Study On Mechanical Characteristics Of Conditioned Gravel Soil By Shield Screw Conveyor

At present,the earth pressure balance shield machine has become the main equipment for tunnel excavation in soft soil.This kind of shield machine uses the muck in the pressured soil bin after excavation as a medium to support the water and earth pressure on the excavation surface.A certain amount of muck in the soil bin is discharged to the outside through a screw conveyor to achieve the purpose of regulating the pressure of the head chamber.During the tunnel excavation process,the screw conveyor balances the muck into the soil bin and the muck from the soil bin to establish the balance of the earth pressure on the face of the construction.The effective control of the tunnel excavation process and the performance of the earth pressure balance shield are mainly determined by the mechanical properties of the excavated muck.In the engineering,it is usually necessary to inject bentonite mud,foaming agent solution,high molecular polymer and other conditioners(such as soapy water,lime,etc.)to condition the muck to change the properties of the excavated soil,thereby forming plastic fluidy muck,wherein bentonite mud,foam solution and polymer materials are the three main conditioners.Effective soil conditioning greatly improves machine performance and improves controllability of flow through the screw conveyor.Although the method of soil conditioning is usually used in engineering practice,due to the low fluidity,high permeability and large internal friction angle of gravel muck,at present,the engineering community does not fully understand the rational additives for the gravel layer soil conditioning,and often encounters problems related to the operation of the earth pressure balance shield machine related to the nature of the muck.In view of this,this paper takes the laboratory test as the starting point,and combines numerical simulation technology,theoretical analysis derivation and model machine test,and carries out related research work.Firstly,the laboratory test of the soil conditioning agent and the gravel soil conditioning was introduced.On the basis of this,the mechanical properties of the shield screw conveyor with the conditioned gravel soil were tested.The main research contents and conclusions are as follows:1.The laboratory test of the parametric ratio of three kinds of soil conditioning agents,namely bentonite mud,foaming agent solution and high molecular polymer,was carried out,and the conditioning test of gravel muck was carried out.The foaming rate and half-life test of the foaming agent solution showed that when the foaming agent concentration was 7%and the foaming pressure was 0.4 MPa,the foaming effect was the best.When the mass ratio of soil to water of sodium bentonite is 1:5,the optimum puffing effect can be obtained at a puffing time of 21 to 24 hours,and the viscosity value at this time is 600 mPa*s.The polymer polymer HPMC concentration is 1%,and the viscosity is about 5000 mPa*s when the dissolution time is about 1 h.The polymer material CMC achieves a high viscosity at a concentration of 1.5%,7800 mPa.s.2.Considering the stress environment of the head chamber where the muck is located in the actual project,a test device capable of compressing and shearing the muck at the same time is designed.The mechanical properties of the muck after adding the soil additives are studied.The test results show that the compressibility of foam-conditioned sand is higher than that of bentonite mud and polymer materials.The sand conditioned by bentonite mud and polymer material has similar compression characteristics.The foaming agent solution with a concentration of 7%has a higher compressibility when the blending ratio is 40%;the shear rate has a significant effect on the shear strength of the sand.The shear rate is higher in the speed range set by the test,the shear strength value is larger and the growth is divided into two stages.When the shear speed is less than 28mm/s,the shear strength increases slowly or is basically flat.When the shear rate is greater than 28 mm/s,the shear strength increases rapidly and linearly.3.According to the shield soil bin and screw conveyor used in the 15th bid of Beijing Metro Line 14,the Solid Works software is used for solid modeling,and the EDEM software of the granular media flow method is used to establish the parameters after laboratory test and numerical test.The calculation model of the screw unloading in the empty state of the soil bin,the calculation model of the screwing machine under the basic steady state of the soil in the soil bin,the calculation model of the screw machine in the ascending/depressing state of the soil bin,and the calculation model of the screw machine when the conveyor is installed at different angles are systematically studied.The relationship between the soil discharge rate of the screw machine,the bulkhead pressure and the screw machine torque is studied systematically.The simulation results reveal that the mass flow rate of the screw section increases with the increase of the screw speed when the particle reaches a certain amount(for example,700,000)in the soil bin and the quantity remains basically stable.When the number of particles in the soil bin is reduced to a certain extent(for example,100,000),the use of a higher screw speed does not make much sense for improving the "dumping effect";In the case of emptying of the soil bin,the torque of the screw machine decreases with the increase of the calculation time,and the negative exponential function can be used to fit the drop.In the case where the particles in the soil bin reach a certain amount and the number is kept stable as the screw machine rotates,the screw machine torque decreases as the screw machine speed increases.Except for the screw speed of 360deg/s,the torque of the screw machine decreases with the increase of the rotational speed.There is a good linear correlation between the installation angle of the screw and its torque.With the increase of the installation angle of the screw machine,the torque of the screw machine increases linearly;in the process of conveying the bulk medium by the shield screw conveyor,although various working performance parameters of the screw machine,such as torque,cross-section mass flow,screw casing pressure and the bulkhead pressure shows a certain "discrete" and "fluctuating" characteristics,but it can still be well described by a continuous function.4.Based on the mechanical environment and stress state of the conditioned muck in the earth pressure balance shield screw conveyor,the force and motion of the soil micro-body are analyzed,and the earth pressure is derived according to the steady state momentum moment equilibrium condition.The equivalent spiral shear stress calculation model of the balanced shield screw conveyor is analyzed.It is concluded that the driving torque of the screw conveyor is closely related to the shear stress generated at the interface between the conveyor shell and the conditioned soil.5.According to the mechanical structure of the screw conveyor,the composition of the driving torque is discussed.The formula of the torque of each component is derived through the mechanical analysis,and the theoretical model of the driving torque of the screw conveyor is obtained.Secondly,under the condition that the filling rate of conditioning soil in the spiral tube is 100%,the calculation formula of the pressure field gradient distribution of the screw conveyor in the steady state is given by the force analysis of the soil micro-body.The pressure gradient and the screw driving torque are related to the spiral geometry and the rotational speed,the soil flow transporting angle and the mechanical properties of the conditioned soil.6.The indoor experimental study on the mechanical properties of the gravel soil after adding the conditioning agent was carried out by using the self-developed shield model screw conveyor.The test results show that the pore water pressure is less affected by the rotation speed,and the pore water pressure can be considered constant when the pressure is constant.The volumetric transporting rate of muck is an important index to evaluate the capacity of the dumping soil of the screw conveyor.The test results show that the volumetric transporting rate increases with the increase of the rotation speed,and the stability of the solid discharge rate at high rotational speed is lower.The situation is not good.The efficiency of dumping soil at different speeds indicates that the dumping efficiency of the screw conveyor is related to the rotational speed of the motor.When the speed becomes faster,the efficiency decreases.When the speed becomes slower,the efficiency increases again.As the effective stress of the average normal stress of the contact surface between the particles,the value directly affects the shear strength of the conditioned soil.The test results of the indoor model machine show that the change of the motor speed has little effect on the effective stress,and the linearity of the effective stress at large speed is obviously better than the corresponding result at the low speed.