True triaxial tests on influence of spherical and deviatoric stresses on deformation of coarse-grained soil

In order to study the influence of p, q and θ_σ on the deformation of coarse-grained soil, several tests in which only one of the three variables changes while the other two keep constant are performed on coarse-grained soil by using the TSW-40 type true triaxial apparatus in Hohai University. The results show that if p decreases with constant q and b, few deviatoric strain but some volumetric dilation will be generated at the preliminary stage. With the decrease of p, the volumetic dilation increases, which causes the increase of deviatoric strain that is smaller than the volumetric strain in absolute value. Later both the volumetric strain and the deviatoric strain will be accelerated until failure. It indicates that the decrease of p induces the volumetric dilation immediately, which loosens the particle structure, and then the deviatoric strain will be generated. If q increases with constant p and b, little volumetric dilation but some deviatoric strain will be produced at the preliminary stage. With the increase of q, the deviatoric strain increases, which causes the increase of the volumetric dilation that is smaller than the deviatoric strain in absolute value. Later both the deviatoric strain and the volumetric strain will be accelerated until failure. It demonstrates that the increase of q induces the deviatoric strain directly which causes particle dislocation, and then the volumetric dilation will be produced. If θ_σ changes with constant p and q, some but very small unrecoverable volumetric and deviatoric strains will be generated. Two parameters s_p (s_p =(p/q-p₀/q₀)/(1/M_f-p₀/q₀)) and s_q (s_p=(q/p-q₀/p₀)/(M_f-q₀/p₀)) are introduced, in which p₀ and q₀ are the initial spheric stress and the deviatoric stress respectively, M_f is the stress ratio at failure. The test results show that when p decreases with constant q and b, dε_v/dp and 1/(1-s_p)^1/2-1 are in direct proportion; dε_s/dp and -s_p1/(1-s_p)^1/2-1 are in direct proportion; stress-dilatancy equation is dε_v/dε_s= -1/s_p. When q increases with constant p and b, dε_s/dq and 1/(1-s_q)^1/2-1 are in direct proportion; dε_v/dq and -s_q1/(1-s_q)^1/2-1 are in direct proportion; stress-dilatancy equation is dε_v/dε_s= -s_q. Finally the characteristics of the flexibility matrix of coarse-grained soil are analyzed according to the test results.