Detailed information on strength index testing of soil
(1) Table O.1 suggests a summary checklist on the test procedures for the strength index tests for soil included in this standard.
|Strength index test||Checklist|
|Any strength index test||Tests provide approximate index of shear strength
There is a large uncertainty in the measurements
Use results cautiously for non-homogeneous and jointed/slickensided soil
All results are affected by the applied testing rate
Test repeatability needs to be checked
|Laboratory vane||Test provides in addition a measure of sensitivity and remoulded shear strength
Check mode of rotation (hand-operated versus motorised)
Tests can be performed on extruded samples or in sampling tube
|Fall cone||Test can be performed on extruded samples or in sampling tube on intact material. This can be supplemented with a test on remoulded material to establish the sensitivity i.e. the ratio between intact and remoulded strength data
Provides in addition a measure of sensitivity on remoulded specimen
Check wear on tip of cone
Check tip cone angle
NOTE Examples of test procedures for the laboratory vane and fall cone strength index tests are given in the documents listed in X.4.3.
Detailed information on strength testing of soil
P.1 Triaxial compression tests
P.1.1 Number of tests
(1) Table P.1 gives guidelines for the minimum number of tests required as a function of the variability of the soil and existing comparable experience with the type of soil. If only one test set is required, the test is run to provide a verification of existing knowledge. If the new test results do not agree with the existing data, more tests should be run.
(2) The number of tests may be reduced if shear stress data are available from other testing methods, for example, field tests.
NOTE Examples of test procedures for the triaxial compression tests are given in X.4.4.
|Recommended number of tests to determine the effective angle of shearing resistance a|
|Variability in strength envelope
Coefficient of correlation r on regression curve
|r ≤ 0,95||4||3||2|
|0,95 < r ≤ 0,98||3||2||1|
|r ≥ 0,98||2||1||1|
|Recommended number of tests to determine the undrained shear strength a|
|Variability in undrained shear strength (for same consolidation stress)||Comparable experience|
|Ratio max/min values > 2||6||4||3|
|1,25 < Ratio max/min value ≤ 2||4||3||2|
|Ratio max/min value ≤ 1,25||3||2||1|
|One recommended test means a set of three individual specimens tested at different cell pressures.|
P.1.2 Evaluation of test results
(1) In addition to a factual evaluation, the undrained shear strength should be checked against correlations with the soil type, plasticity index, etc. The undrained shear strength evaluations should be related to the type of test that produced the results.
(2) The angle of shearing resistance should be checked against correlations with the soil type, plasticity, density index, etc. Laboratory and the in-situ stress conditions (e.g. axi-symmetrical versus plane strain conditions) should be carefully considered and in relevant cases, the angle of shearing resistance should be adjusted. Relations with for example the results of cone penetration tests and existing correlations with angle of shearing resistance should also be included.
P.2 Consolidated direct shear box tests
P.2.1 Test procedures
(1) The direct shear (box or ring shear) test is run preferably for soil and stability conditions where a distinct rupture plane is expected to develop or when the strength characteristics of an interface are to be determined.
(2) Comparative studies show that the test results of direct (translational) shear box and ring shear box tests are in good agreement. The preparation of the specimen is easier in the translational shear box test. The stresses are more homogeneous in the ring shear test but the strains are not uniform. It is easier to produce large strains and thus determine the residual strength of a soil in the ring shear apparatus than in the translational shear box apparatus.
(3) Twice as much material as needed for the number of specimens tested should be taken from the stratum.
NOTE Examples or procedures for direct shear testing are given in documents listed in X.4.4.4.
P.2.2 Number of tests
(1) Table P.2 gives a guideline for the recommended minimum number of tests required as a function of the variability of the soil and existing comparable experience with the type of soil. The recommendation applies to the case when direct shear tests are used alone to determine the shear strength of a soil stratum.
|Recommended number of tests a|
|Variability in strength envelope
Coefficient of correlation on regression curve
|Coefficient of correlation < 0,95||4||3||2|
|0,95 ≤ Coefficient of correlation < 0,98||3||2||2|
|Coefficient of correlation ≥ 0,98||2||2||1 b|
|a One recommended test means a set of three individual specimens tested at different normal stresses.
b A single test and classification tests to verify compatibility with comparable experience. If the test results do not agree with the existing data, additional tests should be run.