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The fluid seepage in local-saturated zone of subgrade promotes the migration of fine particles in the filler, resulting in the change of pore structure and morphology of the filler and the deformation of solid skeleton, which affects the fluid seepage characteristics. Repeatedly, the muddy interlayer, mud pumping and other diseases are finally formed. Based on the theory of two-phase seepage, the theory of porous media seepage, and the principle of effective stress in porous media, a two-phase fluid-solid coupling mathematical model in local-saturated zone of subgrade considering the effect of fine particles migration is established. The mathematical model is numerically calculated with the software COMSOL Multiphysics○R, the two-phase seepage characteristics and the deformation characteristics of the solid skeleton in local-saturated zone of the subgrade are studied. The research results show that due to the continuous erosion and migration of fine particles in local-saturated zone of the subgrade, the volume fraction of fine particles first increases then decreases and finally becomes stable with the increase of time. And the volume fraction of fine particles for the upper part of the subgrade is larger than that for the lower part of the subgrade. The porosity, the velocity of fluid, the velocity of fine particles, and the permeability show a trend of increasing first and then stabilizing with time; the pore water pressure has no significant changes with time. The vertical displacement increase first and then decrease slightly with the increase of time, and finally tend to be stable. For a filler with a larger initial volume fraction of fine particles, the maximum value of the volume fraction of fine particles caused by fluid seepage is larger, and the time required to reach the maximum value is shorter. It can be concluded that in actual engineering, the volume fraction of fine particles in the subgrade filler should be minimized on the premise that the filler gradation meets the requirements of the specification.

With calculation results comparison of different throttle nozzle diameter value, This article analyses the flow field with throttle nozzle diameter variety of 1.0mm, 2.0mm, 2.4mm, 2.6mm, 2.8mm, 4.0mm, while the other parameters take same value (such as the inlet pressure is 20mpa, the outlet pressure is 10MPa, the temperature is 333k, and the well depth is about 2000m). the results present the influence upon downhole choke flow field including pressure, velocity, temperature distribution. The pressure field composed by minimum on the axis shows the pressure dropping change at the throttle nozzle outlet. Velocity field gives the maximum value of velocity and Mach number, which determines the flow field is subsonic or supersonic flow. With hydrate formation conditions, temperature field formed by minimum on the axis gives the existing judgment of hydrate formation under a certain condition. With the throttle nozzle diameter increasing, the outlet flow maximum velocity increases, and the differential pressure before and after nozzle becomes larger, which means the pressure drop effect is better. The downhole choke design of throttle nozzle needs to consider the combination effects of pressure drop and temperature drop. These parameters provide reference for downhole choke structure design and operating performance improvement.

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