The Theory Of Geomechanics Magnetic Model Test And The Equipment Development

Since the beginning of the20thcentury, both structural engineering andgeological engineering model test have developed widely. Geomechanicsmodel test, as an effective method for studying physical and mechanicalproperties of rock-soil mass, is getting more and more attention. The mainmethods of geomechanics model test are conventional model test, geotechnicalcentrifuge model test and water seepage force model test, etc. Especially, thecentrifuge model test, which simulates gravity by centrifugal force, canincrease the gravity acceleration of the model to n times so that the prototypematerial can be used in the test with some appropriate treatment, and thedifficulty of getting the similar material is reduced. Compared with theconventional model test, the centrifuge model test improves the reliability ofthe results with a qualitative leap.Geomechanics magnetic model test simulates gravitational field bymagnetic force field. By mixing ferromagnetic materials, the similar materialin a specific magnetic field will be subject to magnetic force, which canincrease the material body force to n times of its own gravity. Geomechanicsmagnetic model test was put forward in2004as a new kind of model testmethod and won the support by National Natural Science Foundation (grantNo.50679037) in2007. A small magnetic field generator was developedduring2007and2009, but because it does not take advantage of ferromagneticmaterials to form a closed circuit, the utilization rate of the magnetic field isreduced. Therefore, the magnetic flux density and its gradient are both not highvery small, and the uniformity of the gradient is also not ideal, so it is difficultto enhance the magnetic force of the model. Based on the Natural ScienceFoundation Project (No.51279100) and National Basic Research Program ofChina (973Program2011CB013505), this paper researches on the theory andmagnetic field generator optimization to obtain a magnetic circuit which canmeet the requirements of the magnetic model test and simulates the test processby theoretical analysis and numerical simulation. The main achievements areas follows:1) The basic theory of geomechanics model test according to the basicprinciples of electromagnetic is studied.Magnetic force and gravity are both body force, and in the assumption ofthe existence of magnetic monopoles, they both meet the inverse squarelaw, which demonstrates the feasibility of simulation gravity field with magnetic field.2) The arrangement of coils to construct the uniform gradient magnetic fieldis discussed.Because the magnetic force on the ferromagnetic materials is proportionalto the magnetic flux density gradient, if the magnetic force is uniform, amagnetic field with uniform magnetic flux density gradient is needed. ByBiot-Savart law, several solenoid coil layout modes, which can generate agradient magnetic field, are constructed, and the combination of Helmholtzcoils is the most ideal ones.3) The magnetic circuit design theory and method of the magnetic fieldgenerator are put forward.Theoretically, the magnetic field generated by the coils will be full of thewhole space. The magnetic energy will be stored in the surrounding air, soa high current is needed to maintain the magnetic field intensity. Theintroduction of magnetic circuit can increase the utilization rate ofmagnetic field, and in all of the magnetic circuit structures, the cylindricitymagnetic circuit is the most suitable one to be used in the geologicalmechanics model test as the magnetic field generator.4) The method to determine the average relative magnetic permeability of thesimilar material is put forward.The ferromagnetic materials in the similar material of the geomechanicsmagnetic model test can not only increase the density but also be subjectto the magnetic force. The average relative magnetic permeability is one ofthe key factors that influence the magnetic force of the similar material. Byusing the method of equivalent magnetic resistance, the average relativemagnetic permeability is calculated, which can provide reference for latermodel test.5) The feasibility of the theory and method of geomechanics magnetic modeltest are verified according to the similarity principle and basic theory ofelectromagnetism by numerical simulation.A typical slope with classical solutions is selected in the numericalsimulation. The magnetic field generated by the solenoid and the force fieldof the slope are computed at the same time with a coupling relationship inthe finite element software. The results of the simulation is compared withthe classical solution of the typical slope, which shows that the magneticmodel test can reflect the deformation and the failure characteristics of theslope prototype.6) The application of geomechanics magnetic model test in the practical engineering is studied.Taking the left bank high slope of Jinping I Hydropower Station as anexample and designing the process of geomechanics magnetic model test,the safety factor is calculated by over-loading method and revolving modelmethod and the main factors influencing the slope stability is evaluated,which is consistent with the conventional model test results.At present, there still are some insufficient in the geomechanics magneticmodel test that needs further study. How to simulate some complex conditions,such as the water environment, earthquake, excavation and reinforcement inthe geotechnical engineering; how to improve the uniformity and quantity ofthe magnetic flux density; and how to test the model in a high strengthmagnetic field, these all require constant research and exploration. All thesequestions will be improved during the period of my studying for a doctorate.