Design Standard

Slide3 currently supports the Eurocode 7 (EC7) and British Standard (BS) 8006 design codes. The loading and resistance factors stipulated in the codes are automatically applied to the forces and material properties during the analysis of the factor of safety.

Specifying a Design Standard

To specify a design standard for analysis, go to Project Settings > Design Standards. Here you can select from a list of supported design standards along with their associated combinations of partial factors. The partial factors can be viewed by clicking View Partial Factors.

You can also define your own list of partial factors by clicking the Add button, and then selecting an existing standard as a base. These lists can also be imported and exported between Slide3 models.

What are Partial Factors?

In the Eurocode 7 and British Standard, factors must be applied to the loading and resistance components of the slip surface analysis. These factors serve to amplify the actions inducing the slope failure and/or reduce the capacity of resisting materials. Different factors are applied to different components of the analysis – hence the term “partial” is used to describe them.

Depending on the standard, the factors can be multiplicative or divisive. If you are adding your own factors, be sure to review the standard that you are basing the set of factors from.

In some standards, separate factors are required depending on whether a particular force in the analysis is favourable or unfavourable with regards to preventing the slip surface from mobilizing.

A sample list of partial factors from the BS 8006: 2010 is provided below.

For anchorage, the partial load factors can be overridden when defining Support Properties. See the section Specifying Support Capacity later in this document for more information.

Specifying Applied Loads

If a design standard as been specified in Project Settings > Design Standards, then the interface for adding Loads to the model will have an option to specify whether an applied load is Variable or Permanent (Live and Dead, respectively). Depending on the selection and, if applicable, whether the load is favourable or unfavourable to prevent sliding of the slip surface, Slide3 will automatically apply the appropriate partial factor(s) during the analysis.

Single Source Assumption

It is important to note that the distinguishing whether a load is “favourable” or “unfavourable” can sometimes be ambiguous. In particular, the weight of soil above the slip surface can be partitioned into unfavourable and a favourable regions, such as in the example below, where W_f is the favourable weight of soil and W_u is the unfavourable weight of soil. The slices to the right of the dashed line will instigate sliding while the slices to the left will react against sliding.

In such a case, it may be appropriate to use the “single source assumption”, which applies the unfavourable factor to the entire soil mass, regardless of location. Since the soil weight arises from a single source, it may not be appropriate or realistic to apply different factors to different portions of the same component or “source”. This assumption is discussed in Bond et al. (2013).

This single source assumption can be toggled during selection of a Design Standard via the “Use a single factor for all slice weight actions” checkbox. If the checkbox is toggled ON, Slide3 will assume the value of the unfavourable partial factor for Permanent Actions for ALL slices. If it is toggled OFF, then the dip direction of a slice will determine whether the weight of that slice is favourable or unfavourable.

Factoring Soil Weight

The weight of soil in a slice affects many of the terms in the calculation of factor of safety in a slip surface, which are often inter-dependent. These terms include the base normal and shear force for each slice, forces applied on anchors, and pore pressure. As such, there is ambiguity in some design standards with regards to whether the favourable or unfavourable factors of soil weight should be applied for each specific component of the analysis. Specifically, in Eurocode 7, there are two factors for soil weight: γ_γ which reduces the soil weight and γ_G which increases the soil weight.

In the Limit Equilibrium Method (LEM), soil weight has a transcendental relationship with the equilibrium forces used to solve for the factor of safety and thus cannot always be separated into terms which can specifically be designed as actions or resistances in the force balance equations. With exception to anchor calculations (described in the next paragraph), the weight of soil for a given slice is thus generally taken as a single value in Slide3. This means the same value of weight is used to calculate all contributions (base normal and shear forces, pore pressure, etc.) to the analysis. For this reason, γ_γ will be applied globally (i.e. directly to the input values in the Material Properties dialog). If the single source assumption is used, then this single value is the “unfavourable” factored value for every slice, even though a higher value of soil weight may increase the resisting frictional forces. If the single source assumption is not used, then the factor for each slice will depend the dip direction of its base.

For anchor calculations, by default, the factors for permanent applied loading (e.g. γ_G in EC7 and f_fs for BS8006) are not considered during effective stress calculations as these unequivocally increase the effective stress and result in non-conservative anchor forces. However, this can be toggled by checking the “Apply permanent loading factor when calculating effective stress for anchors” checkbox, shown in the interface below. Note that as explained above, for Eurocode 7, the material resistance factor for unit weight, γ_γ, will always be applied to reduce the weight of soil above the anchor.

Specifying Support Capacity

When specifying support capacities in Slide3, there are options in up to three different locations to apply reduction factors to the strengths. The first location is via the Design Standards interface, as previously discussed.

The following image shows the interface that appears when defining Support Properties (Properties > Define Support). The Geosynthetic support type is currently selected.

As shown in the next image, specifying Manufacturer's Library dialog involves defining partial factors (RFcr, RFid, RFd, and Fr). These factors are typically used instead of the Design Standard factors, rather than multiplicatively.

References

Bond, A.J., Schuppener, B., Scarpelli, G. and Orr, T.L.L. (2013). Eurocode 7: Geotechnical Design Worked Examples. [workshop presentation] Luxembourg: Publications Office of the European Union, pp. 16-31.