Effects of porosity on strength and deformation of rockfill materials

In view of the rockfill materials utilized in a 300 m - grade high earth-rock core rockfill dam to be constructed, large static triaxial tests under different porosities are performed to analyze the influences of porosity on strength and deformation of rockfill materials. The test results show that with the increase of porosity, the peak strength of rockfill materials decreases and the volume deformation gradually increases. Also, the shear displacement at the peak point increases, and the shear displacement corresponding to the maximum volumetric deformation also increases. With the increase of the loading confining pressure, the particles will be broken to some extent. Thus, these conditions are reflected in the parameters of the Duncan-Chang and the NHRI Constitutive models as follows: the parameters k and n_d decrease, and n, c_d and R_d increase when the porosity increases. The main reason is that with the increase of porosity, the internal porosity of the particles increases, the interparticle spacing increases, and the contact points between the particles decrease. Under the same confining pressure and shear stress, the volume deformation increases and thus the deviatoric stress corresponding to the maximum reduced volume strain increases. Along with the increasing value of porosity, the spacing between particles increases, the interlocking of particles is weakened, the initial tangent modulus of the rockfill is reduced and the ability of particles to resist external force is weakened, which shows that the internal friction angle between particles and the shear strength decrease. In addition, the content of P_<5 has evident effect on the strength and deformation of rockfill materials. In the case of the same porosity, the peak strength, the maximum volume variable, the shear displacement corresponding to the peak shear strength point, and the shear displacement corresponding to the maximum volume strain increase with the increase of percentage of P_<5 contained. It is obvious that the change of porosity has an influence on the strength and deformation of rockfill materials. It is proposed that the degree of compaction should be strictly controlled during the process of on-site rolling compaction to meet the design requirements and to ensure the safety of the dam.