I have tested point load strength of a gneissic rock and two loading directions were parallel and perpendicular to foliation plain. The rock included fresh to highly weathered samples. In each loading directions, the difference of point load strength between each weathering grade (eg Fresh to slightly, slightly to moderately, so on) is decreasing with increasing weathering grade.
I suppose it may be due to uniform weathering of the rock. Would there be any other explanation to this?
Thank you
The rock weathers from the outside to the inside as well along discontinuities like faults or joints. This leads to a weakening and loosening of the rock structure. With increasing weathering grade the anisotropy of the gneissic rock decreases.
With weathering the strength ought to decrease because of rock softening etc. Now you can compare the decrease in strength at various points parallel to the foliation plane and at various test points perpendicular to foliation plane to confirm your findings on uniform weathering in both the directions. Furthermore, you may also like to compare the decrease in strength values when measured parallel and perpendicular to the foliation plane to ascertain the directional impact. This will help in design of structures in such rocks..
The answer by Cristian Gunter is comletely true. The anisotropy for fresh rock is quite higher, while th degradation of minerals and structure for the highly weathered samples leads to minimising of foliation plane influences. I have similar experiences with other types of foliated metamorphic rocks.
I am agree with Dr. Milorad Jovanovski, Dr. Piyush Rai, and Dr. Christian Günther. Based on my experiences, i think there are no uniform weathering in sedimenatry rock because any laminated rock, they have some different minerals inside the lamination plane. But, u can see on the micro and microcracks inside the rock especially Grade III,IV, and V. try to look at the effective porosity and the minerals alteration especially the presence of clay (due to decomposition of montmorillonite, illite, kaolinite etc.).
I also confirm the explanations of Christian Günther and Milorad Jovanovski. I would suggest to analyze the variation of the anisotropy index ( Ia = Is# / Is// ) with increasing weathering degree. I expect that with increasing weathering degree the anisotropy index will decrease: this will confirm the explanation of Christian Günther. Even the type of fractures forming during the analysis should be more and more undefined with increasing weathering degree (thus the samples do not tend to break along preferred planes anymore but rather in several irregular pieces).
Unweathered gneiss has an strength anisotropy coefficeinet that varies between 1.5 and 2. However, weathering leads to the homoginity in the geniss. It is quite logical that you get uniformity in the test results of weathered sample.
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