Civil Engineering July 2021 | Vol 29 No 6

Q Q The average IRI value for any 1 km long section of road must be less than 4.0 mm/m. Q Q The average IRI value for the entire road must be less than 3.0 mm/m. The average IRI values for the entire road, measured quarterly during the maintenance stage, are plotted in Figure 6. The values are less than the maximum of 3 mm/m specified and, as can be expected, indicate deteriora- tion of the road roughness with time. The most recent (March 2021) average 1 km IRI values, varying along the road length, are plotted in Figure 7. The plot indi- cates higher values in the central portion of the road, roughly coinciding with the lower lying areas of the road, which are located in close proximity to the St. Johns River. Pavement deflection (FWD) measurements In accordance with the contract specifica- tions, the pavement deflections must conform to the following requirement during the maintenance stage: Q Q the 95 th percentile maximum deflec- tion (Y0) of any 1 km section of road must be less than 0.7 mm. The variation and reduction of average deflections with time may be attributed to seasonal and climatic effects, which corre- spond with variation in moisture content. However, based on the plot, higher deflec- tions are not necessarily corresponding with the wet/rainy season. This may be attributed to a lagging effect, as identified by researchers, particularly with respect to deflections of the upper pavement layers. The most recent average 1 km 95 th percentile deflections, varying along the road length, are plotted in Figure 8. The plot indicates that the highest deflections are occurring in the central portion of the road, roughly coinciding with the lower lying areas of the road, which are located in close proximity to the St. Johns River. ASSESSMENT OF TRAFFIC DATA Traffic volumes The average annual daily traffic (AADT) for 2009 applicable to the various vehicle types was provided in the contract documents. As part of the contractual requirements, seven-day traffic counts for various traffic classes were done annually from the start of the contract. These counts were used to determine the AADT for each calendar year Location (SV, km) Yo deflection (micrometer) 250 1 100 900 700 500 100 240 230 220 210 190 170 Figure 9 Variation of 95 th percentile deflections (2021) along the road length 200 180 300 FWD 95th percentile – additional FWD 95th percentile The newly upgraded road between Gbarnga and the Guinea border Table 1 Summary of ELF Vehicle types Contract Based on Liberia legal loads Proposed Goods vehicle and minibus 0.740 2.00 1.11 Bus 1.000 3.00 2.70 Light truck 2.048 3.00 3.00 Medium truck 4.345 3.04 4.35 Heavy truck 5 axles 4.400 5.99 5.99 Heavy truck 6 axles 5.600 7.85 7.85 Civil Engineering July 2021 29