Input Features
Interfaces
These joint elements are needed for calculations involving soil-structure interaction. They may be used to simulate the thin zone of intensely shearing material around tunnel linings or at the contact of footings and retaining walls. Values of interface friction angle and adhesion, that are not necessarily the same as the friction angle and cohesion of the surrounding soil, may be assigned to these elements.
Geogrids
Geogrids are often used for the
construction of reinforced embankments
or retaining soil structures. They can
be simulated in Plaxis by the use of
special tension elements. It is often
convenient to combine these elements
with interfaces to model the interaction
with the surrounding soil.
Automatic mesh generation
Plaxis allows for fully automatic
generation of (2D) unstructured finite
element meshes with options for global
and local mesh refinement.
Graphical input of geometry models
The input of soil layers, structures,
construction stages, loads and boundary
conditions is based on the same
convenient drawing procedures (CAD) as
the Plaxis 2D version 8, which allows
for detailed and accurate modeling of
realistic situations. From the geometry
model a 2D and 3D finite element mesh is
generated automatically.
Screens (interface elements)
Interface elements in Plaxis 3D Tunnel
Version 2, in addition to their existing
functionalities, can be used to simulate
an impermeable screen. An active
interface element is fully impermeable
(separation of head degrees-of-freedom
of node pairs). An inactive screen is
fully permeable (coupling of head
degrees-of-freedom of node pairs).
Plates
Special plate elements are used to model
the bending of tunnel linings, shells,
retaining walls and other slender
structures. The behavior of these
elements is defined using a flexural
rigidity, an axial stiffness and an
ultimate bending moment. A plastic hinge
may develop for elasto-plastic plates,
as soon as the ultimate moment is
mobilized. Plates may be used together
with interfaces to perform realistic
analyses of tunnel projects and other
geotechnical applications.
Structural elements
The Plaxis 3D Tunnel offers a number of
special elements that are dedicated to
model typical structural objects. These
elements are to be used in the
out-of-plane direction (z-direction).
Point loads
appear as point forces in the cross
section model, but in the full 3D model
they can be used both as point loads on
individual vertical cross sections as
well as line loads on volume sections.
Distributed loads
Appear as line loads in the cross
section model, but in the full 3D model
they can be used both as line loads on
individual vertical cross sections
(z-planes) as well as real distributes
loads on volume sections (slices).
Z-loads
Z-loads are loads normal to a cluster in
a vertical cross section. This type of
loads can be applied in the framework of
Staged Construction in individual
vertical cross sections of the 3D model.
Z-loads can be used, for example, to
analyze the tunnel heading stability.
Loads
A convenient option is the possibility
to (de)activate and change input values
of loads per z-plane or per slice in a
Staged Construction phase. In this way
unlimited load combinations can be made.
Z-planes and slices
Tab sheets for each individual Z-plane
or slice have been included in Plaxis 3D
Tunnel Version 2. This facility allows
the user to specify different water
boundary conditions at each plane. The
boundary conditions at the slice
surfaces between successive planes are
linearly interpolated by the program.
Also, individual slices can be selected
and defined as dry. It is possible to
tilt the full model in z-direction to
simulate a dipping of the tunnel.
NATM Tunnel
The creation of any tunnel shape is
possible. Shells (discontinuous plates)
can be added to the outer contour to
simulate tunnel linings composed of more
than one lining layer or sandwich
structures. The properties of the shell,
as contained in plate data sets, need to
be assigned to each shell section
individually. This enables the use of
different data sets for individual shell
sections. A thick and massive lining
that is composed of volume elements
rather than plates can also be added.
When defining a calculation phase in the
framework of Staged Construction, each
section can be (de)activated
individually. A volume strain can be
applied to each section separately to
model a volume loss.
Bored tunnels
A bored tunnel is circular and therefore
allows for the input of only one radius.
A homogeneous and continuous slender
shell (plate) can be added to the full
tunnel cross section. It is possible to
add a thick and massive lining that is
composed of volume elements rather than
plates. To simulate volume loss during
the construction of the Bored Tunnel, a
contraction or volume strain can be
applied.
Tunnels
Plaxis 3D Tunnel offers improved and
extended options to create circular and
non-circular tunnels composed of arcs,
straight lines and corners. Plates and
interfaces may be added to model the
tunnel lining and the interaction with
the surrounding soil. Fully
isoparametric elements are used to model
the curved boundaries within the mesh.
Different practical methods are
implemented to analyze the deformations
that occur due to the construction of
the tunnel. Two commonly used types of
tunnels have predefined settings.
High-order elements
The 15-node wedge element is composed of
6-node triangles in x-y-direction and
8-node quadrilaterals in z-direction.
This type of volume element for soil
behavior gives a second-order
interpolation for displacements and the
integration involves six stress points.
The accuracy of the 15-node wedge
elements in a 3D analysis is comparable
with the 6-node triangular element in a
2D Plaxis analysis.
Creation of the 3D model
The 3D model is created simply by linear
extension of the mesh in z-direction.
The user can specify the number and
thickness of cross section planes
(z-planes). Two successive z-planes form
a slice. The previously generated 2D
mesh is repeated at each z-plane. The 3D
mesh is created by connecting the
corners of the 2D triangular elements to
the corresponding points of the
corresponding elements in the next
z-plane. In this way, a 3D mesh composed
of thousands of 15-node wedge elements
is created.
Updated mesh
This option allows the analysis of large
deformation problems. Typical
applications, where updated mesh
analyses may be necessary, include the
analysis of reinforced soil structures,
the analysis of large offshore footing
collapse problems and the study of
problems where soils are soft and large
deformations occur.
Closed flow boundary
A closed flow boundary is an object that
can be placed at the boundary of the
geometry model to ensure that flow
across this boundary will not occur.
Anchors
Elastoplastic spring elements are used
to model anchors and struts. The
behavior of these elements is defined
using an axial stiffness and a maximum
force. A special option exists for the
analyses of pre-stressed (ground)
anchors and excavation supports