Info

8-14%

Epistemic uncertainty is highly relevant to geotechnical engineers, who rarely have sufficient test data with which to justify a statistical approach to parameter selection.

In 1991, the Ground Board of the UK's Institution of Civil Engineers published a report10 on site investigation, which recommended:

'National guidelines on the extent, intensity and quality of ground investigation should be produced for the benefit of clients, planners and engineers. These guidelines should follow the philosophy of Eurocode 7 on geotechnical categories.'

We hope that the introduction of Eurocode 7 will provide greater encouragement for all parties to procure adequate ground investigation.

5.3.2 Cautious estimate

Because of the inherent difficulties in selecting characteristic geotechnical parameters on the basis of EN 1990's statistical definition, Eurocode 7 'redefines' the characteristic value as:

a cautious estimate of the value affecting the occurrence of the limit state

But what is a cautious estimate? EN 1997-1, unfortunately, gives little guidance, so we must revert to commonly accepted definitions:

cautious [adj.] careful to avoid potential problems or dangers estimate [n.] an approximate calculation or judgement11

Combining these definitions gives:

cautious estimate = approximate calculation/judgement that is careful to avoid problems or dangers

The words 'affecting ... the limit state' are an important rider to the definition of a characteristic geotechnical parameter. It means that there is no such thing as the characteristic value - rather there are potentially several characteristic values, one for each limit state being considered.

For example, when selecting a characteristic angle of shearing resistance for use in the design of a pile foundation, we might select a lower value for calculating end bearing resistance (which involves primarily compression) than for calculating shaft friction (which involves direct shear). Thus in this example: n^ <Vk,shaft .

A consequence of EN 1997-1's definition is that the characteristic value can only be selected during the design of the structure, not before (for example as part of a ground investigation), since determination of limit states is a design activity.

5.3.3 Representative values

Prior to the publication of Eurocode 7, the design of retaining walls in the UK was based on 'representative' soil parameters, defined in BS 800212 as:

conservative estimates ... of the properties of the soil as it exists in situ ... properly applicable to the part of the design for which it is intended

For soil parameters such as weight density, which show little variation in value, the representative value 'should be the mean value of the test results'. Where greater variations occur (or where values cannot be fixed with confidence), the representative value 'should be a cautious assessment of the lower limit ... of the acceptable data'.

In practice, the difference between BS 8002's representative value and Eurocode 7's cautious estimate is merely one of semantics.

5.3.4 Moderately conservative and worst credible values

The terms 'moderately conservative' and 'worst credible' describe two commonly-used UK design approaches, which were originally set out in a seminal report covering the design of embedded retaining walls in stiff clays (CIRIA 104).13

According to CIRIA 104, the safety factors applied to soil parameters should depend on how those parameters, together with loads and geometry, have been selected. The table opposite summarizes the main aspects of the two design approaches and indicates the equivalent level of reliability in Eurocode terms.

At the time CIRIA 104 was written, the moderately conservative design approach was the method most often used by experienced engineers in the UK. Regrettably, CIRIA 104 does not define precisely what is meant by 'moderately conservative'. Referring once again to the dictionary:

moderately [adv.] average in amount, intensity, or degree conservative [adj.] (of an estimate) purposely low for the sake of caution14

Combining these definitions gives:

moderately conservative = a low, cautious average

Ground characterization

0 0

Post a comment