Hiroyuki Maeda

Estimate of Uniaxial Compressive Strength of Hydrothermally Altered Soft Rocks Based on Strength Index Tests

The purpose of this study was to clarify the relationships between results of index tests and uniaxial compressive strength (UCS) in hydrothermally altered soft rocks of the Upper Miocene, which are typical of the soft rock found in northeastern Hokkaido, Japan. Index tests were performed using point load testing machine and needle penetrometer with irregular lump specimens under forced-dry, forced-wet, and natural-moist states. The relationships between irregular lump point load strength (IPLS) index and UCS, and needle penetration (NP) index and UCS were “UCS = approximately 19 IPLS index” and “UCS = 0.848 (NP index)0.619”, respectively, in soft rocks with a UCS below 25 MPa. These relationships could be applied to on-site tests of rocks with natural moisture content. The UCS could be calculated from IPLS and NP tests on soft rocks only when UCS was below 25 MPa, using the equations obtained as a result of this study.

Slip surface clay in a greenstone landslide: An example of the Hokuyo A-160 Slide occurred within the Nikoro Group greenstone in the Tokoro Belt, Hokkaido, Japan@@@－北海道常呂帯仁頃層群緑色岩を基岩とする北陽A-160地すべりの例－

The Hokuyo A―160Slide is one ancient greenstone landslide occurred at the Hokuyo-Toyomi landslide-prone area in Kitami City, northeastern Hokkaido, Japan, and is a large scale having approximately250m in width, approximately300m in length, and approximately30m in thickness. The slide is a weathered and hydrothermally altered rockslide, and the slide body consists primarily of weathered and hydrothermally altered rocks of accretionary greenstone and small amounts of their debris and earth. The inferred slip surface clay at depth of21.25m in core of a borehole excavated in the middle part of the slide body is tan in color, has shear planes, and is primarily composed of smectite and vermiculite, with lesser albite, laumontite, and goethite. Because this tan thin clay layer rich in swelling clay minerals such as smectite and vermiculite is considered to be the hydrothermal alteration and weathering products of the fault gouge in the crush zone based on the mode of occurrence and properties, this suggests a close relationship between the slide and the hydrothermal alteration and weathering of the fault gouge and fault breccia in the crush zone of greenstone.