Mechanism and preventive measures of slurry leakage at shield tail brushes under abnormal shutdown condition—A case study

Based on an underwater shield tunnel project,this study introduces the phenomenon of abnormal shutdown and the measures designed to mitigate problems in construction.On the strength of the analysis of the slurry leakage causes during re-tunneling,the mech-anism of slurry leakage induced by jack retraction was studied,and treatment measures of leakage at shield tail brushes were elaborated upon.The results show that the sudden change of stratum leads to an abnormal shutdown,and a sharp increase in the slurry pressure at the shield cut location leads to the slurry penetrating the gap between the outer wall of the shield shell and the soil,increasing the risk of instantaneous breakdown at shield tail brushes.The radial displacement of the outer wall of the segments caused by retractions of three jacks and a single jack along the circumferential direction distinctly shows three regions and four regions,respectively.The radial shrink-age of the outer wall of the segments at the jack retraction position decreases the tightness between the shield tail brushes and the outer wall of the segments.The expansion of the outer wall of the segments at the surrounding position,especially at the lower position,increases the slurry level to the radial shrinkage position of the segments,which increases the risk of slurry leakage.Furthermore,when the shield machine faces an abnormal shutdown,re-tunneling,and assembly of segments,the jack can be gradually retracted from top to bottom.The slurry leakage degree can be observed when retracting,and the corresponding measures can be taken for timely plugging.According to the leakage degree,it can be determined whether to replace the shield tail brushes to reduce the risk of large-scale leakage accidents.

Mechanical responses of underground carriageway structures due to construction of metro tunnels beneath the existing structure:A case study

To understand the mechanical response pattern of the existing structure and ground due to the construction of metro tunnels underneath,the finite difference method is adopted to study the torsional deformation and stress variation of the existing structure and the effect of underground carriageway structures on the surface subsidence.The curves of the maximum differential subsidence,torsion angle,and distortion of the cross-section of the existing structure show two peaks in succession during traversing of two metro tunnels beneath it.The torsion angle of the existing structure changes when the two tunnels traverse beneath it in opposite directions.The first traversing of the shield tunnel mainly induces the magnitude variation in torsional deformation of the existing structure,but the second traversing of the subsurface tunnel may cause a dynamic change in the magnitude and form of torsional deformation in the existing structure.The shielding effect can reduce the surface subsidence caused by metro tunnel excavation to a certain extent,and the development trend of subsidence becomes slower as the excavation continues.

Effect of inverted T arrangement on AC pollution flashover characteristics of insulator strings

The arrangement of insulator strings has an effect on its pollution flashover characteristics.A new arrangement of insulator strings,inverted T-type arrangement,is proposed in this study.The electric field distribution is simulated and the pollution flashover characteristics are tested in an artificial climate chamber.The test results indicate that the inverted T-type arrangement can greatly improve the flashover voltage of polluted insulator strings,the‘7+2’inverted T-type arrangement can increase the flashover voltage of each insulator by 8.5%on average,and the‘3+3’inverted T-type arrangement can increase the flashover voltage of each insulator by 13.4%.The discharge process is different from that of the normal suspension string when the insulator string is inverted T-shaped,i.e.a local arc is produced at the low voltage end of the insulator string and develops downward along the suspension part.While as for the horizontal tension string parts,it is difficult to generate arc during discharge development.

Influence of pollution chemical components on AC flashover performance of various types of insulators

The pollution on insulators is composed of various chemical components,which threaten the stability of external insulation.However,the existing insulation designs have not taken this issue into consideration.This work aimed to study the influence of pollution chemical components on flashover performance of insulators with various materials.Flashover tests of porcelain,glass and silicone rubber insulators polluted by eight types of common contaminants were conducted.Based on the test data,the influence of chemical components and their solubility on equivalent salt deposit density(ESDD)and insulator flashover performance were analysed;and the electrical properties of different types of insulators were compared.Results show that the ESDD contribution rate changes with chemical components.The dissolution characteristic has a significant influence on the measurement of ESDD;slightly soluble contaminants need to be considered separately.The corresponding insulator flashover gradient for ions can be summarised as Cl^(−)<NO_(3)^(−)<SO_(4)^(2−),K^(+)<Na^(+),NH_(4)^(+)<Ca^(2+).For a glass insulator,the flashover gradient is 4.9-17.0%higher than that of a porcelain insulator.For a silicone rubber insulator,this gap reaches 14.6-24.4 and 83.3-90%for freely soluble pollution and CaSO_(4),respectively,this phenomenon can be explained by the combined action of the slightly soluble character of CaSO_(4)and hydrophobicity transference of silicone rubber.