Triaxial creep tests of glacitectonically disturbed stiff clay – structural, strength, and slope stability aspects

This study concerns the creep impact on strength parameters of the selected very cohesive soils (PI = 30–70%). The analysis refers to Neogene clays characterized by a complex structure, resulting directly from a complicated load history in the geological time scale and identified glacitectonic deformations. In the process of samples’ preparation for strength tests as well as during the interpretation of the post-failure state, particular attention was paid to the soil structure. The imaging methods (X-ray densitometry and computer microtomography) enabled the comparison of the soil structure and the selection of samples with similar characteristics. The completed program of strength tests consisted of two series of tests in the triaxial stress state, differentiated by the occurrence of the initial creep stage, preceding the typical strength test scheme under undrained conditions. This study allowed to obtain a quantitative assessment of the influence of the creep process on the strength parameters of tested soils. Constant stress lower than 60% of the shear stress deviator leads to the deceleration creep course (m parameter 0.64–0.89). As a result, higher values of internal friction angle (20% increase comparing to triaxial tests without creep stage) and cohesion reduction are obtained from triaxial creep tests. Creep parameter m is found to be a valuable indicator for differentiation of landslide activity trend. The tests proved low values of axial strains (1–5%) at failure, which was associated with lithogenesis. By the implementation of obtained strength parameters into the 3D finite element model of the slope, the potential influence of the creep process on the stability of an exemplary cross section of the Warsaw slope could be determined.

Experimental Researches on Long-Term Strength of Granite Gneiss

It is important to confirm the long-term strength of rock materials for the purpose of evaluating the long-term stability of rock engineering. In this study, a series of triaxial creep tests were conducted on granite gneiss under different pore pressures. Based on the test data, we proposed two new quantitative methods, tangent method and intersection method, to confirm the long-term strength of rock. Meanwhile, the isochronous stress-strain curve method was adopted to make sure of the accuracy and operability of the two new methods. It is concluded that the new methods are suitable for the study of the long-term strength of rock. The effect of pore pressure on the long-term strength of rock in triaxial creep tests is also discussed.

A NONLINEAR VISCOELASTIC-PLASTIC RHEOLOGICAL MODEL FOR ROCKS BASED ON FRACTIONAL DERIVATIVE THEORY

The soft-matter element between the ideal solid and the ideal liquid is established and is described based on the definition of the fractional derivatives. By replacing a component in the generalized Kelvin model with the soft-matter component and connecting it in series with a nonlinear visco-plastic body, a nonlinear viscoelasto-plastic rheological model is proposed based on the fractional derivatives in order to describe the rheological behaviors of rocks. The data obtained from the triaxial creep tests of a typical rock are simulated with this model and the fitting result is good. The model can describe well three rheological stages of the rock during the triaxial creep tests. The validity of this model is then discussed. In this model, the fractional order β controls creep strain rate in the stable creep stage under the condition of low stress; while the creep index n controls creep rate of the accelerated rheological stage under the condition of high stress. Few parameters and good simulation results manifest the outstanding performance of the model. The model also adopts the damage theory to describe the progressive deterioration of rock viscous coefficient of the accelerated creep stage. The model can also give an excellent description of the three rheological stages of rocks, especially the accelerated creep stage.

Triaxial Creep Experiment of Red Sandstone

Creep is an important mechanical property of rock, and it is closely associated with the long-term stability of rock landslides. To further discover the sliding mechanism of the horizontal-strata landslides, red sandstone samples were taken from Erchengyan horizontal-strata landslide bedrock in Wanzhou, and triaxial creep tests by step loading technique were conducted on red sandstone with RLM-2000 triaxial rheology test machine. The triaxial creep tests lasted 1180h under the constant temperature of 18.5±0.5. Boltzmann superposition was adopted for test data processing, and an improved translational coordinate method was used to reduce the degree of the data distortion. With the improved method, a complete creep curve of red sandstone strikingly consisted of three stages, namely, attenuated creep stage, steady creep stage and accelerated creep stage. The complete creep curve of red sandstone can be fitted by Burgers model.