Engineering Analysis on Street Pavement Standards
And
Options to Improve Standards
Background
In any discussion on technical issues such as this, a brief summation of the engineering aspects is always helpful. This discussion is limited to pavement considerations only and does not address street geometric standards such as width, right of way width, minimum curve radii, etc.
The primary purpose of roadway pavements are to provide a sound and safe surface for vehicular traffic to move across without being mired in mud, dust and irregularities (potholes). Modern roadway pavements are essentially systems of controlled materials that are placed in such a manner as to work together to distribute heavy wheel loads over soils that, of themselves, could not carry the load.
Pavement Systems
Asphaltic pavements are normally made up of three layers. The visible asphalt serves as both a traction surface for vehicle tires, and as a moisture barrier to keep water out of the underlying layers. The middle layer is normally composed of crushed limestone and serves to distribute the load of the vehicle tires over a greater surface area of the lowest layer. The bottom layer is normally composed of native clays that have been stabilized with lime to increase their load carrying capacity and their resistance to movement and degradation caused by moisture.
Portland Cement pavements, sometimes referred to as 'concrete' streets are normally make up of only two layers. The bottom layer is normally identical to that of the asphaltic pavement system. The top, visible, layer performs both functions of surface traction, water barrier, and load distribution.
In the case of both pavement systems, the thickness of the layers is dependent on the relative costs of the component materials and the quality of the native soils that will support the roadway.
Proper design of pavement systems includes the factors of native soil characteristics, material and labor costs, strength and load transfer characteristics of the pavement layering materials, anticipated traffic over the design life of the improvement, the percentage of heavy trucks within this traffic prediction, and the expected level of maintenance activities over the life of the design life of the improvement. The design must also consider the ability of the roadway to properly drain storm water run-off and eliminate ponding within the roadway.
Failure Mechanisms
In the College Station area, the failure of pavement systems originates in three basic forms.
Structural failures are situations where the vehicular loadings exceed the load transfer capabilities of the pavement layers, which then break down into loose materials. This rarely happens in this area unless the usage of a roadway by heavy trucks increases drastically after construction. Evidence of this occurs when heavy construction traffic utilizes new or existing residential streets when developing subdivisions or building homes.
Material failures occur when one or more of the layers looses its ability to transfer the expected loads because of moisture or chemical changes. As the moisture in the lower layers increases, it's ability to bear the required loads drops quickly. This occurs frequently in this area, with the most common culprit being rain or irrigation water that seeps down through cracks in the top layers and is not able to quickly escape the lowest layer.
Foundation failures occur when the native soil that lies beneath the pavement system loses it's ability to bear the required loading, most commonly due to increases in moisture level. No matter how strong a roadway is, as water intrudes into the underlying soils, the supporting material will become more mud than dirt, and the pavement will fail. This also happens quite frequently in this area, and is most often caused by water standing in bar ditches, and water introduced behind street curbs in dry seasons by leaking water pipes and excessive landscape irrigation.
In College Station, the most common failure mechanism is a combination of the last two listed above. As we move through the seasons, the moisture in the soil under the pavements changes enough to create ground movement causing cracks in the upper pavement layers themselves. These cracks then allow rain and irrigation water to infiltrate into the structural layers, and both reduce their ability to withstand the loadings, as well as add water to the underlying soil that simply accelerates the pavement deterioration. Because each symptom in turn makes the entire situation worse, it becomes a rapidly accelerating problem and requires complete replacement of the pavement to correct.
Current College Station Standards
The City of College Station currently does not have any standard numerical criteria for the engineering design of roadway pavement systems. The City does have minimum pavement sections that are allowed based upon street classification for asphaltic pavements. The City does not have minimum standards for concrete pavements.
Capital roadway projects do include pavement system designs that incorporate the many factors needed to construct a roadway to meet the 20 year design life normally used for roadway design, as well as to meet the City's fiscal policy of constructing facilities that will last longer than the financial instrument used to fund the projects (General Obligation Bonds in most cases). For this reason, the pavements for capital roadway projects have differed around the City mainly due to the differences in native soil characteristics roadway usage and construction material costs.
However, for many years, the City has allowed private developers to construct streets to the thickness' shown in the Typical Pavement Sections that are included in the City's Standard Specifications and Details Document. It is important to note that the thickness' referenced are not actual designs, but established minimums that are expected in order for the City to accept the finished improvement as a public street. The thickness' of the base materials referenced do increase with the width of the proposed street, due to the increased anticipated traffic loadings, but are not related in any way to the soil on which they will be constructed on.
