Study on the Mechanical Characteristics of the Screw Driving Torque during the Emptying of the Shield Soil Bin

This article studies the changing law of the driving torque of the screw conveyor during the emptying process of the earth pressure balance shield chamber. First, the discrete element method of discrete medium theory and 3D software SolidWorks were used to create the research object and the screw conveyor model, and then the model parameters were determined and calibrated through numerical calculations and indoor experiments. The final numerical calculation results show that: 1) the screw torque will drop in waves with the increase of the calculation time. When the screw conveyor rotates at 360 deg/s, the calculated screw torque fluctuation amplitude is small; 2) when the number of particles in the soil bin is reduced to a certain extent, the use of a higher screw speed to improve the “dumping soil effect” is of little significance; 3) the negative exponential function can be used to better fit the decrease of screw torque with time; 4) for the bulk medium, for a given particle size and screw structure, there is a suitable speed, so that the effect of “machine-soil collision” is small, and the torque change of the screw is relatively stable.

Clogging Potential of Earth-Pressure Balance Shield Driven Tunnels

Background: Nowadays, the construction of urban tunnels for rapid transportation in metropolises is necessary. Since these tunnels are excavated at low depths, they are often associated with different problems and hazards. Some of them can reduce the efficiency of the tunnel boring machines and sometimes will stop the project. Among these problems the clogging can cause problems at the cutter head, in the chamber, and in other sections where the material transference occurs. Objective: The main purpose of this paper is to evaluate and determine the risk of clogging in the tunneling boring machine in Line 6 of the Tehran Metro. It includes stations: Amirkabir, Shohada Square, Emam Hossein Square and Sayyadeh Shirazi. This phenomenon induces an adhesion of the shield with the soil, increasing the necessary shear forces and it can eventually leads to the project interruption. Methods: Due to the fact that the criterion for the behavior of fine soils against moisture is Atterberg Limits, therefore, Atterberg Limits and the water content were utilized. For this purpose, the new method proposed by Hollman and Thewes (2013) was used. In this study, in addition to the Atterberg limits, the amount of free water resulting from the machine and from the underground water inflow was included in the calculations. Results: It was found that the water content should be increased carefully as the soil is very sensitive to this parameter. An increase of 15% of the water content permits to reduce the risk of clogging. If the added free water amount 15%, the probability of clogging becomes high. Whereas, in case where the added free water amount reaches 20%, the risk of clogging decreases significantly. Conclusion: According to the performed assessments, it was found that critical areas for the clogging aspect are both the cutter head and the chamber. The sensitivity of the soil is very important to the free water amount. Therefore, due to the behavior of sticky and plastic of clay soils against increasing water, it is necessary to determine the percentage of allowable water used in mechanized excavation projects.

Comparison of predicted and actual states in tunnels: lessons to be learned from the Gerede Water Transmission Tunnel, Turkey

The Gerede Tunnel is a large inter-basin project in Turkey. As with many long tunnels, some of the predictions in the project report and the tender contract have not been realized. Completion of the tunnel has been delayed for more than twice the time given in the tender contract. Unexpectedly complex geological–tectonic–hydrogeological conditions and the selection of tunnel-boring machines (TBMs) are the main reasons for this long delay. There were some discrepancies between the predicted and actual geology. The groundwater pressure encountered was much higher than expected. Plastic clays in the limestone and breccia and loose agglomerates with a high water pressure in the faults decreased the advance rate and led to several long stoppages. One of the three double-shield TBMs became stuck in the tunnel and could not be recovered. The last quarter of the tunnel was excavated more slowly, but without severe problems, by a modified earth pressure balance shield TBM. As a result, the TBM type, drive direction, length, route, cost and completion date of the Gerede Tunnel have all changed.