Numerical Modelling of Tunnels
Until a few years ago tunnel construction was based exclusively on experience. Numerical methods, however, constitute a very valuable supplement. Shown here are the approximations necessary for 2D analysis. The most important load cases, such as dead load
- PDF / 3,612,483 Bytes
- 42 Pages / 482 x 692 pts Page_size
- 12 Downloads / 207 Views
G. Swoboda University of Innsbruck, Innsbruck, Austria
ABSTRACT Until a few years ago tunnel construction was based exclusively on experience. Numerical methods, however, constitute a very valuable supplement. Shown here are the approximations necessary for 2D analysis. The most important load cases, such as dead load or water pressure, are also illustrated. The damage tensor theory needed for realistic simulation of jointed rock is also presented. In future 3D analysis will take on increasing significance, for which reason the pertinent models are also dealt with.
C. S. Desai et al. (eds.), Numerical Methods and Constitutive Modelling in Geomechanics © Springer-Verlag Wien 1990
278
1
G. Swoboda
Introduction
This chapter is intended as a guide to those engineers responsible for th(~ dP.sign of tunnels and aims to give them an overview of Lhe state of the art of practical 1111 merical models for tunneling. For many years the design of tunrwls wa:; hasPd soi('I.Y on experience and sometimes on very simple analytical considerations. This d(H'S not mean that a tunnel cannot be designed without the use of a numeri..K
< FJ( z
>..K
>..K
Free
Free
1
)..1\
Slip
Slip
N-
s > FJ( t
< FJ( z
N-
a.FtK < o a.F{ ~ fl.~< 0
o
)..K >FK N z
)..K
N
>FK z
fl.KN>0
Mesh generation of jointed rock
Jointed rock is described in the finite element mesh as rock along given possible fault lines, where constraint elements make it possible for the failure mechanisms described above to take place. In the framework of interactive mesh generation by means of a digitizer, the NETDIG program [22] includes the possibility of introducing individual fault lines after completion of the mesh topology. For this purpose, the node numbers on both sides of the fault line are reassigned, whereby one node retains its original number and the second, newly introduced node receives the highest node number available. The reference nodes in Fig. 15 have to be included in the generation. Special problems arise in this algorithm when two fault lines intersect. In this case, appropriate constraint element combinations have to be generated, as shown in Fig. 16, in order to permit all elements to move along the given lines. A total of up to eight intersecting lines can be generated in the NETDIG program system.
293
Numerical Modelling of Tunnels
Constraint Element
'\ I
I
I
/':
:/':: /':
Element Joints
Figure 16: Crossing fault lines of jointed rock
4.1.3
Calculation of a tunnel in discontinuum
According to the New Austrian Tunneling Method (NATM) [1], driving is performed in partial excavation, with rapid securement of the cavity using shotcrete. This effects an activation of the surrounding rock and economic calculation of the cavity's securing measures. After excavation of the top heading, the primary stresses in the undisturbed soil or rock are redistributed transverse to the direction of driving and also via the face by arch action. This redistribution of loads causes displacements in the roof, which in shallow tunnels can ex
Data Loading...
