Hongming Tian

Investigating the Permeability of Marble under Moderate Pressure and Temperature

The permeability of intact marble samples collected from the depth of 1.6 km in southwestern China is investigated under moderate confining pressures and temperatures. No microcracks initiate or propagate during the tests, and the variation of permeability is due to the change of aperture of microcracks. Test results show a considerable decrease of permeability along with confining pressure increase from 10 to 30 MPa and temperature increase from 15 to 40°C. The thermal effect on the permeability is notable in comparison with the influence of the stress. A simple permeability evolution law is developed to correlate the permeability and the porosity in the compressive regime based on the microphysical geometric linkage model. Using this law, the permeability in the compressive regime for crystalline rock can be predicted from the volumetric strain curve of mechanical tests.

Estimation of Elastic Moduli of Non-persistent Fractured Rock Masses

Abbreviations E0 Young’s modulus of intact rock G0 Shear modulus of intact rock m0 Poisson’s ratio of intact rock Kn Fracture normal stiffness Ks Fracture shear stiffness E Effective elastic modulus of fractured rock mass E\i (Eki) Effective elastic modulus of fractured rock mass in the normal (shear) direction of the ith fracture sets G Effective shear modulus of fractured rock mass m Effective Poisson’s ratio of fractured rock mass a Half-length of fracture r The distance from fracture center along its length q Fracture density, defined as the number of fracture central points per square meter h Angle between loading direction and normal direction of fracture plane u Normal displacement jump of fracture faces u Average normal displacement jump of fracture faces V Shear displacement jump of fracture faces rn Resolved normal stress on fracture from far-field stress ss Resolved shear stress on fracture from far-field stress rk Tension stress of fractures rp Normal stress acting on fracture faces rp Averaged normal stress acting on fracture faces S Far-field shear stress DR The total excess elastic strain energy due to the existence of fractures DRfn The elastic strain energy stored in fracture due to normal stress DRrn The excess elastic strain energy stored in rock due to normal stress DRrs The elastic strain energy stored in rock due to shear stress DRfs The excess elastic strain energy stored in fracture due to shear stress A * D Parameters of excess strain energy terms for non-uniform deformation mode A D Parameters of excess strain energy terms for uniform deformation mode rn1rk1 The subscript 1 denotes the ultimate quantities in integrals

Application of the orthogonal design method in geotechnical parameter back analysis for underground structures

The back analysis method has been widely used as an indirect method of determining geotechnical parameters based on field measurements. The number of parameters and their initial values greatly influence the reliability and efficiency of back analysis. Therefore, sensitivity analysis is often employed to select high sensitivity parameters that have more greater impact on measured back analysis values. The orthogonal design method was first utilized to select geotechnical parameters for back analysis. The optimized parameter values obtained from an orthogonal design table can be used as the initial back analysis values, so as to avoid optimisation algorithm searching in local parameter spaces. By introducing a penalty function to the objective function, back analysis of the geotechnical parameters is changed into an unconstrained optimisation problem, whereby the Nelder–Mead method can then be employed. To verify the feasibility of the proposed back analysis method, a case study was conducted to determine the rock mass parameters for the Houziyan underground powerhouse complex.

Numerical evaluation of a yielding tunnel lining support system used in limiting large deformation in squeezing rock

Large deformation in squeezing soft rock is a significant challenge that complicates the safety of underground construction engineering. A yielding tunnel support system allows a certain amount of over-excavation, thereby accommodating large deformation in severely squeezing rock. In this study, a special yielding support system has been developed with a type of newly developed foamed concrete material which has a cushion effect. The special yielding support uses pre-cast foamed concrete blocks which are mounted in the primary lining, and an in situ cast foamed concrete layer which is placed between the primary lining and the secondary lining. The effect of the special yielding support on squeezing rock tunnels has been validated by comparing the numerical results with those of a lining-strengthened stiff support system. The incorporation of the foamed concrete blocks can both reduce the maximum and minimum principal stress in the primary lining. Relative to the stiff support, the maximum and minimum principal stress in the primary lining are about 50 and 60% of those of the stiff support, respectively, thereby improving the stress state of the primary lining. Further, compared with that in the stiff support, the plastic zone in the secondary lining in this yielding support is significantly improved, and the deformations at the roof and the sides of the secondary lining are 40 and 46% less than that of the stiff support, respectively, resulting in a better stress state and less deformation in the secondary lining.