Double Shield Research Articles

Optimization investigation on variable-density multi-layer insulation coupled with vapor-cooled shield for liquid hydrogen tank based on sequential quadratic programming

The MLI (multi-layer insulation), commonly used in liquid hydrogen tank, acts to reduce HLF (heat leakage flux) into the tank and slow down tank pressurization. It is usually optimally designed into three or four regions, with the same RS spacing in every region, which is regarded as the discontinuous variable density and called VDMLI (variable-density multi-layer insulation). However, the VCS (vapor-cooled shield) and P-O (para-to-ortho hydrogen conversion) in VDMLI are not considered in the optimal design of VDMLI. Therefore, a kind of nonlinear programming algorithm named SQP (sequential quadratic programming), in which HLF solver of liquid hydrogen tank is embedded, is firstly introduced to obtain the optimized distribution of RS (radiative shield) spacing to minimize HLF of liquid hydrogen storage tank. The HLF solver is a numerical model of VDMLI coupled with VCS and P-O, and SQP can be regarded as the optimization process of searching for an optimal solution in high dimensional space, under inequality constraints. The calculation results show that the best SN (serial number) of RS for SVCS (single vapor-cooled shield) setting to minimize HLF, in three cases of before optimization with P-O, after optimization with P-O, and after optimization without P-O is 16, 11, and 14 respectively. With P-O, the minimum HLF after optimization is reduced by 8.45% compared with the minimum HLF before optimization. When SVCS is set at the 4th RS, P-O leads to the most decreasing range of HLF, which is 28.3%, while the optimization process leads to the most decreasing range of 48.1% under condition that SVCS is set at the 2nd RS and P-O is coupled. Under condition of no P-O, after optimization, the minimum HLF occurs when the SN of RS where DVCS (double vapor-cooled shield) is set is (8,21), and the minimum HLF is reduced by 9.0%, compared to that before optimization with DVCS, and is reduced by 50.43% compared to that after optimization with SVCS.

Segmented RF shield design to minimize eddy currents for low-field Halbach MRI systems

MRI systems have a thin conducting layer placed between the gradient and RF coils, this acts as a shield at the RF-frequency, minimizing noise coupled into the experiment, and decreasing the coupling between the RF and gradient coils. Ideally, this layer should be transparent to the gradient fields to reduce eddy currents. In this work the design of such a shield, specifically for low-field point-of-care Halbach based MRI devices, is discussed. A segmented double layer shield is designed and constructed based on eddy current simulations. Subsequently, the performance of the improved shield is compared to a reference shield by measuring the eddy current decay times as well as using noise measurements. A maximum reduction factor of 2.9 in the eddy current decay time is observed. The segmented shield couples in an equivalent amount of noise when compared to the unsegmented reference shield. Turbo spin echo images of a phantom and the brain of a healthy volunteer show improvements in terms of blurring using the segmented shield.

Deformation characteristics of existing tunnels induced by above-crossing quasi-rectangular shield tunnel

Quasi-rectangular shield (QRS) tunneling, a recent development in tunneling technology, offers the advantage of creating double tunnels with a single excavation. This technology causes significantly less secondary damage to the environment than traditional double shield tunneling. However, the deformation characteristics of existing tunnels caused by the QRS tunnel crossing above them are unknown. This study presents the first application of a QRS tunnel crossing above existing tunnels. It provides a comprehensive overview of the vertical displacement, horizontal displacement, segment convergence, and torsional deformation of existing tunnels when the QRS crosses above and runs parallel to the existing tunnels in the Hangzhou Metro Line 4. The measurement results indicate that the existing tunnels exhibit a maximum vertical displacement of 7.2 mm, maximum horizontal displacement of 2.3 mm, maximum segment convergence of 5.8 mm, and rotation index of up to ±10 × 10-4 at the intersection of the QRS and existing tunnels. All of the measurement results fall within the acceptable range, demonstrating the exceptional efficacy of QRS in controlling the deformations of existing tunnels. The effects of the plane position and distance between the tunnels on the deformation of the existing tunnels is also discussed. The deformation of the existing tunnels is negligible when the distance between the QRS and existing tunnels exceeds 16 m. The maximum vertical displacement, horizontal displacement, segment convergence, and zero-values of the rotation index occur at approximately 5 m in the plane position rather than at the intersection between the QRS and existing tunnels. The primary influence zone where the QRS tunnel crosses above existing tunnels is discussed.

