Defect in Unweathered Bedrocks
The foundation engineer is commonly not seriously troubled by structural defects in the bedrock. However, certain types of rock deverse careful scrutiny.
Limestone and, to a less extent, dolomite are soluble in groundwater. Strata of these rocks may contain large channels, often filled with clay, or even extensive caverns. The absence of surface drainage or the presence of shink holes in limestone region is indicate of solution cavities. Such cavities may endanger the stability of foundations and must be located.
Shales are often relatively compressible and commonly contain partings along which horizontal movements readly occur. Special precautions may be required to provide for lateral forces exerted by the foundations of retaining walls and bridge abutments. The distinction between stiff clay and shale is somewhat arbitrary, and swelling clays may be mistaken for soft shales. Under some condotions, laboratory tests may be needed for proper identification.
Occasionally bridge and building foundations must be located in fault zones where excavations encounter strongly upturned or jointed strata. It is always desirable to obtain at least a rough conseption of the general geology in the vicinity and the special geological fatures at the site if rock excavation is contemplated or if foundations are to rest on rock.
In mineralized regions, mining operations or production by solution from wells may have to removal or weakening of some bedrock strata. This possibility requires investigation, and particular attention should be given to the detection of long-abandoned workings.
Processes of Weathering
The oldest rocks exposed on the face of the earth appear to be metamorphosed sediment. They have been extensively deformed by heat and pessure, and they have in places recrystallized into granite and other typical igneous rocks. Yet, their sedimentary origin bespeaks the effectiveness of the agents of weathering throughout geological time, inasmuch as these ancient rocks are composed of the products of weathering of still older rocks.
Some of the agents of weathering are purely physical. One of the most important of these is the differential expansion and contracction that arises on account of heating and cooling masses of rock containing minerals of different thermal properties. Other are the abrasive action of wind and water, esspecially when these agents are transporting sediments that aid the grinding action; these expansive force of freezing ; and in many localities, esspecially in the past, the plucking and grinding action of glaciers. Important as these physically agents are, however, they are not capable of reducing the size of individual fragments below about 0.01 mm. More complete disintegration can be accomplished only by chemical processes.
The prncipal agents of chemical weathering are oxidation, hydration, carbonation, and solution. Almost all materials combine with oxygen, esspecially in humid climates, but iron compounds are particularly susceptible. The combination with oxygen is usually associated with increased volume, whereupon the oxidized portion is likely to split away from the unaltered material. Similarly, the increse in volume accompanying the formation of new minerals by hydration is responsible for disintegration of the original minerals. Falling rain dissolves small quantities of carbon dioxide from that atmosphere and thus becomes a weak solution of carboni acid. The chemical combination of certain minerals with carbonic acid leads to the formation of softer and weaker minerals ; more important, however, is the fact that weak carbonic acid is a much more effective solvent than pure water. It attracks not only materials containing calcium carbonate but also those containing silicates.
As a rule, physical weathering predominates in frigid or dry climates, whereas chemical weathering is more important in warm humid climates. Plant and animal life contribut to both processes. However, rocks may also be subjected the chemical decomposition when far below the surface, because of the sirculation of magmatic waters charged with chemicals. Granites and gneisses thus may locally have been transformed into soft rocks of claylike character, and hard quartzites into cohesionless sand.
When the processes of weathering have eliminated the cohesive bonds between most of constituents of the rock, the material become soil. Yet, even after this state has been reached, further weathering occurs. Ultimately, the disintegration may be so complete that only certain substances, they clay minerals, are present.
Source: Foundation Engineering. Ralph B. Peck, Walter E. Hanson, Thomas H. Thorburn. Modern Asia Editions