The standard œdometric test (incremental œdometer) is a very old test (Terzaghi, 1925) which is still carried out today. It has several drawbacks, particularly the inaccuracy in the determination of the preconsolidation stress and the length and complexity of the procedure. The constant rate loading test (CRS) avoids these two problems, but it is not yet considered as a standard test.
An initial research project (VSS 1999/285) has shown great interest in this procedure. It has also led to a literature search, develop theories of primary consolidation, conduct comparative tests and develop a test procedure. However this research has been carried out on reconstituted samples. In order to introduce this procedure, as a standard œdometric test and as a basis for the drafting of an SN standard, tests on undisturbed samples are still needed.
An initial research was conducted on reconstituted soil (VSS 1999/285). This additional research will be carried out on undisturbed samples. The advantages of the CRS tests are numerous enough to justify a standardisation procedure in Switzerland.
These steps will be done during the project:
- sample collection intact,
- conducting tests and oedometer CRS standards,
- comparison of the result.
Like in the precedent research, the test results on intact samples show a good overall analogy between the two types of tests. The odometric curves and the characteristic indices (Cc, Cs, cv, k and mv) are in particular very similar.
The determination of the preconsolidation stress was facilitated by the measurement of the pore pressure during the test. The results show that this method is suitable for clayey and silty-clayey soils. In contrast for silty and silty-sandy soils, the loading rate should not be too high, which is a limitation for our apparatus.
The main shortcomings that appeared during tests were linked to manipulation problems on the one hand, and to the heterogeneity of the samples on the other hand. The time span between the two kinds of tests, due to the unavailability of the apparatuses and technicians, was also sometimes sources of confusion when making comparisons.
This research however shows that the CRS test could at present advantageously replace the traditional œdometric test for any kind of compressible Swiss soils. For the argillaceous soils (very plastic clay) and in comparison with the incremental tests, the loading rate had no significant influence on the results.
Consequently and under these conditions, the advantages of the CRS tests are numerous enough to justify a standardisation in Switzerland.