Engineering Geology

E n g i n e e r i n g G e o l o g y
The degree of resistance of sandstone to weathering depends on its mineralogical composition,
porosity, amount and type of cement and the presence of any planes of weakness such as
lamination. Sandstones are composed chiefly of quartz grains that are highly resistant to weathering,
but other minerals present in lesser amounts may be suspect, for example, feldspars may
become kaolinized. Calcareous cements are vulnerable to attack by weak acids. The durability
of weak sandstones, in particular, frequently is suspect. For instance, Yates (1992) referred to
certain sandstones from the Sherwood Sandstone Group, Stokes-on-Trent, England, which
often disaggregated on saturation. In fact, such sandstones had a saturated unconfined compressive
strength, when determined, of less than 0.5 MPa. The clay mineral content of sandstone
can affect its durability. Bell and Culshaw (1998), for example, found a significant
relationship between clay fraction and slake durability in the sandstones from the Sneinton
Formation. The clay fraction ranged from 4 to 18%. Although most of these sandstones had
a high or very high slake durability index, it declined with increasing clay fraction.
Mudrocks
Mudrocks include those rocks that possess a modal grain size within the silt and/or clay grade.
In other words, mudrocks contain more than 50% clastic grains of less than 60 mm size, and
hence their geomechanical behaviour is dominated by the fine material. Therefore, they include
siltstones, shales, mudstones and claystones. In terms of mineralogy, the clay minerals and
quartz are quantitatively the most important, and the quartz–clay minerals ratio influences their
geotechnical properties. For example, the liquid limit of clay shales increases with increasing
clay mineral content, the amount of montmorillonite, if present, being especially important.
Because of their frequent high clay mineral content, mudrocks may possess weak strength, and
their durability may be suspect. Therefore, they can prove problematic as regards engineering.
Shales are characterized by their fissility. Consolidation with concomitant recrystallization on
the one hand and the parallel orientation of platey minerals, notably micas, on the other give
rise to the fissility of shales. An increasing content of siliceous or calcareous material gives
less fissile shale, whereas carbonaceous or organic shales are exceptionally fissile.
Moderate weathering increases the fissility of shale by partially removing the cementing
agents along the laminations or by expansion due to the hydration of clay particles.
The natural moisture content of shales varies from less than 5%, increasing to as high as
35% for some clayey shales (Table 5.31). When the natural moisture content of shales
exceeds 20%, they frequently are suspect as they tend to develop potentially high pore water
pressures. Usually, the moisture content in the weathered zone is higher than in the unweathered
shale beneath. Depending on the relative humidity, many shales slake almost immediately
when exposed to air. Desiccation of shale following exposure leads to the creation of
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