This paper is a study on the mixing technology for improving soil conditioning in a Shield TBM chamber. The mixing efficiency according to the arrangement of the mixing bar in the chamber was evaluated. To evaluate, a scaled model experiment was conducted. The scaled model experiment was conducted in various cases.
First, the effect of gravity was tested. The parameters were the cutter head rotational speed per minute, the size and shape of the mixing bar. When the gravity of the chamber is in the horizontal direction and in the vertical direction, the sizes of RPM 5 and 10 mixing bars were performed for 10 mm, 15 mm, and 20 mm.
The sample composition was made by giving a constant viscosity of 4mm (black), 6mm (white), 8mm red, 10mm (blue) of a plastic material.
As a result of the experiment, it is found that the mixing condition is more efficient when the mixing axis in the chamber is perpendicular to gravitational direction, that is, in the currently operating form. In addition, when the mixing bar is the square shape section with the larger size and the faster RPM, the mixing efficiency is increased in general. However, it is clearly indicated that the mixing efficiency is decreased when RPM is high enough to destroy the shear resistance of soil in chamber. In particular, mixing torque is affected by size rather than shape at low RPM.
Secondly, regarding to the effect of the mixing bar arrangement, the experimental model tests are carried out according the different mixing bar position arrangements and soil particle size in chamber. In this experiments, the square shape of the mixing bar shown high mixing efficiency in the first experimental study is applied. First, the range of influence according to the speed of the square mixing bar was identified. As a result, it is found that there is no difference in the range of influence according to the speed. Based on the results, the mixing bar size is selected as 15mm, which minimizes the overlap of the influence range and mixes all ranges. Various experimental cases are carried out in batches applied to actual equipment (Case 1, 1-1), batches in a row (Case 2. 2-1), batches at 120° intervals (Case 3, 3-1), and batches that are operated individually (Case 4, 4-1). The sample composition is tested under conditions of the same particle size (6mm) (good conditions) and different conditions (poor conditions) of particle sizes (6mm, 8mm, 10mm).
As a result of the experiments, it is concluded that the improved batches, Cases 4 and 4-1, increased the mixing efficiency under all conditions.
When the particle size is the same, the mixing efficiency increases by about 47% compared to Case 1 applied to the actual equipment. If the particle size is different, the mixing efficiency increases by about 10% compared to Case 1-1 applied to the actual equipment.
Through this study, it is concluded that the improved mixing bar increased the overall mixing efficiency. If the improved mixing bar is used, when the Shield TBM is actually operated, the mixing efficiency is increased, and it is uniformly discharge through the screw conveyor. It will contribute to the stability by contributing to maintaining the chamber pressure with a constant discharge.