| Baiihetan Hydropower station in China, following the Three Gorges, Xiluodu hydropower station, is another one million kilowatts hydropower station under survey and design. During the survey, the phenomenon of deep differential weathering is found inside the light weathered- unaltered rock. It is of great significance to analyse its formation mechanism and the rock availability in the research of high-slope stability of Baiihetan Hydropower station.Based on referring to previous study materials,by investigating in field the deep weathering in different adit, combining the analysis of mineral element and chemical element, following the thought"geologic process mechanism analysis", based upon the geologic conditions and combining the tectonic evolution and valley evolution, it expounds the distribution of development of deep differential weathering, ananlyses the factors that effect its distribution of development and the genetic mechamism of diverse positional deep differential weathering, which provides basis for availability analysis of rock mass. Following are the obtained achievements:(1)The distribution of development of deep differential weathering in dam area presents ellipsoid type in space, that is, fewness in both ends and much in middle from upper to lower of Jinsha river.The elevation also has the same regulation. The boundary is F14 on the lelf bank of upper river andâ…¡3 at lower river; F16 on the right bank of upper-iver and F19 at lower river. On the exploration line, it lies between lineâ… and lineâ…©1. For elevation it lies in middle and high elevation about 720m-780m. Its buried depth is in the area away from the bank slope for 100-140m. The extended length is from several meters to several tens meters, even 87m. It develops largerly in the columnar joint basalt, aphanitic basalt and breccia lava far from fault zone for 0-32m, and this phenomenon of left bank is more intense than that of the right bank.(2)The deep differential weathering phenomenon appears as following: moderate to serious rust along staggered zone, fault, structural plane of steep-gentle inclination and lithologic contact zone; rough undulationof structural plane; filled cuttings; calcium-film and a little mud; cataclastic-mosaic structure; dripping-seepage; partial moisture-dripping and seepage; the place of weathering aggreviation between fault zone and lithologic contact zone within 5-30cm.(3)The basalt in the dam area belongs to high-Ti, Si-rich and Al-rich weak alkali-alkali basalt. Compared with slightly new rock, theâ–³SiO2/â–³Al2O3 at differential weathering area is 4.35, and chemical weathering is more intense. Choosing the quantitative index characterizing weathering features to evaluate differential weathering rock, its wave velocity, resilience and RQD are respectively 3720-4480 m/s,31.2-44.11Mpa,0.453-0.59, so it is between weak upper weathering section and weak lower section, and the main is weak upper one.To sum up, the deep differential weathering can be divided into deep differential weathering near fault zone (including deep differential weathering controlled by staggered zone, fault, structural plane of steep-gentle inclination or near lithologic contact zone) and that controlled by lithology.(4) The formation process of deep differential weathering is following: on the basis of rock ingredient difference, during the vertical unloading and lateral unloading, under the structural plane condition of appearance of latent tiny fissure, groundwater as the main weathering stress, matter in fault zone continuously intenerates, fissure further opens and transfixes, then the structural plane weathering constantly becomes rock weathering, thus the deep differential weathering comes into being.Differrent deep differential weathering has different cause. The one controlled by staggered zone depends mainly on the forward displacement, creation of closely spaced fissure zones, component of cuttings and infiltrate mud substance and the disorderly arrangement of these component. The one effected by fault have closely relation with early structural movement and fissure's further opening. The one controlled by structural plane of steep-gentle inclination is closely related to gentle inclination structural plane developed from basic plane, the intersection and transfixion of steep inclination structural plane and the character of rough structural plane. The one controlled by lithology means chiefly that intense interaction happens between lava and cement in the process of diagenesis and weathering aggravates at lithologic contact zone.(5) The deep differential weathering decreases to some extent the mechanical parameters of rock, and makes the quality of rock mass fall toâ…¢2 degree. However, in the scope away from bank slope for the distance of 100-140m, Its extended length, depth and influence scope are limited, and it is not regional large fault. As a result, pertinent disposal will largely improve the quality of project and not threaten the stability of rock in dam area.
|
PDF Full Text Request