1:45 PM - 3:15 PM
[HCG21-P06] Relation between 1m-interval density of boring core and rock grade classification, an example from the Monju site
Keywords:density, rock classification, granite
The rock density or the unit weight of rocks decrease in crush zone along faults or in landslide bodies (e.g., Ikeda et al., 2001; Yamada et al., 2011). Weight measurement of rock cores with wooden case packing several pieces of 1m cores at site can be used for practical sequential rock density logging (Japan Society of Dam Engineers, 2012). We retrieved 2 boring cores of basement rocks of granite from the Monju (prototype FBR at the Tsuruga city, the shore on the Sea of Japan) site. Drilling points are at about 1km from the surface trace of an active fault. Excavation lengths are 100m and 200m. The breakdown of 300m is 251m of granite and 49m of overlying layer. Objectives of the drilling are presence check of weak zones that could became as slip plane for slope failure, and large crush zones. Because of the presence of these zones can be problems for large-building constructions, we were asked for prompt confirmations. We tried, therefore, to measure the weight of each 1m core of granite during geological classification of the rock grades.
We measured the weight (kg) of the 1m core placed on the half-split PVC pipe to the first decimal place and subtracted the weight of the half-split pipe (0.7kg). The core weight was divided by the core volume (assumed dimensions of diameter 6cm and length 1m). Rock classification (class B, CH, CM, CL, D and crush zones are recorded as clay zones Nc (mainly composed of clay), Nb (breccia with clay), and Nj (many joints with thin layers of clay)) is based on combinations of properties of dimension of fractured piece, state of fracture plane, qualitative hardness, and clay distribution. Rock class of each 1m core is represented by the class occupies over 75cm in each, i.e., data of the wet density for the core composed of shorter classified rocks under 75cm is not used. Total 147m density-rock class data was obtained as follows, classes of B; 79m, CH;24m, CM;41m, CL; 3m. There are no class D rocks. Crush zones with the length under 75cm are observed in the classes of CM or CL. Cumulative length of each rock class in the total 251m is as follows, classes B; 108m, CH; 57m, CM;73m, CL;13m.
The average wet density and data range of each rock classes as follows, classes B: wet density 2.53 (g/cm3, same as below); range 0.10, CH: 2.51; 0.11, CM: 2.48; 0.42, CL: 2.42; 0.11. The density gradually increases from classes CL to B. The range of the class CM is rather wide probably due to mixture of highly-fractured harder core and longer (non-separated) softer (weathered) core. The decreased density (less than 2.42 g/cm3) continuous zone of 5m (44-49m in depth of 100m hole) is corresponds to brecciated deterioration. This low-density zone is, however, shorter than the example of an active fault which possesses a few decameter scale zone (Ikeda et al., 2001; read from the diagram), and discontinuous to the next 200m hole (distance between the two holes is about 30m). Therefore, the data identified do not show any large-scale weaknesses that would pose an immediate problem for the construction of heavy structures. Continuous weight measurement of 1m core samples is relatively simple, and it is hoped that it will spread toward the rationalization of geological analysis.
References: Japan Soc. Dam Engineers, 2012, Introduction to Geotechnical Investigation and Evaluation of Rockmass: Manual of Geotechnical Boring and Investigation for Dam Engineers, 529p (in Japanese)., Ikeda, R., et al., 2001, Report of NIED, 61, 141-153 (in Japanese with English abstract). Yamada, M., et al., 2011, https://www.zenchiren.or.jp/e-Forum/2011/084.PDF (in Japanese)
We measured the weight (kg) of the 1m core placed on the half-split PVC pipe to the first decimal place and subtracted the weight of the half-split pipe (0.7kg). The core weight was divided by the core volume (assumed dimensions of diameter 6cm and length 1m). Rock classification (class B, CH, CM, CL, D and crush zones are recorded as clay zones Nc (mainly composed of clay), Nb (breccia with clay), and Nj (many joints with thin layers of clay)) is based on combinations of properties of dimension of fractured piece, state of fracture plane, qualitative hardness, and clay distribution. Rock class of each 1m core is represented by the class occupies over 75cm in each, i.e., data of the wet density for the core composed of shorter classified rocks under 75cm is not used. Total 147m density-rock class data was obtained as follows, classes of B; 79m, CH;24m, CM;41m, CL; 3m. There are no class D rocks. Crush zones with the length under 75cm are observed in the classes of CM or CL. Cumulative length of each rock class in the total 251m is as follows, classes B; 108m, CH; 57m, CM;73m, CL;13m.
The average wet density and data range of each rock classes as follows, classes B: wet density 2.53 (g/cm3, same as below); range 0.10, CH: 2.51; 0.11, CM: 2.48; 0.42, CL: 2.42; 0.11. The density gradually increases from classes CL to B. The range of the class CM is rather wide probably due to mixture of highly-fractured harder core and longer (non-separated) softer (weathered) core. The decreased density (less than 2.42 g/cm3) continuous zone of 5m (44-49m in depth of 100m hole) is corresponds to brecciated deterioration. This low-density zone is, however, shorter than the example of an active fault which possesses a few decameter scale zone (Ikeda et al., 2001; read from the diagram), and discontinuous to the next 200m hole (distance between the two holes is about 30m). Therefore, the data identified do not show any large-scale weaknesses that would pose an immediate problem for the construction of heavy structures. Continuous weight measurement of 1m core samples is relatively simple, and it is hoped that it will spread toward the rationalization of geological analysis.
References: Japan Soc. Dam Engineers, 2012, Introduction to Geotechnical Investigation and Evaluation of Rockmass: Manual of Geotechnical Boring and Investigation for Dam Engineers, 529p (in Japanese)., Ikeda, R., et al., 2001, Report of NIED, 61, 141-153 (in Japanese with English abstract). Yamada, M., et al., 2011, https://www.zenchiren.or.jp/e-Forum/2011/084.PDF (in Japanese)