Engineering Rock Mechanics

Specimen geometry, loading conditions and environmental effects 1 03

6.4.4 Environmental effects

Other factors which affect rock behaviour, in particular moisture content,

time and temperature, can be of importance in engineering and we have

grouped them here under the general title of environmental effects.

Moisture content. The moisture content is known to influence the
complete stress-strain curve because of its effect, in certain rocks, on the
deformability, the compressive strength and post-peak behaviour. For this
reason, it is recommended, for example by the ISRM, that the moisture
content be measured as an integral part of the compressive strength
determination of rocks. It is beyond the scope of this book to provide a
comprehensive discussion of all aspects of the influence of moisture content
and saturation, but the reader is alerted to the following factors which can
be particularly important in rock engineering.

1. Some rocks, and in particular those with high clay mineral contents, may
   experience desiccation when exposed. In situ, the rock may possess a
   stable, but high, moisture content; on exposure after excavation, its
   properties may change as it dries out and it may become friable and
   hence crumble with very little applied stress.


2. Similarly, the same types of rock could be saturated on excavation, and
   be subjected simultaneously to mechanical action as part of the excava-
   tion process. This leads to slaking and there is an associated slake
   durability test to assess the susceptibility of a rock under these condi-
   tions. The rock can then also break down and crumble under a very low
   applied stress. The reader should be aware that slaking behaviour is not
   dissolution.


3. Another moisture related effect is the tendency to swelling as the
   moisture content is changed. This can lead to the generation of addi-
   tional stresses, for example behind tunnel linings. In some cases, the
   stresses thus generated can be of a similar magnitude to those due to
   the in situ stress field, and can initiate failure.


4. If the pore spaces in the rock are connected and the pore fluid is under
   pressure, we can subtract this pressure, or a proportion of it, from all the
   components of normal stress. This leads to the well-known concept of
   effective stress, widely used in soil mechanics and which we will discuss
   in Chapter 9. If the water pressure is increased sufficiently, the effective
   stress can be reduced to such an extent that failure occurs. In the case
   of rocks, the effective stress concept can apply well for materials such
   as sandstone, but be inappropriate for granites, especially over
   engineering rather than geological timescales.

These are some of the main effects but there are many others which occur

as water (or other pore fluids) move through the rock and cause alterations

and effects of different kinds. For example, the chemistry of groundwater

can be important, e.g. its acidity. In materials such as chalk and limestone,

this results in dissolution of the intact rock with complete removal of the

material to produce caves. Freeze-thaw cycles can also degrade intact rock,

usually in a similar fashion to slaking.