Research On The Performance Of Sand And Gravel Formations Controlled By Grouting With Mixed Admixtures

With the acceleration of urbanization,more and more large and medium-sized cities began to construct metro.Due to the constraints of construction sites and minimizing the impact on urban traffic,Shield tunneling is the main method in Metro Construction.Sand and pebble strata are widely distributed in southwest China,Beijing,Shenyang and other large and medium-sized cities.Shield tunneling is still facing many difficulties for this kind of Sandy and pebble strata.Sand and pebble stratum has weak cohesion,large pore and loose structure.In the process of shield construction,sand and pebble stratum is very susceptible to disturbance and subsidence,which can lead to ground collapse and structural deformation of adjacent buildings,thus causing accidents.In order to ensure the safety of construction,grouting is often used to pre-reinforce the sand pebble strata.At present,cement grouting is widely used in gravel stratum reinforcement in Chengdu area.There are still some problems in traditional grouting technology.Therefore,it is necessary to study the grouting fluid and method suitable for gravel stratum reinforcement in shield construction.At present,cement grouting is widely used in sand pebble strata reinforcement in Chengdu area.There are still some problems in traditional grouting technology.Therefore,it is necessary to study the grouting fluid and method suitable for gravel stratum reinforcement in shield construction.According to the characteristics of sand pebble strata,this paper analyses the common grouting methods and grouting fluid in s sand pebble strata,and studies the improvement of grouting property by grouting ratio and mixed admixtures through laboratory tests.The influence of the internal structure of the grout stone body on the property of the grout is analyzed from the microscopic point of view.Combined with Chengdu Metro Line 18 crossing overpass,the influence of sand pebble strata reinforcement on pile foundation condition is analyzed by numerical simulation,and the above research is verified by engineering.The following conclusions are drawn.(1)Combined with some engineering examples,it is analyzed that the main grouting methods used in shield construction are flower tube grouting,sleeve valve tube grouting and deep hole grouting.The grouting fluid used in shield construction is mainly pure cement grouting and cement-sodium silicate double grouting.Among them,cement-sodium silicate grouting is mostly used in groundwater-rich strata to stop water leakage.(2)In order to improve the property of neat cement grouting fluid,various admixtures were added to the grouting fluid,including common fly ash,bentonite and nano-materials such as nano-SiO2 and silica sol.By analyzing the effects of different admixtures on grouting fluid viscosity,water precipitation rate,calculus rate and compressive strength,a kind of mixed admixture in cement grouting fluid was obtained.(3)The micro-structure of the cement paste was observed and analyzed by scanning electron microscopy and micro-CT scanning.Compared with the pure cement paste,the hardened paste with micro-silica powder has better cementation effect and lower porosity.It can be seen that the filling effect of micro-silica powder is better.(4)The surface deformation of shield tunnels in sand pebble strata is studied by numerical simulation.The area with strong influence is within 1 times the diameter of tunnel axis.The influence of pre-grouting reinforcement on pile foundation deformation during shield tunneling construction is analyzed,and the deformation of pile foundation after reinforcement meets the control standard.(5)Combining with the actual situation of Chengdu Metro Line 18 underpass Tianfu interchange project,the grouting reinforcement scheme is designed,the field monitoring data are analyzed,the ground settlement is effectively controlled after grouting reinforcement of gravel stratum,and the settlement of pile foundation meets the control standard,which proves that the grouting field application effect of laboratory test research is good.