ASPHALT OVERLAY DESIGN FOR GBARNGA ROAD, LIBERIA

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ASPHALT OVERLAY DESIGN FOR GBARNGA ROAD, LIBERIA

JG Afrika was appointed by China Henan International Cooperation Group (CHICO) as its consulting engineer for the rehabilitation and upgrade of a strategic section of road in Liberia for the initial implementation and future maintenance stage, as well as the evaluation of the performance of the current pavement design for the future planned overlay.

CHICO was awarded the contract for the design, rehabilitation and maintenance of the 70,16 km section of road between Gbarnga and the Guinea border by the Liberian Ministry of Public Works in 2016. The work scope comprises the initial rehabilitation of the road and its maintenance for the full 10-year contract period. Provision was also made for periodic maintenance in the eighth year and quarterly monitoring of the pavement condition to ensure that the contractual performance criteria are met. Regular measurement of road roughness, deflections and visual condition assessment are included in the monitoring.

Elevation profile of the route.

The new alignment generally follows the original road. It traverses rolling terrain and runs adjacent to the St. Johns River at a low point.

The original road had a carriageway width of between 7 m and 7.4 m and 1 m to 1.5 m wide shoulders. From Gbarnga and Ganta, the asphalt surface of the road had deteriorated significantly leaving a potholed gravel surface and the remaining 1.83 km between Ganta and Guinea had a gravel surface.

The project is in an area that experiences high average temperatures all year round, with high annual rainfall and a wet climate.

As part of the rehabilitation, the existing pavement was strengthened by overlaying it with a crushed aggregate base and asphalt layers while the road was slightly widened.

 

The new pavement comprises a 40 mm asphalt wearing course; a 70 mm asphalt binder course; and 150 mm crushed aggregate base, compacted to 98% Mod AASHTO density. This is in addition to a 250 mm gravel subbase, rip and compacted in situ to 98% Mod AASHTO density; and subgrade.

The original pavement was widened on the left-hand side to achieve a uniform roadway width of 7.5 m and 1.5 m wide surfaced shoulders. As part of the widening, a side fill and new pavement layers were placed. The upper imported layers of the original pavement mainly consist of nodular laterite to sandy, silty gravel. This is underlain by residual material comprising gravelly, silty sand to clayey sandy silt. The imported layer thickness largely exceeds 500 mm and is 300 mm thick at 214km. Laboratory test results for samples of the original pavement taken at depths of between 400 mm and 700 mm indicate CBR values that all exceed 30.

JG Afrika undertook a visual assessment of the pavement in April 2021 to identify any defects. Only small linear and alligator cracks in the outer wheel track and no major rutting were observed.

Road roughness and pavement deflections were also measured and traffic data assessed.

JG Afrika recommended that the contractor proceed with placing the 50 mm asphalt overlay as originally planned.

Although the actual traffic volumes are lower than originally forecasted, it is recognised that uncertainties with regards to future traffic growth and actual load equivalency factors necessitates a conservative approach to determining the 20-year design traffic.

The actual layer coefficient values and associated pavement capacity may be less than what the design was originally based on due to a variation in moisture state, stress state and defects developing.

Back-calculated from the average FWD, the subgrade resilient modulus value is significantly less than the value assumed for the original design, resulting in reduced pavement capacity. Some individual high pavement deflections are already present, exceeding the maximum 95th percentile value of 0.7 mm. This means that the contractor must place the overlay now, as originally planned, to achieve the specified pavement condition at the end of the contract.

 

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