considered, a CBR value of 3 to 5 would be selected. In order to be conservative, a CBR value of 4 was selected. In addition, a Field CBR value of 4 was selected due to the fact that the CBR values range from 3 to 40 in the location of the runway and taxiway.

Once the Field CBR value is determined, the total pavement thickness can then be found. The overall pavement thickness is a function of both the CBR value and the maximum aircraft gross weight. Figure 7.3 illustrates the design curve that was used to determine the pavement thickness. With a Field CBR value of 4 and a maximum aircraft gross weight of 12,500 pounds, the thickness of the pavement was found to be 14 inches.

Table 7.2. Soil characteristics pertinent to pavement foundations per AC 150/5320-6D.

Figure 7.3. Design curves to find the overall thickness required for flexible pavement per AC 150/5320-6D.

7.3 Typical Pavement Layers

There are various types of pavement that the Federal Aviation Administration deems acceptable. These include flexible, ridged, hot mix asphalt overlays, and ridged overlays. A pavement does not have to consist of just one of these specific types, but rather, there can be a combination which would result in a complex pavement ranging somewhere between flexible and ridged.

A typical pavement design consists of four distinct layers: surface, base, subbase, and subgrade. The first layer in the pavement is the surface. The surface courses include portland cement concrete, hot mix asphalt, sand-bituminous mixture, and sprayed bituminous surface treatments. The subsequent layer is the base. The base courses consist of a variety of different materials which generally fall into two main classes: treated and untreated. The untreated bases consist of crushed or uncrushed aggregates, where as the treated bases normally consist of a crushed or uncrushed aggregate that has been mixed with a stabilizer such as cement or bitumen. The subbase course is the next in the pavement design layer. The subbase courses consist of a granular material, a stabilized granular material, or a stabilized soil. The final layer consists of the subgrade. The subgrade is the natural soil or a fill, which is compacted to a specified percent of compaction based on the depth of compacted soil.

7.4 Flexible Pavement for Light Aircraft

Pavement for light aircraft is deemed by the Federal Aviation Administration as pavement intended to serve aircraft with gross weights of less than 30,000 pounds. With the Lake Mathews Airport being an AII-BII category airport, the maximum gross weight that the airport can serve is 12,500 pounds. This gross aircraft weight would put the Lake Mathews Airport in the light aircraft category.

Flexible pavement for light aircraft is composed of hot mix asphalt surfacing, base course, subbase, and prepared subgrade. The hot mix asphalt surface is responsible for preventing water to seep into the base course. In addition, the hot mix asphalt must provide a smooth, well-bonded surface free from loose particles that pose a potential threat to aircraft or persons using the airport. The base course is the primary load carrying component of the flexible pavement. The subbase course is usually required for flexible pavement, except for those with a CBR value of 20 or greater. However, with a CBR value of 4 for the Lake Mathews site, a subbase course is needed.

Figure 7.4. Typical sections for light aircraft pavement per AC 150/5320-6D.Overall Pavement Thickness

The pavement thickness of 14 inches, which was determined previously, must be used upon all areas of airport pavement according to the Federal Aviation Administration. No reduction in thickness should be made for noncritical areas of pavement. With the total pavement thickness already determined, the next step in the pavement design process is to find the thickness of the surfacing and the base.

Surfacing and Base Thickness

According to the Federal Aviation Administration, to find the surfacing and base thickness, the CBR 20-line is used. Using the chart provided by the Federal Aviation Administration, the thickness of the surfacing and base can be found. Using a CBR value of 20, as instructed by the Federal Aviation Administration, and a maximum aircraft gross weight of 12,500 pounds, a value of four and a half inches is obtained for the thickness of the surfacing and base. This thickness value is rounded up to five inches in order to be conservative and also because this base is, in essence, the structural element of the pavement. Figure 7.5 represents how the thickness for the surfacing and base was generated.

The Federal Aviation Administration states that the minimum thickness of the hot mix asphalt surfacing over a granular base is two inches. The reason for the minimum surfacing thickness is that layers thinner than two inches are difficult to place and compact on granular bases. Hot mix asphalt surfacing thickness of less than two inches is permissible on stabilized base materials if proper lay down and compaction can be achieved. However, for the Lake Mathews Airport, a hot mix asphalt surface of two inches will be used. This will have a corresponding base thickness of three inches.

