Assessment of penetration depth of small-diameter EPB-TBM and muck discharge efficiency of screw conveyor

Abstract

Since most tunnels and underground spaces pass through urban areas, the ground excavation method using TBMs is recently preferred due to civil complaints and safety problems caused by noise and vibration during tunnel construction. The millions or tens of millions dollar of expensive TBMs are custom-made according to the excavated ground conditions and are being put into each field. These TBMs can be reused due to economic problems, but most of the reused TBMs have different performance than the specifications at the time of release, so they must be put into the field after the performance assessment is performed. However, due to environmental and economic problems, reuse TBM is being put into the field without performance assessment. In this way, reused TBM put into the field without performance evaluation can cause unexpected downtime in the field with performance lower than the performance at the time of release and may disrupt construction progress. In Korea, the technology to produce TBMs on its own is still insufficient, and many studies on TBM excavation must be conducted. Also, in Korea, bedrock formations are found around 10 m underground, and bedrock formations are also mostly located around 20 m basement, where common areas most in demand in Korea are located. Taking into account these domestic ground conditions and the shortcomings in TBM-related research, in this study, full-scale excavation facility and apparatus for TBM that can verify TBM-related research and development technology including TBM performance evaluation are built. Excavation assessment of full-scale EPB-TBM with 3.54 m diameter is performed for the concrete and mortar samples to replace the rock according to the strength of the samples. Through the above test, the TBM excavation performance is evaluated when the thrust and the cutterhead speed are changed according to the strength of the samples. The penetration depth is almost constant according to the thrust regardless of the cutterhead speed, and it is confirmed that the greater the strength of the sample, the more the thrust has a dominant effect. The larger the penetration depth, the greater the torque of the cutterhead, and through this relationship, penetration depth can be predicted from the torque. The machine factor representing mechanical characteristics of TBM is newly defined and a prediction model composed of machine factor and UCS of rocks to the penetration depth is suggested. In addition, a study on the efficiency of discharging mucks from the rock is conducted through the scale-down model test and numerical analysis by discrete element method in the part of muck treatment. The cause of the difference in the discharge rate of the particles according to the particle characteristics and the mechanical characteristics is analyzed. The closer the particle is to the chip shape and the larger the screw conveyor angle is, the lower the discharge rate is, and when the ratio between the screw diameter and the pitch is close to 1:1 the discharge rate of the particles is the highest considered geometric characteristics of the particles. Through the results of the screw conveyor study, a table and models to predict the discharge efficiency of muck are derived. Lastly, how the discharge efficiency of muck in the rock varies according to the polymer concentration of the polymer solution and what cause it are analyzed. The effect of the use of polymer solution in the rock cannot be overlooked by reducing the wear of the TBM and using the complex system of a pipe and a screw conveyor for discharge of mucks. Therefore, the degree of reduction in the efficiency of discharge of the mucks according to the concentration of the polymer solution is suggested through the study and the decrease in the discharge efficiency is higher with the viscosity of the polymer according to its concentration.