Study On Permeability And Physical Mechanics Characteristics Of Lanzhou Special Red Sandstone

Red sandstone is one of widely distributed rocks in China.The particle composition,mineral composition,chemical composition and micro structure of red sandstone vary from region to region,their physical and mechanical properties and engineering mechanical properties are also different.In actual projects,red sandstone are easily damaged due to their poor water chemistry such as softness,swelling,and disintegration.It is generally considered that the red sandstone formation is not permeable in existing theoretical research and experience,but during the excavation of the foundation pit at the Lanzhou subway station,water inrush occurs in the red sandstone formation within the depth of the foundation pit,resulting in serious water accumulation in the pit.It causes serious harm to the safety of the foundation pit and has also brought great difficulties to the construction of the project.Therefore,it is urgent to conduct in-depth study on the physical and mechanical properties,permeability characteristics of red sandstone in Lanzhou,and then provide a basis guidance for the study of drainage and lowering methods for foundation pits in this area.This paper relies on the deep foundation pit engineering of Oriental Red Metro Station of Line 1 of the Urban Rail Transit of Lanzhou.Based on the study of the physical and mechanical properties of red sandstone,the permeability of red sandstone and its influencing factors were stud ied and the material composition of red sandstone,the causes of water softening disintegration were analyzed by a series of physical and mechanical tests,osmosis tests,on-site pumping tests and in-situ seepage tests.The physical and mechanical properti es and permeability characteristics of red sandstone are studied comprehensively by numerically simulating the seepage field of red sandstone formation with ADINA software,and comparing it with the experimental results.The specific research content and a chievements are as follows:1.The permeability coefficient of red sandstone in this station is between 10^-4and 10^-2 by means of indoor osmosis test?cm/s?,on-site pumping test and in-situ osmosis test,and the transverse permeability coefficient is slightly larger than it.The main permeability direction is horizontal and medium permeability in the permeability class.2.The relationship between the permeability coefficient and particle size at the same porosity,the relationship between the permeability coefficient and porosity at the same particle size are studied by the results of indoor penetration test,particle size distribution analysis test,and porosity test.The linear fitting was performed and the fitting results showed that the correlation was good.The direct shear test was performed on the red sandstone samples with different water contents,the relationship between the cohesion and the water content was obtained.The compression deformation modulus,softening coefficient,non-uniform coefficient,curvature coefficient and other experimental data of the red sandstone was obtained from the relevant physical and mechanical tests which provid e the basis for earth pressure,foundation bearing capacity calculation and foundation pit support optimization under the condition of red sandstone formation saturation in subway station.3.The micro structure of red sandstone was observed by electron mi croscope and the cause of the cementation and red sandstone formation permeability was analyzed.The study found that there are more soluble salt particles in the red sandstone,which are distributed in blocks,with poor density,large gaps between the par ticles,and a large amount of adhesive between the particles and the pores,the decrease of cementation after contact with water reduces the strength of the rock mass,resulting in the permeability of the red sandstone.4.Based on the data from the field pumping test,the following conclusions are obtained by studying the variation curves of the depth drop of each well with time,the depth drop and the cumulative depth drop with the pumping volume.It is found that when the pumping capacity of the well is 2.4 m³/h,the effective influence range of pumping water is 1 to 2,osmosis coefficient is 0.251 m/d.According to the results of the study,the following measures are put forwar d to improve the precipitation:for the geological conditions of large amount of underground water in pebble and sandstone layers,precipitation outside the pit is firstly carried out to reduce the difference of water head inside and outside the pit.Precipitation in the pit is secondly carried out.The vacuum pump is used to reduce the water in the pit with a small thickness and a small area,and the construction method of excavation is adopted when the precipitation is carried out.5.The in-situ seepage test model and red sandstone sand dam model were established by finite element software.Numerical simulation analysis was conducted on the seepage problem of the in-situ seepage test to understand the location,head distribution and seepage field distribution of the red sandstone sand dam.The seepage effect of red sandstone sand dams under different water heads was compared and analyzed.