The degradation evolution of the bonded geomaterials under different loading conditions is investigated by using distinct element method (DEM). First

considering the bonding behavior at contacts between bonded geomaterials

an equation of degradation parameter

which is used to describe the degradation process in bonded geomaterials

is established based on the micromechanics theory. The equation has microscopic physical significance

which cannot be directly used to establish macroscopic constitutive model. Second

a series of isotropic

constant stress ratio and biaxial compression tests are performed on the numerical specimens by using a two-dimensional DEM code

NS2D; and the evolution of the degradation variable varying with macro variables

i.e. volumetric and shear strains

is investigated under the three loading conditions. Finally

a simple evolution equation for the degradation parameter is proposed; and it is a function of the major principal strain. It is shown that three variables

i.e. bond strength

constant stress ratio

and confining pressure

can influence the degradation evolution of the numerical specimens to a certain extent. The evolution of the parameter varying with the volumetric or deviatoric strains can be easily described by functions in the isotropic and constant stress ratio compression tests. The evolution of the parameter varying with the volumetric strain cannot be easily described due to the dilatancy in the specimens under biaxial compression tests. The equation proposed for the degradation parameter can well describe the degradation evolution observed in the three loading conditions.