Prediction method of longitudinal surface settlement caused by double shield tunnelling based on deep learning

The deep learning method faces the challenges of small sample data and high dimensional shield operational parameters in predicting the longitudinal surface settlement caused by shield excavation. In this study, various optimization algorithms were compared, and the slime mould algorithm (SMA) was optimally chosen to optimize the hyperparameters of random forest (RF), and SMA-RF was used for dimensionality reduction and feature contribution analysis. A double-input deep neural network (D-DNN) framework was proposed for the prediction of surface settlement, which considers the influence of twin tunnels and effectively increases the high-fidelity data in the database. The results show that SMA performs best among various optimization algorithms; employing features that have a cumulative contribution value exceeding 90% as input can result in high prediction accuracy; there is significant uncertainty in the feature contribution analysis for small sample data; the reduced shield running parameters show a strong nonlinear relationship with surface settlement; compared with S-DNN, D-DNN takes into account the excavation of twin tunnels and expands the database capacity by more than 1.5 times, with an average increase of 27.85% in the R2 and an average decrease of 53.2% in the MAE.

Transient-state modeling and thermodynamic analysis of self-pressurization liquid hydrogen tank considering effect of vacuum multi-layer insulation coupled with vapor-cooled shield

The transient-state model of self-pressurization of liquid hydrogen tank is constructed. It consists a heat and mass transfer model of fluid domain and a heat conduction model of VMLI (vacuum multi-layer insulation) coupled with VCS (vapor-cooled shield). The vapor consumption factor λv, the dormancy extension factor λd and the unit factor λ are defined. λv is the ratio of vapor consumption with the initial hydrogen mass in tank, and λd is the extension of the dormancy with VCS opened relative to that with VCS closed, and λ is the efficiency of VCS shielding heat leakage. The effects of the mass flowrate in VCS, λv, dimensionless position of VCS, opening moment of VCS on λd and λ are investigated. The results show that, when λv and the operating time of VCS are fixed, the best dimensionless position of SVCS (single vapor-cooled shield) and DVCS (double vapor-cooled shield) that maximizes λd is 0.622 and (0.333,0.644) respectively. Under condition that the duration time of VCS and λv are fixed, for SVCS and DVCS, the best opening moment that maximizes λd is observed to be day 23.26 and day 34.84 respectively, and the maximum of λd with DVCS is 29.5 % larger than that with SVCS.

Analysis of existing railway deformation caused by double shield tunnel construction in soil–rock composite stratum

Subway tunnel construction often crosses existing railways, and the soil loss caused by shield construction will inevitably lead to deformation of the surrounding strata and uneven settlement of the adjacent railway tracks, but there are few studies on the existing railway deformation caused by double shield construction in soil–rock composite strata. In this paper, based on the field monitoring data, the deformation analysis of the railway under-crossed by double shield tunnel in soil–rock​ composite stratum was carried out, and the influence law of hard-layer ratio, tunnel burial depth and railway roadbed reinforcement on the railway track deformation was revealed by numerical simulation. Results show that the track settlement mainly occurred in the middle and late stages of the shield crossing the railway. The track settlement showed an obvious trend of decreasing with the increase of hard-layer ratio or tunnel buried depth. The roadbed reinforcement can significantly reduce the settlement of the railway track, the smaller the hard-layer ratio, the better the roadbed reinforcement effect.

Settlement Prediction of Double Line Shield Tunnel in Upper Soft and Lower Hard Stratas

Settlement Prediction of Double Line Shield Tunnel in Upper Soft and Lower Hard Stratas

Field measurement analysis of the influence of simultaneous construction of river channel and bridge on existing double shield tunnels

This study presents the construction of a river channel and a bridge adjacent to existing metro tunnels in Changzhou. The influence of simultaneous construction on these existing tunnels was investigated via 3D numerical modelling to predict tunnel deformation before construction. Then, a targeted protection scheme was developed according to the obtained numerical results. The full construction period field monitoring scheme can monitor tunnel responses during construction. Subsequently, the safety of the tunnel structures was evaluated according to the monitoring results, and the evolution of tunnel deformations was analysed. The analytical results can help to clarify the influence characteristics of different construction stages, verify the effect of the proposed protection scheme, and determine the disturbance mechanism of short-distance pile construction. According to the results, the tunnel deformation mainly occurred during pile construction and river channel excavation, and the tunnel vertical displacement and convergence were mainly affected by the construction. The anti-floating scheme of the partition excavation and casting effectively controlled the tunnel heave with an alarm value of approximately 6 mm. The penetration of the short-distance casing resulted in a tunnel deformation. The main construction influence area of the casing pile was within 6D (D is the pile diameter).

