Range of studies – asset management for aged foundations

World War in France, will reach the age of 85. To prevent this network from losing its reliability, altered and corroded OHL will have to be rebuilt, involving the replacement of many old non-galvanised pylons.

In order to comply with regulatory procedures for pylon replacements when kept on location, the French Transmission System Operator (TSO) has decided to evaluate the risk of reusing old foundations before deciding whether to demolish, refurbish or reinforce the existing asset.

To adopt the best approach on this matter, the French TSO has launched a range of studies regrouped under the label “Plan Fondations”. It aims at addressing 5 topics of which the 3 following:

Topic#1: Failure is defined by a situation in which the foundation resistance is lower than the designed stresses it is supposed to undergo, independently of the fact that it is already altered or not. Looking into the records available in the French TSO, a low rate of failures is due to foundation failures, it is estimated between 0,2 and 1% of global incidents.

Topic#2: From a benchmark performed by the French TSO with TSOs from other countries, foundation alteration is found to be mostly occurring in environments which are aggressive to concrete piling or blocks. The mapping of aggressiveness factors mostly linked to frost/defrost cycles, salted roads proximity and chemicals contained in the soil or water table, shows where they are existing and where foundations are likely to deteriorate quicker. Through studies of the same type, a similar map was created with a GIS software for the French OHL network using water table chemical contents to identify where concrete could potentially be altered. During a concrete core sampling and lab testing survey done throughout the country (approx. 455 samples taken in 2023 and 2024), an unexpected population of altered foundations was discovered, different from a basic survey of common criteria such as age, location and soil type.

This discovered population was about 10% of the global concrete samples and held compression crush limits below the required standards values.

Topic#3: 3 solutions of foundation reinforcements have been developed and their merits evaluated:

• 2 adding concrete and reinforcement bars to existing infrastructures to widen its ground surface and provide additional support to OHL towers, • 1 using precast concrete structures for narrow based pylons. Series of studies were performed by the French TSO, addressing these 3 topics, some of which are still in progress. They have already shown that foundations may lead to a low rate of OHL failures on the French grid. Regarding the 10% of foundations that did not meet the standard, it is assumed that, during climate event, other parts of the OHL are likely to fail before the foundation and therefore avoid the failure on the foundation itself. Ongoing works are to be pursued, looking at foundation casualty rates to check if the first conclusions of this study remain valid. The development of other new types of tower foundation is also an ongoing topic in current studies.