3.3 Evaluation of geotechnical parameters

3.3.1 General

(1) In the following requirements concerning the evaluation of geotechnical parameters, only the most commonly used laboratory and field tests have been referred to. Other tests may be used provided their suitability has been demonstrated through comparable experience.

3.3.2 Characterisation of soil and rock type

(1)P The character and basic constituents of the soil or rock shall be identified before the results of other tests are interpreted.

(2)P The material shall be examined, identified and described in accordance with a recognised nomenclature. A geological evaluation shall be made.

(3) Soils should be classified and soil layers described according to an acknowledged geotechnical soil classification and description system.

(4) Rock should be classified in terms of the quality of the solid (stone) material and jointing. Stone quality should be described in terms of weathering, particle organisation, dominant grain size of minerals, and hardness and toughness of the main mineral. Jointing should be characterised in terms of joint type, width, spacing and fill quality.

(5) In addition to visual inspection, a number of tests for classification, identification and quantification of soils and rocks may be used (see EN 1997-2), such as

for soils:

  • grain size distribution;
  • weight density;
  • porosity;
  • water content;
  • grain shape;
  • grain surface roughness;
  • density index;
  • Atterberg limits;
  • swelling;
  • carbonate content;
  • organic matter content.

for rocks:

  • mineralogy;
  • petrography;
  • water content;
  • weight density;
  • porosity;
  • sound velocity;
  • quick water absorption;
  • swelling;
  • slake-durability index;
  • uniaxial compressive strength.

3.3.3 Weight density

(1)P The weight density shall be determined with sufficient accuracy to establish design or characteristic values of the actions that derive from it.

(2) The weight density should be determined on specimens of soil and rock taken from undisturbed samples (see EN 1997-2). Alternatively, it may be derived from wel established or documented correlations based on, for example, penetration tests.

3.3.4 Density index

(1)P The density index shall express the degree of compaction of a non-cohesive soil with respect to the loosest and densest condition as defined by standard laboratory procedures.

3.3.5 Degree of compaction

(1)P The degree of compaction of natural ground or fill shall be expressed as the ratio between dry weight density and maximum dry weight density obtained from a standard compaction test.

3.3.6 Shear strength

(1)P In assessing the shear strength of soil, the influence of the following features shall be considered:

  • the stress level imposed on the soil;
  • anisotropy of strength, especially in clays of low plasticity;
  • fissures, especial y in stiff clays;
  • strain rate effects;
  • very large strains where these may occur in a design situation;
  • pre-formed slip surfaces;
  • time effects;
  • sensitivity of cohesive soil;
  • degree of saturation.

(2) When the shear strength assessment is based on test results, the level of confidence in the theory used to derive shear strength values should be taken into account, as well as the possible disturbance during sampling and heterogeneity of samples.

(3) As to time effects, it should be considered that the period for which a soil will be effectively undrained depends on its permeability, the availability of free water and the geometry of the situation.

(4)P The values of effective shear strength parameters c' and tan φ' shall be assumed to be constant only within the range of stresses for which they have been evaluated.

3.3.7 Soil stiffness

(1)P In assessing the soil stiffness, the following features shall be considered:

  • drainage conditions;
  • level of mean effective stress;
  • natural or artificial pre-consolidation;
  • level of imposed shear strain or induced shear stress, this latter often normalised with respect to the shear strength at failure.

(2) Reliable measurements of the stiffness of the ground are often very difficult to obtain from field or laboratory tests. In particular, owing to sample disturbance and other effects, measurements obtained from laboratory specimens often underestimate the in-situ stiffness of the soil. Observations of the behaviour of previous constructions should therefore be analysed wherever available.

Eurocode 7 Geotechnical design Part 1 : General rules