stabilized after both tunnels have passed through, ΔV is the increased
ground loss from the initial time to the final stablizaed stabilized
time, p is a parameter that determines the shape of the curve and is
relevant to the geological conditions along with the construction
conditions.
The selected settlement array shown in Fig. 17 was located at
chainage DK25+192. It was put into use from January 31st, 2009
(when the tunnel face of the preceding tunnel was 28 m before the
array) to May 19th, 2009 (when the tunnel face of the preceding tunnel
was 75.2 m after the array). These 109 days are mapped to a normalized
time scale, which also indicates the 104.2 m advancement of
the preceding tunnel face.
It could be concluded that the value of Vl at a certain area will increase
sharply while the tunnel face is approaching and passing
through, whereas the growth rate of Vl will decrease after the tunnel
passes through. Finally, the value of Vl stops increasing when it
reaches a certain value. As shown in Fig. 18(a), the first derivative
of Vl at DK25+192 reflects the growing rate of Vl, which also reflects
the evolution of safety risk. The moment with the highest growing
rate of Vl which indicates the highest safety risk, can be easily fixed
by the second derivative of Vl (as shown in Fig. 18(b)). By combining
this finding with the records of construction progress in the DW, it
can be concluded that the moment subjected to the highest safety
risk is the moment when the preceding tunnel is just arriving beneath
the settlement array. These revealed evolution laws of safety risk are
considered to be valuable references for safety management in future
Wuhan Metro construction project.
Additionally, by dividing the Vl of stabilized ground settlement
troughs by the excavation area of twin tunnels, the ration of ground
loss can be derived. Most of the ground loss rations from different
areas in this project fall between 0.5% and 1%, as is shown in Fig. 19.
This may be a reference for further work.