Traffic data and permanent deformation mesurement

Traffic data was gathered for the LTPP roads as well as the N7 highway. The past traffic these pavements accommodated was determined at various points in time. The rut depth measurements at various points in time was related to traffic data. By comparing the traffic data with deformation measurements, accumulation of permanent deformation in these pavements could be determined.

3.2.3.1 LTTP Traffic data and deflection measurements

Traffic counts and economic growth rates were used to obtain an estimation of the amount of standard axles that each of the LTPP pavements has accommodated. Although this provides only an estimation, it is referred to the actual traffic (Nactual) in this study. Details about the traffic

counts for each of the LTPP pavements can be found in Appendix B. These traffic counts were obtained from Long and Jooste (2007). Table 3.3 provide a summary of the amount of standard axles that each pavement had accommodated at specific points in time. Traffic in this Table and further in this study is measured in million equivalent standard axles (MESA). A standard axle was defined as an axle with an 80 kN load. It was assumed that the pavements had accommodated zero traffic at the time of opening.

Rut depth measurements were taken at various stages of each of these pavements’ design life. Traffic data, from similar points in time, were related to the rutting depth measured for each individual pavement. In the case where the pavements were rehabilitated or resurfaced, rutting was determined cumulatively. This research focuses on the deformation of the BSM layers, therefore, levelling the pavement with a seal or asphalt overlay did not have an effect on the deformation that has accumulated in the base layer. Figure 3.11 illustrates the assumed accumulation of rutting if the measurements decreased over time.

The use of rutting to define deformation in the BSM layer is further discussed in Section3.6. Table

3.3provides a summary of the average rut depth measurements with the amount of traffic for each of the LTPP pavements.

Figure 3.11: Accumulated rutting as for MR 27 NB

3.2.3.2 N7 traffic data and deflection measurements

Traffic count data was obtained for the N7 highway at a number of stations over its length. Data was available for fourteen counting stations based on data obtained form the Roads Network Information System (RNIS) with an additional nine stations available from Syntell. The data from the RNIS was based on twelve hour counts, while the Syntell data was obtained from seven day counts. Therefore, the Syntell data was used to estimate the cumulative traffic on the N7 for this study.

Table 3.4: N7 traffic loading assumptions

Table3.4shows the assumptions made regarding growth rates and standard axles per heavy vehicle. The number of standard axles per heavy vehicle at the base year (2002 or 2007) was assumed as 1.8 and increased to a maximum of 2.2 by 2017 at a growth rate of 1.5%. The average daily

traffic (ADT) and average daily heavy traffic (ADTT) growth rates varied between the different stations. The growth rate used for each station was based on the data obtained from RNIS data. The station number, location, ADTT, growth rate and cumulative traffic for 2017 is summarized in Table 3.5. The cumulative traffic estimates the total number of standard axles the pavement has accommodated in either 50 (2002 - 2017) or 10 years (2007 - 2017).

Uniform sections were identified for the N7 based on the FWD results. The FWD results and uniform section allocation is discussed in Section 4.2.2 and 4.2.3 respectively. The traffic count station in these sections were used to estimate the cumulative traffic for each section at various points in time. Traffic data was assigned to each section based on the nearest counting station. In cases where the uniform section was between two stations or contained two counting stations, the average of the two stations was used.

Table 3.5: N7 cumulative traffic 2017

The average rut depth for each uniform section was also determined from the available data. Deformation measurements were related to the traffic data as shown in Table M.2 and M.3 for

the Southbound carriageway and in Table M.1 for the Northbound carriageway in Appendix M. Deformation measurements showed a much higher deformation before the 2002 count than in 2002. It was therefore assumed that the deformation measurements taken in 2002 were after rehabilitation of the foam sections. The measurements in 2002 were assumed to be a result of the first year’s traffic.

Average deformation measurements were used for each stage of each uniform section. In certain cases the average measured deformation decreased over time. The allocation of cumulative traffic to each of the uniform sections is shown in TableN.1for the three uniform sections on the Northbound carriageway. Table N.2 and Table N.3 in Appendix N show the allocation for the eight uniform section on the Southbound carriageway.

The accumulation of deformation of the BSM layer cannot be negative, as this would suggest that the pavement was expanding. The reduction in average deformation may be due to a wider rut forming, additional settling of the materials or patching that occurred. Therefore, the rut measurements were added cumulatively if the rut depth decreased over time. Adjustments were made to ensure that the accumulation of permanent deformation reflected the actual accumulation of deformation in the base layer.

The traffic data and deformation measurements calculated in this section was further analysed to calculate the deformation in the BSM. This analysis and results thereof is discussed in Section5.1.