Investigation of Increase in Roughness Due to Environmental Factors in Flexible Pavements Using Profile Data From Long-Term Pavement Performance Specific Pavement Studies 1 Experiment

CHAPTER 4. CLIMATIC AND SUBGRADE INFORMATION FOR SPS-1 PROJECTS

This chapter discusses the available climatic and subgrade information regarding SPS-1 projects.

CLIMATIC INFORMATION FOR SPS-1 PROJECTS

Climatic information at SPS-1 project locations computed from virtual weather stations data was available in the PPDB. Annual summaries of precipitation information were available in the table entitled CLM VWS PRECIP ANNUAL. The information in this table included total annual precipitation, intense precipitation days per year, wet days per year, and total snowfall per year. An intense precipitation day is defined as a day when more than 0.5 inches of precipitation fell, while a wet day is defined as a day when precipitation was more than 0.01 inches. Annual summaries of temperature-related information were available in the PPDB table CLM VWS TEMP ANNUAL. The information in this table included mean annual temperature, days above 90° F per year, days below 32 °F per year, FI per year, and freeze-thaw cycles per year. The annual information in these two tables was averaged for each SPS-1 project over the time period for which IRI data were available for that project. Table 36 shows the average annual values for precipitation, wet days per year, and intense precipitation days per year for each SPS-1 project, and the years over which the data were averaged.

Table 36. Average annual precipitation-related parameters for SPS-1 projects.

Table 37 shows the average annual values for mean temperature, days when the maximum temperature was above 90 °F per year, days when the minimum temperature was below 32 °F per year, FI, and the number of freeze-thaw cycles. A freeze-thaw cycle occurs when, during a day, the air temperature goes from less than 32 °F to greater than 32 °F. This table also shows the years over which the data were averaged.

Table 37. Average annual temperature-related parameters for SPS-1 projects.

In the LTPP Program, the boundary between a wet and a dry region was defined as an annual precipitation of 20 inches. An area receiving an annual precipitation of less than or equal to 20 inches was considered to be a dry region, while an area receiving an annual precipitation greater than 20 inches was considered to be a wet region. In the LTPP Program, the boundary between a freezing and a nonfreezing region was defined as an FI of 190 °F days/year. A region having an FI of less than or equal to 190 °F days/year was considered to be a nonfreezing region, while a region having an FI greater than 190 °F days/year was considered to be a freezing region. The SPS-1 projects classified according to the LTPP environmental zones are shown in table 38. This table also shows the average annual values of precipitation, FI, and freeze-thaw cycles at the project location.

Table 38. SPS-1 projects classified according to environmental zones.

SUBGRADE INFORMATION FOR SPS-1 PROJECTS

In the LTPP Program, the subgrade at all test sections was considered as Layer Number 1. The PPDB table entitled SPS-1 Layer provided a description of the subgrade at each test section, the layer type and thickness of any subsurface layers present between the SPS-1 pavement structure and the subgrade, and the layer type and thickness of the various layers of the pavement structure. This table identified that an embankment that was designated as Layer Number 2 was present at four SPS-1 projects. Table 39 shows the projects where an embankment was present and also indicates the subgrade type in the project, the material type of the embankment, and the average thickness of the embankment.

Table 39. SPS-1 projects where an embankment is present.

The subgrade at six projects was treated. The subgrade at these projects might have been treated to increase the strength of the subgrade or to address constructability issues. Table 40 shows the projects where the subgrade was treated and also indicates the subgrade type in the project, the treatment type, and the average thickness of the treatment. In all of these projects, except at the Nevada project, the SPS-1 pavement sections were placed on the treated subgrade. In the Nevada project, a subbase layer with an average thickness of 20 inches was placed on the treated subgrade, and the SPS-1 pavements sections were placed on the subbase layer.

Table 40. SPS-1 projects where the subgrade was treated.

At several projects, a subbase layer was placed on the subgrade before constructing the SPS-1 pavement sections. Table 41 shows the projects where a subbase was placed and also shows the subgrade type of the project, subbase type, and the average thickness of the subbase. In the Nevada project, the subbase was placed on the treated subgrade layer.

Table 41. SPS-1 projects where a subbase was placed.

