Designing reinforced soil structures for the long-term

by Craig Roberts, on June 19, 2020

Assessing the long term performance of geogrid used in reinforced soil walls and slopes is crucial, with some structures having a design life of up to 120 years.

Unlike roads, which have to support constantly changing loads, reinforced soil walls and slopes are typically subjected to permanent loads throughout their working lives. As a result, the design of a structure and its load-bearing components (including the geogrids) needs to reflect this.

Creep strength

The polymers used in geogrids are viscoelastic, which means their strength and stiffness are affected by temperature and how frequently or how long they bear a load. 

Creep strength can be assessed by subjecting geogrids to long-term loading. This is carried out in a controlled laboratory environment, by suspending different-sized weights from the geogrid, in temperature-controlled conditions.

The tests can vary in duration, depending on the loads and requirements, but Tensar has many tests lasting more than 12 years (or more than 100,000 hours). This gives actual test data that is within one log cycle of 1,000,000 hours, giving confidence in 120 year creep strength predictions.

Ground Coffee – Episode 19. Andrew Lees visits the ‘Geogrid torture lab’ (otherwise known as the creep testing facility), at Tensar’s Technical Centre in Blackburn

Creep strength is used to calculate the long term design strength (LTDS), which is the predicted strength of the geogrid at the end of its serviceable life (for example, 120 years). Note that partial reduction factors - including the effect of chemical degradation, biological degradation, soil temperature and installation damage are applied to the creep strength in LTDS calculations.

Long term performance of Tensar geogrids

Tensar RE500 geogrids have been awarded HAPAS (Highways Authority Product Approval Scheme) approval, allowing their design and specification in highways structures and bridge abutments as part of a TensarTech system with a 120-year design life. They are also approved for use in strengthened embankments with 60-year design life.

Learn more about the physical properties of geogrids here.

Want to learn more about Retaining Walls?

Learn more on our Introduction to Retaining walls page here.