Anchors, soil nails, piles, geosynthetics and walls add a force where a slip surface crosses them. In OGR Slip2D a support is a line from its head, at the slope face, to its tail, inside the ground, and it takes its properties from a named support type defined once for the project. The force of a support can enter the equilibrium as active or as passive, and the back analysis finds the force a design needs to reach a target factor of safety. How each type develops its force is described in Section 3.10.
2.6.1Defining support types#
Supports cannot be placed until the project has at least one support type. Choose Properties → Define Support… to open Define Support Properties. The list on the left, Support Types, holds the named property sets of the project, each shown with its kind in brackets; + Add creates one, Duplicate copies the selected one and − Remove deletes it (the last one cannot be removed). Opened on a project without types, the dialog starts with Type 1, an End Anchored support.
| Field | Meaning |
|---|---|
| Name | Name of the property set, used in every list of supports |
| Support Type | One of nine kinds (Table 2.6.2). Changing it resets the parameters to the defaults of the new kind. |
| Force Application | Active or Passive (Section 2.6.2); set to the default of the kind when the kind is chosen |
| Force Orientation | Direction of the support force (Table 2.6.3); set to the default of the kind |
| User Angle (from horizontal) | Angle of the force, enabled only with User-defined angle |
| Parameters | The properties of the kind, grouped in tabs for some of them; a short description of the kind is shown above them |
Parameters are labelled with their stored names in words (Out Of Plane Spacing, Bond Length Percent…) and carry their units: kN, kN/m, kPa, m, degrees or per cent. These units are fixed and are not converted to the unit system of the project. The capacities of anchors, tiebacks, nails and piles are per support, and the program divides them by the out-of-plane spacing to obtain a force per metre of slope; a geosynthetic is described per metre of sheet width, with its strip coverage.
| Kind | Defaults | Parameters |
|---|---|---|
| End Anchored | Active, parallel to support | Anchor capacity 200 kN; out-of-plane spacing 1.5 m. A constant force, the capacity divided by the spacing, wherever the surface crosses the bolt. |
| Grouted Tieback | Active, parallel to support | Tensile / Plate: tensile capacity 600 kN, plate capacity 300 kN, shear capacity 0 kN (0 = no shear). Pullout: bond strength 150 kN/m, bond length 30 % of the length, at the tail. Geometry: out-of-plane spacing 2.0 m. |
| Grouted Tieback with Friction | Active, parallel to support | As the Grouted Tieback, with Pullout (Friction) instead: interface model Linear (Mohr-Coulomb) or Hyperbolic, adhesion 60 kPa, friction angle 25°, grout diameter 0.15 m, bond length 30 %. |
| Soil Nail | Passive, parallel to support | Tensile / Plate: tensile capacity 100 kN, plate capacity 50 kN, shear capacity 0 kN. Pullout: bond strength 30 kN/m, along the whole length. Geometry: out-of-plane spacing 1.5 m. |
| Pile / Micropile | Passive, tangent to slip surface | Failure mode Shear or Ito & Matsui; pile shear strength 100 kN; out-of-plane spacing 2.0 m; pile diameter 0.6 m (Ito & Matsui only); force location at the slip surface or at the centroid of the pressure diagram. |
| Geosynthetic | Passive, parallel to support | Tensile: tensile capacity 50 kN/m, strip coverage 100 %, connection strength 0 kN/m, anchorage None, Slope face, Embedded end or Both ends. Pullout & Stripping: adhesion and friction angle (0 kPa, 25°), coefficient of interaction Ci (0.8) or friction factor F* (0.6). F* vs Depth: F* constant or a function of depth. |
| User Defined | Active, parallel to support | Out-of-plane spacing 1.0 m and a table of force per support against distance from the head, interpolated linearly and constant beyond its ends (by default 100, 200 and 100 kN at 0, 5 and 10 m). |
| Retaining Wall (EFP) | Active, horizontal | Profile Uniform, Triangular (default), Trapezoidal or Custom; pressure 20 kPa (uniform); equivalent fluid pressure 25 kN/m³ (triangular and trapezoidal); flat part 60 % of the height (trapezoidal); force location. Cannot be placed as a pattern. |
| Helical Anchor | Active, tangent to slip surface | Capacities: tensile capacity 150 kN, head assembly capacity 100 kN, shear capacity 0 kN. Anchor: shaft Round or Square, shaft width 0.09 m, 3 helices of average diameter 0.25 m at 1.5 m spacing. Geometry: out-of-plane spacing 2.0 m. Its capacity is computed from the soil. |
The tieback, nail, geosynthetic and anchor capacities combine several failure modes (tensile rupture, pullout behind the slip surface, stripping of the head or plate), and the force at a crossing is the smallest that applies there. The Ito & Matsui mode of the pile derives its force from the soil flowing between the piles of a row (Ito & Matsui, 1975). The equations are in Section 3.10.
OK stores the list. Supports already placed follow their property set: a change of kind or of parameters applies to them at once. A support whose set has been removed falls back to the first set of the same kind or, when there is none, to the default properties of that kind, and the analysis notes say so.
2.6.2Force application and orientation#
The force of a support can enter the equilibrium of the sliding mass in two ways (Duncan & Wright, 2005):
- Active: the force reduces the driving action, as a prestressed anchor does before any movement: .
- Passive: the force adds to the resistance and is mobilised only as the mass moves, as an untensioned nail: , so the support force is divided by the factor of safety like the strength of the soil.
