Detailed information on compressibility testing of soil
Q.1 Number of tests
(1) For a soil stratum which contributes significantly to the settlement of a structure, Table Q.1 gives a guideline for the minimum number of oedometer tests required as a function of the variability of the soil and the existing comparable experience with the type of soil.
(2) The number of specimens tested should be increased if the structure is very sensitive to settlements. In Table Q.1, a specification of only one test represents a verification of the existing knowledge. If the new test results do not agree with the existing data, additional tests should be run.
|Variability in oedometer modulus Eoed (in the relevant stress range)||Comparable experience|
|Range of values of Eoed > 50 %||4||3||2|
|≈20 % < Range of values of Eoed <≈ 50 %||3||2||2|
|Range of values of Eoed <≈20 %||2||2||1a|
|a One oedometer test and classification tests to verify compatibility with comparable knowledge (see Q.1 (2)).|
Q.2 Evaluation of compressibility characteristics
(1) There are four widely used methods to determine the compressibility of a soil:
– back-calculations of measured settlements;
– empirical evaluation of indirect in-situ investigations such as soundings;
– measurements by in-situ tests, such as plate load and pressuremeter tests;
– compression tests with soil samples in the laboratory.
(2) Back-calculation from measured settlements under comparable stresses can be a reliable method to assess the compressibility characteristics (layered ground, load redistribution and time effects may be difficult to take into account). For foundations on sand and gravel, field tests such as soundings are often used: these are interpreted empirically, most often based on comparable experience. In cases where sands, coarser soil, silts and clays are expected, a combination of field and laboratory methods is desirable. Laboratory compressibility tests are most reliable for fine and organic soil, when quality class 1 samples are relatively easy to obtain.
NOTE Examples of test procedures are given in the documents listed in X.4.5.
Detailed information on compaction testing of soil
R.1 Test procedures applicable to both test types
(1) The minimum number of soil specimens to be tested for one soil stratum is three. The number of tests specified should be based on engineering judgement.
(2) The number of tests to be carried out should be selected considering the variation of the particle size distribution, the consistency limits and the quantity of material to be compacted. For dams, road construction etc., the number of tests to be run may be found in the relevant standards.
NOTE Examples of test procedures for compaction testing of soil are given in the documents listed in X.4.6.
R.2 Requirements specific to compaction tests
(1) The most frequently used compaction tests are the Standard and the Modified (Proctor) Compaction Tests.
(2) Some highly permeable soils such as clean gravels, uniformly graded and coarse clean sands do not yield a well defined maximum density. Therefore optimum water content might be difficult to obtain.
(3) For stiff fine soil, suggested methods are to shred the soil so that it can pass through a 5 mm test sieve, or to chop it into pieces to pass through a 20 mm test sieve.
(4) For stiff fine soil which needs to be shredded or chopped into small lumps, the results of a compaction tests depend on the size of the resulting pieces. The densities obtained from the test will not necessarily be directly related to densities obtained in-situ.
(5) For soil not susceptible to crushing, only one sample may be used for testing. The sample can be used several times after increasing progressively the amount of water. The departure from the common procedure should be mentioned in the report.
(6) For soil containing particles that are susceptible to crushing, separate batches at different water contents should be prepared.
R.3 Requirements specific to California Bearing Ratio (CBR) test
(1) In-situ tests may be carried out, but the laboratory test is the definitive procedure.
(2) Tests may be carried out on either undisturbed or re-compacted material.
(3) The moisture content of the soil should be chosen to represent the design conditions for which the test results are required.
(4) The CBR test should be carried out on material passing through the 20 mm test sieve. If the soil contains particles retained on the 20 mm sieve, these particles should be removed and weighed before preparing the test specimen. If the fraction retained on the 20 mm sieve is greater than 25 % by mass of the fraction passing through the 20 mm sieve, the CBR test is not applicable.
(5) Where a range of water contents is to be investigated, water should be added to or removed from the natural soil after disaggregation. The sample should not be allowed to dry.
NOTE Examples of test procedures are given in the documents listed in X.4.2.