Convergence Confinement (CV-CF) Method is currently used in geo-engineering practice as a plane strain approximate tunnel calculation, for example, in the scope of a NATM construction procedure. However, although improvements have been recently achieved for some simple cases, this method is impeded by undetermination of the deconfining ratio value λo at the moment when support is laid. A back analysis method has been proposed on the basis of the Virtual Work Principle, in which the original CV-CF model is compared through an iterative process to a modified model in which the field measurements are prescribed. The parameter λo and the lateral stress ratio k0 of soil mass are back analysed. This method is very robust and works also for non linear rock mass behavior. For illustration of this back analysis method, 2D shallow tunnel calculation data are used as experimental data.