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[SCG58-P19] Changes in physical properties associated with weathering of granite at Mt. Gagara, Hiroshima
Keywords:Weathering, Granite, Hardness, Permeability, Elastic wave velocity, Porosity
Granitic rocks are widely distributed in Hiroshima Prefecture. Many of these are weathered and brittle, often causing slope failure. We analyzed the unweathered to weathered portions of borehole cores drilled in the Hiroshima Gagara Mountains in order to understand the changes in physical properties of granite during weathering.
The borehole cores analyzed were drilled to a depth of 20 m below the ground surface. First, hardness was measured at approximately 10 cm intervals in the direction of core depth using Yamanaka soil hardness tester and a rebound hardness tester. Based on the hardness measurements, core samples and polished samples were prepared at five different depths (2.8 m, 5.2 m, 6.1 m, 7.5 m, and 9.3 m), each with a different degree of weathering. Using these specimens, scanning electron microscopy, modal composition analysis, hardness, solid density, bulk density, open porosity, total porosity, permeability, elastic wave velocity (Vp, Vs), electrical conductivity, and crack density were measured. For modal composition analysis, 1,200 point counts were performed over an area of approximately 12 mm x 16 mm. Solid density was measured by the pycnometer method. Open porosity was determined from the difference between water saturated weight and dry weight. Permeability was measured by a water permeability test (for low permeability) or an air permeability test (for high permeability). The elastic wave velocity was obtained from the time taken for the waves generated by a function generator to pass through the sample.
The unweathered part of the granite consists of about 32% quartz, 32% plagioclase, 32% potassium feldspar, and 4% biotite. Based on borehole core observations, the 0-2.5 m depth region was excluded from the analysis because it was assumed to be a sedimentary layer not directly related to the weathering profile of the granite. At depths of 2.5-5 m, hardness was generally low and considered to be well weathered. From 5 m to 8.3 m depth, hardness increased with depth. The hardness remained almost constant at depths greater than 8.3 m and was considered to be the unweathered zone. The hardness of the unweathered portion was about six times greater than the most weathered portion. For 5-8.3 m depths, bulk density and elastic wave velocity increased with depth, while open porosity, total porosity, electrical conductivity, crack density, and permeability decreased with depth. In particular, the total porosity differed by a factor of 22 and the permeability by a factor of 3 × 107 between the unweathered and the most weathered portions. The differences in open porosity and total porosity decreased with increasing weathering. These changes may indicate that with the progress of weathering, the dissolution of minerals progressed and the bonds between mineral grains were broken and the connection of pore spaces improved. The change in permeability was particularly large in the 5-6 m depth range, while the permeability was much smaller at depths greater than 6 m, suggesting that there may be differences in the velocity and direction of water flow around the 5-6 m depth range. As a whole, hardness showed a good correlation with each property. The present study provides a rough prediction equation for each property using hardness, which can be easily measured.
The borehole cores analyzed were drilled to a depth of 20 m below the ground surface. First, hardness was measured at approximately 10 cm intervals in the direction of core depth using Yamanaka soil hardness tester and a rebound hardness tester. Based on the hardness measurements, core samples and polished samples were prepared at five different depths (2.8 m, 5.2 m, 6.1 m, 7.5 m, and 9.3 m), each with a different degree of weathering. Using these specimens, scanning electron microscopy, modal composition analysis, hardness, solid density, bulk density, open porosity, total porosity, permeability, elastic wave velocity (Vp, Vs), electrical conductivity, and crack density were measured. For modal composition analysis, 1,200 point counts were performed over an area of approximately 12 mm x 16 mm. Solid density was measured by the pycnometer method. Open porosity was determined from the difference between water saturated weight and dry weight. Permeability was measured by a water permeability test (for low permeability) or an air permeability test (for high permeability). The elastic wave velocity was obtained from the time taken for the waves generated by a function generator to pass through the sample.
The unweathered part of the granite consists of about 32% quartz, 32% plagioclase, 32% potassium feldspar, and 4% biotite. Based on borehole core observations, the 0-2.5 m depth region was excluded from the analysis because it was assumed to be a sedimentary layer not directly related to the weathering profile of the granite. At depths of 2.5-5 m, hardness was generally low and considered to be well weathered. From 5 m to 8.3 m depth, hardness increased with depth. The hardness remained almost constant at depths greater than 8.3 m and was considered to be the unweathered zone. The hardness of the unweathered portion was about six times greater than the most weathered portion. For 5-8.3 m depths, bulk density and elastic wave velocity increased with depth, while open porosity, total porosity, electrical conductivity, crack density, and permeability decreased with depth. In particular, the total porosity differed by a factor of 22 and the permeability by a factor of 3 × 107 between the unweathered and the most weathered portions. The differences in open porosity and total porosity decreased with increasing weathering. These changes may indicate that with the progress of weathering, the dissolution of minerals progressed and the bonds between mineral grains were broken and the connection of pore spaces improved. The change in permeability was particularly large in the 5-6 m depth range, while the permeability was much smaller at depths greater than 6 m, suggesting that there may be differences in the velocity and direction of water flow around the 5-6 m depth range. As a whole, hardness showed a good correlation with each property. The present study provides a rough prediction equation for each property using hardness, which can be easily measured.