Key geological issues - Geotechnical Engineering of Dams - page 17

Figure 2.5. Sheet-joints, formed by induced tensile failure (as in an unconfined compression test with

zero friction at the plattens).

joints is often 0.3 to 1 m near the ground surface, and becomes progressively wider with
depth.
From all of these characteristics, sheet joints are considered to be stress-relief features.
The linear decrease in horizontal stresses as the ground surface is approached (Lines A
and B, Figure 2.2 ) is due to the progressive relief of horizontal stresses partly by buckling
and spalling, as the overlying rock load is removed by erosion.
Holzhausen (1989) provides a comprehensive account of the characteristics and origin
of sheet joints.
In fractured rock masses, i.e. those which are already weakened by defects of tectonic ori-
gin, the effects of horizontal stress relief are not so obvious but are always present, as open-
ing up of the existing defects, as shown in Figure 2.6b . The destressing effects usually extend
to greater depths in jointed rock than in massive rock. However, in most geological situa-
tions it can be assumed that beyond the near-surface effects, the tectonically induced
joints will become progressively tighter (less open) with greater depth (Snow, 1970).

2.5.3 Effects in claystones and shales

Claystones and shales (i.e. “mudrocks”) are formed mainly by consolidation of clay-rich sed-
iments, but may be strengthened further by partial recrystallization and cementation. Such
rocks have much higher porosities than igneous and metamorphic rocks. They usually show
expansion, spalling and fretting on unloading and exposure. It is believed that these effects
are due partly to the release of stored strain energy (Bjerrum, 1967) but also to the absorp-
tion of water and subsequent swelling of the clays. (See Section 2.9.1 and Section 3.5.1).

2.5.4 Special effects in valleys

Gentle anticlines, in some cases with associated thrust faults as shown in Figure 2.7 , have
been recorded across many river valleys cutting through near-horizontal sedimentary