Figure 7.5. Design curves to find the surfacing and base thickness required for flexible pavement per AC 150/5320-6D.Subbase Thickness

The difference between the total pavement thickness required and the CBR 20-line thickness yields the thickness of the subbase. With a total pavement thickness of 14 inches and a surfacing and base thickness of five inches, the total subbase thickness is required to be nine inches. Using engineering judgment a surface thickness of four inches will be used, and a base thickness of three inches will be used. Therefore the final thickness of the flexible pavement is 16 inches.

Subgrade

The subgrade materials need to be compacted to a specific percentage depending upon whether the soil is noncohesive or cohesive and the design aircraft gross weight. For the Lake Mathews Airport, with cohesive soil and a maximum gross aircraft weight of 12,500 pounds, it was determined that the subgrade needs to be compacted to 85 percent for eight to twelve inches or 90 percent for four to eight inches in depth. If the above compaction requirements cannot be met, then the base will need to be increased in order to accommodate the aircraft. Table 7.3 depicts how the subgrade compaction requirements for the Lake Mathews Airport were obtained.

Table 7.3. Subgrade compaction requirements for light load flexible pavement per AC 150/5320-6D.

Final Flexible Pavement Design:

Figure 7.6. Expansion Cross-section of flexible pavement (Total Thickness).

7.5 Rigid Pavement for Light Aircraft

Rigid pavements for light aircraft are composed of Portland cement concrete surfacing, subbase, and prepared subgrade.

Portland Cement Surfacing

When determining the required thickness for the various rigid pavement layers, design curves provided by the Federal Aviation Administration prove to be exceptionally helpful. However, there are no design curves for light aircraft ridged pavement. There are only two thickness requirements for the Portland cement surfacing; that for pavement designed to serve aircraft weight 12,500 pounds or less and that for aircraft weighing between 12,501 pounds and 30,000 pounds. For ridged pavements designed to serve aircraft weighing 12,500 pounds or less, a Portland cement surfacing thickness of five inches is required. Rigid pavement designed to serve aircraft weighing between 12,501 pounds and 30,000 pounds should be six inches thick. With the Lake Mathews Airport serving aircraft with a gross weight less than 12,500 pounds, the Portland cement surfacing thickness for rigid pavement needs to be five inches.Subbase

No subbase is required for rigid pavement designs intended to serve aircraft weighing 12,500 pounds or less, except when soil types OL, MH, CH, or OH is encountered. If a subbase is needed, the Federal Aviation Administration specifies when any of the above soil types are present, a minimum four inch subbase should be provided. Due to the fact that CH soil type was discovered at the Lake Mathews Airport site, a subbase of four inches for the rigid pavement design is required.Subgrade

The Federal Aviation Administration stipulates that a subgrade comprised of cohesive soils used in fill sections, the entire fill shall be compacted to 90 percent of the maximum density. For cohesive soils in cut sections, the top six inches of the subgrade shall be compacted to 90 percent of the maximum density. The Federal Aviation Administration specifies different standards for noncohesive soils, however due to the fact that the Lake Mathews Airport consists only of cohesive soils, these standards do not apply. For the Lake Mathews Airport, entire fill section and the top six inches of the cut section needs to be compacted to 90 percent.

Jointing of Light Load Rigid Pavements

The maximum spacing of joints for light load rigid pavements should be 12.5 feet for longitudinal joints and 15 feet for transverse joints. Jointing details for expansion joints, contraction joints, and construction joints are illustrated in Figure 7.6, Figure 7.7, and Figure 7.8 respectively.

Figure 7.7. Expansion joint details for light load rigid pavement per AC 150/5320-6D.

Figure 7.8.Contraction joint details for light load rigid pavement per AC 150/5320-6D.

Figure 7.9. Construction joint details for light load rigid pavement per AC 150/5320-6D.The Federal Aviation Administration notes that several differences exist between light load and heavy load rigid pavement joints. For example, butt-type construction and expansion joints are permitted when an asphalt or cement stabilized subbase is provided. Also, half round-keyed joints are permitted even though the slab thick nesses are less than nine inches. Odd-shaped slabs should be reinforced with 0.05% steel in both directions. The Federal Aviation Administration defines odd-shaped slabs as slabs that are not rectangular in shape, or rectangular slabs which length-to-width ratios exceed 1.25.

For the Lake Mathews Airport, longitudinal joints are going to be placed every 12.5 feet, for a total of six sections. In the transverse direction, joints will be placed every 15 feet. These transverse joints will extend for the entire length of the runway.

Rigid Pavement Design:

Figure 7.10. Construction joint details.Chapter 7: Pavement Design Airport Design California State Polytechnic University / Page 1