The results from gradation tests performed on subsurface material were stored in PPDB table TST SS01 UG01 UG02, while the results from hydrometer tests are stored in PPDB table TST SS02 UG03. Gradation information for subgrade was available at one or more test locations at some test sections, while no information was available for other test sections. A similar situation was noted for the results from the hydrometer test. If results from more than one sample were available at a test section, the results were averaged to obtain a single value for a test section.

From the information available in PPDB table TST SS01 UG01 UG02, the median value and the standard deviation of the percentage of material passing through the No. 200 sieve were computed for each project using the test results available at each test section for the subgrade, embankment (if present), and subbase (if present). From the information in PPDB table TST SS02 UG03, the median values of the percent clay material, percent silt material, and combined clay and silt material were computed for each project using the test results available at each test section for the subgrade, embankment (if present), and subbase (if present).

Table 42 shows the following values for the subgrade, embankment (if present), and subbase (if present) for each SPS-1 project: median value and the standard deviation of the material passing through the No. 200 sieve computed from the data at each test section, the number of test sections for which data were available to compute these values, and the material classification based on the median of the material passing through the No. 200 sieve. No information was available in the PPDB for the Delaware project.

Table 42. Percentage of material passing through the No. 200 sieve for subgrade at each SPS-1 project.

Table 43 shows the following values for the subgrade, embankment (if present), and subbase (if present) for each SPS-1 project: median values of the percent silt material, median value for the percent clay material, median value of the combined silt and clay material, number of test sections for which data were available to compute these values, and material classification based on the median value of the combined clay and silt material.

Table 43. Amount of silt and clay in subgrade at SPS-1 projects.

The material classifications shown in table 42 and table 43 for each layer agreed with each other. For the projects where an embankment was present (i.e., Iowa, Kansas, Louisiana, and Nebraska), the material type for the embankment was similar to the material type of the subgrade. For the Delaware, Kansas, Nevada, and Wisconsin projects where a subbase was placed on the subgrade, the material type for the subbase was similar to the material type for the subgrade.

The results from the Atterberg limits tests were stored in the PPDB table TST UG04 SS03. The results for tests on the subgrade were available at one or more test locations at some test sections, while no information was available for other test sections. If results from more than one sample were available at a test section, the results were averaged to obtain a single value for a test section. Table 44 shows the average values for the liquid limit, plastic limit, and PI for the SPS-1 projects that had fine-grained subgrade. This table also shows the number of test sections in each project that had Atterberg test data, which were used for computing the averages.

Table 44. Results from Atterberg limits test for projects built on fine-grained subgrade.

Results from Atterberg limit tests performed on coarse-grained subgrade were also available in the PPDB table TST UG04 SS03. Some results indicated that the material was not plastic while other results provided a PI value. Table 45 summarizes the test results for the projects that had coarse-grained subgrade and shows the following information: number of test sections in each project where Atterberg limits tests were conducted, number of test sections where the material was not plastic, number of sections for which a PI value was available, and average PI value for the sections where the PI value was greater than 0.

Table 45. Results from Atterberg limits test for projects built on coarse-grained subgrade.

Table 46 summarizes the information presented in table 42 through table 44 and presents the following information for each SPS-1 project: classification of subgrade, embankment (if present), and subbase (if present); average value of percent silt, clay, and silt and clay; and PI (for fine-grained material only).

Table 46. Summary of subsurface information for each SPS-1 project.

As seen in table 42 through table 44, test data were not available for all test sections in a particular project. Therefore, for analysis purposes, a single value of a test parameter had to be used for all test sections in an SPS-1 project.

CLASSIFICATION OF SPS-1 PROJECTS ACCORDING TO SUBGRADE TYPE AND ENVIRONMENTAL ZONES

Table 47 shows the following information for the SPS-1 projects: environmental zone, subgrade type, average age of the sections in the project at the last profile date used for analysis, and average time span over which center of the lane data were collected at the sections in the project.

Table 47. Summary of subgrade type, environmental region, and time period over which center of the lane data were collected at SPS-1 projects.

Table 48 shows the SPS-1 projects classified according to the subgrade type and environmental zone. This table shows the SPS-1 projects are not balanced over the subgrade types and the environmental zones. Eleven SPS-1 projects are located on coarse-grained subgrade compared to seven projects that are located on fine-grained subgrade. A third of the SPS-1 projects are located in the WNF zone on a coarse-grained subgrade.

Table 48. SPS-1 projects classified according to subgrade type and environmental region.