Civil Engineering Reference
In-Depth Information
3.6.3 Simulation of uncoupled partial excavations
If a tunnel is driven with a number of partial excavations, for example a top heading
advance (first partial drift) with subsequent excavation of the bench and invert (second
partial excavation), then the stress and displacement state of the ground changes with the
driving of the top heading. The advance of the bench and invert that follows the top head-
ing occurs in a state that has changed from the original primary state, in which the ground
was untouched. The main characteristic of this state is the cavity that is now present in
the ground, which is wholly or partially supported with shotcrete. The loads, which were
transferred through the area of the cavity before the advance of the top heading, are di-
verted around the shotcrete layer. This loads the shotcrete layer due to the displacements
resulting from the advance of the top heading after the layer was applied. As a result of the
subsequent excavation of bench and invert, the state of displacement and stress present in
the rock mass and support is changed again.
For the calculation of the changed state of displacement and stress due to a further partial
excavation, the stepwise simulation of the tunnel advance using the step-by-step technique
is basically suitable. The partial excavation in this case is decoupled from the previous
partial excavation. When two or more partial excavations are calculated, the location of
the calculation section in the direction of advance should be selected so that each follow-
ing partial advance lies outside the area affected by the face of the previous advance. In
the discretisation of the calculation section, the excavation cross-section of each partial
advance is to be modelled in the planes perpendicular to the advance direction. Different
round lengths are normally considered for the element slices in the direction of advance.
This leads to a size of FE meshes and thus to equation systems, which can often no longer
be calculated at an economically justifiable expense. Should nonetheless many partial
advances be calculated using the step-by-step technique, then this will only be possible
with relatively coarsely discretised calculation sections with the various round lengths are
considered in a simplified form.
The method for the simulation of a number of successive partial advances is based on the
idea that the stresses and displacements determined for the preceding advance are taken
as a basis for the initial state of the following partial advance. The displacement and stress
state of a zone, which is no longer affected by the events at the face, are decisive. For the
calculation of the individual partial advances, the step-by-step technique and the itera-
tion process are equally suitable. This, however, only applies when the tunnel profile is
constant and the ground is homogeneous in the area to be investigated and the overburden
depth and the primary stress state in the ground do not change along the tunnel. Another
precondition is that calculation sections with the same dimensions and discretisation in
the plane perpendicular to the advance direction are selected for the investigation of the
individual partial advances. In the discretisation, the partial excavation cross-section of
each partial advance is to be considered. The discretisation is to be selected for each partial
advance independent of the associated round length. For the dimensions of the calculation
section, the remaining criteria stated for the step-by-step technique apply. Fig. 3-14 shows
the discretisation for an already described calculation.
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