The unconfined compressive strength (UCS) of a rock is a basic parameter for many characterization systems, strength criteria and calculation methods. It is well-known fact that it depends on the water content of the samples, and decrease when the water content increases. The paper discusses the possible causes of this reduction. From published data by Vasarhelyi and co-workers and others authors some empirical tentative guidelines for this reduction are proposed, which can be used in rock engineering problems where changes in water content occur regularly (dam and bridges foundations, harbors...).
The unconfined compressive strength (UCS) is probably the most used of the rock index properties for their characterization. So all the standards have detailed regulations on the test and many authors have published on the effect of the sample size on the results of the test. The standards detail also the form and dimensions of the sample, the conditions of parallelism of the faces, even the speed of load application. But almost none of the standards say anything about the humidity of the samples. This is a surprising lack because the samples can be absolutely dry, air dry, semi saturated or saturated.And the water content, or the saturation state, has a clear influence on the results of the test. As a rule the strength diminishes when the water content increase, with a minimum in saturated samples. So some experienced engineers advise to test the rock in the same humidity conditions in which the rock mass is going to stay. This is especially important in dam foundations (which are going to be saturated) or in rockfills. Some rules of thumb have been proposed to cope with this problem (Romana, 2003) when working with geomechanics classifications. There are a scarcity of published data on the unconfined compressive strength (UCS) of saturated samples with the exception of the work by Vasarely and co-workers. For instant Vasarhely and Ledniczky (1999) say that "it is known that saturated materials have lower strengths…than air-dry ones". The aim of this paper is to point at the problem, to recollect the scarce published data, and to offer a first tentative quantitative approximation of the reduction in unconfined compressive strength of saturated rocks.
Figure 1 (Pells, 1993) shows a Deere-Miller diagram (failure strength vs. deformation modulus at 50% of failure strength) containing data from compression tests in dry and saturated Hawkesbury sandstone. Saturation implies a reduction almost Figure 1. Strength data on Hawkesbury Sandstone (Pells, 1993). proportional in both parameters, but the relationship between them would remain approximately constant. Unfortunately no numerical result can be deduced due to the lack of numerical definition of the data. Hsu and Nelson (1993), in a preliminary research for the not built Super Collider, correlated the unconfined compressive strength of many types of shale (from Canada and USA) with thewater content.Their results (Fig 2), showa marked negative correlation between water content and compressive strength.