and the southbound tunnel respectively. x is the horizontal distance
from the northbound tunnel centerline in the transverse direction. a1
and a2, as replacements of the original parameter (i.e. i) in Eq. (1), are
the shape parameters of the settlement trough. u, which is already
recorded in the DW, is the distance between the two tunnels. A typical
ground settlement trough over twin tunnels is shown in Fig. 6.
While the non-linear single Gaussian model (i.e. Eq. (1)) could be
easily transformed to a linear one by log transformation, the proposed
double Gaussian model (i.e. Eq. (2)) could not. Consequently, in order
to fit the proposed model to the instrumentation data in the DW, a
non-linear least-squares based optimization method (i.e. Levenberg–
Marquardt method) is implemented to solve this problem.
According to Eq. (2), the objective function F will be