Slope Stability Da3 Eurocode

Soil or rock?

No (STR)

Yes (GEO)

Yes (GEO)

No (STR)

No (STR)

Below ground level

Yes (STR)

Yes (STR)

Yes (STR)

No (GEO)

No (GEO)

Magnitude uncertain?

No (GEO)

No (GEO)

No (GEO)

Yes (STR)

Yes (STR)

tChoice made in Eurocode 7 Designers' Guide3 ?It is unclear whether this should be Yes or No tChoice made in Eurocode 7 Designers' Guide3 ?It is unclear whether this should be Yes or No

6.3.4 Choice of design approach by different European countries

Eurocode 7 Part 1 allows each country to specify in its National Annex which design approach must be used within its jurisdiction. The choices made by the countries within CEN4 are summarized in Figure 6.18 (for slopes) and Figure 6.19 (for other geotechnical structures).

As Figure 6.18 shows, the most popular design approach (DA) for slopes is DA3 (adopted by 65% of CEN countries), followed by DA1 (25%). Only one country (Spain) has chosen DA2, while Ireland permits any of the design approaches to be used. As discussed in Chapter 10, DAs 1 and 3 produce almost identical results when applied to slope stability, so almost all of Europe has adopted a common approach for this issue.

Figure 6.19 illustrates the choice of design approach for structures other than slopes. DA2 has been adopted by 55% of CEN countries, followed by DA1 (30%), and then DA3 (10%). Ireland permits any of the design approaches to be used. This division between the various European countries reflects their respective traditions and practice in geotechnical engineering, rather than any judgment regarding the philosophical and engineering merits of each design approach.

Design Approach adopted for slopes

M! DA2 DA3 ] Unconfirmed

Design Approach adopted for slopes

M! DA2 DA3 ] Unconfirmed

Figure 6.18. National choice of Design Approach for slopes
Eurocode Da1 Geotechj

v Also:

Figure 6.19. National choice of Design Approach (other than for slopes)

v Also:

Figure 6.19. National choice of Design Approach (other than for slopes)

6.4 Alternative ways of dealing with design uncertainty

The following sub-sections briefly review alternative ways of dealing with uncertainties in geotechnical design.

6.4.1 Allowable or working stress design (ASD or WSD)

Early codes of practice for geotechnical design ensured reliability in calculations by the introduction of a 'factor of safety', applied in some cases to the structure's dimensions (e.g. to the width of a footing or the embedded depth of a retaining wall) and in other cases to the structure's load-bearing capacity (e.g. to ultimate bearing capacity or passive earth pressure).

According to Meyerhof,5 the concept of a factor of safety was first used in geotechnical design by Belidor and Coulomb in the 18th Century. During the first half of the 20th Century, it was common practice in Europe and North America to utilize a single, 'global' factor of safety F in geotechnical design, defined as the ratio of the ultimate resistance of the foundation Qult to the applied load P:

Qult p

Re-arranging this equation enables the allowable load Pa to be determined:

Values of F typically vary between 1.3 and 3.0, depending on the structure and the type of failure considered.

6.4.2 Load and strength factor design

In the middle of the 20th Century, first Taylor6 and then Brinch Hansen7 introduced the concept of partial factors into geotechnical design, with separate factors being applied to different types of load, shear strength parameters for soils, and on the components of pile capacity. The partial factors recommended by Brinch Hansen, summarized in the table below, are not too dissimilar to the partial factors that have found their way into Eurocode 7 (see Section 6.2.5) and were chosen to give about the same design outcomes as conventional global factors of safety.

The philosophy of material factor design is to apply partial factors as close as possible to the source of uncertainty.

Parameter

0 0

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