Research On Softening Mechanism And Model Of Gypsum Surrounding Rock In Li Rang Tunnel

The LiRang tunnel of Liang Zhong highway in ChongQing gets through the layer of gypsum, and the length of tunnel is 300 m. The gypsum rock is softened easily by water, and poses a serious threat to security in the tunnel construction. For the disease characteristics of this project, in this paper, a series of softening experimental study of gypsum rock have been carried out, and in order to solve the practical engineering problems, the corresponding theoretical model and application model have been established. In this thesis, the following several aspects of research work have been done:①Current researches about gypsum rock and similar soft rock have been analysed and summaried, prepared main contents and technical route of this article; LiRang tunnel engineering geological characteristics were analyzed, especially for the hydrogeology condition.②The mechanical properties of gypsum surrounding rock of tunnel were studied including uniaxial tests, triaxial tests and shear tests, the gypsum rock uniaxial compressive strength, elastic modulus, shear strength, poisson’s ratio and other basic mechanical parameters have been obtained.③According to a series of softening test of gypsum rock, this paper gets the gypsum rock mechanics parameters variation with immersion time, and fitting out of the equation corresponding to soften, gypsum rock uniaxial compressive strength, shear strength, elasticity modulus of immersion time was a negative exponential trend decline, while gypsum rock Poisson ratio under certain conditions increases with immersion time; the mechanics on the basis of injury, damage evolution deduced based on the time factor softening gypsum rock equation, reveal coefficient and soften brittle relationship between the variables of damage, establishes plaster rock softened depth model in different freshwater immersion time. Analysis of mercury extrusion experimental data and found gypsum rock porosity increases with increasing immersion time, the macro performance of gypsum rock pore compaction stage is growth in uniaxial compression, increasing the amount of deformation; with the growth of immersion time, the gypsum rock properties transfer from brittle to ductile.④By analyzing the orthogonal softening test results, the influence of factors of soaking time,temperature,and SO42- concentration on the gypsum rock softened has been obtained, and the weighting factors have been calculated by AHP, the results show that time and temperature play a decisive role in the gypsum rock softening process, the sum of the influence weight of soaking time and temperature greater than 97%, the influence weight of soaking time is the largest, more than 80%, followed by temperature, and the influence of SO42- concentration is minimum, which is almost zero.⑤In consideration of the gypsum rock softening properties, under the premise of have gotten elastic modulus and Poisson’s ratio changes with immersion time, based on the elastic solution to axisymmetric circular tunnel, this paper established a surrounding rock self stabilizing time evaluation model. The model parameters is analyzed, in the same span, self stabilizing time decreases with the increase of the tunnel buried depth, increases with the augment of integrity coefficient, and decreases with groundwater coefficient increased, the surrounding rock elastic modulus more larger and the self stabilizing time more longer, the above results verified the scientific rationality of the model. And for the model to simplify the issue of distribution of groundwater were discussed, we deduced the value considering groundwater distribution formula elastic modulus and Poisson’s ratio. Based on field data and softening test results, using the model to evaluate the surrounding rock self stabilizing time of gypsum segment of LiRang tunnel.