NPA Tough-Cell and Geogrid Hybrid Geosynthetic Solution for Rail Track on Expansive Clay, Geo-Americas 2016 Conference Proceedings, Kief (2016)
Challenge – Reducing Railway Track Maintenance on Expansive Clay
Areas of expansive weak clay subgrades invariably change as a result of seasonal changes in moisture content. For rail track in these areas, the resultant heaving and shrinking causes severe degradation of rail alignment, requiring extensive maintenance. One such area is northern Israel, where the Israel Railway Authority planned to double the existing Nahariya – Acre line, to meet increased transport demand. The conventional solution of horizontal and vertical moisture barriers was not satisfactory as it required frequent and expensive maintenance.
The Hybrid Solution (Geogrid and NPA Tough-Cell)
The Israel Railway Authority standards already call for the use of the PRS-Neoloy (NPA) Tough-Cell as its standard reinforcement solution for weak soils. Now they implemented it with the newly devised Hybrid Geosynthetic Solution. This combines a stiff biaxial geogrid located on the subgrade, which provides a stable working platform for the stiff PRS-Neoloy Tough-Cell layer embedded in the unbound granular layer. The PRS-Neoloy Tough-Cell material is made of novel polymeric alloy (NPA) – Neoloy – which provides a high Modulus Improvement Factor (MIF) for the subballast layer. The stiff biaxial geogrid integrated with stiff Tough-Cell acts like an “I” shape steel girder. Its high resistance to swelling means it is unaffected by variable moisture in the expansive soil subgrade. Furthermore, there was no need for subgrade excavation.
Validation – Restrain Heaving
Monitoring by Israel Railway Authority compared the performance of both the new Eastern Track, utilising the Hybrid Geosynthetic Solution and a parallel Western Track, which uses conventional moisture barriers. After 3 years, vertical heave and track surface deformation was evident on the Western Track, but none was evident on the Eastern Track. The monitoring period included a long period of drought, followed by unusually high precipitation. This would be expected to trigger the volumetric change potential of the clay subgrade under the newly built Eastern Track. No such change was observed.
Benefits – Maintenance Free Track
The results demonstrate that the Hybrid Geosynthetic Solution is more than a suitable solution for rail tracks over expansive clay subgrades.
• More than 50% reduction in vertical stress and settlement on new section compared to unreinforced track
• Significant reduction in maintenance cycles and costs on order of 3x or more
• Higher allowed daily traffic tonnage
• Proven effective solution for expansive clay soils