Environmental Engineering Reference
In-Depth Information
Figure 2.13.
Possible effects of high fluid pressures on valleys in groundwater discharge areas (from
Patton & Hendron, 1972).
rock mass more permeable and facilitates access for groundwater to large surface areas of
rock substance.
Destressing, in particular the formation of sheet joints and the opening up of existing
“tectonic” joints near the ground surface, is the primary and generally the most significant
mechanical weathering process (See
Figures 2.6
,
2.7
,
2.9
and
2.11b
). The other processes,
in order of their (generally) decreasing significance, are as follows:
- gravitational creep (e.g. of slabs and wedges, and toppling);
- oint water thrusting and uplifting during extreme rainfall events;
- earthquake induced displacements;
-growth of tree roots in joints;
- expansion of clays in joints;
- reezing of water in joints;
-
extreme temperature changes causing differential expansion and contraction of
exposed rock faces.
Patton and Hendron (1972) suggested possible mechanical weathering effects beneath
the floor and lower sides of valleys in groundwater discharge areas. This type of situation
(Figure 2.13) may be relatively rare, but if it occurs the hydraulic uplift and thrust effects
shown are clearly possible.
2.6.2
Chemical decomposition
The term chemical decomposition as used here includes all of the chemical (and to a minor
extent physical) processes which cause mineral changes resulting generally in weakening
of rock substances, so that eventually they assume soil properties.
Throughout this topic, the products of chemical decomposition are described using the
terms defined in Section 2.8.1,
Tables 2.3
to
2.6
.
Chemical decomposition of rocks can be caused by either near-surface (weathering)
processes or deep-seated (alteration) processes. Recognition of this distinction is impor-
tant in civil and mining engineering because the nature and distribution of weathered
materials are generally different from those of altered materials (
Figure 2.14
)
.
2.6.3
Chemical weathering
Chemical weathering is caused mainly by circulating groundwater which gains access to
low-porosity rock substances via cleavage micro-cracks, open joints and fractures associated
