Civil Engineering Reference
In-Depth Information
3D: For a supporting crown, the ground is capable of creating a
supporting ring, i.e. the ground can form an arch and transfer loads around
the tunnel void.
In the range 2D
C
3D, the ground above the tunnel crown can be
acting either as a support or not depending upon the geological conditions,
i.e. bedding arrangements.
Further details on the design of shield tunnel linings, segmental linings
for example, can be found in ITA (2000). Further details on structural
design models for tunnels in soft ground can be found in Duddeck and
Erdman (1985).
C
3.5.2 Bedded-beam spring method
The tunnel support is idealized as an elastically supported circular ring.
The elastic bedding is achieved through radial and potentially tangentially
arranged springs. The spring stiffness simulates the support behaviour of
the ground. The important parameters of the ground are the stiffness
modulus E
s
(which is included in the spring stiffness) and the coefficient
of lateral earth pressure K
0
(which is included in the loading). The calcula-
tion is carried out elastically. As the ground is only represented by springs,
the analysis cannot provide any information with regard to the settlement
at the ground surface and to the possible stress and deformation behaviour
of the ground (secondary stress situation). Figure 3.6a shows the model
used within the bedded-beam spring method with an unsupported crown,
the so called 'partially bedded method' for shallow tunnels (ITA 1988).
For deep tunnels the bedded-beam spring method is generally not used
because even with a supporting crown area, the supporting nature of the
ground is not sufficiently taken into account.
The bedded-beam spring method is the fastest and simplest calculation
method. Therefore it is often applied even though it has limited poten-
tial for interpretation with respect to the real situation due to the many
simplifications made. It is often used to determine thickness and required
reinforcement of the supporting circular ring following the results of a more
sophisticated calculation method. The usage is mainly for shallow tunnels
in soft ground or weak rock.
3.5.3 Continuum method
The ground, in which the tunnel is constructed, is idealized as a continuum,
i.e. there are no discontinuities in the material. The method assumes that
the ground is an infinitely large thin section with a hole at the centre (Figure
3.6b). This calculation method allows the interpretation of the deformation
and strains in the ground. In addition, this method allows the construction
phases to be simulated. The elastic modulus, E, is required as a parameter
for the ground. The structural system can be established for both an
