Environmental Engineering Reference
In-Depth Information
minerals. These minerals are weaker and in some cases larger in volume than the original
minerals, resulting in microcracking, increase in porosity and weakening of the rock. The
altered rock is usually greenish in colour.
Where these effects are pronounced, and particularly if the clay mineral montmoril-
lonite is produced, the altered rock is obviously much weaker than the original rock and
is likely to deteriorate or even disintegrate on exposure to air or immersion in water.
However if the alteration effects are relatively minor, e.g. only the margins of the original
crystals are changed to secondary minerals, then the visual appearance of the rock sub-
stance may be little changed from that of fresh, unaltered rock. The altered rock will usu-
ally appear a little dull and have a slightly higher porosity and absorption than the fresh
rock. Rocks such as this, showing only very minor and subtle alteration effects, often
deteriorate rapidly in pavements, are likely to be unsuitable for crushing to produce filters
and may prove to be unsuitable for concrete aggregate and rockfill. Because of this, before
adopting volcanic rocks for use as construction materials, they should always be subjected
to very thorough checking, by local past performance and field observations, petro-
graphic analysis and laboratory tests. For further details readers are referred to Shayan and
Van Atta (1986), Van Atta and Ludowise (1976), Cole and Beresford (1976), Cole and
Sandy (1980) and Hosking and Tubey (1969).
As the alteration is caused by hot water and gases which move through permeable fea-
tures such as joints and highly vesicular zones, the more altered rock is usually located
along and adjacent to such features. Quite commonly, secondary minerals occur as veins,
seams or irregular masses filling previously gaping joints or voids. Such features, particu-
larly where of large extent and composed of clay, serpentine or chlorite, represent signifi-
cant rock mass defects with low shear strength.
3.2.4
Weathering of volcanic rocks
Although all unaltered volcanic rocks are highly durable within the life-span of normal
engineering structures, the more basic varieties, particularly basalt, are quite susceptible
to chemical weathering in a geological time frame. The distribution of weathered materi-
als in volcanic rocks is governed by the distribution of any previously altered material, as
well as by the pattern of joints and vesicular zones.
When extremely weathered, all volcanic rocks are clayey soils in the engineering sense.
The acidic types tend to produce low or medium plasticity clays and the basic types high
plasticity clays. Basalts commonly display spheroidal weathering profiles, with spheroids
of fresh to distinctly weathered basalt surrounded by extremely weathered basalt, which
is clay of high plasticity. Extremely weathered basalts and the surface residual soils devel-
oped on them are usually highly expansive and fissured.
3.2.5
Landsliding on slopes underlain by weathered basalt
The presence of a well developed pattern of slickensided fissures causes the shear strength
of the mass to be significantly lower than that of the intact material. Slopes steeper than
about 10 degrees which are underlain by such materials often show geomorphological
evidence of past or current landsliding.
Such landsliding occurs commonly at the steep margins of plateaus or hills capped by
basalt flows which overlie old weathered land surfaces, as shown in
Figure 3.9
.
The slid-
ing occurs in some cases simply as a result of over-steepening of the hillside by erosion,
and in others due to pressure from groundwater exiting from a permeable zone beneath
the extremely weathered basalt. The permeable zone may be jointed, less weathered basalt
as in Figure 3.9, or alluvial sands or gravels on the old buried land surface. Examples are
given in Fell (1992).
