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

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

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

4. How accurate is LimitState:GEO?

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?

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

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

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

9. What units does LimitState:GEO work in?

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

11. How do I model tension cracks?

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

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 (c_u and tanφ). 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 sliplines from which the solver will select the critical solution. Increasing the nodal resolution will increase the solution accuracy (except in rare cases - see Q6).

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). 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 and
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 Section 5.2 of the User Manual). Selected known issues with plasticity analysis are discussed in Section 6.3.5 of the User Manual.

LimitState:GEO is checked against a wide range of standard closed form solutions in the literature. Typical accuracy is within 1-3% when expressed as the variation in terms of cu or tan  required to give the exact benchmark result. It can however rise to 5-6% or higher for certain problems. Accuracy in terms of collapse load can be higher. 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 problems with no or minimal rotational kinematic constraints, the method gives good results. For problems where rotation is constrained to occur such as a wall rotating around a pivot, LimitState:GEO provides a partial ability to allow rotations of solid bodies. This may lead to larger overestimates of collapse load.

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.

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

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, it is necessary to represent this as a thin 2D Solid. 1D Boundary lines only alter the yield condition on that line. The exception to this rule is a Soil Nail. With correct use of parameters and geometry, this could be used as a 1D rod.

9. What units does LimitState:GEO work in?

LimitState:GEO is designed to work in SI 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 it will not solve

This is typically due to one of two problems:

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.

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.