where S is ground settlement, Smax is the maximum settlement above
the tunnel centerline, x is the horizontal distance from the tunnel centerline
in the transverse direction, and i is the distance from the tunnel
centerline to the inflexion point of the curve, which determines
the shape of the curve, as is shown in Fig. 5.
However, twin tunnels have been usedmore often than a single tunnel,
especially in urban metro construction. This requires a redefinition
of the ground settlement trough over twin tunnels. Suwansawat and
Einstein [14] used a superposition technique to describe the settlement
trough over twin tunnels. First, when the preceding tunnel just passed
through, the ground settlement was fitted by a Gaussian function,
which was referred to as the settlement trough induced by the preceding
tunnel. Then, the additional settlementwas obtained by subtracting
this portion of settlement from the final settlement after the following
tunnel passed. Then the additional settlement was fitted by another
Gaussian function. Finally, the two curves were superposed to present
the overall ground settlement trough. The hypothesis implies that the
additional settlement is only induced by the following tunnel but has
nothing to do with the preceding tunnel. Hence, the hypothesis may
lead to non-optimal fitting results.
In this study, a double Gaussian model is proposed to represent
the settlement trough over twin tunnels as follows: