Recently, many papers have been devoted to the theoretical study of the deformation of some 3D crack front during its coplanar propagation. This works generally showed an increase in time of the geometrical disorder. This studies only considered a single crack. The aim of these thesis is to extend them to the coalescence of coplanar cracks, using the Bueckner-Rice weight functions theory. Under certain conditions, it permits to determine the distribution of the stress intensity factors along the crack fronts, without solving the entire elasticity problem. This work is divided into two parts. The first one is devoted to an analytical study of a system of two tunnel-cracks. The goal is to quantify the rapidity of the geometrical disorder development during the propagation. The major result is that, contrary to the single crack, the disorder continuously increases during the propagation. The second part also deals with coalescence but with a numerical approach. We extend the program, developed by Lazarus (2003) for a single crack under mode I, to the case of two circular coplanar cracks. Simulations show that the mutual attraction of the two crack fronts (i) is sensitive only if the fronts are closed and (ii) has a weak impact on the loading.
