In the Cigéo project for deep geological of radioactive waste, the project manager has to follow the convergence of tunnel (cells) cross-section built at 490 m depth. This convergence is due to the mechanical pressure in the rock layer. Vibrating Wire Extensometers (VWE) are ised to measure the strain at their locations. Our objective is to optimize the location of sensors to estimate the horizontal stress due to strain observations. This issue is solved using an inverse problem, which first requires the creation of a direct model that represents the bahviour of a cross-section. From rock data measured on site, thanks to an underground demonstrator, a numerical model is developed to generate s strain database for different VWE locations with different rock stresses and rigidities. The theoritical orientation of the sensors is orthoradial, but they can have angle and intrinsic errors. Considering various types of uncertainties, an inverse model based on Bayesian approach is developped to calculate the probability distribution of stresses. The last step is to use a genetic algorithm to determine the optimal sensor distribution. The best sensor placements is found to be near the kidneys, i.e. at more or less 45° around 0° and 180°.