The proposed ī is considered to be a measurement of the scope of
safety risk areas, since most of the ground settlement happened within
the distance of 3ī on each cross-section above the twin tunnels.
And the area within the distance of ī deserves the most safety attention.
Consequently, the derived factor from the DW was implemented
in the safety decisions on the Wuhan Metro Project.
By combining the spatial analysis with temporal dimension, the
proposed safety-oriented DW facilitates the evaluation of the evolving
scope of risk. As shown in Fig. 14, the time variations of all the
trough width parameters (i.e. i1, i2 and ī) are investigated. All the
parameters are listed by normalized time t (a value from 0 through
1). Different symbols stand for different ground arrays. Because different
ground arrays are not in use or are not monitored at the same time,
the normalized time makes the comparison more convenient. In addition,
since the ground array is put into service before the preceding
tunnel passes beneath it and goes out of monitoring after both tunnels
pass under the array and the ground has stabilized, the normalized
time can also illustrate the whole tunneling process around each
array. Consequently, the results can also reflect the relationship between
trough width parameters and the tunneling process.
As shown in Fig. 14, during the tunneling process, there is little change
in the width of overall settlement trough and that of each tunnel, but the
settlement trough has been enlarged indeed, as shown in Figs. 10 and 12.
Therefore, the width of settlement trough may be mainly determined by
some time-invariant factors like the geological conditions (i.e. soil condition,
ground water table, etc.). Also, the tunneling process of each tunnel
only deepens the settlement trough, but does not widen it notably.
This finding also implies that ī can only be regarded as a measurement
of static risk. Although a good control of tunneling process may
be helpful in reducing the ground settlement in a certain area, the
scope of the influenced area may be predetermined by the geological
conditions or some other objective conditions.
Some researchers discussed the factors affecting the width of settlement
trough. Based on detailed instrumentation data, Mahmutoglu,
Y [20] reported that the variations in the settlement trough are related
to the soil type and the thickness of the tunnel's overburden. According
to the Gaussian model of a single tunnel, O'Reilly and New
[8] made an analysis of case history data and proposed the formula