Fatigue of rocks

Cyclic loading on civil structures can lead to a reduction of strength of the used materials. For the materials concrete and especially steel, there are clear design codes about how to account for the reduction of the material shear strength due to this cyclic loading, which is called fatigue. For the material rock, however, there are no design codes or standards for fatigue, in terms of shear strength reduction. For this reason, a large number of laboratory triaxial tests have been performed, in order to evaluate the fatigue of rocks by comparing the shear strength parameters obtained in cyclic triaxial tests with the static shear strength. Tests have been performed on artificial gypsum, a mixture of sand and cement (mortar) and soft sedimentary limestone. Correlations of the fatigue, for both the number of cycles and the cyclic stress ratio, have been obtained.

All triaxial tests were conducted on dry samples (no pore pressure) in the natural state. The range of the confining pressure was between 0 MPa and 0.5 MPa. The frequency was kept low to allow for a precise application of the cyclic load and also accurate readings. The number of applied cycles was from a few cycles up to a few hundred thousand.

The imperfections in the artificial gypsum have a significant impact on the results of the (especially cyclic) strength tests. Therefore another man made material was used – a mixture of sand and cement (mortar). As the first static test results were very promising, mortar was used in further tests. The cyclic tests, however, presented a similar, high scatter of results as for artificial gypsum.

Due to the complex behaviour of the cohesive materials and high scatter of the results, many tests were required. Two different strategies were used to investigate the fatigue of the cohesive geomaterials:
1. the remaining shear strength curve: after a given number of cycles, a final single load test until failure, measures the remaining shear strength of the sample.
2. the typical S-N curve (Wöhler curves): one counts the number of constant loading cycles until failure.

The fatigue of rocks can be seen as a reduction of the cohesion. In this way, the fatigue of a cohesive geomaterial can be described by (a reduction of) the remaining cohesion.
An unexpected observation for both artificial gypsum and mortar was that unlike the number of cycles, the size of the cyclic stress ratio has little influence on the remaining shear strength, and therefore on fatigue.
The remaining shear strength (or strength reduction) curve has been compared with the standard S-N curve, and is found to be rather similar for both artificial gypsum and mortar. The reason for this is this unexpected observation. Because of this, the S-N curve and the remaining shear strength curve should be theoretically identical.
The results of the triaxial tests show however that, the S-N curve gives a bit steeper slope than the remaining shear strength curve, which would imply a shorter life and a faster reduction in comparison to the remaining shear strength curve, but this is only because, the prematurely failed samples are not included in the remaining shear strength curve, while a significant number of the S-N samples are prematurely failed samples from the remaining shear strength tests.