You are reading the online version of the LimitState:GEO manual, which is also available as a pdf.

Frequently Asked Questions

1. Is the Adequacy Factor calculated by LimitState:GEO the same as the ‘Factor of Safety’ calculated by other software?

   In LimitState:GEO, an Adequacy Factor may be applied to any load or to the self weight of any body of material. The Adequacy Factor that is returned by LimitState:GEO when it has completed solving is the factor by which all the specified loads/self weights must be multiplied by to cause collapse. The Adequacy Factor is similar to a Factor of Safety on load. However in geotechnics, the Factor of Safety is often a value applied to the material strengths ( and tan). In this case the Adequacy Factor is not the same as the Factor of Safety.

2. How do I change the accuracy of the solution?

   Solution accuracy in LimitState:GEO is modified by changing the nodal resolution. This in turn alters the number and range of the set of potential slip-lines from which the solver will select the critical solution. Increasing the nodal resolution will increase the solution accuracy (except in rare cases - see FAQ later in this section). Further information on changing the nodal resolution may be found here.

3. I should get a symmetrical failure mode but I don’t, or the slip-lines are symmetrical but the animation is not.

   For certain classes of problems (e.g. certain bearing capacity problems) the plastic collapse load for a symmetrical failure mechanism is identical to an asymmetrical mechanism. In many cases LimitState:GEO will generate the symmetrical failure mechanism, however any asymmetry in the initial set up may skew the result to the asymmetrical case (but generate the same collapse load). Were the problem geometry is symmetrical, the most likely cause of asymmetry will be due to numerical tolerance issues.

4. How accurate is LimitState:GEO?

   This is a common issue for numerical analysis codes. Uncertainties exist all the way through geotechnical design calculations from the site investigation data through to construction. The common question at the stage of the numerical calculation is ‘what is the accuracy’. This depends on two issues: (1) the theoretical model underpinning the numerical analysis (2) the accuracy of the numerical method itself. The theoretical model underpinning the numerical analysis is the theory of plasticity which has had a long history of application in Geotechnical Design. The majority of text book stability calculations are based on this theory or on simplifications of this theory (see Stability analysis). Selected known issues with limit analysis are discussed here. LimitState:GEO is regularly benchmarked against a known set of limit analysis solutions from the literature. These tests are described in more detail here and may be accessed via the internet. Reference to these results can provide useful guidance as to the expected accuracy of the software over a range of problem types. Accuracy does of course depend on nodal resolution. The core analysis method used in LimitState:GEO is based on finding the critical translational mechanism that will cause collapse. For a wide range of problem types this leads to good results. For problems where rotational failure mechanisms are likely to be critical (e.g. consider a wall rotating around a pivot), LimitState:GEO currently provides the (optional) capability to model rotations at the edges of pre-defined solid objects. However, though this will lead to a more conservative prediction of the collapse load, as rotations are not permitted to occur within pre-defined solid objects, in certain highly confined problems this may still overestimate the true collapse load significantly. Further details regarding solution accuracy may be found here.

5. I have made a change to the problem geometry that should theoretically have no effect on the collapse load, yet the adequacy factor computed by LimitState:GEO has changed. Why is this?

   This is a common issue that relates to the nodal distribution used in the model. It should in general only result in small (a few %) change in adequacy factor. The default setting for LimitState:GEO is to utilise a fixed number of nodes for the problem. If the geometry is altered, for example by deleting a body of soil that plays no part in the collapse mechanism, then the nodes that would have been placed in this body are redistributed elsewhere and should increase the accuracy of the solution (cause a minor reduction in adequacy factor). Nodes are also used on boundaries and with the water table, so modification of these can lead to similar issues. To avoid this issue, change the nodal settings in LimitState:GEO to fixed spacing rather than a a constant overall number. For further details concerning setting nodal spacing see here.

6. I have increased the number of nodes but the adequacy factor has increased rather than decreased.

   This will normally only occur when a coarse nodal resolution is being used. With a small increase in the number of nodes, the available slip-line locations may move to slightly less favourable locations for collapse, thus leading to an increase in adequacy factor. A large increase in nodal resolution should lead to the expected decrease in adequacy factor.

7. I am benchmarking LimitState:GEO results against known precise solutions, but am unable to get a good match.

   Some problems e.g. the bearing capacity problem in cohesionless soils are extremely sensitive to change in parameters. A change in the angle of shearing resistance by a few degrees can double the capacity in certain circumstances. The results generated by LimitState:GEO are subject to similar sensitivity. A more logical question to ask is what is the change in input soil strength parameters required to generate the result produced by LimitState:GEO. Considered this way, the discrepancies become much smaller (typically of the order of a few percent).

8. Can LimitState:GEO model 1D structural elements or loads?

   LimitState:GEO works in general with 2D bodies. To model what is essentially a 1D strut or rod that is able to transmit loads, there are two options: (i) it can be represented as a thin 2D Solid (ii) It can be modelled as a Engineered Element.

9. What units does LimitState:GEO work in?

   LimitState:GEO is designed to work in either Metric or Imperial units and these are displayed in the Wizards, Property Editor, and Reports. If parameters are only known in other units, then the built in Calculator provided common conversions for all relevant units.

10. I think I have set the problem geometry and loads up correctly, but the problem will not solve

    This is typically due to one of two problems: The problem is geometrically locked. Either there is nowhere for the soil to deform into or the soil has a high angle of shearing resistance and no feasible mechanism can be found for the given number of nodes. To correct this problem,check that the boundary conditions are correctly specified and/or increase the number of nodes. The problem can also arise when a slope is modelled that is shallower than the angle of shearing resistance of the soil. In this case no factor on self weight will cause collapse. There is no fixed (or possibly symmetry) boundary to which the soil is anchored. Any applied load thus cause the whole soil mass to fly off into space. To correct this problem, check that there is at least one fixed boundary (in certain circumstances a symmetry boundary condition can fulfil this function, in that it is, in effect, a fixed but perfectly smooth boundary). The problem can also arise when a slope is modelled that is steeper than the angle of shearing resistance of the soil. In this case no factor on self weight can prevent collapse. Further guidance on Troubleshooting problems can be found here.

11. How do I model tension cracks?

    LimitState:GEO provides a tension cutoff material type. This can be assigned to either Boundaries and Solids. A tension cutoff might be added to a Boundary of a footing for example to permit breakaway of a cohesive soil from the footing. Assigning tension cutoff to a Solid permits tension cracks to form within a body of soil. This might be used for example in a slope stability analaysis. Further information on the tension cutoff material type may be found here.

12. Can LimitState:GEO model water filled tension cracks?

    If a water table lies above any crack that forms, then LimitState:GEO will assume that the crack fills with water and will apply water pressures to the side of the crack. The crack will therefore normally be longer in this case compared to a case with no water present. It is not necessary for the crack to extend to the surface for it to fill with water.

13. Can LimitState:GEO model rotational failures about a single point?

    Yes. For example, if you want to model a retaining wall failing by rotation about a single point, LimitState:GEO allows you to do this by placing the retaining wall on a rigid material and applying a ’No-tension cutoff’ material in the interface between the wall and the rigid base. Assuming ’Allow rotations’ is set to ’Along edges’, then this setup will normally ensure a rotational failure mode is predicted.