5.3.2 Calculation and Analysis of Carrying Capacit...
1）Calculation of carrying capacity
The sand carrying capacity of mud is closely related to the flow rate of mud.The Durand formula is used to calculate the boundary velocity of soil particles transported by slurry to ensure that there is no precipitation in the pipe.The calculation results are shown in Table 5.15.
The calculation results show that the current construction technology can basically ensure that no large amount of sand is precipitated in the pipe.In actual shield construction，there have been no accidents such as pebble precipitation，unable to discharge，pipe blockage and so on.
Table 5.15 Calculation table of limit flow rate of mud sand particles
2）Pebble starting speed calculation
The particle size of pebbles is relatively large，and the mud must have a certain flow rate to have enough kinetic energy to carry pebbles.The starting velocity of pebbles is calculated by the Shamoff formula，and the calculation results are shown in Fig.5.36.
It can be seen from Fig.5.36 that the size of pebble particles that the slurry can carry increases with the increase of slurry flow rate.When the slurry flow rate exceeds 1.6 m/s，a small amount of pebbles with a diameter of 18 cm can be carried.At present，the slurry flow rate of slurry discharge pipe in shield construction is much higher than 1.6 m/s，which has strong carrying capacity.Studies have shown that under normal construction conditions，the discharge pipe of slurry shield in Nanjing Yangtze River Tunnel will not have the accidents of pebble precipitation，discharge and pipe blockage.
Fig.5.36 Pebble particle size versus starting speed
Mud circulation system is a key factor in construction，which affects the density index of mud and has an important influence on mud film formation and excavation surface stability.The circulation system screening slag removal effect should be controlled properly.If the fine particles less than 75μm in the formation are removed by the screener more than 30％（especially the particles less than 50 μm），the density of the mud will be low and the properties of the mud will be affected.If the screening machine cannot screen most particles with particle size greater than 50μm，large particles will circulate to the slurry returning tank，resulting in a large amount of precipitation in the slurry returning tank，affecting the normal production of slurry.Through calculation and field investigation，the slag carrying capacity and circulation system of sludge discharge pipe can meet the construction requirements when slurry shield tunneling in sandy gravel composite stratum.It is suggested that the setting parameters of the system should be properly adjusted according to the actual working conditions of the site when the shield passes through the gravel composite stratum to improve the efficiency.