Here and are the resisting and driving sums and the support force. For the same force the two give different factors of safety; the defaults of each kind are listed in Table 2.6.2.
| Force Orientation | Direction of the force at the crossing |
|---|---|
| Tangent to slip surface | Along the slip surface, resisting the movement |
| Parallel to support | Along the support, from head to tail: a bolt in tension pulls the sliding mass towards its anchor |
| Bisector (tangent + parallel) | Halfway between the two above |
| Horizontal | Horizontal, against the failure direction |
| Perpendicular to pile | Across the support, on the side that resists the movement |
| User-defined angle | At User Angle (from horizontal) |
Each placed support keeps its own force application and orientation. Add Support copies them from the support type when the support is placed; Add Support Pattern… takes them from its own dialog, where they start at Active and Tangent To Slip whatever the type says. Changing them later in Define Support does not change the supports already placed: edit those with Properties… in their right-click menu, or with Modify Pattern… for a row.
2.6.3Placing supports#
One support at a time#
- Choose Support → Add Support.
- Click the head, at the slope face, then the tail, the end inside the ground. Clicks are snapped like any drawing.
- Repeat for the next support; press Esc to finish.
The order of the clicks matters: the stripping length is measured from the head and the pullout length from the tail, and a force parallel to the support pulls from head to tail. Add Support always uses the first support type of the list in Define Support; to give a support another type, use Support → Modify Support… or Properties… in its right-click menu.
A pattern of supports#
Support → Add Support Pattern… places a row of identical supports along the slope. Fill in the dialog, press OK, then click two points along the slope face: the first support has its head on the first point and the others follow at the in-plane spacing along the segment, as many as fit.
| Field | Meaning | Default |
|---|---|---|
| Type | Support type of the row; Retaining Wall (EFP) types are not offered | First type |
| Length | Length of every support | 6.0 m |
| In-plane spacing | Distance between heads along the picked segment | 1.5 m |
| Mode | Angle from horizontal: every support at Angle. Perpendicular to boundary: normal to the picked segment. Vertical (depth): straight down. | Angle from horizontal |
| Angle | Angle of the supports from the positive axis, counter-clockwise; negative values point downwards. Only for Angle from horizontal. | −15° |
| Flip angle 180° | Reverses the direction from head to tail | off |
| Application, Force direction | Force application and orientation given to every support of the row | Active, Tangent To Slip |
The default angle of −15° points down and to the right, into a slope whose crest is on the right (failure from right to left); for a slope that fails from left to right use an angle such as −165°. With Perpendicular to boundary, click the two points from left to right so that the supports point into the ground.
2.6.4Editing supports#
| Command | What it does |
|---|---|
| Right-click → Properties… | Opens Support Properties for that support: name, support type, force application, force orientation, user angle, head and tail coordinates and colour, with its length and axis angle. |
| Right-click → Modify Pattern… | Finds the supports of the same kind and similar length laid at a regular spacing, and edits them together: type, length, in-plane spacing, angle from horizontal, application and orientation. |
| Support → Modify Support… | Choose a support from a list, then its support type. |
| Support → Move Support… | Choose a support, then a displacement in and in ; both ends move, so length and angle are kept. |
| Support → Stretch Support or right-click → Stretch | Click near an end of a support, then the new position of that end. The end moved is the one nearest the first click. |
| Support → Ungroup Support Pattern | Releases every pattern of the model at once, so that each support can be edited on its own. |
| Support → Delete Support or right-click → Delete | Click supports to delete them; Esc finishes. |
Changes to supports are not on the undo stack. Supports are not moved when the geometry is edited, so check them after reshaping the slope. Tools → Property Tables → Support Properties Table lists the supports of the model; in the Interpret window, Data → Support Force Analysis… and Data → Support Force Diagram… show the force each support develops along its length and which failure mode governs (Section 2.8).
2.6.5Back analysis of the support force#
Support → Back Analysis of Support Force… answers the design question the other way round: what horizontal force is needed for every slip surface to reach a target factor of safety? The factor is fixed at the target, so the force comes out of the resisting and driving sums without iteration, under both the active and the passive assumptions; the surface that needs the largest force is reported (Duncan & Wright, 2005).
| Field | Meaning | Default |
|---|---|---|
| Target factor of safety | Factor of safety every surface must reach (0.001 to 100) | 1.3 |
| Elevation (y) | Elevation of the horizontal force. It affects only Bishop, which takes moments; for Janbu the position of a horizontal force does not matter. | 0.0 |
| Analysis method | bishop_simplified, janbu_simplified or janbu_corrected; the method is not defined for the others | bishop_simplified |
The search configured in Surface Options is run on the model, with the design standard if one is on. The status bar reports the required force for the target factor of safety, the active and passive forces of the governing surface, its factor of safety without support and the number of surfaces analysed. The passive value is the larger of the two, so it is the conservative one. In the Interpret window, Data → Back Analysis… computes the same quantities for the critical surface shown there. The back analysis neither uses nor changes the results of Analysis → Compute.
Each kind of support is a plug-in that declares its parameters, its default application and orientation and the force it can develop at a distance from its head; the dialog is built from those declarations. Every placed support names its property set, so two sets of the same kind with different capacities stay distinct in the analysis.
Source: ogr_core/support/support.py · ogr_slip2d/support_integration.py · ogr_slip2d/back_analysis.py