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 modelling 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 behaviour of these elements is defined using a flexural rigidity, an axial stiffness and an ultimate bending moment. A plastic hinge may develop for elastoplastic plates, as soon as the ultimate moment is mobilised. 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 analyse 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 seperately 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 analyse 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 behaviour 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 behaviour 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.
Soil behaviour
Advanced soil models
In the Plaxis 3D Tunnel Version 2 the following soil models are available: Linear elastic, Mohr-Coulomb, Hardening Soil and Soft Soil Creep model. In addition to these soil models an anistropic model for rock is offered: The Jointed Rock model: This is an anistropic elasto-plastic model where plastic shearing can only occur in a limited number of shearing directions. This model can be used to simulate the behaviour of stratified or jointed rock.
Calculation features
Plastic calculation
In a plastic calculation, load multipliers are used to activate: prescribed loads (point loads, distributed loads), prescribed displacements and soil weight. A special feature exists for the simulation of a construction process.
The Calculations program considers only deformation analyses and can only perform Plastic calculations. An efficient and robust iterative solution procedure is used to solve large sets of equations in short time using a minimum amount of RAM.
Top view
To easily define calculation phases in staged construction, a top view of the model can be shown, in which z-planes and slices can be selected interactively.
Steady state groundwater flow
This feature enables the generation of pore pressures as a result of groundwater flow. By adding this feature, the user will be able to generate pore pressures due to hydrostatic phreatic levels (the existing feature) or groundwater flow. Groundwater heads can be conveniently specified at boundary lines of the front plane. The results of the groundwater flow calculation include also total head distribution and Darcy flow (directions and magnitudes).
Consolidation
This feature enables the calculation of the generation and dissipation of excess pore pressures as a result of sudden application of external loads on a wet soil mass. Plaxis 3D Tunnel provides automatic time stepping procedures, which make the analysis robust and easy-to-use.
Copy option
This useful Plaxis 3D Tunnel option enables easy copying of staged construction settings from a slice or a plane to another slice or plane. Copying can also be done for a group of slices or planes.
Staged construction
Staged construction is a very versatile type of loading input and enables an accurate and realistic simulation of various loading, construction and excavation processes. In this special Plaxis feature it is possible to:
- Change the geometry configuration by de-activating or re-activating volume clusters or structural objects as created in the geometry input to simulate for instance the building process of a tunnel or an excavation.
- Change the load configuration by de-activating or re-activating loads and prescribed displacements as created in the geometry input. For example z-loads can be applied to analyse the tunnel heading stability.
- Enter an internal pressure in volume clusters. This option may be used to simulate mechanical processes that result in artificial pressures in the soil, such as compensation grouting.
- Apply a volume strain to a volume cluster in order to simulate processes such as compensation grouting.
- Reassignment of material data sets to simulate for instance soil improvements, i.e. removing ‘soft’ soil and replacing it with ‘reinforced’ soil.
- Change the water pressure distribution in the geometry.
3D inspection view
The 3D model can be visualised from any angle and rotated freely in the staged construction mode to inspect the selections made.
Output features
Load displacement curves & stress paths
A special tool is available for drawing load-displacement curves, stress paths and stress-strain diagrams for preselected nodes and stress points.
Prespective view
By default, a 3D model in the Output program is presented in perspective view. This facility makes the appearance of the 3D model realistic and natural, although it is presented on a flat screen. The arrow keys may be used to change the orientation of a 3D model on the screen.
Partial geometry
To enhance the ‘inside’ visualisation of a 3D model, parts of the geometry can be selected as transparent.
The Plaxis post-processor has enhanced graphical features for displaying computational results. Such as displacements, stresses, strains, distributions of the groundwater heads, (excess) pore pressures. And Darcy flow. Values of these quantities can be obtained from the output tables. Plots and tables can be send to output devices or to the Windows‚ clipboard to export them to other software. |