Several years ago, a draft design manual was prepared by the Engineering Staffs of the Cities of College Station and Bryan that was to be adopted by both cities in efforts to standardize the design and construction of public facilities. The draft manual included a more detailed breakdown of minimum thickness' for streets to be dedicated to the City, and differentiated between four ranges of soil types. In this manner, private development of streets could still be performed without detailed site design, but developments that were on poorer soil would be required to construct more capable streets. With the failure of the Cities to be able to adopt the same manual, the City of Bryan moved forward and adopted increased standard thickness' but without the variable standards based upon soil conditions, while allowing that lesser thickness' would be considered under approved site specific designs. The idea is to obtain a higher standard of performance and pavement life. Standards for concrete streets were also adopted.
The City of College Station currently does not have a specification for the constructed compaction of asphaltic pavement layers. During the same cooperative process with the City of Bryan, standard specifications for the construction of the pavement layers were also reviewed and discussed. It was apparent to the Engineering Staffs in both cities that the upper asphalt layers could perform better with some modifications to the compaction requirements. A standard was prepared by College Station Engineering Staff which has been adopted by the City of Bryan, but is not yet adopted in College Station.
The City of College Station has allowed the use on a trial basis of cement treated base, and a course asphalt mix as substitutes for normal limestone flexbase in attempts to determine if greater performance life can be obtained.
Options for Increasing Service Life
Based on our experience with local failures of pavement systems, and our current standard of development requirements, there exist the following options.
Option #1 Increased Minimum layer Thickness'
Modify the current minimum pavement layer thickness' of both surface course and base course to address soil characteristics at the proposed construction sites. The draft manual broke down the local soils into four ranges based on a simple and inexpensive site soils test. This would reduce the cases where the pavement structure was unable to bear the loading because the underlying soil was too weak, and would also reduce seasonal cracking due to expansion and contraction of the underlying soils, and thus help to protect the load carrying layers from damaging moisture intrusion.
Option #2 Increased Asphalt layer Compaction
Adopt a revised asphalt pavement specification identical to the City of Bryan. This would increase the density of the asphalt layer thus increasing it's ability to keep water from infiltrating into the lower levels. This would also help contractors attain better consistency with their efforts since they would be required to produce the same finished product in both cities.
Option #3 Site Specific Designs of Pavement Systems
Revise the street development regulations to require engineered designs for each development based on the specific characteristics of the soils, traffic, materials, and cost considerations for each project. The City would need to specify what the desired design life, and maintenance level would be, and the obligation to meet these criteria would be born by the Developers of the Streets.
Option #4 Moisture Barriers
Adopt requirements for a moisture barrier system to be used with current pavement standards. An effective moisture barrier lies under the top pavement layer, and extends downward from the edges of the pavement system to a depth between 4 and 6 feet. These improvements are intended to isolate the load bearing soils from significant changes in moisture level. Less moisture changes mean stronger materials and less seasonal cracking. Moisture barriers are most commonly thick plastic sheeting or asphalt-saturated fabrics.
Option #5 Require Portland Cement 'Concrete' Streets
Many cities that are underlain my expansive clays such as the City of College Station have moved towards discouraging asphaltic pavement systems in favor of 'concrete' streets. Because Portland cement pavement systems are much more resistant to soil cracking, much more impervious to water infiltration, and require much less maintenance efforts, the minor increase in initial investment is usually well worth the effort. It is for this very reason that many recent Capital Projects have constructed 'concrete streets' such as Sebesta Road, Victoria Avenue, College Main, Graham Road, and all of the Business Center. Concrete pavement will also provide the ability of the pavement to bridge over small weak subgrade areas.
Option #6 Increased Proactive Street Maintenance
While rarely a preferred long-term solution, street pavement life can be extended by a significant increase in proactive maintenance activities. Specifically, each street would need to be re-sealed against water intrusion each year. Processes that spray the entire road are dangerous because they reduce the traction for motor vehicles. However the sealing of individual cracks is most labor intensive, and most detrimental to the appearance of the roadway. In many cases, crack sealing done to help preserve a street's life is seen by the public as something bad that has happened. Southwest Parkway is an example of this situation.
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