Impact of Double Shield Tunnel Construction on Adjacent Pile Foundation in Silty Fine Sand Strata

Impact of Double Shield Tunnel Construction on Adjacent Pile Foundation in Silty Fine Sand Strata

Excavation rate “predicting while tunnelling” for double shield TBMs in moderate strength poor to good quality rocks

This article presents a case study on adopting the method of “predicting while tunnelling” on a rock tunnel project in Shenzhen China. It is applied to the construction by providing advance rate and the construction scheduling of engineering activities based on the probability density functions of TBM thrust, torque, rotational speed and torque penetration index in different rock masses. The study showed that the torque penetration index is a better parameter in predicting the rock excavation penetration rate than the field penetration index for the project. The study also showed that using statistical models of the operational parameters and boreability indexes in different rock masses can predict the rock excavation penetration and advance rates accurately. The method could also probabilistically predict the construction period considering different variations of the operational parameters and boreability indexes.

Deformation Characteristics of Existing Twin Tunnels Induced by Double Shield Undercrossing with Prereinforcement: A Case Study in Hangzhou

This paper is based on the case of the earth pressure balance (EPB) shield tunnelling project of the new Metro Line 2 undercrossing the existing Metro Line 1 in the soft soil urban area of Hangzhou. Because the EPB shield must break through a plain concrete wall before undercrossing the existing tunnels, the pipe roof prereinforcement was adopted to stabilize the soil between the existing tunnels and the new shield tunnel. The deformation characteristics of the existing tunnels in the process of double shield undercrossing were discussed. According to the variation of shield position, the settlement development could be divided into three stages: shield approaching subsidence, shield crossing heave, and shield leaving subsidence. The horizontal displacement shows a back and forth variation characteristic consistent with the direction of shield tunnelling. At the junction of tunnel and station, the shield undercrossing caused considerable differential settlement between the existing tunnel and the station. The construction of pipe roof prereinforcement will lead to the presettlement of the existing tunnels. The settlement of the existing tunnels caused by the attitude deviation of pipe roof and grouting disturbance should be reduced in reasonable ranges. In addition, the maximum longitudinal settlement of the existing tunnel during the shield second undercrossing was also discussed. It was considered that the influence of wall breaking is greater than the sequence of shield undercrossing. The driving parameters of shield tunnelling should be optimized before the second crossing.

Looking inside TBM parameters: feedback from the Tunnel 4 case study, Angat Water Transmission Improvement Project (Philippines)

The use of TBMs in the tunnel industry has undergone a continuous growth over the last years, with application in ever expanding ranges of geotechnical conditions. Nevertheless, the feedback from completed projects with respect to the use of machine data for assessing geotechnical conditions are not common: there are only a few attempts to correlate machine data with geotechnical conditions during excavation and in most cases the use of machine data is related to assessment of TBM performance for prediction of advance rate. The analysis of TBM parameters may represent an interesting tool for monitoring of geotechnical conditions at the tunnel face and early detection of adverse face conditions. This work presents the feedback from the Tunnel 4, a 6.4 km long water transfer tunnel excavated by double shield TBM in the Philippines. The machine data have been analysed for identification of most sensitive parameters to geomechanical properties of the rock mass. In this respect, the use of specific-excavation parameters provides best results for assessing geotechnical conditions during excavation. Specific energy and Field Penetration Index displayed a good correlation with RMR values. The combined use of Net Advance Rate and Drillability Index allows to highlight how unstable face conditions and / or tough rock can influence the TBM advance.

