5.12 Tests for classification of rocks

5.12.1 General

(1) The following tests are included in this standard:

  • rock, identification and description:
  • water content;
  • density and porosity.

(2) Classification relates to the division of identified rock into specific types defined for particular civil engineering purposes. The classification is related to mineralogical components, structure, induration, rock density, water content, porosity and rock strength.

NOTE Annex U provides more details and guidelines on the classification tests.

5.12.2 Requirements for all classification tests

(1)P The classification test results shall be reviewed together, compared with drilling logs, corresponding geophysical logs, photographs of cores and comparable experience.

(2) The soil and rock classification should be compared with available geological background information in order to provide an engineering geological model.

(3) Geological maps should be used as a guide for the classification of rock and rock masses, whenever available.

(4) Second opinion evaluations and the use of typical examples with rock comparisons may be required in order to achieve a consistent description.

5.12.3 Rock identification and description Objective and requirements

(1) The identification and description of rock material and mass are carried out on the basis of mineralogical composition, predominant grain size, genetic group, structure, weathering and other components. The description may be carried out on cores and other samples of natural rock and on rock masses in-situ.

(2)P The laboratory procedure shall conform to EN ISO 14689-1.

(3) More detailed description of rock may be applied. Then the following should be specified:

  • the rock classification system;
  • the need for advanced geological analyses;
  • the format of reporting.

(4) Rock identification and description should be carried out on all samples received in the laboratory, regardless of rock homogeneity, as the identification and description constitutes the framework for all testing and evaluations. Evaluation of results

(1) The classification of the rock mass using cores should be based on the highest possible core recovery to identify discontinuities and possible cavities.

(2) The disturbance of the core from the drilling process should be evaluated since most rock mass quality designations relate to the fractures found in the cores and their quality.

5.12.4 Water content determination Objective and requirements

(1)P With the exception of rocks mentioned in (2), water content of rock shall be determined by oven-drying at (105 ± 5) °C.

(2)P Measures shall be specified to retain water during sampling and storage, if relevant.

(3)P The following shall be specified:

  • the selection of test samples;
  • the storage in the laboratory before testing;
  • possible re-saturation of desiccated samples using vacuum saturation technique:
  • the number of tests per strata;
  • the number of tests to be run in parallel with other tests from the same formation;
  • the number of accuracy checks to be run.

(4) A minimum sample of at least 50 g or a core piece with a dimension of at least 10 times the maximum grain size of mineral components should be used.

(5)P The report shall state whether the measured water content corresponds to the in-situ water content.

NOTE As no ISO/CEN standard for rock testing is currently available, the laboratory methods described in U.3, with reference to X.4.9.2 can be applied. Evaluation of test results

(1) The results of the determination of the water content should be compared with the fully-saturated water content as a function of the density (or porosity) of the test specimen. Anomalous results should be investigated by repeated testing.

(2) Rock types with significant amounts of gypsum should be tested at 50 °C as the bound water may partly dehydrate at 105 °C.

(3) For rock types in which the pore water contains dissolved salts or rock types with closed pores, the reported water content should be evaluated in this respect.

(4) The water content should be used for correlation of strength and deformation characteristics of the rock types in boreholes and at test sites.

(5) Comparisons with available correlations of water content and rock type should be made.

5.12.5 Density and porosity determination Objective and requirements

(1) The test is used to determine the bulk and dry density to obtain the porosity and related properties of a rock sample. The bulk and dry density is derived based on weight analyses provided reliable determination of the sample volume is available.

(2) The pore volume may be calculated based on the dry density and the particle density determined using methods as for soil, provided no closed pores exist in the rock specimen. Porosity is the ratio of pore volume to total volume.

(3)P The following shall be specified:

  • the selection of test samples;
  • the conditions of storage before testing;
  • whether desiccated samples are to be re-saturated and by which technique;
  • the number of tests required pet formation;
  • whether parallel tests are to be run on the same formation.

(4) A test specimen of at least 50 g, with a minimum dimension of 10 times the maximum grain size of the mineral components, should be tested.

NOTE As no ISO/CEN standard for testing of rock is currently available, the laboratory methods described in U.4 with re Terence to the bibliography in X.4.9.3 can be applied. Evaluation of test results

(1) The density and porosity should be integrated in the reporting of rock description and established strength and deformation characteristics of the rock types in boreholes and at test sites.

(2) The density and porosity results should be used for comparison of rock strength and deformation properties and to establish correlations for the different rock types.

(3) The existence of closed pores can influence the porosity. The determination of the total pore volume should be based on the density of solids of a powdered sample.

NOTE Further information on a procedure, presentation and evaluation of density and porosity determination can be found in CEN ISO/TS 17892-3, (see X.4.9.3).

Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing