Deriving geotechnical parameters Overview

The derived value of a geotechnical parameter is defined in Eurocode 7 as:

[the] value... obtained by theory, correlation or empiricism from test results

As the flow-chart of Figure 5.2 illustrates, test results may be converted into derived values X by use of correlations (such as that between cone penetration resistance and angle of shearing resistance in sand), theoretical considerations (such as conversion of triaxial compression into plane strain strengths for clays), or through empirical rules (such as those between standard penetration test blow count and undrained strength for clays).

Figure 5.2. Deriving geotechnical parameters

When available, test results may be supplemented by other relevant data, such as that from nearby sites (i.e. comparable experience) or from research studies of the materials encountered.

Derived values may also be assessed directly, provided the engineer knows for which limit state the derived value is required.

Figure 5.3 gives an example of a geotechnical parameter (the undrained strength of two clays) derived from two different tests: standard penetration tests, with blow counts suitably converted into undrained strength (black symbols, SPT); and undrained triaxial compression tests (white symbols, TX).2 This data is discussed further in Section 5.3.5 to illustrate the selection of characteristic values.

5.2.2 Correlations

Eurocode 7 Part 23 gives correlations for common field investigation methods and widely-used geotechnical parameters: Annex D, cone penetration test: 9, E', Eoed Annex F, standard penetration test: ID, 9 Annex G, dynamic probing test: ID, 9 , Eoed Annex H, weight sounding test: 9, E' Annex J, flat dilatometer test: Eoed Annex K, plate loading penetration test: cu, EPLT, k where 9 is the soil's angle of shearing resistance, E' its drained Young's modulus, Eoed the one-dimensional (oedometer) modulus, ID relative density, cu undrained strength, EPLT the plate-loading modulus, and k the coefficient of subgrade reaction.

Eurocode Clay Shear Strength
Figure 5.3. Derived values of undrained strength in London clay (squares) and Lambeth clay (triangles)

Correlations may be derived from a theoretical relationship between one parameter and another (e.g. drained Young's modulus E' and shear modulus G) or from some form of empirically derived relationship. Although theories exist that relate field test results to geotechnical parameters, these theories do not fully model the soil behaviour and engineers therefore rely more commonly on empirical rules.

Most correlations are limited to particular ground strata and/or geotechnical situations. Care must be exercised when extrapolating these correlations to strata and/or situations which differ from those on which they are based.

Figure 5.4 shows the correlation between SPT blow count and relative density of a coarse soil that is included in Annex F to EN 1997-2.

5.2.3 Theory

A theory is 'a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena'.4

A theory allows the derivation of a parameter from a measured value or it describes how one geotechnical parameter may be related to one or more other parameters.

The field vane test is widely used to determine the Figure 5.4. Correlation between SPT blow count undrained shear strength of and relative density of coarse soil low strength fine soils (e.g.

soft clays). The test involves measuring the torque required to continuously turn the blades of the vane through the soil. The soil's undrained strength cu is obtained from:

0 0

Post a comment