Based on the experience gained over 20 years of testing, there are a number of essential points to be borne in mind when mastering this method.

The first concerns the fixing of the geophone on the element to be tested. This fixing must enable the dynamic stiffness of the tested element to be measured, not the detachment of the geophone under repeated strikes on the element.

The compression waves generated by the hammer during the 8 acquisitions improve the relative uncertainty of the 6 results selected, and also enable us to assess the behavior of the tie-rod, its sensitivity to shocks and, by the same token, the possibility of re-tensioning it.

The other important point is sampling. For example, on a site where the tie rods have 8 or 10 strands, it is necessary to establish two formulas linking dynamic stiffness and force, as the inertias of the tie rods are different. The same applies to the thickness of the retaining wall, shotcrete and any other elements that may influence the relationship between dynamic stiffness and tension force.

As far as corrosion is concerned, the Swiss standard used for new tie rods can also be used for old tie rods.

Last but not least, re-tensioning tie rods is an important factor in optimizing maintenance operations. To facilitate testing and maintenance of tie rods, it would be advisable to upgrade the tie-head protection system.

Finally, all the results show the influence of fatigue phenomenon on load losses and tie-rod embrittlement.

The non-destructive method presented here has been in operation for over 20 years. It can be used to diagnose the condition of tie rods used to reinforce structures.

These tests enable us to.

  • Determine whether the tie-rod is broken or not

  • Calculate total tie rod length

  • the free length

  • the internal tension force at the time of testing.

By analyzing the curves, we can check whether the tie rods conform to the construction drawings, when they exist, and also calculate the diameters of the tie rods with the grout bonded to the ground, which leads us to check the dimensioning of the tie rods.

A limited number of static tests calibrate these non-destructive tests, and electrical isolation measurements complete the diagnosis.

All these tests lead to an evaluation of the existing situation, and to proposals for maintenance operations that are both technically and financially optimized.

This diagnosis makes it possible to identify the tie rods where a re-tensioning is possible and those too fragile which will not support this type of operation.

This technical contribution leads to the optimization of the work to be done.

The diagnostics contribute to the maintenance of the retaining walls, allow to follow the evolution of the tie rods and to plan the re-tensioning.

The ultimate goal is to guarantee the stability of the structures over time.