Our MacGrid® WG geogrids solve the issue of differential settlement on black cotton soil
The construction of a new highway from Bagamoyo (70km north of Dar es Salaam) to Msata started in September 2010, providing surfaced road for 64 km. The swelling of the black cotton soil soon resulted in longitudinal cracks in the pavement. Our MacGrid® WG 5S geogrids provided a cost-effective solution and in solving the problem became what is thought to be the first project in Tanzania where ground stabilization techniques were used.
The road is expected to advance road transport between the Tanzania and Kenya coastlines. This venture will also enable easier cargo movement from Mombassa and the Tanga port to the landlocked countries of Uganda, Rwanda, Burundi and South Sudan.
Tanzania is known for the presence of black cotton soil, that is characterised by high shrinkage and swelling properties. The black cotton soil is very hard when dry, but loses its strength completely in wet conditions. Therefore, it presents a challenge to the highway engineers since the cyclical wetting and drying of the soil causes movement leading to failure of the pavement in the form of differential settlement, heavy depression, cracking and unevenness.
The initial design for the embankment was as per the ‘Tanzania Road Manual 2000’; 150mm G7, 150 mm G15, 200mm Cement stabilized sub base C1 (UCS min1.0 MPa), 150mm CRR and 50mm asphalt. After the first rainy season, longitudinal cracks developed on the shoulder due to the swelling of the Black Cotton soil with cracking developing through the whole pavement.
The most cost-effective remediation was found using a geogrid to absorb the tensile strains in the granular layers, developed by the movement of the black cotton soil with the variation of moisture content. The asphalt surface was removed and a geogrid type MacGrid® WG 5S was then placed over the recompacted CRR layer and another 150mm CRR layer was placed, finished by 50mm asphalt.
Our engineers visited the site prior to commencement of the installation of the geogrid to ensure it complied with the installation guidelines. The MacGrid® WG 5S was unrolled over the entire width of the road, ensuring 100mm overlap between the rolls. Trucks backfilled the CRR over the geogrid ensuring no direct contact was made between the tyres and the geogrid in order to avoid damaging it. A dozer spread the gravel layer which was then shaped and compacted as per standard construction practice.
This is believed to be the first project in Tanzania where ground stabilization techniques were used, and it drew the attention of TanRoads, Ministry of Works, Transport and Communications.
For more information, please download our Case History.