Experimental study on the mechanical properties and creep-related behaviors of peridotite under gradient of loading

Peridotite, due to its high strength and low brittleness, exhibits good deformability and significant creep characteristics under the continuous action of surrounding rock pressure. Tunnels passing through such strata are prone to large deformation disasters. To study the creep mechanical properties of Yanshan period peridotite, systematic mechanical tests were carried out on peridotite specimens taken from the AnDing Tunnel of the China-Laos Railway using Chen's gradient of loading method. On this basis, a nonlinear viscoelastic creep constitutive model suitable for the creep characteristics of peridotite was established. The results show: (1) The water softening property of the peridotite specimens collected on-site is not obvious, with a softening coefficient of 0.814. The ratio of the elastic modulus of the saturated peridotite specimen to that of the dry specimen is 0.802, which is close to its softening coefficient. The Poisson's ratios are respectively 0.30–0.35 and 0.34–0.39, with no significant difference between them. (2) The transient strain produced by the peridotite specimens after loading includes elastic strain and plastic strain, where elastic strain predominates. Even under load conditions close to creep failure, it accounts for more than 90 %. (3) The peridotite specimens exhibit three-stage creep behavior, including decay creep, steady creep, and accelerated creep. There are thresholds for peridotite creep and accelerated creep. With the increase of confining pressure, both the creep initiation threshold qdcc and the accelerated creep threshold qacc of peridotite show a linear increasing trend. (4) Based on the creep-related mechanical properties of peridotite, a seven-element nonlinear viscoelastic-plastic constitutive model was developed to describe its creep-related behavior. When the stress is less than the creep initiation threshold σ≤qdcc, the constitutive model degenerates to an elastic body. when qdccqacc, the model can characterize the nonlinear accelerated creep behavior of peridotite close to creep failure. (5) The correctness of the constitutive model was verified in conjunction with the experimental results, and the parameters of the instantaneous strain phase and the corresponding parameters of the three creep phases of the peridotite specimens were calibrated. •The creep properties of