A case study on severe damage at a tunnel in serpentinite rock mass

Face squeezing, floor heave, and buckling of invert were found after a 10-day holiday in 2014 around the serpentinite face of a tunnel in Hokkaido, Japan. The damage continued for over 2 months, extending 400 m toward the entrance causing a massive roof fall. The tunnel was excavated again with a circular section and extra-thick shotcrete, and the face had crossed the damaged part 6 years after the damage occurred. Uniaxial and triaxial compression tests were carried out to obtain the mechanical properties of the serpentinite to clarify the severe damage mechanism at the tunnel. The main experimental findings are as follows. The uniaxial compressive strength of the serpentinite samples was very low, and the ratio of the strength to the estimated overburden pressure was extremely low. The parameter n indicated that the time-dependent deformation of the serpentinite was not large but the same as ordinary rocks. All specimens showed strain-hardening in the triaxial compression test, and the friction angle was very low by the brucite content. Only primary creep was observed in the multistage triaxial creep test. The pressure on the shotcrete from rock mass for the damaged tunnel was enough to cause creep deformation and failure of shotcrete. From the above findings, designing the concrete lining that can support the earth and water pressure is recommended for tunnel excavation in such a weak serpentinite rock mass, particularly with a very low friction angle by brucite. Highlights Face squeezing, floor heave, and buckling of invert were found after a 10-day holiday around the serpentinite face in Hokkaido, Japan. The time-dependent deformation of the serpentinite was not large but the same as ordinary rocks. All specimens showed strain-hardening in the triaxial compression test, and the friction angle was very low by the brucite content. Only primary creep was observed in the multistage triaxial creep test. The severe damage to the tunnel was not a brittle creep failure of the serpentinite rock mass itself but the shotcrete lining.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

Study of Creep Mechanical Properties and A Rheological Model of Sandstone under Disturbance Loads

The stress environments of rock masses are complex. To explore the mechanical properties of sandstone under earthquake or disturbance loads, laboratory triaxial creep tests under different disturbance loads were conducted on sandstone from Fuxin, Liaoning Province, China. Given the disturbance load, a creep deformation pattern for sandstone was analyzed, and the influence of the disturbance load on the mechanical properties of rock was considered. Thus, a constitutive model of rock under creep disturbance load was established. The results show that (1) the creep curve can be divided into four stages: attenuation creep, steady creep, disturbance creep, and acceleration creep; the increment of disturbance creep varies for different disturbance loads and the larger the disturbance load, the larger the disturbance creep deformation; (2) with increasing disturbance loads, the long-term strength, failure time, and elastic modulus of sandstone decreases linearly, while the peak strain increases; and (3) considering the influence of the disturbance load and introducing an acceleration element to modify the Nishihara model, a constitutive model describing the whole deformation process of sandstone under creep disturbance load was established. The accuracy of the model was verified by test data and provides a theoretical basis for rock mass stability analysis.

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.