Features
Incorporated Stability Checks
Frew allows the user to run limit equilibrium analyses on each construction stage within the program. This allows the user to determine the wall toe depth more easily. This feature considers the fixed or free earth mechanism. The analysis considers groundwater levels, groundwater flow, surcharges and multiple prop positions.
The output shows the calculated toe level for each stage and the lowest calculated toe level can be used to automatically generate nodes.
Alternatively, if the user would like to specify the toe level, this feature could be bypassed.
More detail about the analysis methodology and underlying theory can be found in the manual.
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Analysis Methods
Frew is a program to analyse the soil structure interaction problem of a flexible retaining wall, for example a sheet pile or diaphragm wall.
The wall is represented as a line of nodal points and three stiffness matrices relating nodal forces to displacements are developed. One represents the wall in bending and the others represent the soil on each side of the wall.
The soil behaviour is modelled using one of three methods:
- "Safe" flexibility method - the soil is represented as an elastic solid with the soil stiffness matrices being developed from pre-stored stiffness matrices calculated using the "Safe" finite element program. This method is ideally limited to a soil with linearly increasing stiffness with depth, but empirical modifications are used for other cases.
- Mindlin method - the soil is represented as an elastic solid with the soil stiffness based on the integrated form of the Mindlin Equations. This method can model a wall of limited length in plan but is ideally limited to a soil with constant stiffness with depth but again empirical modifications are used for other cases.
- Subgrade reaction method - the soil is represented as a series of non interactive springs. This method is considered to be unrealistic in most circumstances.
The program analyses the behaviour for each stage of the construction sequence. At each stage it calculates the force imbalance at each node imposed by that stage and calculates displacement and soil stresses using the stiffness matrices. If the soil stresses are outside the active or passive limiting pressures correction forces are applied and the problem solved iteratively until the stresses are acceptable. Allowance can be made for arching within the soil body when calculating the active and passive limiting pressures.
The following input parameters are included in the analysis:
- problem geometry including dig depths, distances to remote boundaries
- wall profile bending stiffness and creep
- soil stratification, strength, density and stiffness
- struts (or anchors) including prestress, stiffness, inclination and a lever arm (to represent rotational fixity).
- surcharges including depth and extent
- groundwater levels and pore pressures each side of wall.
The program gives results for earth pressures, shear forces and bending moments in the wall, strut forces and displacements. These are presented in tabular form and can be plotted diagrammatically. In addition the number of iterations, the displacement error between successive interactions and the maximum earth pressure error are output.
More detail about the analysis methodology and underlying theory can be found in the manual.
A video giving an overview of the analysis methodology and advanced analysis options is available at: http://www.youtube.com/watch?v=wNCmXRbA8hw
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Advanced Analysis Options - Undrained to Drained Modeling
Frew will now calculate the undrained pore pressure based on the Mohr Coulomb or Modified Cam Clay models. The user can do this by assigning a drained material to the undrained material in the Property Table (see below) and the program uses this as a basis for its pore pressure calculation.
This enables the user to bypass the manual sequence (outlined below) that was required for the modeling of drained to undrained soils and vice versa.
Typical Modelling Sequence:
- initialise stresses with drained materials
- Switch to undrained soils
- Model sequence
- Apply estimated undrained pore pressure profile to undrained materials (no movement)
- Switch to drained parameters (no movement)
- Switch to long term pore pressures
More detail about this method and the underlying theory can be found in the manual.
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Advanced Analysis Options - Wall relaxation and creep
Frew enables the user to specify the wall relaxation which can be used to model creep.
Creep is modeled by applying the relaxation and change in Youngs Modulus.
More detail about this method and the underlying theory can be found in the manual.
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Advanced Analysis Options - Modelling Axi-Symmetric Problems
Frew provides facilities for analysis of a plain strain excavation (e.g. an infinitely long trench) by enabling the user to apply a rigid boundary at the centerline of the problem.
More detail about this method and the underlying theory can be found in the manual.
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Modeling of Props and Anchors
Struts may be inserted (or removed) at any node at any stage in the analysis. More than one strut may be defined at a particular node, and not all struts need to act simultaneously.
Modeling an applied moment or moment restraint
An applied moment or a moment restraint at a node can be modelled by using a strut with an inclination of 90 degrees and a non-zero lever arm together with an applied pre-stress force or a stiffness respectively.
Modelling Anchors
Stressing an anchor may be modelled by specifying a strut with a pre-stress force equal to the stressing force and a zero stiffness. The stiffness of zero would maintain a constant force at the point of application throughout the analysis.
In subsequent stages after the anchor is 'locked off' it is usually convenient to remove this strut and insert a strut that models both the pre-stress and stiffness of the anchor
Inclined anchors are modeled by specifying an inclination to the horizontal and if they are not applied at the vertical axis of the wall a lever arm can be specified to allow for this.
More detail about this method and the underlying theory can be found in the manual.
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Easy Inputs - Model Wizard and Graphical Input
The New Model Wizard is accessed by selecting the `File | New´(Ctrl+N) option from the main menu, or by clicking the 'New' button on the Frew toolbar. The New Model Wizard is designed to ensure that some basic settings and global data can be easily entered.
Clicking "Finish" completes the wizard and creates Stage 0 with the input data.
The graphical input view will open to allow entry of node levels (if these are being created manually). If automatic node generation was selected, the graphical input view will show a single soil zone extending the full depth of the problem.
More soil zones can be added as required to set up the initial ground profile for Stage 0.
Strut and surcharge data is added separately, and additional stages created
A video showing how the wizard and graphical input can be used to create a file is available at the following link: http://www.youtube.com/watch?v=dAPEMNJzS5o
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Easy Inputs - Automatic Node Generation
Frew now generates the nodes automatically based on inputted levels (of stratigraphy, struts etc.). This option also allows the user to input the toe level of the wall or use the toe level calculated from the stability check.
The whole process, including the setting up of the problem, the stability analysis and the automatic node generation is demonstrated in a tutorial video: http://www.youtube.com/watch?v=UjN7-2QuppM
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Easy Inputs - Batch Analysis for Multiple Toe Levels
Frew allows the user to specify a number of toe levels.
By doing this, they can create a number of files with wall toe depths at different levels. Consequently, the outputs and performance of the walls at different toe depths can be compared.
A demonstration showing how to set this up and create the multiple files are available at http://www.youtube.com/watch?v=oS5VGhDqE0E
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Partial Factors
Frew allows the user to specify partial factors, which can be applied to material properties or surcharge input parameters.
A pre-defined list of code compliant partial factors are also available:
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Graphical and Tabular Outputs
The graphical and tabular outputs in Frew have been developed for use by Eningeers in design and as images in reports.
The tabular outputs can be printed by stage or exported to excel for further analysis.
The graphical output enables the user to zoom and scale. Specific outputs can be chosen (e.g. displacements or bending moments) and the envelope of results can be viewed.
A demonstration of the graphical and tabular outputs is available at http://www.youtube.com/watch?v=JL8N6oNNP4I
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Verification with Site Data and FE
Frew has been used for a number of years and there are numerous examples of where Frew results have been compared to 2D FE and empirical results.
Various illustrations of this can be found in literature:
Ciria C580 (Appendix A10)
Rocoe, H. (2003), Retaining Wall Movements, CTRL Contract 430 – Ashford Tunnels, BGA International Conferenve on Foundations
An example of where Frew has been compared to site data can be found in the following case study: http://www.oasys-software.com/casestudies?id=56/
A further example of where Frew has been used to obtain data for 3D FE analysis and its results compared can be found in the following case study: http://www.oasys-software.com/casestudies?id=63
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