SOUTH-EAST VIADUCT OF THE NEW TOULOUSE METRO LINE: CFA PILES BEHAVIOR IN THE TOULOUSE MOLASSE

Abstract

In the context of the construction of the viaducts of the new Toulouse metro line, it was decided to follow an experimental approach to verify the feasibility and to optimize the design of CFA piles in a geological and geotechnical context still not so well apprehended. Indeed, the Toulouse molasse, a very hardened soil whose behavior is not perfectly known and which is locally considered as a marl even if laboratory tests identify it as mainly silt, is a complex geological horizon, consisting mainly of alternating silty to sandy silt passages, and whose state can vary from very altered to sound, depending on the depth and location. CFA piles, on which only one load test was carried out in this context, are known to show many advantages but also have limitations, which could hinder the schedule of the project if this technique proved to be not adapted to the geological context. This publication states the problematic of the project, and summarizes the geological and geotechnical context of the test plot. Then, the instrumentation (using fiber optics and vibrating wire strain gauges) and the results obtained are presented. In particular, it was shown that the q_s achieved during the tests are much higher than the maximum value allowed by the French standard, but that the base resistance is on the contrary smaller than expected. The analysis of these results made it possible to propose in the conclusions optimized values to retain for the design of the future foundations of the project.