Analytical solutions of stress and displacement in strain softening rock mass around a newly formed cavity

The closed form solutions of the stress and displacement in strain softening rock mass around a newly formed cavity are derived with a three step-wise elasto-plastic model. Hoek-Brown criterion is adopted as the yielding criterion of rock mass. Damage factors are proposed to account for degradation of the material parameters to reflect the degree of strain softening. The surrounding rock mass around the cavity is divided into three regions: elastic region, strain softening region and residual state region. The analytical solutions of stress, strain, displacement and radius of each region are obtained. The effects of the strain softening and shear dilatancy behavior on the results are investigated with parametric studies. The results show that the radii of the residual state region and strain softening region in the surrounding rock mass with higher damage degree are larger. The radii of the residual state region and strain softening region are 1-2 times and 1.5-3 times of the cavity radius, respectively. The radial and tangential stresses decrease with the increase of the damage factor. The displacement of the cavity wall for the case with maximum plastic bulk strain is nearly twice than that with no dilation. Rock mass moves more toward the center for the case with larger damage factor and shear dilation. The area of the plastic region is larger when the damage factors are considered. The displacements in the surrounding rock mass increase with the increase of the damage factors and shear dilation factors. The solutions can be applied to the stability analysis and support design of the underground excavation.

Research on novel pressurized fiber Bragg grating static level based on circular structure

The static levelling system has significant value in the settling measurement of the structures.A novel pressurized static level based on circular structure is designed by using fiber Bragg grating(FBG)sensing technology in this research.The variation of the level pressure in the connecting vessel can be transmitted through the diaphragm to the FBG of the circular ring.The Ritz method is used to calculate the lateral deformation of the circular ring under the concentrated force,constructing the theoretical sensitivity model of the sensor.Further,finite element simulation and static experiments are applied to modify and verify it.According to the experimental data,two FBGs arranged horizontally along the circular ring,multiplying the sensitivity of the FBG static level to about 4.75 pm/mm,while the resolution can reach 0.02 mm.In addition,the temperature compensation of the sensor can be realized by the synergistic change of the measurement point and the reference point within the system of connecting vessel.