A Combined Discrete Element-Finite Difference Model for Simulation of Double Shield TBM excavation in Jointed Rocks

A Combined Discrete Element-Finite Difference Model for Simulation of Double Shield TBM excavation in Jointed Rocks

Development of an automatic measurement device for double laser shield tail gap based on image recognition technology

Shield tail clearance is an important basis for the selection of segment assembly and shield attitude control. It is the most direct factor affecting shield tail brush wear. Poor shield tail clearance is prone to segment damage and shield tail seal failure. Using manual measurement of the shield tail gap, the measurement data has a certain hysteresis, and can not directly reflect the changing trend of the shield tail gap. Based on image recognition technology, a dual laser shield tail gap measurement device was developed to measure the changes in the shield tail gap during shield tunneling. The field test shows that: 1 The measurement device can monitor the shield tail gap in real time, and can simulate the ellipticity of the shield tail, monitor the deformation of the shield tail and the space state of the shield tail and the tube. 2 The acquisition device adopts a dual laser transmitter, meanwhile, the attitude of the shield machine cannot be controlled in real time, Can accurately establish the geometric relationship between the actual distance and the pixel distance, improve the test accuracy of the measuring device, and control the monitoring accuracy at 3.8 mm.

Performance Analysis of Tunnel Boring Machines for Rock Excavation

The study takes into account different classes of tunnel boring machines (TBM), with the aim of identifying correlation models which are meant to estimate, at a preliminary design phase, the construction time of a tunnel and to evaluate the mechanical and operational parameters of the TBMs, starting from the knowledge of the tunnel length and the excavation diameter. To achieve this goal, first of all a database was created, thanks to the collection of the most meaningful technical parameters from a large number of tunnels; afterwards, it was statistically analyzed through Microsoft Excel. In a first phase, forecasting models were identified for the three types of machines investigated, separately for compact rocks (open TBM) and fractured rocks (single and double shield TBM). Then, the mechanical parameters collected through the database were analyzed, with the aim of obtaining models that take into account, in addition to the type of TBM, the geological aspect and the type of rock characterizing the rock mass. Finally, the validation of the study was proposed in a real case, represented by the Moncenisio base tunnel, a work included in the new Turin–Lyon connection line. The estimated values were compared with the real ones, in order to verify the accuracy of the experimental models identified.

Research on Guidance Method of Double Shield TBM Based on Monocular Vision

Research on Guidance Method of Double Shield TBM Based on Monocular Vision

Actual performance analysis of a double shield TBM through sedimentary and low to medium grade metamorphic rocks of Ghomrood water conveyance tunnel project (lots 3 and 4)

In this paper, after a brief description of engineering geological properties of Ghomrood water conveyance tunnel project (lots 3 and 4 with a total length of 18 km), the actual performance of the machine during the excavation of tunnel through sedimentary and low to medium grade metamorphic rocks has been analyzed. Subsequently, using data collected during pre-construction and construction phases of the project, the relationships between rock mass properties and measured machine performance parameters have been investigated and proposed as a prediction model which can be used in future projects in which similar geological conditions are encountered. Results of analyses indicated that there are strong relationships between TBM performance and engineering geological conditions of the tunnel alignment.

Research on Water Inrush Disaster Control Technology in Double Shield TBM Excavation Process

Taking the TBM water inrush accident in An-Xue section of Qingdao Metro Line 1 as the engineering background, the research is carried out from two aspects of TBM water inrush emergency measures and grouting reinforcement treatment technology, which can ensure the strength of stratum reinforcement and water shutoff, and at the same time prevent the slurry from damaging the cutter head. For different areas, the grouting construction technology is proposed. After the reinforcement, the water stopping effect is obvious, the water inrush point has no water flow out, the hole drilling effect is good, the core removal rate is 75%∼85%, and the strength meets TBM excavation requirements.

DPavatar: A Real-Time Location Protection Framework for Incumbent Users in Cognitive Radio Networks

Dynamic spectrum sharing between licensed incumbent users (IUs) and unlicensed wireless industries has been well recognized as an efficient approach to solving spectrum scarcity as well as creating spectrum markets. Recently, both US and European governments called a ruling on opening up spectrum that was initially licensed to sensitive military/federal systems. However, this introduces serious concerns on operational privacy (e.g., location, time, and frequency of use) of IUs for national security concerns. Although several works have proposed obfuscation methods to address this problem, these techniques only rely on syntactic privacy models, lacking rigorous privacy guarantee. In this paper, we propose a comprehensive framework to provide real-time differential location privacy for sensitive IUs. We design a utility-optimal differentially private mechanism to reduce the loss in spectrum efficiency while protecting IUs from harmful interference. Furthermore, we strategically combine differential privacy with another privacy notion, expected inference error, to provide double shield protection for IU's location privacy. Extensive simulations are conducted to validate our design and demonstrate significant improvements in utility and location privacy compared with other existing mechanisms.

双护盾TBM超前支护系统设计

双护盾TBM超前支护系统设计