Iga Concrete Designs

The design is unacceptable if the degree of utilization is > 100%

© In DA2, the partial factor yg = 1.35 increases the actions but the material properties are not changed, since ym = 1.0. An additional partial factor YRe = 1.1 is applied to resistance.

© It has been assumed that the resistance factor should be applied to both c (i.e. c' or cu) and to tan 9.

& As for DA1, the drained analysis is critical. DA2 has a marginally lower degree of utilization than DA1.

© In DA3, partial factors greater than one are only applied to material properties.

© DA3 is identical to DA1-2, since for slopes the actions are treated as 'geotechnical' and are therefore not modified.

® Traditional calculations apply different factors of safety in undrained and drained analyses (e.g. 1.5 and 1.3, respectively). Using these traditional factors of safety produces a marginally more conservative design.

Charts do not enable the application of different partial factors to permanent actions (e.g. self-weight) and variable actions (e.g. surcharges). This makes their use for anything but the simplest of cases difficult. Although the ready availability of slope stability software largely precludes the need to use charts, they do provide a simple and quick means of assessing the likely safe angle for a slope and to check the results obtained from software.

9.8.4 Road cutting (using slope stability software)

Example 9.4 considers the 8m deep motorway cutting shown in Figure 9.13, which passes through 1.5m of granular head deposits overlying firm to stiff, medium strength CLAY. The groundwater table is just below the head deposits and is kept below the carriageway level by drainage. Figure 9.13 indicates the steady state position of the phreatic surface. The cutting is to be widened and land ownership constraints mean that it would be preferable to steepen the slope to a 1:2.25 gradient.

Head deposits

1.5m

Head deposits

1.5m

Figure 9.13. Motorway cutting

Such problems are not amenable to simple analysis using charts so the problem has been analysed using Bishop's Simplified Method.10 This example demonstrates the issues of applying standard slope stability software in the context of design to EN 1997-1 and highlights the difficulties involved in applying DA2 to slope stability problems.

Notes on Example 9.4

O In Design Approach 1 Combination 1 (DA1-1), partial factors yg = 1.35 and Yq = 1.5 are applied to permanent and variable actions, respectively, and partial factors on material properties and resistances are 1.0. In slope stability software, this can be achieved by factoring the soils' weight densities by 1.35 and any applied surcharges by 1.5. A search is then made for the critical slip circle with a 'target' factor of safety of 1.0.

© In Design Approach 1 Combination 2 (DA1-2), partial factors yg = 1.0 and Yq = 1.3 are applied to permanent and variable actions, respectively; partial factors on resistances are 1.0; and partial factors on material properties are Y<p = Yc = 1.25. This can be achieved by factoring the soils' effective strengths down by 1.25 and any applied surcharges up by 1.3. A search is then made for the critical slip circle with a target factor of safety of 1.0.

© In Design Approach 2 (DA2), partial factors yg = 1.35 and yq = 1.5 are applied to permanent and variable actions, respectively; partial factors on material properties are ym = 1.0; and on sliding resistance YRe = 1.1. If the 'single source' principle is applied, then the same value of yg is applied to both favourable and unfavourable actions. This can be achieved by factoring the soils' weight densities by 1.35 and any applied surcharges by 1.5. A search is then made for the critical slip circle with a target factor of safety of 1.1.

© In the variation of Design Approach 2 known as DA2*, favourable and unfavourable actions are meant to be treated separately, with YG,fav = 1.0 applied to the former and yg = 1.35 to the latter.11 However, this cannot easily be achieved with existing software, so instead we apply YG,fav = 1.0 to all permanent actions and an 'intermediate' factor yQ/G = yq/ YG = 1.5/1.35 = 1.11 to any applied surcharges, and then search for the critical slip circle with a target factor of safety of yr x yg = 1.1 x 1.35 = 1.485.

Example 9.4. Road cutting (using slope stability software)

Input parameters

0 0

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