hossein mirmehrabi

 In mechanized tunneling of soft grounds with boulders, several types of cutting tools are used on the cutter head of the machine. The main cutting tools used in this condition are rippers, disks or a combination of them. In the tunnel of the eastern part of the Mashhad urban railway line 3 project, due to fine grain layers the tunnel section, the main tools used were the rippers. The discs were used only in the cutter head gauge. The addition of a gravel layer containing boulders in the tunnel section has reduced the durability of the rippers and increased the specific excavation energy up to 25%. In order to increase durability, NFM designed rippers were replaced with heavy rippers. In addition, various alloys have been investigated by test and error method. Changes in dimensions and alloy increased the durability if the tools up to 37%. By replacing the NFM designed rippers with heavy rippers, the performance of the rippers have increased from 183 meters to 250 meters. The result of tool replacement in the eastern part of Mashhad urban railway line 3 (about 10 km from the route) shows that the consumption of cutting tools in the boulder area was 10 times the area before the collision with the boulder. In general, 280 tools have been replaced in the investigated boulder area (about 1.7 km), which has resulted in saving about 60 billion rials by using heavy rippers instead of NFM rippers.

One of the limitations of mechanized tunnel excavation compared to traditional excavation methods is the creation of tight curve. In addition, excavation problems and limitations are doubled if small radius curves are combined with surface and subsurface obstacles. Typically, there are options available to designers to deal with these obstacles, such as rerouting, proposing remodeling and improvement, and protection plans in the pre-construction phase. According to the initial design of the plan and profile in the western tunnel of the Mashhad urban railway line 3 project, some part of the implementation of the route has faced important challenges, including the synchronization of the 300-meter curve with passing high-rise residential and commercial structures in the region. After studies and investigations, the implementation of the curve with an unusual radius of 270 meters was put on the agenda. Necessary actions have been taken to prevent excavation problems in the tight curve. Some of these actions include the implementation of protection plans for the southern buildings of Ferdowsi Avenue and the Ziba Sazeh building, replacement of brushes and cutting tools, determining the correct position of the key in the ring, special arrangement of inverts and conveyor design that resulted in high quality excavation in the curve. In terms of value engineering, the selection and implementation of a curve with a radius of 270 meters, is equivalent to 20 billion rials in savings.

Excessive and unpredicted tool wear is one of the major challenges in mechanized excavation in tunnel projects.This problem, due to more demand for maintenance, increases excavation time in addition to cost overruns. For obtaining of a proper view about relation between performance parameters of the TBM and abrasivity, a new laboratory test machine is built in Engineering Geological Laboratory in Ferdowsi University of Mashhad, Iran. In this paper the results that achieved from performing tests on silica samples by the laboratory machine is presented. For this purpose 36 tests with various amount of surcharge and rotation speed at different times on coarse silica sand carried out. The obtained results have shown direct relation between test duration, surcharge and rotation speed versus weight loss. For example a direct linear relation between the tools wear and surcharge and/or rotation speed was observed. Indeed a logarithmic relation was gained for the time effect.

Encountering hydrogen sulfide gas (H2S) during excavation is one of the important engineering, geological and environmental hazards during the tunnelling. Tackling this hazard and solving its challenges solving are very difficult and costly. During investigation of the situation, one of the main tasks is prediction or evaluation of the risk of H2S gas and selection of the best methods to tackle its engineering and environmental problems. In this study, water conveyance tunnel of Aspar excavated in H2S bearing environment is discussed. This tunnel is excavated in the hydrocarbon formations. In this paper, hazards, characteristics, safety regulations, and geological sources of H2S as well as the methods to decrease the risks and problems in excavation of the tunnel are presented in brief.

In order to determine the source of gas and to select the best methods to mitigate its hazards and problems, in addition to investigation of the same experiences, concentration of various gases was recorded by fixed stations on the machine and by mobile sensors at the beginning, midpoint and end of each working shift. Moreover, sampling of the polluted air and water was implemented. The samples were sent to a specific laboratory for chemical analysis. At the same time, concentration of the gas in the air and water of the tunnel was measured.

Experiments raleted to the tunnel showed that the gas caused an unacceptable condition for workers. For predicting the risk of H2S gas in underground spaces, it is possible to use some evidences such as sulfur springs, organic traces, organic shales, exposure of H2S odor from fresh surface of rock, and smelling of H2S during boreholes drilling. Results of the analysis show that the gas enters the tunnel along with water, dominantly. Also considerable amount of the gas is released to the air at the beginning. According to the investigations, the source of the gas is relevant to hydrocarbon formations inthe area.

Geological formations related to hydrocarbon resources are very important in the formation and reservation of H2S gas. Since the gas is in solution form and is emitted promptly, controlling the inflow of groundwater into the excavation, diluting the concentration of H2S and training the workers are a series of methods used to decrease the risks and problems associated with tunnel excavation in an H2S-bearing environment.