7.2.2　Calculation of Tunneling Parameters

7.2.2　Calculation of Tunneling Parameters

Due to the rapid advance，rapid assembly and rapid passage of the advancing scheme adopted in the process of shield tunneling in the middle reaches of the Yangtze River，in order to ensure the smooth and safe implementation of the construction scheme，it is necessary to calculate and set the various tunneling parameters in advance during the shield tunneling，including tunneling speed，cutterhead speed，jack thrust，incision pressure，post-wall grouting parameters，mud parameters，mud flow rate and mud pipe flow rate.Therefore，how to establish a benign and dynamic slurry cycle balance model in a short period of time is the key to determine the smooth progress.

The tunneling speed of the shield directly determines the excavation rate of the machine（i.e.，the excavation amount per unit time）.The faster the tunneling speed is，the higher the excavation rate is，which leads to the increase of the accumulation speed of the slag and soil in the excavation chamber.Under the condition of constant inflow and outflow of slurry，the slurry level in the excavation chamber increases，and ultimately leads to the increase of the incision water pressure，namely the support pressure in front of the shield，which is extremely unfavorable for the stability of the excavation face.How to establish a benign cycle between these three main parameters should be carried out in accordance with the following points：The first step，according to the geological characteristics of the rock ahead，the thickness of the tunnel，the height of the water level using the method of setting the water pressure of the incision mentioned above.The second step is to set the pressure of the bubble chamber according to the set water pressure of the incision and the liquid level on the shield 0 scale line.The third step，set reasonable penetration and cutter head speed，so as to obtain the shield propulsion speed.In the fourth step，when other factors（such as the uneven geology ahead）lead to significant changes in speed，the thrust of each group of jacks should be adjusted in time，or the speed should be stabilized by adjusting the penetration and rotational speed.The fifth step，when the speed changes，should actively adjust the amount of slurry in and out so as to stabilize the excavation tank level，adjust the amount of slurry in and out is mainly achieved by adjusting the speed of each pump.

Among the various tunneling parameters，the tunneling speed，cutterhead speed，post-wall grouting parameters，mud parameters，and mud pipe flow rate can be obtained according to experience or laboratory tests，while the incision pressure，mud flow rate and jack thrust need to be calculated.The calculation methods of three parameters，namely，incision pressure，mud flow rate and jack thrust，are introduced below.

When calculating the cutting water pressure of excavation face，the stress analysis is shown in Fig.7.7.

（1）The upper limit formula of incision water pressure is as follows.

Pfu—upper limit of incision pressure（kPa）.

P1—Groundwater pressure（kPa）.

P2—Static earth pressure（kPa）.

P3—Variable earth pressure，generally 20 kPa.

P4—River water pressure is determined according to different water depths.

γw—Bulk density of water（kN/m3）.

H—Tunnel buried depth below groundwater level（calculated to tunnel center，m）.

K0—Static earth pressure coefficient.

γ—Soil bulk density（kN/m3）.

H—Tunnel buried depth（calculated to tunnel center，m）.

γwater—Bulk density of river water（kN/m3）.

hwater—Depth of river water（m），the actual water depth above the shield cut shall be determined according to the tidal gauge.

（2）The lower limit of incision water pressure is as follows.

Pf1—cut water pressure lower limit（kPa）.

P'2—Active earth pressure（kPa）.

Ka—Active earth pressure coefficient.

Cu—Soil cohesion（kPa）.

2）Sludge flow rate

According to Duroude limit velocity formula：

vL—Minimum limit velocity（m/s）.

FL—Depends on the constant of soil particle concentration and diameter，the value of feeding pipe is 0.7，the value of discharging pipe is 1.35.

ds—The proportion of soil particles flowing through the mud pipe.

D—Pipe diameter.

d—The proportion of mud.

g—Gravity acceleration，9.8 m/s2.

vd—Design minimum velocity.

3）The thrust calculation principle of shield jack

The shield jack should have sufficient thrust to overcome the resistance encountered during shield tunneling.These propulsion resistance mainly include：slurry pressure acting on the shield reaction F1.Soil friction to shieldF2.The traction forceF3of the supporting system behind the shield.The component force of shield gravity in heading directionF4.Cutting tool resistance F5.Shield tail seal tension F6.

The sum of the above various propulsion resistances is expressed by the following formula.In use，the specific situation of the shield machine should be considered，and a certain amount of surplus should be left，namely，the total thrust of the shield jack：