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Observational Methodand Eurocode 7

Antun Szavits-Nossan

University of Zagreb, Croatia

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Observational Method in Eurocode 7 (1995) Acceptable limits of behaviour shall be

established before construction commences; It shall be shown that there is an acceptable

probability that the actual behaviour will be within these limits;

… A plan of contingency actions shall be devised

which can be adopted should the monitoring reveal behaviour outside these limits.

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What is the acceptable probability ? Eurocode 7 does not refer to acceptable

probability any further. The acceptable probability determines the

degree of conservatism for the initial design prior to construction. The higher the chosen probability, the more conservative will be the initial design.

The lower bound of acceptable probability corresponds to the design based on characteristic values from Eurocode 7(a cautious estimate of ground conditions).

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… continued

Eurocode 7 implicitly requires a margin of conservatism for the design based on characteristic values, because the actual behaviour may step outside acceptable limits. If this happens, there should be provisions for the application of planned contingency actions. The required margin of conservatism is, however, not specified.

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How conservative should one be?

Peck (1969) lists eight ingredients of the complete application of the observational method, and one of them is: ‘(c) establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions.’(Which should correspond to characteristic values of Eurocode 7).

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… continued

Powderham (1994) supports a moderately conservative initial design based on more probable conditions and introduces the progressive modification of design through the application of the observational method, which proved to be a very efficient approach.

Muir Wood (2000) suggests a conservative initial design for tunnels based on a simplified economical analysis.

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Initial design: constant probability (Muir Wood 2000)

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… continued

The relative cost of contingency actions is defined as:

contingency actions cost for 0initial design construct. cost for 1

pk

p=

==

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Probability distribution (Muir Wood)

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Triangular probability distribution

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Initial design: triangular probability

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Construction cost

Probability distribution

Initial design Contingency actions

Constant

Muir Wood (2000)

Triangular

(this study)

f( )p g( )p

Ap (1 )kA p-

11

2p

Aæ ö- ÷ç - ÷ç ÷çè ø

12p

kA-

f( )p g( )p

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Minimal cost and optimal probability for adequate design

Total estimated cost

Minimal cost

Solution for both the constant and the triangular distribution

f( ) (1 )g( )C p p p= + -

min 0dC

Cdp

® =

minCp = -

k1

12

(k : relative cost of contingency actions)

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Optimal probability of adequate design

= -pk1

12

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Conclusions

Eurocode 7 provides no guidance for the selection of the acceptable probability on which to base the initial design when using the observational method.

Many agree that the initial design should be more conservative than the Peck proposal based on most probable conditions.

The optimal degree of conservatism for the initial design depends on the relative cost of contingency actions (k).

This may be the way to acceptable probability.

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References

Eurocode 7 (1995). ENV 1997-1 Eurocode 7, Geotechnical design, Part 1: General rules (together with the United Kingdom National Application Document). British Standards Institution, London.

Muir Wood, A. (2000). Tunnelling: Management by design. E & FN Spon, London.

Peck, R. B. (1969). Advantages and limitations of the Observational Method in applied soil mechanics. Géotechnique, 44 (4), 619-636.

Powderham, A. J. (1994). An overview of the observational method: development in cut and cover bored tunnelling projects. Géotechnique, 19 (2), 171-187.