Field measurements of anchored flexible systems for slope stabilisation: Evidence of passive behaviour

Flexible systems anchored to the ground constitute a technique for slope surface stabilisation. They are formed by membranes (cable nets, wire mesh, ring net), reinforcement cables and bolts tightly anchored to the ground forming regular patterns. Most manufacturers and independent researchers assume active behaviour of these systems when they propose their design models, which means that the system avoids occurrence of instability. To consider active behaviour, two conditions have to be fulfilled: the membrane has to be initially prestressed and the slope must present a convex shape. Neither of these two conditions has been verified so far, therefore, there are no guarantees that current design methods are adequate. Review of the technical brochures of manufacturers shows that applied prestress force on membranes and reinforcement cables is nonexistent or very low.In this paper, three field systems in north Spain with cable nets anchored to the ground have been instrumented. Net cables, reinforcement cables and bolt heads have been instrumented by using sensors based on electrical extensometry. Tensile forces on cables and compression forces on bolt heads have been monitored for more than a year from the beginning of installation. Initial tensile forces in net and reinforcement cables induced an average value of 0.7% and 2.1%, respectively, of the ultimate strength of each component. In relation to bolts, average compression force on bolt heads was 5.3% of their ultimate strength.These measurements demonstrate that prestress force on flexible systems is very low, and nearly negligible. Therefore, flexible systems anchored to the ground cannot be considered as active, but passive, which means that most current design methods are not adequate.

► Monitoring forces in anchored flexible systems in three field systems in North Spain
► Measuring for the first time pretension force on a membrane (cable net)
► Findings: very low initial pretension force on these systems
► Conclusions: flexible systems behaviour is passive (retains instabilities), not active.
► Recommendations: not to use current design methods (based on active behaviour)