Rock is classified into hard rock, moderate rock, soft rock, weathered rock based on the degree of weathering. However, this standard does not reflect the changes due to weathering process because it is qualitative standard based on visual observation. Also, the ground investigation at the time of designing is based on the condition at the time of construction which often fails to reflect the decrease in the durability of the rock during maintenance. The accurate information on rock characteristics at the time of construction and also for maintenance is very significant.
The geo-structures construction including the road, railroad, urban railroad, and urban area is increased with advanced excavation technology. But accidents by collapse due to uncertainties of rock is increasing as well. Based on the investigation these accidents, most of the damages caused by collapse take place on sedimentary rock rather than granite. Unfortunately, however, there are only few studies conducted on the weathering of sedimentary rock.
The clastic sedimentary rocks are classified into sandstone, mudstone, and shale based on their deposition environment, and they have different types of lamination depending on the grain size as well as the arrangement pattern of composition particle of their rocks. The lamination causes anisotropy of mudstone and shale. Based on the test results obtained from magnetic susceptibility test, sandstone showed just less than 5% of anisotropy, but mudstone and shale showed about 20% of anisotropy. The anisotropy of mudstone and shale was higher than that of rock at fault zone in Kyungsang basin.
Weathering indices were estimated using whole rock analysis(X-ray fluorescence analysis) on clastic sedimentary rocks. In the cases of clastic sedimentary rocks, the correlation of weathering indices based on feldspar weathering was higher than weathering indices using Si. The indices with significant correlation in sandstone, mudstone, and shale (all three kinds of samples) were CIA and CIW in the weathering indices correlation analysis. Especially CIA, the most widely used weathering index of feldspar, was easy to apply as it suggests the index range of rock types and minerals, and easy to verify the weathering pathway. In addition, to make the accurate measure of weathering indices of clastic sedimentary rocks, samples with a high volume of carbonate minerals should be excluded through naked eye examination and X-ray diffraction analysis.
The weathering indices of sandstone, mudstone and shale indicated linear increase through weathering process. The weathering pathway followed from fresh sedimentary rock to illite and then kaoline. This indicated that the engineering problem of the mudstone and shale was related to their low critical load. Each rock had its unique critical load and when a rock reaches its critical load, its physical strength decreased and the rock property changed from the designing condition.
Accelerated weathering tests(7days which indicates 49years)through soaking in acid water and freezing-thawing tests changed the properties of hard and soft rocks of sandstone, mudstone and shale; seismic velocity decreased, absorption ratio increased, unconfined compressive strength decreased with weathering. Change of those properties in mudstone and shale were significantly varied compared to the sandstone. It was closely related to anisotropy magnitude of each rock material.