5.7 Strength index testing of soil
(1) The purpose of strength index tests is to determine in a rapid and simple manner the undrained shear strength cu of clayey soil.
(2) This standard covers the following strength index tests:
- laboratory vane test;
- fall cone test.
(1)P The tests shall be performed on undisturbed specimens of quality class 1.
NOTE 1 Further in formal ion on a procedure, presentation and evaluation for strength index testing can be found in CEN ISO/TS 17892-6, (sec X.4.3).
NOTE 2 Annex O gives information on the strength index tests considered and a checklist on the test procedures for the tests.
5.7.3 Use of test results
(1) It should be considered that the values of cu represent the undrained shear strength of the soil at its state in the laboratory. They need not represent the undrained strength of the soil in-situ.
(2) Depending on the characteristics of the soil and on the particular strength index lest selected, the test results can be only an approximate estimate of cu.
(3) Strength index tests should only be used for design where well-documented, comparable experience with similar soil exists.
(4) If well-documented, comparable evidence exists, the undrained shear strength derived from the strength index tests may be used if the sample analytical method of EN 1997-1:2004, D.3 is applied.
(5) The test results may be used to check variability of undrained shear strength within a stratum.
5.8 Strength testing of soil
5.8.1 Objective and scope
(1) The objective of the test is to establish the drained and/or undrained shear strength parameters,
(2) The following strength tests are covered:
- unconfined compression test;
- unconsolidated undrained triaxial compression test;
- consolidated triaxial compression test;
- translational shear box lest and torsional shear box (ring shear) test.
(3) Translational and ring shear box tests should be used for testing soil under drained conditions.
NOTE Under quick strain rate, clays of very low permeability may sometimes be considered loaded undrained in both types oldie shear box apparatus. The test then gives an indication of the undrained strength.
(4) In this section, only strength tests on fully saturated or dry soil are dealt with.
5.8.2 General requirements
(1) For the determination of the shear strength of clay, silt and organic soil, undisturbed samples (Quality Class 1) should be used. For certain soil or special purposes, tests can be carried out on reconstituted or remoulded specimens.
(2) For coarse silts and sands, the test specimens may be re-compacted or reconstituted. Care should be taken to select a preparation method that reproduces as closely as possible the structure and density relevant for the design at hand.
(3)P For re-compacted or reconstituted specimens, the composition, density and water content of the prepared specimens relevant for the in-situ conditions and the specimen preparation method shall be specified.
(4)P For a strength test, the following shall be evaluated or specified:
- the number of tests required;
- the selection of location of test specimens in the recovered samples:
- the required quality of sample;
- the specimen preparation method;
- the orientation of specimen;
- the type of test;
- the classification tests that need to be done;
- the consolidation stresses (if applicable);
- the time for consolidation increments (if applicable);
- the shearing rate;
- the failure criteria;
- the criteria for terminating tests (e.g., strain at which the test shall be stopped) ;
- the acceptability criteria (e.g. saturation, scatter);
- the accuracy of measurements;
- the format for presentation of test results;
- any procedure used in addition to those referenced in an accepted standard.
(5) The shear strength of a sample should be determined by a set of three or more tests under different normal stresses.
(6)P When determining the shear strength of a soil stratum, the following shall be taken into account:
- the type of shearing;
- the specimen preparation method;
- the need for additional classification tests.
(7) If samples of Quality Class 2 are tested, the effects of the sample disturbance should be considered in the interpretation of the results.
NOTE Annex P provides a guideline for the minimum number of samples and tests for one stratum, as well as additional information on the test and its evaluation.
5.8.3 Evaluation and use of test results
(1)P The presentation of the test results shall include, where applicable:
- the effective stress paths;
- the Mohr circles;
- the stress-strain curves;
- the pore pressure-strain curves;
- the pore pressure parameters.
NOTE Linear extrapolation of the test results can give erroneous values of the strength of the soil as the failure envelope is generally not a straight line, especially at low normal stresses.
(2)P The stress range over which the strength parameters have been determined shall be given.
(3) There are several methods to obtain the stress-strain and strength parameters of soil in the laboratory and in-situ. If appropriate, the results from these different tests should be compared when evaluating the test results.
(4) The results should be evaluated taking into account the strain rate used for testing.
(5) The compression and direct shear tests provide commonly accepted strength parameters which may be applied to routine design methods, but which are not necessarily applicable to other analyses.
(6) It should be considered that unconfined compression and unconsolidated, undrained compression tests need not represent the undrained strength of the soil in-situ.