5.8.4 Unconfined compression test

5.8.4.1 Requirements

(1) The unconfined compression test should be carried out on specimens of soil of sufficiently low permeability to maintain undrained conditions during the test.

(2) Delays between trimming and testing should be avoided to prevent change of water content of specimens.

NOTE Further information on a procedure, presentation and evaluation for the unconfined compressison test can be found in CEN ISO/TS 17892-7, (see X.4.4.1).

5.8.4.2 Evaluation and use of test results

(1) The test result is the approximate value of the unconfined compressive strength of the tested soil.

(2) The undrained shear strength cu may be determined as one half of the measured unconfined compressive strength.

(3) The effective stresses within the laboratory specimen may deviate from the effective stresses in-situ. The test result need not represent the undrained strength of the soil in-situ due to this difference.

5.8.5 Unconsolidated, undrained triaxial compression test

5.8.5.1 Requirements

(1)P The test shall be carried out without allowing any drainage from the specimens.

(2)P During preparation and testing, soil specimens shall not be given access to water (e.g. from the drainage leads or pore pressure transducers, etc.).

(3)P Water content before and after the test and bulk density before the test shall be determined for each specimen.

NOTE Further information on a procedure, presentation and evaluation for the unconsolidated undrained triaxial compression test can be found in CEN ISO/TS 17892-8, (see X.4.4.2).

5.8.5.2 Evaluation and use of test results

(1) The test result is the undrained shear strength cu of the tested soil.

(2) The effective stresses within the laboratory specimen may deviate from the effective stresses in-situ. The test result need not represent the undrained strength of the soil in-situ due to this difference.

5.8.6 Consolidated triaxial compression test

5.8.6.1 Requirements

(1)P The tests shall be performed on undisturbed specimens of quality class 1.

NOTE Further information on a procedure, presentation and evaluation of the consolidated triaxial compression test can be found in CEN ISO/TS 17892-9, (see X.4.4.3).

(2)P For a consolidated triaxial compression test, the following shall be evaluated or specified further to 5.8.2:

  • the saturation method and saturation criterion;
  • the back-pressure required;
  • any procedure used in addition to those referenced in an accepted standard (e.g. lubricated ends, local measurements of strains or pore pressure).

(3)P For consolidated, undrained triaxial tests, the requirements on measuring pore pressures, and the total stress path for shearing, shall be specified.

(4)P For consolidated drained tests, the volume change measurement device and the stress path for shearing shall be specified.

(5)P Water content before and after the test and bulk density before the test shall be determined for each specimen.

(6) One consistency limit determination and particle size analysis should be made per set of triaxial tests in one stratum.

(7)P The results shall clearly slate the type of test that was carried out, which strength parameters are given, the shearing rate and the failure criterion used to select the shear strength (e.g. peak deviator stress, maximum stress ratio).

(8)P The report shall indicate any known deviation from the standard testing procedure, for example in the degree of saturation of the test specimens, testing procedures, composition of the specimen or in any other aspect.

(9) In conformity to 2.4.2.3 (4), more advanced laboratory strength tests, such as triaxial extension tests, simple shear tests, plane strain compression and extension tests, true triaxial tests, directional shear tests, all with the possibility of anisotropic instead of isotropic consolidation, should be considered.

5.8.6.2 Evaluation and use of test results

(1) The evaluation of the test results should take into account that undrained shear strength, pore pressure parameters and stress-strain relationships are affected by sample disturbance to a greater extent than the drained strength parameters.

NOTE Reliable stress-strain modulus values, especially in the case of stiff soil, can only be obtained from advanced tests with special procedures to measure strains and axial force accurately (see 5.9).

(2) Depending on the type of test, drained or undrained strength of the soil may be derived. Accordingly, the values are effective angle of shearing resistance (φ') and effective cohesion (c'), or undrained shear strength (cu).

(3) The values can be used in both drained and undrained stability analyses

NOTE See EN 1997-1:2004, Annex D.

5.8.7 Consolidated direct shear box tests

5.8.7.1 Requirements

(1)P The tests shall be performed on undisturbed specimens of Quality Class 1.

NOTE Further information on a procedure, presentation and evaluation of the consolidated direct shear box lest can be found in CEN ISO/TS 17892-10, (see X.4.4.4).

(2)P The location and orientation of the specimen shall be considered carefully so as to duplicate as closely as possible the in-situ conditions. In the translational shear box and ring shear tests, failure is forced to occur on or near a horizontal plane in the middle of the test specimen.

(3)P Negative or positive pore water pressures due to shear shall be avoided during the test as they cannot be measured and accounted for in the interpretation of the test. To maintain drained conditions, the rate of shearing shall be slow enough so that pore water pressures may dissipate.

5.8.7.2 Establishment and use of values

(1) The results of standard shear box tests represent the strength under drained conditions. The values are effective angle of shearing resistance and effective cohesion (see NOTE in 5.8.1 (3) ).

(2) The values can be used in stability analyses.

NOTE See EN 1997-1:2004, Annex D.

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