The City of Bogotá identified that 127th street is due for rehabilitation. Due to the City being built on an old lake-bed, the underlying materials consist of deep sedimentary deposits of clays and silts with poor engineering properties. To save on construction cost the City requested Loudon International to design a pavement structure which recycles the existing materials as far as possible whilst providing a durable pavement structure.
Agricultural activities in the city outskirts, particularly flowers grown for export, extract water from boreholes. This practice has significantly lowered the water table to the extent that the materials close to the surface have dried out. Being cohesive in nature, these materials exhibit large volume changes with varying moisture content and the drying out has resulted in enormous vertical cracking in excess of 150mm width at the surface, reaching to depths of over 5m. When load is applied to the surface, slip-circle failures tend to occur in the material adjacent to these cracks resulting in large settlements.
The pavement design philosophy adopted for this project entails the “unloading” of the underlying subgrades by removing the existing structure and replacing it with relatively light-weight materials. Ash (with a compacted density of ± 600 kg/m3 ) imported from a power-station some 50 km from the city is used as backfill material due to its relative light nature, constructed in 500 mm thick layers separated by a woven geofabric interlayer that acts as a reinforcing membrane. Such a structure is designed to bridge across future desiccation cracking in the subgrade and reduce the potential for deep-seated slip-circle failures. This light-weight structure is topped with a new 250 mm thick subbase constructed from foamed bitumen stabilized material recycled in-plant from the upper layers of the existing pavement. This treated material is placed and compacted on a combined geofabric / geogrid reinforcing membrane placed on the surface of the upper ash layer. Finally, a 60 mm thick hot-mix asphalt is constructed as the final road surface.