Review on the Construction Stability of Shield Method Employed in Composite Strata
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The difficulties in the design of water-conveying shield tunnels are discussed,with a water-conveying shield tunnel in Nanchang as an example.The loading of the water-conveying shield tunnel is analyzed and the design of the tunnel is described.Main study contents and conclusions drawn are as follows: 1) The design of the alignment,longitudinal profile,segments and shield receiving shaft of the tunnel are studied on basis of the geological conditions.2) The major difference among water-conveying shield tunnels,highway shield tunnels and railway shield tunnels is that water-conveying shield tunnels have internal water pressure.The difficulty in the design of water-conveying shield tunnels is how to determine the internal water pressure.3) The maximum ground water level,the minimum ground water level,the usual internal water pressure and the unusual internal water pressure need to be considered in the design of water-conveying shield tunnels.In this way,the internal force envelope diagram of the tunnel is achieved and the steel bar arrangement is worked out,so as to achieve safe and economic design.4) The water-conveying shield tunnel crossing Ganjiang river in Nanchang is one of the few water-conveying shield tunnels located in rocks built in China,which provides a solution for the construction of small-diameter water-conveying tunnels in the future.
Water pressure
Water tunnel
Bar (unit)
Envelope (radar)
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The kinematic shield model had been proposed in order to simulate the shield behaviour during excavation based on the equilibrium conditions, taking into account ground displacement around the shield. Based on the proposed model, the simulation of the slurry shield behaviour during excavation was carried out using the actual shield operational control data, of which were obtained at the curve alignment in the multilayer of the ground. Furthermore, the analysis results were compared with the observations in order to validate the model performance. As a result, it was found that the model simulates the observed shield behaviour reasonably well. The result also indicates that the excavated area including the area generated by the copy cutter is a predominant factor affecting shield behaviour during shield tunnel excavation.
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Line (geometry)
Lateral earth pressure
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As an important component of shield tunnel,segments have a significant impact on the construction cost and safety of the tunnel structure.At present,Metro shield tunnel segment whose diameter reaches 6 m,is relatively rare in China.This article briefly describes the structural parameters of this kind of segment such as thickness,width,block,etc.Its calculation model and calculation method are also described.The article takes the shield tunnel on Shenzhen Metro Line 11 as an example and makes a numerical simulation analysis and research.The result shows that the force behavior of large-diameter shield tunnel segment is similar to the common diameter shield tunnel segment.And the structural reinforcement is controlled mainly by the crack.The reinforcement of some special parts of the segment should be strengthened so as to prevent the segment crack.
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The authors describe the purposes of the shields (conductor shield, insulation shield, and metal shield) in medium-voltage cables. Shield losses, shield grounding, and shield terminations are also discussed. It is noted that, in essence, the conductor shield is an electrical extension of the conductor and the insulation shield is an electrical extension of the outer metal shield; together they ensure that all the voltage is insulated from the outer surface of the conductor shield to the inner surface of the insulation shield (i.e. within the insulation).< >
Shields
Shielded cable
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Introduction: The surface of Venus displays > 550 clusters of small (< 20 km basal diameter) shield volcanoes. Aubele [2] proposed the existence of 2 morphological classes of these Venusian shield clusters: shield fields (Fig. 1) and shield plains (Fig. 2). Shield fields consist of small shield volcanoes that are contained within a 10 3 – 150 2 km 2 region, whereas shield plains are shield volcanoes distributed over 10 3
Shield volcano
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To study the influence of shield excavation on the surrounding soil, and to control the influence of the shield excavation of the subway tunnel on the surface settlement and the deformation of the surrounding buildings, it is necessary to study the additional stress of the surrounding soil caused by the tunnel shield excavation and the law of surface settlement. Combined with a shield construction project of a subway tunnel, this article uses the Mindlin solution in elastic mechanics to obtain three-dimensional calculation formulas for the additional thrust on the shield cutter head surface and the additional stress and displacement field caused by the friction force between the shield shell and the soil contact interface on the surrounding soil. The Loganathan ground movement pattern is used to determine the soil movement mode. The source–sink method is used to study the formula for calculating the three-dimensional additional stress of the soil around the shield tunnel caused by ground loss. The results show that when the shield is digging, the ground goes through five stages, namely the uplift of the ground before the arrival of the shield cutter head, surface settlement when the shield reaches the surface subsides when the shield tail of the shield passes through, the passage of the shield tail of the shield, surface settlement when the lining is separated from the shield, and consolidation and settlement after the shield passes through.
Settlement (finance)
Consolidation
Stress field
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Abstract : A parameter study was conducted for a space nuclear reactor radiation shield. The focus of this research was to explore alternatives to current radiation shield designs to reduce the mass while maintaining the same shielding performance. MCNP4C was used to determine the parameters necessary to build an optimum shield. A design known as the split scatter shield offered some potential for reductions in shield mass In theory, less material is required for this type of shield, which uses thin shield sections to scatter radiation away from the dose plane. The parameters for this shield design are the shield geometry, number of shield sections, and material selection. Split scatter shielding offers a potential for reducing the shield mass by allowing the gamma shield material to be moved closer to the source plane. Further research needs to be conducted on this shielding technique, however, to isolate optimum shield values. Once these optima have been identified, a split shield can be developed and compared to the original shield performance. Finally, an energy deposition study indicates that the split scatter shield will absorb less energy than the unit shield, implying that there may be less thermal stress on a scatter shield.
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Abstract Due to ongoing development of shield tunnelling technologies, usage of more complicated tunnel alignments is increasing. Therefore, more proper steering of shield machines is expected. Shield behaviour is controlled mainly by copy cutter, articulation mechanism, and jack force, but these three shield steering parameters are not easy to be determined, because of their high co-linearity and varying geological conditions. A discharging tunnel with a sharp curve was planned by an articulated EPB type shield. Therefore, as a preliminary analysis, the required shield steering parameters for the planned tunnel were calculated by shield behaviour simulation, and the obtained shield steering parameters were compared with the shield equipped capacity. Furthermore, as a post-analysis, the shield behaviour simulation was carried out by using the site measured data, and the input parameters at the preliminary analysis were examined. Through this research, the followings were found: 1) in the preliminary analysis, the shield equipped capacity is enough to construct the tunnel; 2) in the post-analysis, it was required to increase the face resistance parameter and to decrease the effective rates of overcut and copy cutter length, to match the analysis shield behaviour to the measured one; and 3) the shield kinematic model can simulate shield behaviour reasonably.
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Shields
Compressed air
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