The numerous works devoted to the phenomena involved in the compaction of sediments have led to a good understanding and to mechanically consistent modellings of this process. During compaction, large reduction of porosity occurs and the sediments undergo large strains: in this context, the usual linearized Green-Lagrange strain tensor is no longer relevant. Using the constitutive law of a poroelastic medium (made of incompressible constituents) at large strains, we discuss the influence of geometrical nonlinearities in the study of purely mechanical compaction.