Deep weathering

Extensive, horizontal sandstone plateaus occur in tropical shield areas. Well-known examples are the Precambrian Roraima sandstone formations on the Guiana Shield and the Voltaian sandstone formations in Western Africa. Major occurrences of consolidated sands are found in Northern Africa, in Guyana and Surinam, eastern Peru, northeastern Brazil and in Liberia (western Africa). These sandstone formations have a history of tropical weathering in common they all have a deep weathering mantle of bleached, white sands that are very rich in quartz, poor in clay and excessively drained. Electrolyte contents differ by region In arid and semi-arid areas where evaporation exceeds precipitation, salts and carbonates may accumulate at or near the surface of the soil. 

Many exposed BIF sequences have been subjected to deep weathering and metamorphism, where hematite and goethite are, for the most part, secondary alteration products (e.g., Beukes and Klein 1992). In a few cases, however, primary hematite is found as finely disseminated... 

Because UV radiation does not penetrate wood more than a few cells deep, weathering is considered a surface phenomenon. Over time, it can account for a significant loss in surface fiber (Figure 18). As the degradative process continues, the loss in fiber may eventually cause a reduction in the material s load-carrying capacity. 

Two hidden assumptions implicit in the model of Sato and Mooney (1960) are (1) that the conductor consists of a single phase, such as graphite or pyrite and (2) that oxidation of the conductor would result in its conversion to a non-conductive phase. Thomber (1975a, 1975b) presents a reactive conductor model in which the conductor itself is the reducing agent, which is in apparent contrast to the model of Sato and Mooney. However, the reactive conductor model is based on the presence of one oxidised phase and at least one reduced phase relative to the first phase, all of which are electronically conductive. Such scenarios have been noted in terrain with deep weathering profiles due to the phase conversion of reduced sulphide minerals to more oxidised forms. 

Benbow, M.C., Callen, R.A., Bourman, R.P. Alley, N.F. (1995) Deep weathering, ferricrete and silcrete. In Drexel, J.F. Preiss, W.V. (Eds) The Geology of South Australia, Volume 2 the Phanerozoic. Bulletin 54, Geological Survey of South Australia, pp. 201-207. 

Gilkes, R.J., Scholz, G. Dimmock, G.M. (1973) Lateritic deep weathering of granite. Journal of Soil Science 24, 523-536. 

Dury, G.H. Knox, J.C. (1971) Duricrusts and deep-weathering profiles in southwestern Wisconsin. Science 174, 291-292. 

DEEP WEATHERING AND ALTERATION IN GRANITES - A PRODUCT OF COUPLED... 

Drilled shafts, which are also known as drilled piers, or more commonly referred to as bored piles in Malaysia, have been found to be suitable in urban areas not only because of environment requirements but also to cater for the deep weathered formation. For heavier loads and larger diameters drilled shaft (up to 1.5 m), equipments have been developed to core suitable depths in underlying rock formations to form the pile socket (Ting et al. 2004). Hand dug caissons are sometimes used in steep terrain with limited space. 

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